Achondroplasia

Mendelian MONDO:0007037 Pathograph 36 Show in embeddings browser FGFR3-related skeletal dysplasia Rhizomelic limb shortening syndrome

Achondroplasia is the most common cause of disproportionate short stature. It is caused by heterozygous gain-of-function variation in FGFR3, almost always p.Gly380Arg, which prolongs inhibitory FGFR3 signaling in growth-plate chondrocytes and impairs endochondral ossification. Rhizomelic limb shortening, macrocephaly, frontal bossing, midface retrusion, and trident hands are characteristic. Important complications include craniocervical-junction compression in infancy, sleep-disordered breathing, middle-ear dysfunction, limb bowing, obesity, and symptomatic spinal stenosis in adulthood. Daily vosoritide and once-weekly navepegritide increase growth velocity in eligible children with open epiphyses; both US indications use annualized growth velocity as an accelerated-approval endpoint, so final-height and complication-prevention benefits remain under study.

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Classifications

ISDS Skeletal Nosology

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Inheritance

Autosomal dominant HP:0000006

Achondroplasia has complete-penetrance autosomal-dominant inheritance. About 80% of affected individuals have average-stature parents and a de novo FGFR3 variant. In informative sporadic cases, the recurrent variant arose on the paternal chromosome; increased paternal age is an established association.

Autosomal dominant inheritance Penetrance: COMPLETE De novo rate: approximately 80%

Parent-of-origin effect: De novo variants are predominantly paternal in origin and associated with increased paternal age

Show evidence (5 references)

PMID:8078586 SUPPORT Human Clinical

"Affected individuals are fertile and achondroplasia is transmitted as a fully penetrant autosomal dominant trait, accounting for rare familial forms of the disease (10%)."

Establishes autosomal dominant inheritance with full penetrance for achondroplasia.

PMID:8078586 SUPPORT Human Clinical

"More than 90% of cases are sporadic and there is an increased paternal age at the time of conception of affected individuals,"

An older series documents a high sporadic fraction and paternal-age association; the structured estimate is calibrated to the current approximately 80% estimate.

url:https://www.ncbi.nlm.nih.gov/books/NBK1152/ SUPPORT Other

"Approximately 80% of individuals with achondroplasia have parents of average stature and have achondroplasia as the result of a"

The 2026 GeneReviews revision gives the current approximately 80% de novo estimate.

+ 2 more references

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Mechanistic Hypotheses

FGFR3 Growth-Plate Signaling Model

canonical_fgfr3_gain_of_function_chondrocyte_model CANONICAL

Evidence balance 3 support

Recurrent heterozygous FGFR3 p.Gly380Arg gain-of-function prolongs receptor signaling in growth-plate chondrocytes. Experiments support delayed receptor down-regulation while G380R activation remains predominantly ligand dependent. MAPK-mediated inhibition of hypertrophic differentiation and a qualified STAT-p21 branch impair endochondral ossification.

Strict constitutive activation is not asserted as the sole receptor-level mechanism. CNP-analogue growth-velocity responses support therapeutic counter-regulation but do not establish final-height or complication benefit.

Pathograph links

FGFR3 p.Gly380Arg gain-of-function with impaired receptor down-regulation -> Prolonged FGFR3 signaling in growth-plate chondrocytes DIRECT Delayed down-regulation expands mutant-receptor signaling in growth plate.

Prolonged FGFR3 signaling in growth-plate chondrocytes -> Growth-plate chondrocyte dysregulation DIRECT Prolonged signaling engages MAPK and STAT-p21 growth-inhibitory programs.

Growth-plate chondrocyte dysregulation -> Impaired endochondral ossification and chondrodysplasia DIRECT Dysregulated growth-plate maturation impairs endochondral bone growth.

Impaired endochondral ossification and chondrodysplasia -> Reduced longitudinal growth of endochondral bones DIRECT Impaired hypertrophic differentiation reduces longitudinal growth.

Impaired endochondral ossification and chondrodysplasia -> Cranial-base and vertebral synchondrosis dysplasia UNKNOWN The endochondral process affects synchondroses, but ordinary-heterozygous causal evidence is incomplete.

Impaired endochondral ossification and chondrodysplasia -> Congenital spinal-canal constraint with superimposed degeneration UNKNOWN Impaired vertebral growth plausibly contributes to congenital constraint; adult degeneration adds a second component.

Impaired endochondral ossification and chondrodysplasia -> Genu varum UNKNOWN Genu varum is observed during childhood in the chondrodysplasia; the specific growth-plate route is not established by the cited source.

Impaired endochondral ossification and chondrodysplasia -> Bowing of the legs UNKNOWN Lower-limb bowing overlaps clinically with genu varum; the specific growth-plate route is not established by the cited source.

Reduced longitudinal growth of endochondral bones -> Disproportionate short stature DIRECT Reduced longitudinal growth produces disproportionate stature.

Reduced longitudinal growth of endochondral bones -> Rhizomelia INDIRECT KNOWN INTERMEDIATES Proximal long-bone undergrowth produces rhizomelia. Intermediates: Preferential undergrowth of proximal long-bone segments

Reduced longitudinal growth of endochondral bones -> Limb undergrowth DIRECT Reduced longitudinal growth is expressed as limb undergrowth.

Reduced longitudinal growth of endochondral bones -> Brachydactyly INDIRECT KNOWN INTERMEDIATES Shortening of endochondral hand bones produces brachydactyly. Intermediates: Reduced longitudinal growth of metacarpals and phalanges

Reduced longitudinal growth of endochondral bones -> Trident hand INDIRECT KNOWN INTERMEDIATES Hand-bone shortening contributes to the trident configuration. Intermediates: Phalangeal shortening and separation of the third and fourth digits

Cranial-base and vertebral synchondrosis dysplasia -> Foramen magnum stenosis INDIRECT KNOWN INTERMEDIATES Skull-base growth restriction contributes to narrowing. Intermediates: Restricted endochondral growth of the skull base; Reduced bony dimensions of the foramen magnum

Cranial-base and vertebral synchondrosis dysplasia -> Jugular-foramen venous outflow impairment INDIRECT KNOWN INTERMEDIATES Skull-base undergrowth can narrow jugular foramina. Intermediates: Reduced skull-base dimensions; Jugular-foramen narrowing

Cranial-base and vertebral synchondrosis dysplasia -> Craniofacial and upper-airway restriction UNKNOWN Skull-base fusion co-occurs with retrusion; direction remains incompletely resolved.

Cranial-base and vertebral synchondrosis dysplasia -> Caudal interpedicular narrowing UNKNOWN Vertebral dysplasia is associated with the caudal interpedicular pattern.

Jugular-foramen venous outflow impairment -> Hydrocephalus INDIRECT KNOWN INTERMEDIATES Venous hypertension and impaired CSF resorption can produce hydrocephalus. Intermediates: Raised intracranial venous pressure; Impaired cerebrospinal-fluid resorption

Craniofacial and upper-airway restriction -> Midface retrusion UNKNOWN Midface retrusion is an anatomical component of restriction.

Craniofacial and upper-airway restriction -> Obstructive sleep apnea UNKNOWN Retrusion contributes to, but does not solely determine, obstruction.

Congenital spinal-canal constraint with superimposed degeneration -> Spinal canal stenosis DIRECT Congenital constraint and degeneration produce recognized stenosis.

Congenital spinal-canal constraint with superimposed degeneration -> Cervical spinal canal stenosis DIRECT The congenital component can affect the cervical canal.

Deep research

OpenScientist citations 29 citations 2026-05-23T16:17:13.173615

Show evidence (3 references)

PMID:10611230 SUPPORT In Vitro

"We found that dimerization and activation of the G380R mutant receptor are predominantly ligand dependent."

Direct receptor experiments qualify strict constitutive-activation shorthand.

PMID:10611230 SUPPORT In Vitro

"Metabolic pulse-chase experiments, cell surface labeling, and kinetics of uptake of radiolabeled ligand demonstrated a selective delay in the down-regulation of the mutant receptor."

Direct experiments identify delayed receptor down-regulation.

DOI:10.1038/s41574-021-00595-x SUPPORT Other

"The condition is caused by a common, recurring, gain-of-function mutation in FGFR3, the gene that encodes fibroblast growth factor receptor 3. This mutation leads to impaired endochondral ossification of the human skeleton."

International consensus links FGFR3 gain of function to impaired endochondral ossification.

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Pathophysiology

FGFR3 p.Gly380Arg gain-of-function with impaired receptor down-regulation

Nearly all individuals have one of two substitutions producing p.Gly380Arg. The receptor is gain of function, but experiments show predominantly ligand-dependent activation with delayed down-regulation and resistance to ligand-mediated internalization.

FGFR3 Growth-Plate Signaling Model

FGFR3 hgnc:3690

fibroblast growth factor receptor signaling pathway GO:0008543 ↑ INCREASED

transmembrane receptor protein tyrosine kinase activity GO:0004714 ↑ INCREASED

Downstream

Prolonged FGFR3 signaling in growth-plate chondrocytes Delayed down-regulation expands mutant-receptor signaling in growth plate.

Show evidence (3 references)

PMID:7847369 SUPPORT Human Clinical

"All but one, an atypical case, were found to have a glycine-to-arginine substitution at codon 380. Of these, 150 had a G-to-A transition at nt 1138, and 3 had a G-to-C transversion at this same position."

A 154-person series demonstrates the near-uniform recurrent p.Gly380Arg genotype.

PMID:10611230 SUPPORT In Vitro

"Moreover, this receptor was now resistant to ligand-mediated internalization, even at saturating ligand concentrations."

Directly supports impaired internalization.

CGGV:assertion_0adafa1b-e609-4b99-8e90-f73473b70771-2023-09-28T040000.000Z SUPPORT Other

ClinGen classifies the autosomal-dominant FGFR3 relationship as definitive.

Prolonged FGFR3 signaling in growth-plate chondrocytes

Delayed internalization expands mutant FGFR3 immunoreactivity in epiphyseal growth plates, where signal kinetics regulate chondrocyte maturation.

FGFR3 Growth-Plate Signaling Model

growth plate cartilage chondrocyte CL:1000217

MAPK cascade GO:0000165 ↑ INCREASED ERK1 and ERK2 cascade GO:0070371 ↑ INCREASED cell surface receptor signaling pathway via STAT GO:0097696 ↑ INCREASED

Downstream

Growth-plate chondrocyte dysregulation Prolonged signaling engages MAPK and STAT-p21 growth-inhibitory programs.

Show evidence (1 reference)

PMID:10611230 SUPPORT Model Organism

"transgenic mice expressing the human G380R mutant receptor under the mouse receptor transcriptional control demonstrated a markedly expanded area of FGFR3 immunoreactivity within their epiphyseal growth plates, compatible with an in vivo defect in receptor down-regulation."

Transgenic mice provide in vivo evidence for expanded receptor persistence.

Growth-plate chondrocyte dysregulation

MAPK inhibits hypertrophic differentiation in models. Human fetal growth plates support altered FGFR3/STAT/p21 expression and premature cell-cycle exit. STAT1 is not treated as sufficient to explain all growth inhibition.

FGFR3 Growth-Plate Signaling Model

growth plate cartilage chondrocyte CL:1000217 prehypertrophic chondrocyte CL:0020022

MAPK cascade GO:0000165 ↑ INCREASED chondrocyte differentiation GO:0002062 ↕ DYSREGULATED cell population proliferation GO:0008283 ↓ DECREASED

Downstream

Impaired endochondral ossification and chondrodysplasia Dysregulated growth-plate maturation impairs endochondral bone growth.

Show evidence (2 references)

PMID:14871928 SUPPORT Model Organism

"These observations indicate that the MAPK pathway inhibits hypertrophic differentiation of chondrocytes and negatively regulates bone growth without inhibiting chondrocyte proliferation."

Active MEK1 supports inhibition of hypertrophic differentiation by MAPK.

PMID:14751560 SUPPORT Human Clinical

"Immunohistochemical analysis of fetal growth plates showed a phenotype-related reduction of the collagen type X-positive hypertrophic zone."

Human fetal histology documents a reduced hypertrophic zone.

Impaired endochondral ossification and chondrodysplasia

Altered growth-plate maturation reduces the hypertrophic zone and impairs endochondral ossification. Anatomical consequences are represented through calibrated branches rather than an unsupported all-phenotype fan-out.

FGFR3 Growth-Plate Signaling Model

hypertrophic chondrocyte CL:0000743 columnar chondrocyte CL:0000744

endochondral ossification GO:0001958 ↓ DECREASED growth plate cartilage development GO:0003417 ↕ DYSREGULATED

Downstream

Reduced longitudinal growth of endochondral bones Impaired hypertrophic differentiation reduces longitudinal growth.

Cranial-base and vertebral synchondrosis dysplasia The endochondral process affects synchondroses, but ordinary-heterozygous causal evidence is incomplete.

Congenital spinal-canal constraint with superimposed degeneration Impaired vertebral growth plausibly contributes to congenital constraint; adult degeneration adds a second component.

Genu varum Genu varum is observed during childhood in the chondrodysplasia; the specific growth-plate route is not established by the cited source.

Bowing of the legs Lower-limb bowing overlaps clinically with genu varum; the specific growth-plate route is not established by the cited source.

Show evidence (1 reference)

DOI:10.1038/s41574-021-00595-x SUPPORT Other

"This mutation leads to impaired endochondral ossification of the human skeleton."

International consensus identifies the central consequence.

Reduced longitudinal growth of endochondral bones

Reduced long-bone growth produces disproportionate short stature, rhizomelic limb shortening, and characteristic hand shortening.

FGFR3 Growth-Plate Signaling Model

Downstream

Disproportionate short stature Reduced longitudinal growth produces disproportionate stature.

Rhizomelia Proximal long-bone undergrowth produces rhizomelia.

Limb undergrowth Reduced longitudinal growth is expressed as limb undergrowth.

Brachydactyly Shortening of endochondral hand bones produces brachydactyly.

Trident hand Hand-bone shortening contributes to the trident configuration.

Show evidence (1 reference)

url:https://www.ncbi.nlm.nih.gov/books/NBK1152/ SUPPORT Other

"Individuals with achondroplasia have short stature with rhizomelic shortening of the limbs, macrocephaly, characteristic facies with frontal bossing and midface retrusion, exaggerated lumbar lordosis, limitation of elbow extension and rotation, genu varum, brachydactyly, and trident appearance..."

The current review documents the characteristic pattern.

Cranial-base and vertebral synchondrosis dysplasia

FGFR3-MAPK activation promotes premature closure in models and severe human FGFR3 dysplasias. CT in heterozygous achondroplasia documents skull-base fusion and age-associated craniofacial severity.

FGFR3 Growth-Plate Signaling Model

growth plate cartilage chondrocyte CL:1000217

Downstream

Foramen magnum stenosis Skull-base growth restriction contributes to narrowing.

Jugular-foramen venous outflow impairment Skull-base undergrowth can narrow jugular foramina.

Craniofacial and upper-airway restriction Skull-base fusion co-occurs with retrusion; direction remains incompletely resolved.

Caudal interpedicular narrowing Vertebral dysplasia is associated with the caudal interpedicular pattern.

Show evidence (2 references)

PMID:18923003 SUPPORT Model Organism

"Chondrocyte-specific activation of Fgfr3 in mice induced premature synchondrosis closure and enhanced osteoblast differentiation around synchondroses."

Chondrocyte-specific activation establishes a causal model mechanism.

PMID:37072824 SUPPORT Human Clinical

"All patients with available CT-scan had premature fusion of skull base synchondroses."

CT documents skull-base fusion in a clinical cohort.

Jugular-foramen venous outflow impairment

Narrow jugular foramina can obstruct cerebral venous drainage, increasing venous pressure and impairing cerebrospinal-fluid resorption.

FGFR3 Growth-Plate Signaling Model

Downstream

Hydrocephalus Venous hypertension and impaired CSF resorption can produce hydrocephalus.

Show evidence (1 reference)

PMID:33579320 SUPPORT Human Clinical

"This condition causes a partial obstruction of venous flow, which increases intracranial venous pressure and causes problems with the venous resorption of cerebrospinal fluid (CSF)"

The study states the venous and CSF-resorption mechanism.

Craniofacial and upper-airway restriction

Midface and maxillomandibular retrusion reduce upper-airway dimensions; lymphoid hypertrophy, airway malacia, and hypotonia can add obstruction.

FGFR3 Growth-Plate Signaling Model

Downstream

Midface retrusion Midface retrusion is an anatomical component of restriction.

Obstructive sleep apnea Retrusion contributes to, but does not solely determine, obstruction.

Show evidence (1 reference)

PMID:37072824 SUPPORT Human Clinical

"We report a significant correlation between the severity of maxillo-mandibular retrusion and obstructive sleep apnea syndrome (p < 0.01)."

Clinical morphometry links retrusion with obstructive sleep apnea.

Congenital spinal-canal constraint with superimposed degeneration

Altered vertebral geometry constrains the canal; disc degeneration and sagittal deformity add to adult upper-lumbar narrowing.

FGFR3 Growth-Plate Signaling Model

Downstream

Spinal canal stenosis Congenital constraint and degeneration produce recognized stenosis.

Cervical spinal canal stenosis The congenital component can affect the cervical canal.

Show evidence (1 reference)

PMID:32170149 SUPPORT Human Clinical

"Unlike the general population, spinal stenosis and disc degeneration involve the upper part of the lumbar spine in adults with achondroplasia, associated with thoraco-lumbar kyphosis and loss of lumbar lordosis."

Adult imaging supports combined stenotic and degenerative components.

CNP-NPR2 counter-regulation of FGFR3-MAPK signaling

CNP signaling through NPR2/GC-B counteracts FGFR3-MAPK activity. CNP corrected matrix synthesis and bone shortening in a mouse model without correcting the STAT1 branch.

growth plate cartilage chondrocyte CL:1000217

receptor guanylyl cyclase signaling pathway GO:0007168 MAPK cascade GO:0000165 ↓ DECREASED

Show evidence (2 references)

PMID:14702637 SUPPORT Model Organism

"CNP prevented the shortening of achondroplastic bones by correcting the decreased extracellular matrix synthesis in the growth plate through inhibition of the MAPK pathway of FGF signaling."

Mouse data support CNP counter-regulation of MAPK.

PMID:14702637 SUPPORT Model Organism

"CNP had no effect on the STAT-1 pathway of FGF signaling that mediates the decreased proliferation and the delayed differentiation of achondroplastic chondrocytes."

The model defines the limitation on the STAT1 branch.

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Pathograph

Use the checkboxes to hide or show graph categories. Hover nodes for evidence and cross-linked metadata.

Referential integrity issues (1):

Target 'Genu varum' (from 'Impaired endochondral ossification and chondrodysplasia') not found in named elements

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Phenotypes

Ear 1

Conductive hearing impairment HP:0000405

Show evidence (3 references)

PMID:39660705 SUPPORT Human Clinical

"Among 53 patients who underwent audiometry, 26 showed conductive hearing loss, 2 had mixed-type hearing loss, and 4 had sensorineural hearing loss."

Pediatric multicenter cohort shows that hearing loss in achondroplasia is usually conductive or mixed.

PMID:34736503 SUPPORT Human Clinical

"The majority (15/24) had conductive hearing loss, or a combination of conductive and sensorineural hearing loss (8/24)."

Adult population-based cohort shows conductive or mixed hearing loss remains common beyond childhood.

ORPHA:15 SUPPORT Other

"HP:0000365 | Hearing impairment | Frequent (79-30%)"

Orphanet supports frequent hearing impairment generally; the conductive subtype is established by the cohort evidence above.

Head and Neck 7

Macrocephaly FREQUENT HP:0000256

Show evidence (2 references)

PMID:29972438 SUPPORT Human Clinical

"The most prevalent clinical findings were short stature, high forehead, trident hands, genu varum and macrocephaly."

Clinical cohort data identify macrocephaly as one of the most prevalent physical findings.

ORPHA:15 SUPPORT Other

"HP:0000256 | Macrocephaly | Frequent (79-30%)"

Orphanet classifies macrocephaly as frequent in achondroplasia.

Prominent forehead HP:0011220

Show evidence (1 reference)

PMID:37072824 SUPPORT Human Clinical

"Craniofacial phenotype was characterized by maxillo-zygomatic retrusion, deep nasal root, and prominent forehead."

Multimodal craniofacial study directly documents prominent forehead in pediatric achondroplasia.

Midface retrusion HP:0011800

Show evidence (1 reference)

PMID:37072824 SUPPORT Human Clinical

"Craniofacial phenotype was characterized by maxillo-zygomatic retrusion, deep nasal root, and prominent forehead."

Maxillo-zygomatic retrusion directly supports midface retrusion as a core craniofacial phenotype.

Anteverted nares FREQUENT HP:0000463

Show evidence (1 reference)

ORPHA:15 SUPPORT Other

"HP:0000463 | Anteverted nares | Frequent (79-30%)"

Orphanet classifies anteverted nares as frequent in achondroplasia.

Foramen magnum stenosis Small foramen magnum HP:0002677

Show evidence (2 references)

PMID:32883660 SUPPORT Human Clinical

"Of 36 infants (M:F, 18:18), 2 (5.6%) did not have FMS (AFMS0); 13 (36.1%) had FMS with preservation of the cerebrospinal fluid (CSF) spaces (AFMS1); 3 (8.3%) had FMS with loss of the CSF space but no spinal cord distortion (AFMS2); 13 (36.1%) had FMS with flattening of the cervical cord without..."

Infant MRI cohort shows that most screened infants had foramen magnum stenosis, including many with cord flattening or signal change.

PMID:38554024 SUPPORT Human Clinical

"All patients were monitored by magnetic resonance imaging; 73.0% had foramen magnum stenosis, while 54.1% had Achondroplasia Foramen Magnum Score 3 or 4."

Independent pediatric cohort confirms that foramen magnum stenosis is a frequent early complication.

Depressed nasal bridge FREQUENT HP:0005280

Show evidence (1 reference)

ORPHA:15 SUPPORT Other

"HP:0005280 | Depressed nasal bridge | Frequent (79-30%)"

Orphanet classifies depressed nasal bridge as frequent in achondroplasia.

Frontal bossing FREQUENT HP:0002007

Show evidence (1 reference)

ORPHA:15 SUPPORT Other

"HP:0002007 | Frontal bossing | Frequent (79-30%)"

Orphanet classifies frontal bossing as frequent in achondroplasia.

Integument 1

Acanthosis nigricans OCCASIONAL HP:0000956

Show evidence (2 references)

ORPHA:15 SUPPORT Other

"HP:0000956 | Acanthosis nigricans | Occasional (29-5%)"

Orphanet classifies acanthosis nigricans as occasional in achondroplasia.

url:https://www.ncbi.nlm.nih.gov/books/NBK1152/ SUPPORT Other

"may be seen in about 10% of individuals with achondroplasia"

The 2026 clinical review supports a peripheral association while not placing it in the core skeletal mechanism graph.

Limbs 5

Trident hand FREQUENT HP:0004060

Show evidence (2 references)

PMID:29972438 SUPPORT Human Clinical

"The most prevalent clinical findings were short stature, high forehead, trident hands, genu varum and macrocephaly."

Clinical cohort data identify trident hands as one of the most prevalent physical findings.

ORPHA:15 SUPPORT Other

"HP:0004060 | Trident hand | Frequent (79-30%)"

Orphanet classifies trident hand as frequent in achondroplasia.

Brachydactyly FREQUENT HP:0001156

Show evidence (1 reference)

ORPHA:15 SUPPORT Other

"HP:0001156 | Brachydactyly | Frequent (79-30%)"

Orphanet classifies brachydactyly as frequent in achondroplasia.

Short middle phalanx of finger FREQUENT HP:0005819

Show evidence (1 reference)

ORPHA:15 SUPPORT Other

"HP:0005819 | Short middle phalanx of finger | Frequent (79-30%)"

Orphanet classifies short middle phalanx of finger as frequent in achondroplasia.

Limited elbow extension FREQUENT HP:0001377

Show evidence (1 reference)

ORPHA:15 SUPPORT Other

"HP:0001377 | Limited elbow extension | Frequent (79-30%)"

Orphanet classifies limited elbow extension as frequent in achondroplasia.

Limb undergrowth VERY_FREQUENT HP:0009826

Show evidence (1 reference)

ORPHA:15 SUPPORT Other

"HP:0009826 | Limb undergrowth | Very frequent (99-80%)"

Orphanet classifies limb undergrowth as very frequent in achondroplasia.

Musculoskeletal 6

Thoracolumbar kyphosis VERY_FREQUENT HP:0005619

Show evidence (3 references)

PMID:37493935 SUPPORT Human Clinical

"Thoracolumbar kyphosis (TLK) is common in children with achondroplasia and resolves in 90% by 10 years of age."

Supports both childhood onset and the usual tendency toward spontaneous improvement by age 10.

PMID:27927547 SUPPORT Human Clinical

"Thoracolumbar kyphosis was observed in 79%, with 52% exhibiting moderate to severe curvature."

Large orthopedic cohort documents thoracolumbar kyphosis as a common spinal phenotype.

ORPHA:15 SUPPORT Other

"HP:0005619 | Thoracolumbar kyphosis | Very frequent (99-80%)"

Orphanet classifies thoracolumbar kyphosis as very frequent in achondroplasia.

Scoliosis HP:0002650

Show evidence (1 reference)

PMID:27927547 SUPPORT Human Clinical

"Scoliosis was observed in 60%."

An orthopedic cohort documents scoliosis in 60% of its selected patient population.

Spinal canal stenosis FREQUENT HP:0003416

Show evidence (3 references)

PMID:32864841 SUPPORT Human Clinical

"Cervical and lumbar stenosis were diagnosed in children and adults while, genu varum, elbow contractures, and radial head dislocations were identified during childhood."

Longitudinal chart review supports spinal stenosis as a cross-lifespan complication affecting both cervical and lumbar regions.

PMID:32170149 SUPPORT Human Clinical

Adult imaging study refines the adult lumbar phenotype by showing upper-lumbar predominance with associated degenerative change.

ORPHA:15 SUPPORT Other

"HP:0003416 | Spinal canal stenosis | Frequent (79-30%)"

Orphanet classifies spinal canal stenosis as frequent in achondroplasia.

Lumbar hyperlordosis FREQUENT HP:0002938

Show evidence (1 reference)

ORPHA:15 SUPPORT Other

"HP:0002938 | Lumbar hyperlordosis | Frequent (79-30%)"

Orphanet classifies lumbar hyperlordosis as frequent in achondroplasia.

Floppy infant FREQUENT HP:0008947

Show evidence (1 reference)

ORPHA:15 SUPPORT Other

"HP:0008947 | Floppy infant | Frequent (79-30%)"

Orphanet lists floppy infant as frequent in achondroplasia, supporting infantile muscular hypotonia.

Thoracic hypoplasia OCCASIONAL HP:0005257

Show evidence (1 reference)

ORPHA:15 SUPPORT Other

"HP:0005257 | Thoracic hypoplasia | Occasional (29-5%)"

Orphanet classifies thoracic hypoplasia as occasional in achondroplasia.

Nervous System 4

Obstructive sleep apnea FREQUENT HP:0002870

Show evidence (5 references)

PMID:40675782 SUPPORT Human Clinical

"Sleep-disordered breathing subtypes included obstructive sleep apnoea in 81% (55/68), central sleep apnoea in 3% (2/68), mixed sleep apnoea in 7% (5/68) and primary snoring in 9% (6/68)."

In a pediatric polysomnography cohort, 55 of 80 children had obstructive sleep apnoea and 5 additional children had mixed apnoea with an obstructive component.

PMID:40675782 SUPPORT Human Clinical

"A total of 85% (68/80) had sleep-disordered breathing and 21% reported no prior symptoms."

Pediatric polysomnography data show that sleep-disordered breathing can be clinically silent before screening.

PMID:32864841 SUPPORT Human Clinical

"Central sleep apnea and obstructive sleep apnea were present in children, while the diagnosis of obstructive sleep apnea was shown to recur in adulthood."

Longitudinal review supports both childhood occurrence and adult recurrence of obstructive sleep apnea.

+ 2 more references

Hydrocephalus VERY_RARE HP:0000238

Show evidence (4 references)

PMID:33579320 SUPPORT Human Clinical

"Cervical myelopathy and hydrocephalus occasionally occur in young children with achondroplasia."

Supports hydrocephalus as a recognized pediatric complication without overstating its frequency.

PMID:29972438 SUPPORT Human Clinical

"One patient developed hydrocephalus at 10 years old."

Natural-history cohort provides direct longitudinal evidence that hydrocephalus can occur during follow-up.

ORPHA:15 SUPPORT Other

"HP:0000238 | Hydrocephalus | Very rare (<4-1%)"

Orphanet classifies hydrocephalus as very rare in achondroplasia.

+ 1 more reference

Delayed gross motor development HP:0002194

Show evidence (2 references)

PMID:22409389 SUPPORT Human Clinical

"children with achondroplasia are delayed in development of gross motor and ambulatory skills."

Prospective developmental cohort directly supports delayed gross motor development.

PMID:29972438 SUPPORT Human Clinical

"There was motor developmental delay in 18 patients and speech delay in 16 patients."

Independent natural-history cohort corroborates developmental delay affecting motor milestones.

Delayed speech and language development HP:0000750

Show evidence (2 references)

PMID:29972438 SUPPORT Human Clinical

"There was motor developmental delay in 18 patients and speech delay in 16 patients."

Natural-history cohort directly reports speech delay in a substantial subset of patients.

PMID:22409389 SUPPORT Human Clinical

"While delays were seen in development of later communication items, there were fewer delays seen across development of early communication, fine motor, and feeding skills."

Prospective developmental study supports delayed later communication skill acquisition.

Respiratory 1

Restrictive ventilatory defect OCCASIONAL HP:0002091

Show evidence (1 reference)

ORPHA:15 SUPPORT Other

"HP:0002091 | Restrictive ventilatory defect | Occasional (29-5%)"

Orphanet classifies restrictive ventilatory defect as occasional in achondroplasia.

Constitutional 1

Limb pain HP:0009763

Show evidence (1 reference)

PMID:29972438 SUPPORT Human Clinical

"The most common clinical intercurrences were middle ear dysfunction, sleep apnea, limb pain and obesity from 2 to 9 years of age."

Natural-history cohort lists limb pain among the most common clinical intercurrences in childhood.

Growth 3

Disproportionate short stature Disproportionate short-limb short stature HP:0008873

Show evidence (1 reference)

PMID:32803853 SUPPORT Human Clinical

"Achondroplasia is a genetic disorder that results in disproportionate short stature."

Supports disproportionate short stature as a core phenotype of achondroplasia.

Rhizomelia HP:0008905

Show evidence (2 references)

PMID:29972438 SUPPORT Human Clinical

"The most prevalent radiographic findings were rhizomelic shortening of the long bones and narrowing of the interpediculate distance of the caudal spine."

Supports rhizomelic long-bone shortening as a characteristic skeletal manifestation.

ORPHA:15 SUPPORT Other

"HP:0008905 | Rhizomelia | Occasional (29-5%)"

Orphanet classifies rhizomelia as occasional in achondroplasia.

Obesity HP:0001513

Show evidence (1 reference)

PMID:3228140 SUPPORT Human Clinical

"Obesity is a significant and potentially serious health problem in achondroplasia. Body mass indices, weight-to-square of the height ratio (W/H2), and triceps skinfold measurements show that obesity is common. It begins in early childhood and is prevalent at all ages."

Classic cohort study directly supports obesity as an early and persistent complication.

Other 14

Parietal bossing FREQUENT HP:0000242

Show evidence (1 reference)

ORPHA:15 SUPPORT Other

"HP:0000242 | Parietal bossing | Frequent (79-30%)"

Orphanet classifies parietal bossing as frequent in achondroplasia.

Wide anterior fontanel OCCASIONAL HP:0000260

Show evidence (1 reference)

ORPHA:15 SUPPORT Other

"HP:0000260 | Wide anterior fontanel | Occasional (29-5%)"

Orphanet classifies wide anterior fontanel as occasional in achondroplasia.

Bowing of the legs VERY_FREQUENT HP:0002979

Onset: CHILDHOOD

Show evidence (4 references)

PMID:29972438 SUPPORT Human Clinical

"The most prevalent clinical findings were short stature, high forehead, trident hands, genu varum and macrocephaly."

Clinical cohort data identify lower-limb deformity (including genu varum) as one of the most prevalent physical findings.

ORPHA:15 SUPPORT Other

"HP:0002979 | Bowing of the legs | Very frequent (99-80%)"

Orphanet classifies bowing of the legs as very frequent in achondroplasia.

PMID:32864841 SUPPORT Human Clinical

"genu varum, elbow contractures, and radial head dislocations were identified during childhood"

Chart-review cohort places recognition of genu varum in childhood, preserving the onset nuance of the merged entry.

+ 1 more reference

Cervical spinal canal stenosis FREQUENT HP:0008445

Show evidence (2 references)

PMID:32864841 SUPPORT Human Clinical

"Cervical and lumbar stenosis were diagnosed in children and adults while, genu varum, elbow contractures, and radial head dislocations were identified during childhood."

Longitudinal chart review explicitly reports cervical stenosis in affected patients.

ORPHA:15 SUPPORT Other

"HP:0008445 | Cervical spinal canal stenosis | Frequent (79-30%)"

Orphanet classifies cervical spinal canal stenosis as frequent in achondroplasia.

Central sleep apnea HP:0010536

Show evidence (2 references)

PMID:32864841 SUPPORT Human Clinical

"Central sleep apnea and obstructive sleep apnea were present in children, while the diagnosis of obstructive sleep apnea was shown to recur in adulthood."

Longitudinal natural-history data explicitly document central sleep apnea in children with achondroplasia.

PMID:40675782 SUPPORT Human Clinical

"Sleep-disordered breathing subtypes included obstructive sleep apnoea in 81% (55/68), central sleep apnoea in 3% (2/68), mixed sleep apnoea in 7% (5/68) and primary snoring in 9% (6/68)."

Polysomnography cohort confirms central sleep apnoea as a measured pediatric sleep phenotype, although much less common than obstructive disease.

Functional abnormality of the middle ear FREQUENT HP:0011452

Show evidence (2 references)

PMID:29972438 SUPPORT Human Clinical

"The most common clinical intercurrences were middle ear dysfunction, sleep apnea, limb pain and obesity from 2 to 9 years of age."

Natural-history cohort lists middle-ear dysfunction among the most common childhood clinical intercurrences.

ORPHA:15 SUPPORT Other

"HP:0011452 | Functional abnormality of the middle ear | Frequent (79-30%)"

Orphanet classifies functional abnormality of the middle ear as frequent in achondroplasia.

Otitis media with effusion HP:0031353

Show evidence (1 reference)

PMID:39660705 SUPPORT Human Clinical

"Fifty-one patients (72.9%) had middle ear effusion at least once."

Multicenter otology cohort shows otitis media with effusion in most evaluated patients.

Elbow contracture HP:0034391

Show evidence (1 reference)

PMID:32864841 SUPPORT Human Clinical

"Cervical and lumbar stenosis were diagnosed in children and adults while, genu varum, elbow contractures, and radial head dislocations were identified during childhood."

Longitudinal chart review identifies elbow contractures as a childhood orthopedic phenotype.

Radial head dislocation Proximal radial head dislocation HP:0005070

Show evidence (1 reference)

PMID:32864841 SUPPORT Human Clinical

"Cervical and lumbar stenosis were diagnosed in children and adults while, genu varum, elbow contractures, and radial head dislocations were identified during childhood."

Longitudinal chart review identifies radial head dislocation as a childhood orthopedic phenotype.

Short proximal phalanx of finger FREQUENT HP:0010241

Show evidence (1 reference)

ORPHA:15 SUPPORT Other

"HP:0010241 | Short proximal phalanx of finger | Frequent (79-30%)"

Orphanet classifies short proximal phalanx of finger as frequent in achondroplasia.

Knee joint hypermobility FREQUENT HP:0045086

Show evidence (1 reference)

ORPHA:15 SUPPORT Other

"HP:0045086 | Knee joint hypermobility | Frequent (79-30%)"

Orphanet classifies knee joint hypermobility as frequent in achondroplasia.

Hip joint hypermobility FREQUENT HP:0045087

Show evidence (1 reference)

ORPHA:15 SUPPORT Other

"HP:0045087 | Hip joint hypermobility | Frequent (79-30%)"

Orphanet classifies hip joint hypermobility as frequent in achondroplasia.

Cervical myelopathy HP:0002318

Show evidence (1 reference)

PMID:33579320 SUPPORT Human Clinical

"Thirteen (20%) of the 65 patients showed T2 HSI on MRI, long tract sign, or respiratory difficulty that corresponded to cervical myelopathy"

A selected tertiary pediatric MRI cohort documented cervical myelopathy in 13 of 65 children; this is not used as a population frequency.

Caudal interpedicular narrowing HP:0008457

Show evidence (1 reference)

PMID:29972438 SUPPORT Human Clinical

"The most prevalent radiographic findings were rhizomelic shortening of the long bones and narrowing of the interpediculate distance of the caudal spine."

Natural-history cohort identifies caudal interpedicular narrowing as one of the most prevalent radiographic findings in achondroplasia.

🧬

Genetic Associations

FGFR3 (Causative)

Gene: FGFR3 hgnc:3690 relationship_type: CAUSATIVE variant_origin: GERMLINE

Show evidence (2 references)

CGGV:assertion_0adafa1b-e609-4b99-8e90-f73473b70771-2023-09-28T040000.000Z SUPPORT Other

ClinGen classifies the FGFR3-achondroplasia relationship as definitive.

ORPHA:15 SUPPORT Other

"FGFR3 | fibroblast growth factor receptor 3 | hgnc:3690 | Disease-causing germline mutation(s) (gain of function) in"

Orphanet identifies germline FGFR3 gain of function.

Variants (2)

c.1138G>A (p.Gly380Arg)

The predominant recurrent allele; 150 of 153 p.Gly380Arg-positive cases in one 154-person series carried c.1138G>A.

Show evidence (1 reference)

PMID:7847369 SUPPORT Human Clinical

The large series directly supplies the allele counts.

c.1138G>C (p.Gly380Arg)

A less common nucleotide substitution producing the same p.Gly380Arg protein change; three cases were observed in the same series.

Show evidence (1 reference)

PMID:7847369 SUPPORT Human Clinical

The series directly documents the c.1138G>C minority allele.

💊

Medical Actions

Vosoritide (Voxzogo)

Category: Therapeutic Action: Pharmacotherapy NCIT:C15986

Agent: vosoritide NCIT:C152918

Daily CNP analogue indicated in the United States to increase linear growth in pediatric patients with achondroplasia and open epiphyses. Accelerated approval is based on annualized growth velocity; final adult height and prevention of non-growth complications are not established by the approval. A three-year external-control comparison estimated 5.75 cm additional height, explicitly not a three-year randomized comparison.

Mechanism Target:

ACTIVATES CNP-NPR2 counter-regulation of FGFR3-MAPK signaling — Vosoritide activates the CNP-NPR2 counter-regulatory branch.

Show evidence (2 references)

PMID:31269546 SUPPORT Human Clinical

"Vosoritide is a biologic analogue of C-type natriuretic peptide, a potent stimulator of endochondral ossification."

The clinical trial identifies vosoritide as a CNP analogue.

PMID:14702637 SUPPORT Model Organism

"These results demonstrate that activation of the CNP-GC-B system in endochondral bone formation constitutes a new therapeutic strategy for human achondroplasia."

Preclinical data support the CNP-GC-B/NPR2 mechanism.

Target Phenotypes: Limb undergrowth HP:0009826

Show evidence (4 references)

url:https://www.accessdata.fda.gov/drugsatfda_docs/label/2024/214938s004lbl.pdf SUPPORT Other

"VOXZOGO is a C type natriuretic peptide (CNP) analog indicated to increase linear growth in pediatric patients with achondroplasia with open epiphyses."

The current US label has no obsolete five-year minimum-age clause.

url:https://www.accessdata.fda.gov/drugsatfda_docs/label/2024/214938s004lbl.pdf SUPPORT Other

"This indication is approved under accelerated approval based on an improvement in annualized growth velocity."

The label identifies the surrogate endpoint and accelerated-approval basis.

PMID:39740666 SUPPORT Human Clinical

"Data from the CLARITY achondroplasia study provided an external untreated control population and reference data."

This specifies that the comparator was external and untreated.

+ 1 more reference

Navepegritide (Yuviwel)

Category: Therapeutic Action: Pharmacotherapy NCIT:C15986

Agent: navepegritide

Once-weekly CNP analogue indicated in the United States to increase linear growth in patients aged two years and older with achondroplasia and open epiphyses. The 2026 accelerated approval is based on annualized growth velocity; continued approval may be contingent on verification and description of clinical benefit in confirmatory trials.

Mechanism Target:

ACTIVATES CNP-NPR2 counter-regulation of FGFR3-MAPK signaling — CNP released from navepegritide binds NPR-B and inhibits the MAPK component of FGFR3 signaling.

Show evidence (1 reference)

url:https://www.accessdata.fda.gov/drugsatfda_docs/label/2026/219164Orig1s000lbl.pdf SUPPORT Other

"CNP released from navepegritide has the same receptor binding affinity and activity as endogenous CNP. CNP binds to natriuretic peptide receptor-B (NPR-B), which inhibits the mitogen activated protein kinase signaling (MAPK) pathway."

The current label directly supports NPR-B activation and MAPK counter-regulation.

Target Phenotypes: Limb undergrowth HP:0009826

Show evidence (4 references)

url:https://www.accessdata.fda.gov/drugsatfda_docs/label/2026/219164Orig1s000lbl.pdf SUPPORT Other

"YUVIWEL is a C-type natriuretic peptide (CNP) analog indicated to increase linear growth in pediatric patients 2 years of age and older with achondroplasia with open epiphyses."

The 2026 US label supplies the indication and eligible population.

url:https://www.accessdata.fda.gov/drugsatfda_docs/label/2026/219164Orig1s000lbl.pdf SUPPORT Other

"This indication is approved under accelerated approval based on an improvement in annualized growth velocity."

The label identifies the accelerated-approval endpoint.

url:https://www.accessdata.fda.gov/drugsatfda_docs/label/2026/219164Orig1s000lbl.pdf SUPPORT Other

"Continued approval for this indication may be contingent upon verification and description of clinical benefit in confirmatory trial(s)."

The label qualifies the confirmatory-evidence condition for continued approval.

+ 1 more reference

Infigratinib (oral FGFR1-3 tyrosine kinase inhibitor)

Category: Therapeutic Action: Pharmacotherapy NCIT:C15986

Agent: infigratinib CHEBI:63451

Once-daily oral ATP-competitive FGFR1-3 tyrosine kinase inhibitor, repurposed from oncology at roughly a hundredth of the oncology dose (0.25 mg/kg/day). It is mechanistically distinct from the approved CNP analogues: vosoritide and navepegritide act on a parallel counter-regulatory arm (NPR-B/cGMP braking MAPK downstream of the receptor), whereas infigratinib inhibits the mutant receptor's kinase output directly — the first drug in achondroplasia to do so. In the 52-week phase 3 PROPEL 3 trial (NCT06164951) it increased annualized height velocity by 1.74 cm/year over placebo, and it is oral rather than injected. Two caveats belong with the result: the upper-to-lower segment ratio, the measure of *disproportion* as opposed to height, did not change, and the drug is a pan-FGFR1-3 rather than FGFR3-selective inhibitor, so long-term off-target skeletal effects are not yet characterized in children. Not approved at time of curation.

Mechanism Target:

INHIBITS Prolonged FGFR3 signaling in growth-plate chondrocytes — Infigratinib inhibits the FGFR kinase domain directly, reducing the sustained downstream signal from the constitutively active p.Gly380Arg receptor rather than opposing it from a parallel pathway.

Show evidence (1 reference)

PMID:42370681 SUPPORT Human Clinical

"Infigratinib, an oral FGFR1-3 tyrosine kinase inhibitor, down-regulates key pathways in the pathogenesis of achondroplasia."

States the receptor-directed mechanism that distinguishes infigratinib from the CNP-analogue class.

Target Phenotypes: Limb undergrowth HP:0009826

Show evidence (3 references)

PMID:42370681 SUPPORT Human Clinical

"In children with achondroplasia, treatment with once-daily oral infigratinib for 52 weeks resulted in a significantly greater increase from baseline in the annualized height velocity than placebo."

The phase 3 primary result.

PMID:42370681 SUPPORT Human Clinical

"The difference between infigratinib and placebo in the least-squares mean change from baseline to week 52 was 1.74 cm per year (95% confidence interval [CI], 1.31 to 2.17; P<0.001) for the annualized height velocity, 0.32 (96% CI, 0.23 to 0.41; P<0.001) for the height z score, and -0.02 (96% CI,..."

Quantifies the effect and records that the upper-to-lower segment ratio, the disproportion measure, was not improved.

PMID:42370681 SUPPORT Human Clinical

"Adverse events occurred in 71 of 74 patients (96%) in the infigratinib group and in 37 of 39 patients (95%) in the placebo group; serious adverse events occurred in 4 of 74 patients (5%) and 1 of 39 patients (3%), respectively."

The 52-week safety comparison; longer-term skeletal safety is untested.

Surgical limb lengthening

Category: Therapeutic Action: Limb lengthening surgery Ontology label: Surgical Procedure NCIT:C15329

Extended limb lengthening is an elective option for selected individuals. Decisions require specialized multidisciplinary counseling about goals, prolonged treatment, complications, and patient preferences; the evidence cited here supports use, not a universal recommendation.

Show evidence (1 reference)

url:https://www.ncbi.nlm.nih.gov/books/NBK1152/ SUPPORT Other

"Additional treatment for short stature may include extended limb lengthening;"

The current clinical review lists limb lengthening as an optional intervention.

Foramen magnum decompression

Category: Therapeutic Action: Foramen magnum decompression Ontology label: Surgical Procedure NCIT:C15329

Experienced neurosurgical teams may perform suboccipital or cervicomedullary decompression when clinical, respiratory, and imaging findings indicate significant craniocervical compression. The procedure is not equated with lumbar laminectomy.

Target Phenotypes: Small foramen magnum HP:0002677

Show evidence (1 reference)

url:https://www.ncbi.nlm.nih.gov/books/NBK1152/ SUPPORT Other

"suboccipital decompression as indicated for signs and symptoms of craniocervical junction compression;"

The current clinical review supports symptom- and sign-based surgical selection.

Surgery for symptomatic spinal stenosis

Category: Therapeutic Action: spinal decompression surgery Ontology label: Surgical Procedure NCIT:C15329

Adults with symptomatic spinal stenosis may require decompressive spinal surgery. Selection depends on neurologic symptoms and specialist assessment.

Target Phenotypes: Spinal canal stenosis HP:0003416

Show evidence (1 reference)

url:https://www.ncbi.nlm.nih.gov/books/NBK1152/ SUPPORT Other

"surgery to correct spinal stenosis in symptomatic adults;"

The current clinical review supports surgery for selected symptomatic adults.

Lifelong multidisciplinary surveillance

Category: Monitoring Action: clinical monitoring Ontology label: Clinical Evaluation NCIT:C124351

Age-specific follow-up continues across the lifespan, with visits at least every six to twelve months in infancy and childhood and every three to five years in adulthood.

Show evidence (3 references)

url:https://www.ncbi.nlm.nih.gov/books/NBK1152/ SUPPORT Other

"Infants and children should be seen at a minimum of every six to 12 months."

The review specifies regular pediatric surveillance.

PMID:32457214 SUPPORT Other

"This document is intended to provide guidance for health care providers to help identify individual patients at high risk of developing serious sequelae and to enable intervention before complications develop."

AAP guidance supports anticipatory health supervision.

url:https://www.ncbi.nlm.nih.gov/books/NBK1152/ SUPPORT Other

"Adults should be seen every three to five years."

The current review specifies continued adult follow-up.

Middle-ear and hearing management

Category: Therapeutic Action: supportive care Ontology label: Supportive Care NCIT:C15747

Recurrent effusion and hearing impairment require hearing surveillance and individualized ENT management, including pressure-equalizing tubes when indicated.

Target Phenotypes: Otitis media with effusion HP:0031353 Conductive hearing impairment HP:0000405

Show evidence (1 reference)

url:https://www.ncbi.nlm.nih.gov/books/NBK1152/ SUPPORT Other

"aggressive management of frequent otitis media including long-lasting pressure-equalizing tubes for middle ear dysfunction;"

The current review explicitly recommends active middle-ear management.

Obstructive sleep apnea management

Category: Therapeutic Action: supportive care Ontology label: Supportive Care NCIT:C15747

Treatment is individualized and may include adenotonsillar surgery, positive airway pressure, or rarely tracheostomy after appropriate sleep and airway assessment.

Target Phenotypes: Obstructive sleep apnea HP:0002870

Show evidence (1 reference)

url:https://www.ncbi.nlm.nih.gov/books/NBK1152/ SUPPORT Other

"adenotonsillectomy, positive airway pressure, and, rarely, tracheostomy to correct obstructive sleep apnea;"

The current review lists the principal individualized modalities.

Genetic counseling

Category: Counseling / Informational Action: genetic counseling Ontology label: Genetic Counseling NCIT:C15240

Counseling addresses the 50% transmission risk from an affected heterozygous parent with an average-stature partner.

Show evidence (1 reference)

url:https://www.ncbi.nlm.nih.gov/books/NBK1152/ SUPPORT Other

"If an individual with achondroplasia has a reproductive partner of average stature, each child has a 50% chance of having achondroplasia."

The current review directly states the recurrence risk.

🔬

Diagnosis

Characteristic physical examination

Examination identifies disproportionate short-limb stature with rhizomelic shortening, macrocephaly, frontal bossing and midface retrusion, brachydactyly, and trident hands.

physical examination NCIT:C20989

Results: Characteristic disproportionate skeletal and craniofacial pattern.

Show evidence (1 reference)

url:https://www.ncbi.nlm.nih.gov/books/NBK1152/ SUPPORT Other

The current review defines the characteristic examination pattern.

Skeletal radiographic evaluation

Characteristic clinical and radiographic findings establish the diagnosis in most individuals; radiographs define long-bone, hand, pelvic, and spinal morphology.

radiograph imaging procedure NCIT:C38101

Results: Rhizomelic long-bone shortening and characteristic spinal and pelvic pattern.

Show evidence (1 reference)

url:https://www.ncbi.nlm.nih.gov/books/NBK1152/ SUPPORT Other

"Achondroplasia can be diagnosed by characteristic clinical and radiographic findings in most affected individuals."

The current review explicitly supports clinical-radiographic diagnosis.

FGFR3 molecular confirmation

Targeted testing for c.1138G>A and c.1138G>C is highly efficient because both produce p.Gly380Arg and account for nearly all typical cases.

molecular genetic testing NCIT:C19770

Results: Heterozygous pathogenic FGFR3 variant, usually c.1138G>A or c.1138G>C (p.Gly380Arg).

Show evidence (2 references)

PMID:9718331 SUPPORT Human Clinical

"More than 97% of achondroplasia cases are caused by one of two mutations (G1138A and G1138C) in the fibroblast growth factor receptor 3 (FGFR3) gene, which results in a specific amino acid substitution, G380R."

The study supports targeted testing for the two recurrent nucleotide substitutions.

CGGV:assertion_0adafa1b-e609-4b99-8e90-f73473b70771-2023-09-28T040000.000Z SUPPORT Other

ClinGen supports molecular confirmation through the definitive gene-disease relationship.

🩻

Imaging Findings

Rhizomelic long-bone shortening on skeletal radiographs

Skeletal radiographs show disproportionate proximal long-bone shortening. This is part of the characteristic diagnostic pattern rather than a stand-alone molecular confirmation.

Xray

Rhizomelia HP:0008905

Show evidence (1 reference)

PMID:29972438 SUPPORT Human Clinical

"The most prevalent radiographic findings were rhizomelic shortening of the long bones and narrowing of the interpediculate distance of the caudal spine."

A clinical cohort directly documents the radiographic pattern.

Caudal interpedicular narrowing on spinal radiographs

The interpedicular distance narrows toward the caudal spine, a characteristic vertebral radiographic feature.

Xray

Caudal interpedicular narrowing HP:0008457

Show evidence (1 reference)

PMID:29972438 SUPPORT Human Clinical

"The most prevalent radiographic findings were rhizomelic shortening of the long bones and narrowing of the interpediculate distance of the caudal spine."

A clinical cohort directly documents caudal narrowing.

Foramen-magnum stenosis and cord compression spectrum on MRI

Craniocervical MRI can show foramen-magnum narrowing, loss of surrounding CSF space, cord flattening, and T2 signal change. This evaluates a complication and is not required to establish the skeletal diagnosis.

Mri

Small foramen magnum HP:0002677

Show evidence (1 reference)

PMID:32883660 SUPPORT Human Clinical

The cohort defines an MRI severity spectrum from no stenosis to cord signal change.

Premature skull-base synchondrosis fusion on CT

CT can demonstrate premature fusion of skull-base synchondroses. This finding informs craniofacial mechanism and complication assessment but is not presented as a universal diagnostic requirement.

Show evidence (1 reference)

PMID:37072824 SUPPORT Human Clinical

"All patients with available CT-scan had premature fusion of skull base synchondroses."

CT in a small clinical cohort directly documents premature fusion.

Upper-lumbar stenosis and disc degeneration on adult MRI

Adult imaging shows preferential upper-lumbar canal narrowing and disc degeneration, often with thoracolumbar kyphosis and loss of lordosis.

Mri

Show evidence (1 reference)

PMID:32170149 SUPPORT Human Clinical

Adult MRI data define the characteristic distribution.

📈

Progression

Infancy

Age: Birth through the first two years

Hypotonia and achondroplasia-specific motor delay are typical. Craniocervical-junction compression is a distinct high-acuity risk and increases the risk of death in infancy.

Show evidence (2 references)

url:https://www.ncbi.nlm.nih.gov/books/NBK1152/ SUPPORT Other

"In infancy, hypotonia is typical, and acquisition of developmental motor milestones is often both aberrant in pattern and delayed."

The current clinical review describes the typical infant developmental course.

url:https://www.ncbi.nlm.nih.gov/books/NBK1152/ SUPPORT Other

"Intelligence and life span are usually near normal, although craniocervical junction compression increases the risk of death in infancy."

The current clinical review identifies the phase-specific mortality risk.

Early childhood

Age: Approximately two to ten years

Middle-ear dysfunction, sleep apnea, limb pain, and excess weight are common clinical burdens. Thoracolumbar kyphosis often improves after independent walking, although persistent deformity needs orthopedic follow-up.

Show evidence (2 references)

PMID:29972438 SUPPORT Human Clinical

"The most common clinical intercurrences were middle ear dysfunction, sleep apnea, limb pain and obesity from 2 to 9 years of age."

A natural-history cohort identifies the principal early-childhood burdens.

PMID:37493935 SUPPORT Human Clinical

"Thoracolumbar kyphosis (TLK) is common in children with achondroplasia and resolves in 90% by 10 years of age."

Longitudinal follow-up supports usual improvement of kyphosis in childhood.

Later childhood and adolescence

Age: Approximately ten years through skeletal maturity

Lower-limb bowing can persist as a complex deformity involving lateral bowing, differential tibial and fibular growth, internal tibial torsion, and dynamic knee instability.

Show evidence (1 reference)

url:https://www.ncbi.nlm.nih.gov/books/NBK1152/ SUPPORT Other

""Bowing" is actually a complex deformity arising from a combination of lateral bowing, differential growth between the tibia and fibula, internal tibial torsion, and dynamic instability of the knee"

The current clinical review describes the interacting growth and mechanical components of lower-limb bowing.

Adulthood

Age: Skeletal maturity onward

Symptomatic spinal stenosis involving L1-L4 is the most common adult medical complaint. Imaging shows preferential upper-lumbar stenosis and disc degeneration.

Show evidence (2 references)

url:https://www.ncbi.nlm.nih.gov/books/NBK1152/ SUPPORT Other

"The most common medical complaint in adulthood is symptomatic spinal stenosis involving L1-L4"

The 2026 review identifies symptomatic upper-lumbar stenosis as the leading adult complaint.

PMID:32170149 SUPPORT Human Clinical

Adult imaging data define the characteristic upper-lumbar distribution.

📊

Prevalence

Global live births

Birth Prevalence 4.6 per 100,000 1–9 per 100,000

A systematic review and meta-analysis estimated worldwide birth prevalence at 4.6 per 100,000 live births.

Show evidence (2 references)

PMID:32803853 SUPPORT Human Clinical

"Based on the meta-analysis, the worldwide birth prevalence of achondroplasia was estimated to be 4.6 per 100,000."

This meta-analysis provides the strongest aggregate global birth-prevalence estimate for achondroplasia.

ORPHA:15 SUPPORT Other

"1-9 / 100 000 | Worldwide | Prevalence at birth | PMID:32803853"

Orphanet epidemiology data confirm worldwide birth prevalence in the 1-9 per 100,000 range, consistent with published estimates.

Europe

Birth Prevalence 3.72 per 100,000 1–9 per 100,000

EUROCAT population-based surveillance identified 434 cases and estimated birth prevalence at 3.72 per 100,000 births.

Show evidence (1 reference)

PMID:31294928 SUPPORT Human Clinical

"The study population consisted of 434 achondroplasia cases with a prevalence of 3.72 per 100,000 births (95%CIs: 3.14-4.39)."

A large population-based European study provides the regional estimate.

🔀

Differential Diagnoses

Conditions with similar clinical presentations that must be differentiated from Achondroplasia:

Hypochondroplasia MONDO:0007793

Overlapping Features A milder FGFR3 skeletal dysplasia that can overlap mild achondroplasia.

Distinguishing Features

Skeletal and medical manifestations tend to be milder than in achondroplasia.

Show evidence (1 reference)

url:https://www.ncbi.nlm.nih.gov/books/NBK1152/ SUPPORT Other

"The skeletal features are very similar to those seen in achondroplasia but tend to be milder."

The current review states the principal clinical distinction.

Pseudoachondroplasia Not Yet Curated MONDO:0008322

Overlapping Features A clinically and genetically distinct skeletal dysplasia whose similar name can cause diagnostic confusion.

Distinguishing Features

It is genetically distinct from FGFR3 p.Gly380Arg achondroplasia.
The radiographic and craniofacial pattern should be assessed rather than relying on nomenclature.

Show evidence (1 reference)

url:https://www.ncbi.nlm.nih.gov/books/NBK1152/ SUPPORT Other

"is a clinically and genetically distinct skeletal dysplasia but the similar nomenclature may cause confusion with achondroplasia."

GeneReviews explicitly warns that the similar name masks a distinct disorder.

Thanatophoric dysplasia type 2 MONDO:0008547

Overlapping Features A much more severe, usually life-limiting FGFR3 short-limb skeletal dysplasia.

Distinguishing Features

Severe, usually lethal short-limb skeletal dysplasia favors thanatophoric dysplasia.

Show evidence (1 reference)

PMID:25800480 SUPPORT Human Clinical

"Thanatophoric dysplasia (TD) is a lethal form of skeletal dysplasia with short-limb dwarfism."

The report supports the severe life-limiting short-limb phenotype.

SADDAN MONDO:0014658

Overlapping Features SADDAN is a severe FGFR3 allelic disorder in which developmental delay, structural brain abnormalities, hearing loss, and acanthosis nigricans accompany the skeletal dysplasia.

Distinguishing Features

Marked developmental and neurologic abnormalities with acanthosis nigricans favor SADDAN.
SADDAN is associated with FGFR3 p.Lys650Met rather than p.Gly380Arg.

Show evidence (2 references)

PMID:24352917 SUPPORT Human Clinical

"Severe achondroplasia with developmental delay and acanthosis nigricans (SADDAN) is an extremely rare severe skeletal dysplasia characterized by significant developmental delay, brain structural abnormalities, hearing loss, and acanthosis nigricans."

The report directly states the distinguishing multisystem phenotype.

PMID:24352917 SUPPORT Human Clinical

"The disorder is the result of a single missense mutation at codon 650 (p.Lys650Met) in the fibroblast growth factor receptor 3 gene (FGFR3)."

The report identifies the distinct SADDAN allele.

🔬

Clinical Trials

NCT06164951 PHASE_III ACTIVE_NOT_RECRUITING

PROPEL 3 — the pivotal 52-week randomized, double-blind, placebo-controlled trial of once-daily oral infigratinib in children aged 3 to <18 years with achondroplasia. It is the first phase 3 result for a drug acting on the FGFR3 receptor kinase itself rather than on the CNP counter-regulatory arm.

Target Phenotypes: Limb undergrowth HP:0009826 Short stature HP:0004322

Show evidence (2 references)

clinicaltrials:NCT06164951 SUPPORT Human Clinical

"This is a Phase 3, multicenter, double-blind, randomized, placebo-controlled study to evaluate the efficacy and safety of infigratinib in children and adolescents with achondroplasia (ACH)"

The registry record establishes the trial's design and population.

PMID:42370681 SUPPORT Human Clinical

"In this phase 3, multicenter, double-blind, placebo-controlled trial, we randomly assigned children with achondroplasia (3 to 17 years of age) in a 2:1 ratio to receive infigratinib (at a dose of 0.25 mg per kilogram of body weight) or placebo once daily for 52 weeks."

The publication reporting this trial's results.

🐁

Animal Models

Human FGFR3 G380R transgene under mouse Fgfr3 transcriptional control Mouse

Mutant-receptor immunoreactivity extends across a larger growth-plate area, supporting impaired in vivo receptor down-regulation. This record does not claim an unsupported full phenotypic inventory.

Species

Mouse

Genotype

Human FGFR3 G380R transgene under mouse Fgfr3 transcriptional control

Show evidence (1 reference)

PMID:10611230 SUPPORT Model Organism

The experiment directly supports the modeled receptor-persistence phenotype.

Activated Fgfr3 achondroplasia model with chondrocyte CNP overexpression Mouse

Targeted CNP overexpression counteracted dwarfism and corrected reduced matrix synthesis through MAPK inhibition, providing proof of concept for CNP analogues.

Species

Mouse

Genotype

Activated Fgfr3 achondroplasia model with chondrocyte CNP overexpression

Show evidence (2 references)

PMID:14702637 SUPPORT Model Organism

"targeted overexpression of CNP in chondrocytes counteracts dwarfism in a mouse model of achondroplasia with activated fibroblast growth factor receptor 3 (FGFR-3) in the cartilage."

The intervention directly demonstrates rescue of dwarfism.

PMID:14702637 SUPPORT Model Organism

"CNP prevented the shortening of achondroplastic bones by correcting the decreased extracellular matrix synthesis in the growth plate through inhibition of the MAPK pathway of FGF signaling."

The experiment supplies the matrix and MAPK mechanism.

Constitutively active MEK1 driven by Col2a1 in chondrocytes Mouse

Chondrocyte MEK1 activation produces achondroplasia-like dwarfism and inhibits skeletal overgrowth in Fgfr3-deficient mice, supporting MAPK as a downstream effector.

Species

Mouse

Genotype

Constitutively active MEK1 driven by Col2a1 in chondrocytes

Show evidence (2 references)

PMID:14871928 SUPPORT Model Organism

"We generated transgenic mice that express a constitutively active mutant of MEK1 in chondrocytes. These mice showed a dwarf phenotype similar to achondroplasia, the most common human dwarfism, caused by activating mutations in FGFR3."

Directly demonstrates sufficiency of chondrocyte MAPK activation for an achondroplasia-like phenotype.

PMID:14871928 SUPPORT Model Organism

"Expression of a constitutively active mutant of MEK1 in chondrocytes of Fgfr3-deficient mice inhibited skeletal overgrowth, strongly suggesting that regulation of bone growth by FGFR3 is mediated at least in part by the MAPK pathway."

The epistasis experiment positions MAPK downstream of FGFR3.

{ }

Source YAML

click to show

name: Achondroplasia
synonyms:
- Achondroplasia, FGFR3-related
creation_date: '2026-02-02T00:16:36Z'
category: Mendelian
description: >-
  Achondroplasia is the most common cause of disproportionate short stature. It is
  caused by heterozygous gain-of-function variation in FGFR3, almost always
  p.Gly380Arg, which prolongs inhibitory FGFR3 signaling in growth-plate
  chondrocytes and impairs endochondral ossification. Rhizomelic limb shortening,
  macrocephaly, frontal bossing, midface retrusion, and trident hands are
  characteristic. Important complications include craniocervical-junction
  compression in infancy, sleep-disordered breathing, middle-ear dysfunction,
  limb bowing, obesity, and symptomatic spinal stenosis in adulthood. Daily
  vosoritide and once-weekly navepegritide increase growth velocity in eligible
  children with open epiphyses; both US indications use annualized growth
  velocity as an accelerated-approval endpoint, so final-height and
  complication-prevention benefits remain under study.
disease_term:
  preferred_term: achondroplasia
  term:
    id: MONDO:0007037
    label: achondroplasia
parents:
- FGFR3-related skeletal dysplasia
- Rhizomelic limb shortening syndrome
inheritance:
- name: Autosomal dominant
  inheritance_term:
    preferred_term: Autosomal dominant inheritance
    term:
      id: HP:0000006
      label: Autosomal dominant inheritance
  penetrance: COMPLETE
  de_novo_rate: approximately 80%
  parent_of_origin_effect: De novo variants are predominantly paternal in origin and associated with increased paternal age
  description: >-
    Achondroplasia has complete-penetrance autosomal-dominant inheritance. About
    80% of affected individuals have average-stature parents and a de novo FGFR3
    variant. In informative sporadic cases, the recurrent variant arose on the
    paternal chromosome; increased paternal age is an established association.
  evidence:
  - reference: PMID:8078586
    reference_title: "Mutations in the gene encoding fibroblast growth factor receptor-3 in achondroplasia."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Affected individuals are fertile and achondroplasia is transmitted as a fully penetrant autosomal dominant trait, accounting for rare familial forms of the disease (10%)."
    explanation: Establishes autosomal dominant inheritance with full penetrance for achondroplasia.
  - reference: PMID:8078586
    reference_title: "Mutations in the gene encoding fibroblast growth factor receptor-3 in achondroplasia."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "More than 90% of cases are sporadic and there is an increased paternal age at the time of conception of affected individuals,"
    explanation: An older series documents a high sporadic fraction and paternal-age association; the structured estimate is calibrated to the current approximately 80% estimate.
  - reference: url:https://www.ncbi.nlm.nih.gov/books/NBK1152/
    reference_title: Achondroplasia - GeneReviews® - NCBI Bookshelf
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "Approximately 80% of individuals with achondroplasia have parents of average stature and have achondroplasia as the result of a"
    explanation: The 2026 GeneReviews revision gives the current approximately 80% de novo estimate.
  - reference: PMID:9718331
    reference_title: Mutations in fibroblast growth-factor receptor 3 in sporadic cases of achondroplasia occur exclusively on the paternally derived chromosome.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "In this population, the achondroplasia mutation occurred on the paternal chromosome in all 40 cases in which parental origin was unambiguous."
    explanation: Direct parental-origin analysis supports the paternal origin of recurrent de novo FGFR3 variants.
  - reference: ORPHA:15
    reference_title: Achondroplasia
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "Autosomal dominant"
    explanation: Orphanet classifies achondroplasia inheritance as autosomal dominant.
classifications:
  isds_skeletal_category:
  - classification_value: fgfr3_chondrodysplasia
    notes: >-
      ISDS Nosology and Classification of Genetic Skeletal Disorders, 2019
      revision (Mortier et al., PMID:31633310), Table 1 group 1 "FGFR3
      chondrodysplasia group"; listed as "Achondroplasia".
prevalence:
- population: Global live births
  measure_type: BIRTH_PREVALENCE
  prevalence_class: BAND_1_9_PER_100000
  rate_per_100000: 4.6
  notes: >-
    A systematic review and meta-analysis estimated worldwide birth prevalence
    at 4.6 per 100,000 live births.
  evidence:
  - reference: PMID:32803853
    reference_title: "Birth prevalence of achondroplasia: A systematic literature review and meta-analysis."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Based on the meta-analysis, the worldwide birth prevalence of achondroplasia was estimated to be 4.6 per 100,000."
    explanation: This meta-analysis provides the strongest aggregate global birth-prevalence estimate for achondroplasia.
  - reference: ORPHA:15
    reference_title: Achondroplasia
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "1-9 / 100 000 | Worldwide | Prevalence at birth | PMID:32803853"
    explanation: Orphanet epidemiology data confirm worldwide birth prevalence in the 1-9 per 100,000 range, consistent with published estimates.
- population: Europe
  measure_type: BIRTH_PREVALENCE
  prevalence_class: BAND_1_9_PER_100000
  rate_per_100000: 3.72
  notes: >-
    EUROCAT population-based surveillance identified 434 cases and estimated
    birth prevalence at 3.72 per 100,000 births.
  evidence:
  - reference: PMID:31294928
    reference_title: "Epidemiology of achondroplasia: A population-based study in Europe."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The study population consisted of 434 achondroplasia cases with a prevalence of 3.72 per 100,000 births (95%CIs: 3.14-4.39)."
    explanation: A large population-based European study provides the regional estimate.

progression:
- phase: Infancy
  age_range: Birth through the first two years
  notes: >-
    Hypotonia and achondroplasia-specific motor delay are typical.
    Craniocervical-junction compression is a distinct high-acuity risk and
    increases the risk of death in infancy.
  evidence:
  - reference: url:https://www.ncbi.nlm.nih.gov/books/NBK1152/
    reference_title: Achondroplasia - GeneReviews® - NCBI Bookshelf
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "In infancy, hypotonia is typical, and acquisition of developmental motor milestones is often both aberrant in pattern and delayed."
    explanation: The current clinical review describes the typical infant developmental course.
  - reference: url:https://www.ncbi.nlm.nih.gov/books/NBK1152/
    reference_title: Achondroplasia - GeneReviews® - NCBI Bookshelf
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "Intelligence and life span are usually near normal, although craniocervical junction compression increases the risk of death in infancy."
    explanation: The current clinical review identifies the phase-specific mortality risk.
- phase: Early childhood
  age_range: Approximately two to ten years
  notes: >-
    Middle-ear dysfunction, sleep apnea, limb pain, and excess weight are common
    clinical burdens. Thoracolumbar kyphosis often improves after independent
    walking, although persistent deformity needs orthopedic follow-up.
  evidence:
  - reference: PMID:29972438
    reference_title: Natural history of 39 patients with Achondroplasia.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The most common clinical intercurrences were middle ear dysfunction, sleep apnea, limb pain and obesity from 2 to 9 years of age."
    explanation: A natural-history cohort identifies the principal early-childhood burdens.
  - reference: PMID:37493935
    reference_title: "Walking status and spinopelvic parameters in young children with achondroplasia: 10-year follow-up."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Thoracolumbar kyphosis (TLK) is common in children with achondroplasia and resolves in 90% by 10 years of age."
    explanation: Longitudinal follow-up supports usual improvement of kyphosis in childhood.
- phase: Later childhood and adolescence
  age_range: Approximately ten years through skeletal maturity
  notes: >-
    Lower-limb bowing can persist as a complex deformity involving lateral
    bowing, differential tibial and fibular growth, internal tibial torsion,
    and dynamic knee instability.
  evidence:
  - reference: url:https://www.ncbi.nlm.nih.gov/books/NBK1152/
    reference_title: Achondroplasia - GeneReviews® - NCBI Bookshelf
    supports: SUPPORT
    evidence_source: OTHER
    snippet: '"Bowing" is actually a complex deformity arising from a combination of lateral bowing, differential growth between the tibia and fibula, internal tibial torsion, and dynamic instability of the knee'
    explanation: The current clinical review describes the interacting growth and mechanical components of lower-limb bowing.
- phase: Adulthood
  age_range: Skeletal maturity onward
  notes: >-
    Symptomatic spinal stenosis involving L1-L4 is the most common adult medical
    complaint. Imaging shows preferential upper-lumbar stenosis and disc
    degeneration.
  evidence:
  - reference: url:https://www.ncbi.nlm.nih.gov/books/NBK1152/
    reference_title: Achondroplasia - GeneReviews® - NCBI Bookshelf
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "The most common medical complaint in adulthood is symptomatic spinal stenosis involving L1-L4"
    explanation: The 2026 review identifies symptomatic upper-lumbar stenosis as the leading adult complaint.
  - reference: PMID:32170149
    reference_title: Lumbar spinal stenosis and disc alterations affect the upper lumbar spine in adults with achondroplasia.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Unlike the general population, spinal stenosis and disc degeneration involve the upper part of the lumbar spine in adults with achondroplasia, associated with thoraco-lumbar kyphosis and loss of lumbar lordosis."
    explanation: Adult imaging data define the characteristic upper-lumbar distribution.
mechanistic_hypotheses:
- hypothesis_group_id: canonical_fgfr3_gain_of_function_chondrocyte_model
  hypothesis_label: FGFR3 Growth-Plate Signaling Model
  status: CANONICAL
  description: >-
    Recurrent heterozygous FGFR3 p.Gly380Arg gain-of-function prolongs receptor
    signaling in growth-plate chondrocytes. Experiments support delayed receptor
    down-regulation while G380R activation remains predominantly ligand
    dependent. MAPK-mediated inhibition of hypertrophic differentiation and a
    qualified STAT-p21 branch impair endochondral ossification.
  notes: >-
    Strict constitutive activation is not asserted as the sole receptor-level
    mechanism. CNP-analogue growth-velocity responses support therapeutic
    counter-regulation but do not establish final-height or complication benefit.
  evidence:
  - reference: PMID:10611230
    reference_title: The transmembrane mutation G380R in fibroblast growth factor receptor 3 uncouples ligand-mediated receptor activation from down-regulation.
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "We found that dimerization and activation of the G380R mutant receptor are predominantly ligand dependent."
    explanation: Direct receptor experiments qualify strict constitutive-activation shorthand.
  - reference: PMID:10611230
    reference_title: The transmembrane mutation G380R in fibroblast growth factor receptor 3 uncouples ligand-mediated receptor activation from down-regulation.
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "Metabolic pulse-chase experiments, cell surface labeling, and kinetics of uptake of radiolabeled ligand demonstrated a selective delay in the down-regulation of the mutant receptor."
    explanation: Direct experiments identify delayed receptor down-regulation.
  - reference: DOI:10.1038/s41574-021-00595-x
    reference_title: International Consensus Statement on the diagnosis, multidisciplinary management and lifelong care of individuals with achondroplasia
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "The condition is caused by a common, recurring, gain-of-function mutation in FGFR3, the gene that encodes fibroblast growth factor receptor 3. This mutation leads to impaired endochondral ossification of the human skeleton."
    explanation: International consensus links FGFR3 gain of function to impaired endochondral ossification.

pathophysiology:
- name: FGFR3 p.Gly380Arg gain-of-function with impaired receptor down-regulation
  role: trigger
  conforms_to: "fgfr_gain_of_function_skeletal_dysplasia#Constitutive FGFR Activation"
  description: >-
    Nearly all individuals have one of two substitutions producing p.Gly380Arg.
    The receptor is gain of function, but experiments show predominantly
    ligand-dependent activation with delayed down-regulation and resistance to
    ligand-mediated internalization.
  gene:
    preferred_term: FGFR3
    term:
      id: hgnc:3690
      label: FGFR3
  molecular_functions:
  - preferred_term: transmembrane receptor protein tyrosine kinase activity
    term:
      id: GO:0004714
      label: transmembrane receptor protein tyrosine kinase activity
    modifier: INCREASED
  biological_processes:
  - preferred_term: fibroblast growth factor receptor signaling pathway
    term:
      id: GO:0008543
      label: fibroblast growth factor receptor signaling pathway
    modifier: INCREASED
  evidence:
  - reference: PMID:7847369
    reference_title: Achondroplasia is defined by recurrent G380R mutations of FGFR3.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "All but one, an atypical case, were found to have a glycine-to-arginine substitution at codon 380. Of these, 150 had a G-to-A transition at nt 1138, and 3 had a G-to-C transversion at this same position."
    explanation: A 154-person series demonstrates the near-uniform recurrent p.Gly380Arg genotype.
  - reference: PMID:10611230
    reference_title: The transmembrane mutation G380R in fibroblast growth factor receptor 3 uncouples ligand-mediated receptor activation from down-regulation.
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "Moreover, this receptor was now resistant to ligand-mediated internalization, even at saturating ligand concentrations."
    explanation: Directly supports impaired internalization.
  - reference: CGGV:assertion_0adafa1b-e609-4b99-8e90-f73473b70771-2023-09-28T040000.000Z
    reference_title: FGFR3 / achondroplasia (Definitive)
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "FGFR3 | HGNC:3690 | achondroplasia | MONDO:0007037 | AD | Definitive"
    explanation: ClinGen classifies the autosomal-dominant FGFR3 relationship as definitive.
  downstream:
  - target: Prolonged FGFR3 signaling in growth-plate chondrocytes
    hypothesis_groups:
    - canonical_fgfr3_gain_of_function_chondrocyte_model
    causal_link_type: DIRECT
    description: Delayed down-regulation expands mutant-receptor signaling in growth plate.
    evidence:
    - reference: PMID:10611230
      reference_title: The transmembrane mutation G380R in fibroblast growth factor receptor 3 uncouples ligand-mediated receptor activation from down-regulation.
      supports: SUPPORT
      evidence_source: MODEL_ORGANISM
      snippet: "transgenic mice expressing the human G380R mutant receptor under the mouse receptor transcriptional control demonstrated a markedly expanded area of FGFR3 immunoreactivity within their epiphyseal growth plates, compatible with an in vivo defect in receptor down-regulation."
      explanation: In vivo receptor distribution supports prolonged growth-plate signaling.

- name: Prolonged FGFR3 signaling in growth-plate chondrocytes
  conforms_to: "fgfr_gain_of_function_skeletal_dysplasia#Sustained MAPK/STAT Signaling"
  role: central_effector
  description: >-
    Delayed internalization expands mutant FGFR3 immunoreactivity in epiphyseal
    growth plates, where signal kinetics regulate chondrocyte maturation.
  cell_types:
  - preferred_term: growth plate cartilage chondrocyte
    term:
      id: CL:1000217
      label: growth plate cartilage chondrocyte
  biological_processes:
  - preferred_term: MAPK cascade
    term:
      id: GO:0000165
      label: MAPK cascade
    modifier: INCREASED
  - preferred_term: ERK1 and ERK2 cascade
    term:
      id: GO:0070371
      label: ERK1 and ERK2 cascade
    modifier: INCREASED
  - preferred_term: cell surface receptor signaling pathway via STAT
    term:
      id: GO:0097696
      label: cell surface receptor signaling pathway via STAT
    modifier: INCREASED
  evidence:
  - reference: PMID:10611230
    reference_title: The transmembrane mutation G380R in fibroblast growth factor receptor 3 uncouples ligand-mediated receptor activation from down-regulation.
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: "transgenic mice expressing the human G380R mutant receptor under the mouse receptor transcriptional control demonstrated a markedly expanded area of FGFR3 immunoreactivity within their epiphyseal growth plates, compatible with an in vivo defect in receptor down-regulation."
    explanation: Transgenic mice provide in vivo evidence for expanded receptor persistence.
  downstream:
  - target: Growth-plate chondrocyte dysregulation
    hypothesis_groups:
    - canonical_fgfr3_gain_of_function_chondrocyte_model
    causal_link_type: DIRECT
    description: Prolonged signaling engages MAPK and STAT-p21 growth-inhibitory programs.
    evidence:
    - reference: PMID:14871928
      reference_title: Constitutive activation of MEK1 in chondrocytes causes Stat1-independent achondroplasia-like dwarfism and rescues the Fgfr3-deficient mouse phenotype.
      supports: SUPPORT
      evidence_source: MODEL_ORGANISM
      snippet: "These observations suggest a model in which Fgfr3 signaling inhibits bone growth by inhibiting chondrocyte differentiation through the MAPK pathway and by inhibiting chondrocyte proliferation through Stat1."
      explanation: Genetic mouse experiments support MAPK and a parallel STAT1 branch.

- name: Growth-plate chondrocyte dysregulation
  conforms_to: "fgfr_gain_of_function_skeletal_dysplasia#Growth-Plate Chondrocyte Dysregulation"
  role: effector
  description: >-
    MAPK inhibits hypertrophic differentiation in models. Human fetal growth
    plates support altered FGFR3/STAT/p21 expression and premature cell-cycle
    exit. STAT1 is not treated as sufficient to explain all growth inhibition.
  cell_types:
  - preferred_term: growth plate cartilage chondrocyte
    term:
      id: CL:1000217
      label: growth plate cartilage chondrocyte
  - preferred_term: prehypertrophic chondrocyte
    term:
      id: CL:0020022
      label: prehypertrophic chondrocyte
  biological_processes:
  - preferred_term: MAPK cascade
    term:
      id: GO:0000165
      label: MAPK cascade
    modifier: INCREASED
  - preferred_term: chondrocyte differentiation
    term:
      id: GO:0002062
      label: chondrocyte differentiation
    modifier: DYSREGULATED
  - preferred_term: cell population proliferation
    term:
      id: GO:0008283
      label: cell population proliferation
    modifier: DECREASED
  evidence:
  - reference: PMID:14871928
    reference_title: Constitutive activation of MEK1 in chondrocytes causes Stat1-independent achondroplasia-like dwarfism and rescues the Fgfr3-deficient mouse phenotype.
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: "These observations indicate that the MAPK pathway inhibits hypertrophic differentiation of chondrocytes and negatively regulates bone growth without inhibiting chondrocyte proliferation."
    explanation: Active MEK1 supports inhibition of hypertrophic differentiation by MAPK.
  - reference: PMID:14751560
    reference_title: Overexpression of FGFR3, Stat1, Stat5 and p21Cip1 correlates with phenotypic severity and defective chondrocyte differentiation in FGFR3-related chondrodysplasias.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Immunohistochemical analysis of fetal growth plates showed a phenotype-related reduction of the collagen type X-positive hypertrophic zone."
    explanation: Human fetal histology documents a reduced hypertrophic zone.
  downstream:
  - target: Impaired endochondral ossification and chondrodysplasia
    hypothesis_groups:
    - canonical_fgfr3_gain_of_function_chondrocyte_model
    causal_link_type: DIRECT
    description: Dysregulated growth-plate maturation impairs endochondral bone growth.
    evidence:
    - reference: PMID:14871928
      reference_title: Constitutive activation of MEK1 in chondrocytes causes Stat1-independent achondroplasia-like dwarfism and rescues the Fgfr3-deficient mouse phenotype.
      supports: SUPPORT
      evidence_source: MODEL_ORGANISM
      snippet: "These observations indicate that the MAPK pathway inhibits hypertrophic differentiation of chondrocytes and negatively regulates bone growth without inhibiting chondrocyte proliferation."
      explanation: The model links differentiation impairment to reduced bone growth.
    - reference: PMID:14751560
      reference_title: Overexpression of FGFR3, Stat1, Stat5 and p21Cip1 correlates with phenotypic severity and defective chondrocyte differentiation in FGFR3-related chondrodysplasias.
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "Immunohistochemical analysis of fetal growth plates showed a phenotype-related reduction of the collagen type X-positive hypertrophic zone."
      explanation: Human fetal histology corroborates the impaired hypertrophic compartment.

- name: Impaired endochondral ossification and chondrodysplasia
  conforms_to: "fgfr_gain_of_function_skeletal_dysplasia#Impaired Endochondral Ossification and Chondrodysplasia"
  role: consequence
  description: >-
    Altered growth-plate maturation reduces the hypertrophic zone and impairs
    endochondral ossification. Anatomical consequences are represented through
    calibrated branches rather than an unsupported all-phenotype fan-out.
  cell_types:
  - preferred_term: hypertrophic chondrocyte
    term:
      id: CL:0000743
      label: hypertrophic chondrocyte
  - preferred_term: columnar chondrocyte
    term:
      id: CL:0000744
      label: columnar chondrocyte
  biological_processes:
  - preferred_term: endochondral ossification
    term:
      id: GO:0001958
      label: endochondral ossification
    modifier: DECREASED
  - preferred_term: growth plate cartilage development
    term:
      id: GO:0003417
      label: growth plate cartilage development
    modifier: DYSREGULATED
  evidence:
  - reference: DOI:10.1038/s41574-021-00595-x
    reference_title: International Consensus Statement on the diagnosis, multidisciplinary management and lifelong care of individuals with achondroplasia
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "This mutation leads to impaired endochondral ossification of the human skeleton."
    explanation: International consensus identifies the central consequence.
  downstream:
  - target: Reduced longitudinal growth of endochondral bones
    hypothesis_groups:
    - canonical_fgfr3_gain_of_function_chondrocyte_model
    causal_link_type: DIRECT
    description: Impaired hypertrophic differentiation reduces longitudinal growth.
    evidence:
    - reference: PMID:14871928
      reference_title: Constitutive activation of MEK1 in chondrocytes causes Stat1-independent achondroplasia-like dwarfism and rescues the Fgfr3-deficient mouse phenotype.
      supports: SUPPORT
      evidence_source: MODEL_ORGANISM
      snippet: "These observations indicate that the MAPK pathway inhibits hypertrophic differentiation of chondrocytes and negatively regulates bone growth without inhibiting chondrocyte proliferation."
      explanation: The model links impaired differentiation to reduced bone growth.
  - target: Cranial-base and vertebral synchondrosis dysplasia
    hypothesis_groups:
    - canonical_fgfr3_gain_of_function_chondrocyte_model
    causal_link_type: UNKNOWN
    description: The endochondral process affects synchondroses, but ordinary-heterozygous causal evidence is incomplete.
    evidence:
    - reference: PMID:18923003
      reference_title: FGFR3 promotes synchondrosis closure and fusion of ossification centers through the MAPK pathway.
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "We observed premature synchondrosis closure in the spine and cranial base in human cases of homozygous achondroplasia and thanatophoric dysplasia"
      explanation: Severe human FGFR3 dysplasias support the branch but do not directly establish it in ordinary heterozygous achondroplasia.
    - reference: PMID:18923003
      reference_title: FGFR3 promotes synchondrosis closure and fusion of ossification centers through the MAPK pathway.
      supports: SUPPORT
      evidence_source: MODEL_ORGANISM
      snippet: "Chondrocyte-specific activation of Fgfr3 in mice induced premature synchondrosis closure and enhanced osteoblast differentiation around synchondroses."
      explanation: Chondrocyte-specific mouse activation supplies causal model support for the branch.
  - target: Congenital spinal-canal constraint with superimposed degeneration
    hypothesis_groups:
    - canonical_fgfr3_gain_of_function_chondrocyte_model
    causal_link_type: UNKNOWN
    description: Impaired vertebral growth plausibly contributes to congenital constraint; adult degeneration adds a second component.
    evidence:
    - reference: PMID:32864841
      reference_title: "Natural history of achondroplasia: A retrospective review of longitudinal clinical data."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "Cervical and lumbar stenosis were diagnosed in children and adults while, genu varum, elbow contractures, and radial head dislocations were identified during childhood."
      explanation: Longitudinal data establish stenosis but not the entire upstream route.
  - target: Genu varum
    hypothesis_groups:
    - canonical_fgfr3_gain_of_function_chondrocyte_model
    causal_link_type: UNKNOWN
    description: Genu varum is observed during childhood in the chondrodysplasia; the specific growth-plate route is not established by the cited source.
    evidence:
    - reference: PMID:32864841
      reference_title: "Natural history of achondroplasia: A retrospective review of longitudinal clinical data."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "Cervical and lumbar stenosis were diagnosed in children and adults while, genu varum, elbow contractures, and radial head dislocations were identified during childhood."
      explanation: Longitudinal data record genu varum appearing during childhood in achondroplasia, but document its timing rather than establishing the causal route.
  - target: Bowing of the legs
    hypothesis_groups:
    - canonical_fgfr3_gain_of_function_chondrocyte_model
    causal_link_type: UNKNOWN
    description: Lower-limb bowing overlaps clinically with genu varum; the specific growth-plate route is not established by the cited source.
    evidence:
    - reference: PMID:32864841
      reference_title: "Natural history of achondroplasia: A retrospective review of longitudinal clinical data."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "Cervical and lumbar stenosis were diagnosed in children and adults while, genu varum, elbow contractures, and radial head dislocations were identified during childhood."
      explanation: The same childhood angular-deformity observation supports lower-limb bowing (which overlaps clinically with genu varum) as to timing, not causal route.

- name: Reduced longitudinal growth of endochondral bones
  description: >-
    Reduced long-bone growth produces disproportionate short stature,
    rhizomelic limb shortening, and characteristic hand shortening.
  evidence:
  - reference: url:https://www.ncbi.nlm.nih.gov/books/NBK1152/
    reference_title: Achondroplasia - GeneReviews® - NCBI Bookshelf
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "Individuals with achondroplasia have short stature with rhizomelic shortening of the limbs, macrocephaly, characteristic facies with frontal bossing and midface retrusion, exaggerated lumbar lordosis, limitation of elbow extension and rotation, genu varum, brachydactyly, and trident appearance of the hands."
    explanation: The current review documents the characteristic pattern.
  downstream:
  - target: Disproportionate short stature
    hypothesis_groups:
    - canonical_fgfr3_gain_of_function_chondrocyte_model
    causal_link_type: DIRECT
    description: Reduced longitudinal growth produces disproportionate stature.
    evidence:
    - reference: PMID:32803853
      reference_title: "Birth prevalence of achondroplasia: A systematic literature review and meta-analysis."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "Achondroplasia is a genetic disorder that results in disproportionate short stature."
      explanation: The review identifies the defining result.
  - target: Rhizomelia
    hypothesis_groups:
    - canonical_fgfr3_gain_of_function_chondrocyte_model
    causal_link_type: INDIRECT_KNOWN_INTERMEDIATES
    intermediate_mechanisms:
    - Preferential undergrowth of proximal long-bone segments
    description: Proximal long-bone undergrowth produces rhizomelia.
    evidence:
    - reference: PMID:29972438
      reference_title: Natural history of 39 patients with Achondroplasia.
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "The most prevalent radiographic findings were rhizomelic shortening of the long bones and narrowing of the interpediculate distance of the caudal spine."
      explanation: Cohort radiographs document rhizomelic shortening.
  - target: Limb undergrowth
    hypothesis_groups:
    - canonical_fgfr3_gain_of_function_chondrocyte_model
    causal_link_type: DIRECT
    description: Reduced longitudinal growth is expressed as limb undergrowth.
    evidence:
    - reference: ORPHA:15
      reference_title: Achondroplasia
      supports: SUPPORT
      evidence_source: OTHER
      snippet: "HP:0009826 | Limb undergrowth | Very frequent (99-80%)"
      explanation: Orphanet documents limb undergrowth as a core feature.
  - target: Brachydactyly
    hypothesis_groups:
    - canonical_fgfr3_gain_of_function_chondrocyte_model
    causal_link_type: INDIRECT_KNOWN_INTERMEDIATES
    intermediate_mechanisms:
    - Reduced longitudinal growth of metacarpals and phalanges
    description: Shortening of endochondral hand bones produces brachydactyly.
    evidence:
    - reference: url:https://www.ncbi.nlm.nih.gov/books/NBK1152/
      reference_title: Achondroplasia - GeneReviews® - NCBI Bookshelf
      supports: SUPPORT
      evidence_source: OTHER
      snippet: "Individuals with achondroplasia have short stature with rhizomelic shortening of the limbs, macrocephaly, characteristic facies with frontal bossing and midface retrusion, exaggerated lumbar lordosis, limitation of elbow extension and rotation, genu varum, brachydactyly, and trident appearance of the hands."
      explanation: The review supports brachydactyly within the pattern.
  - target: Trident hand
    hypothesis_groups:
    - canonical_fgfr3_gain_of_function_chondrocyte_model
    causal_link_type: INDIRECT_KNOWN_INTERMEDIATES
    intermediate_mechanisms:
    - Phalangeal shortening and separation of the third and fourth digits
    description: Hand-bone shortening contributes to the trident configuration.
    evidence:
    - reference: url:https://www.ncbi.nlm.nih.gov/books/NBK1152/
      reference_title: Achondroplasia - GeneReviews® - NCBI Bookshelf
      supports: SUPPORT
      evidence_source: OTHER
      snippet: "Individuals with achondroplasia have short stature with rhizomelic shortening of the limbs, macrocephaly, characteristic facies with frontal bossing and midface retrusion, exaggerated lumbar lordosis, limitation of elbow extension and rotation, genu varum, brachydactyly, and trident appearance of the hands."
      explanation: The review supports the characteristic trident hand.

- name: Cranial-base and vertebral synchondrosis dysplasia
  description: >-
    FGFR3-MAPK activation promotes premature closure in models and severe human
    FGFR3 dysplasias. CT in heterozygous achondroplasia documents skull-base
    fusion and age-associated craniofacial severity.
  cell_types:
  - preferred_term: growth plate cartilage chondrocyte
    term:
      id: CL:1000217
      label: growth plate cartilage chondrocyte
  evidence:
  - reference: PMID:18923003
    reference_title: FGFR3 promotes synchondrosis closure and fusion of ossification centers through the MAPK pathway.
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: "Chondrocyte-specific activation of Fgfr3 in mice induced premature synchondrosis closure and enhanced osteoblast differentiation around synchondroses."
    explanation: Chondrocyte-specific activation establishes a causal model mechanism.
  - reference: PMID:37072824
    reference_title: "Craniofacial growth and function in achondroplasia: a multimodal 3D study on 15 patients."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "All patients with available CT-scan had premature fusion of skull base synchondroses."
    explanation: CT documents skull-base fusion in a clinical cohort.
  downstream:
  - target: Foramen magnum stenosis
    hypothesis_groups:
    - canonical_fgfr3_gain_of_function_chondrocyte_model
    causal_link_type: INDIRECT_KNOWN_INTERMEDIATES
    intermediate_mechanisms:
    - Restricted endochondral growth of the skull base
    - Reduced bony dimensions of the foramen magnum
    description: Skull-base growth restriction contributes to narrowing.
    evidence:
    - reference: PMID:32883660
      reference_title: "Achondroplasia Foramen Magnum Score: screening infants for stenosis."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "Of 36 infants (M:F, 18:18), 2 (5.6%) did not have FMS (AFMS0); 13 (36.1%) had FMS with preservation of the cerebrospinal fluid (CSF) spaces (AFMS1); 3 (8.3%) had FMS with loss of the CSF space but no spinal cord distortion (AFMS2); 13 (36.1%) had FMS with flattening of the cervical cord without signal change (AFMS3); and 5 (13.9%) had FMS resulting in cervical cord signal change (AFMS4)."
      explanation: MRI establishes the stenosis spectrum; the upstream link remains partly inferential.
  - target: Jugular-foramen venous outflow impairment
    hypothesis_groups:
    - canonical_fgfr3_gain_of_function_chondrocyte_model
    causal_link_type: INDIRECT_KNOWN_INTERMEDIATES
    intermediate_mechanisms:
    - Reduced skull-base dimensions
    - Jugular-foramen narrowing
    description: Skull-base undergrowth can narrow jugular foramina.
    evidence:
    - reference: PMID:33579320
      reference_title: Predictors of cervical myelopathy and hydrocephalus in young children with achondroplasia.
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "the size of the cranial base, which is formed by endochondral ossification, is smaller, and the jugular foramen is also smaller. This condition causes a partial obstruction of venous flow, which increases intracranial venous pressure and causes problems with the venous resorption of cerebrospinal fluid (CSF)"
      explanation: The study states the skull-base-to-venous-obstruction mechanism.
  - target: Craniofacial and upper-airway restriction
    hypothesis_groups:
    - canonical_fgfr3_gain_of_function_chondrocyte_model
    causal_link_type: UNKNOWN
    description: Skull-base fusion co-occurs with retrusion; direction remains incompletely resolved.
    evidence:
    - reference: PMID:37072824
      reference_title: "Craniofacial growth and function in achondroplasia: a multimodal 3D study on 15 patients."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "3D morphometric analyses showed more severe craniofacial phenotypes associated with increasing patient age, predominantly regarding the midface-with increased maxillary retrusion in older patients-and the skull base-with closure of the spheno-occipital angle."
      explanation: Cross-sectional associations support coupled skull-base and midface morphology.
  - target: Caudal interpedicular narrowing
    hypothesis_groups:
    - canonical_fgfr3_gain_of_function_chondrocyte_model
    causal_link_type: UNKNOWN
    description: Vertebral dysplasia is associated with the caudal interpedicular pattern.
    evidence:
    - reference: PMID:29972438
      reference_title: Natural history of 39 patients with Achondroplasia.
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "The most prevalent radiographic findings were rhizomelic shortening of the long bones and narrowing of the interpediculate distance of the caudal spine."
      explanation: Radiographs establish the finding but not the complete upstream route.

- name: Jugular-foramen venous outflow impairment
  description: >-
    Narrow jugular foramina can obstruct cerebral venous drainage, increasing
    venous pressure and impairing cerebrospinal-fluid resorption.
  evidence:
  - reference: PMID:33579320
    reference_title: Predictors of cervical myelopathy and hydrocephalus in young children with achondroplasia.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "This condition causes a partial obstruction of venous flow, which increases intracranial venous pressure and causes problems with the venous resorption of cerebrospinal fluid (CSF)"
    explanation: The study states the venous and CSF-resorption mechanism.
  downstream:
  - target: Hydrocephalus
    hypothesis_groups:
    - canonical_fgfr3_gain_of_function_chondrocyte_model
    causal_link_type: INDIRECT_KNOWN_INTERMEDIATES
    intermediate_mechanisms:
    - Raised intracranial venous pressure
    - Impaired cerebrospinal-fluid resorption
    description: Venous hypertension and impaired CSF resorption can produce hydrocephalus.
    evidence:
    - reference: PMID:33579320
      reference_title: Predictors of cervical myelopathy and hydrocephalus in young children with achondroplasia.
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "This condition causes a partial obstruction of venous flow, which increases intracranial venous pressure and causes problems with the venous resorption of cerebrospinal fluid (CSF)"
      explanation: The source supplies both named intermediates.

- name: Craniofacial and upper-airway restriction
  description: >-
    Midface and maxillomandibular retrusion reduce upper-airway dimensions;
    lymphoid hypertrophy, airway malacia, and hypotonia can add obstruction.
  evidence:
  - reference: PMID:37072824
    reference_title: "Craniofacial growth and function in achondroplasia: a multimodal 3D study on 15 patients."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "We report a significant correlation between the severity of maxillo-mandibular retrusion and obstructive sleep apnea syndrome (p < 0.01)."
    explanation: Clinical morphometry links retrusion with obstructive sleep apnea.
  downstream:
  - target: Midface retrusion
    hypothesis_groups:
    - canonical_fgfr3_gain_of_function_chondrocyte_model
    causal_link_type: UNKNOWN
    description: Midface retrusion is an anatomical component of restriction.
    evidence:
    - reference: PMID:37072824
      reference_title: "Craniofacial growth and function in achondroplasia: a multimodal 3D study on 15 patients."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "Craniofacial phenotype was characterized by maxillo-zygomatic retrusion, deep nasal root, and prominent forehead."
      explanation: Multimodal assessment directly documents retrusion.
  - target: Obstructive sleep apnea
    hypothesis_groups:
    - canonical_fgfr3_gain_of_function_chondrocyte_model
    causal_link_type: UNKNOWN
    description: Retrusion contributes to, but does not solely determine, obstruction.
    evidence:
    - reference: PMID:37072824
      reference_title: "Craniofacial growth and function in achondroplasia: a multimodal 3D study on 15 patients."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "We report a significant correlation between the severity of maxillo-mandibular retrusion and obstructive sleep apnea syndrome (p < 0.01)."
      explanation: The association supports a contribution without proving sole causation.

- name: Congenital spinal-canal constraint with superimposed degeneration
  description: >-
    Altered vertebral geometry constrains the canal; disc degeneration and
    sagittal deformity add to adult upper-lumbar narrowing.
  evidence:
  - reference: PMID:32170149
    reference_title: Lumbar spinal stenosis and disc alterations affect the upper lumbar spine in adults with achondroplasia.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Unlike the general population, spinal stenosis and disc degeneration involve the upper part of the lumbar spine in adults with achondroplasia, associated with thoraco-lumbar kyphosis and loss of lumbar lordosis."
    explanation: Adult imaging supports combined stenotic and degenerative components.
  downstream:
  - target: Spinal canal stenosis
    hypothesis_groups:
    - canonical_fgfr3_gain_of_function_chondrocyte_model
    causal_link_type: DIRECT
    description: Congenital constraint and degeneration produce recognized stenosis.
    evidence:
    - reference: PMID:32864841
      reference_title: "Natural history of achondroplasia: A retrospective review of longitudinal clinical data."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "Cervical and lumbar stenosis were diagnosed in children and adults while, genu varum, elbow contractures, and radial head dislocations were identified during childhood."
      explanation: Longitudinal records establish cervical and lumbar stenosis.
  - target: Cervical spinal canal stenosis
    hypothesis_groups:
    - canonical_fgfr3_gain_of_function_chondrocyte_model
    causal_link_type: DIRECT
    description: The congenital component can affect the cervical canal.
    evidence:
    - reference: PMID:32864841
      reference_title: "Natural history of achondroplasia: A retrospective review of longitudinal clinical data."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "Cervical and lumbar stenosis were diagnosed in children and adults while, genu varum, elbow contractures, and radial head dislocations were identified during childhood."
      explanation: The study explicitly reports cervical stenosis.

- name: CNP-NPR2 counter-regulation of FGFR3-MAPK signaling
  conforms_to: "fgfr_gain_of_function_skeletal_dysplasia#CNP-NPR2 Counter-Regulation and FGFR-Pathway Antagonist Therapy"
  role: therapeutic_vulnerability
  description: >-
    CNP signaling through NPR2/GC-B counteracts FGFR3-MAPK activity. CNP
    corrected matrix synthesis and bone shortening in a mouse model without
    correcting the STAT1 branch.
  cell_types:
  - preferred_term: growth plate cartilage chondrocyte
    term:
      id: CL:1000217
      label: growth plate cartilage chondrocyte
  biological_processes:
  - preferred_term: receptor guanylyl cyclase signaling pathway
    term:
      id: GO:0007168
      label: receptor guanylyl cyclase signaling pathway
  - preferred_term: MAPK cascade
    term:
      id: GO:0000165
      label: MAPK cascade
    modifier: DECREASED
  evidence:
  - reference: PMID:14702637
    reference_title: Overexpression of CNP in chondrocytes rescues achondroplasia through a MAPK-dependent pathway.
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: "CNP prevented the shortening of achondroplastic bones by correcting the decreased extracellular matrix synthesis in the growth plate through inhibition of the MAPK pathway of FGF signaling."
    explanation: Mouse data support CNP counter-regulation of MAPK.
  - reference: PMID:14702637
    reference_title: Overexpression of CNP in chondrocytes rescues achondroplasia through a MAPK-dependent pathway.
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: "CNP had no effect on the STAT-1 pathway of FGF signaling that mediates the decreased proliferation and the delayed differentiation of achondroplastic chondrocytes."
    explanation: The model defines the limitation on the STAT1 branch.
phenotypes:
- name: Disproportionate short stature
  description: >
    Disproportionate short-limb short stature is a defining skeletal manifestation
    of achondroplasia.
  phenotype_term:
    preferred_term: Disproportionate short-limb short stature
    term:
      id: HP:0008873
      label: Disproportionate short-limb short stature
  evidence:
  - reference: PMID:32803853
    reference_title: "Birth prevalence of achondroplasia: A systematic literature review and meta-analysis."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Achondroplasia is a genetic disorder that results in disproportionate short stature."
    explanation: Supports disproportionate short stature as a core phenotype of achondroplasia.
- name: Rhizomelia
  description: >
    Shortening is most pronounced in the proximal long bones. Orphanet codes
    the strict HPO term HP:0008905 (Rhizomelia) as Occasional; the broader
    rhizomelic shortening pattern is characteristic but captured under
    Disproportionate short stature and Limb undergrowth.
  phenotype_term:
    preferred_term: Rhizomelia
    term:
      id: HP:0008905
      label: Rhizomelia
  evidence:
  - reference: PMID:29972438
    reference_title: "Natural history of 39 patients with Achondroplasia."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The most prevalent radiographic findings were rhizomelic shortening of the long bones and narrowing of the interpediculate distance of the caudal spine."
    explanation: Supports rhizomelic long-bone shortening as a characteristic skeletal manifestation.
  - reference: ORPHA:15
    reference_title: Achondroplasia
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "HP:0008905 | Rhizomelia | Occasional (29-5%)"
    explanation: Orphanet classifies rhizomelia as occasional in achondroplasia.
- name: Macrocephaly
  frequency: FREQUENT
  description: >
    Macrocephaly is part of the characteristic cranial phenotype.
  phenotype_term:
    preferred_term: Macrocephaly
    term:
      id: HP:0000256
      label: Macrocephaly
  evidence:
  - reference: PMID:29972438
    reference_title: "Natural history of 39 patients with Achondroplasia."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The most prevalent clinical findings were short stature, high forehead, trident hands, genu varum and macrocephaly."
    explanation: Clinical cohort data identify macrocephaly as one of the most prevalent physical findings.
  - reference: ORPHA:15
    reference_title: Achondroplasia
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "HP:0000256 | Macrocephaly | Frequent (79-30%)"
    explanation: Orphanet classifies macrocephaly as frequent in achondroplasia.
- name: Parietal bossing
  frequency: FREQUENT
  description: >
    Parietal bossing is part of the cranial-vault expansion phenotype that
    accompanies macrocephaly in achondroplasia.
  phenotype_term:
    preferred_term: Parietal bossing
    term:
      id: HP:0000242
      label: Parietal bossing
  evidence:
  - reference: ORPHA:15
    reference_title: Achondroplasia
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "HP:0000242 | Parietal bossing | Frequent (79-30%)"
    explanation: Orphanet classifies parietal bossing as frequent in achondroplasia.
- name: Wide anterior fontanel
  frequency: OCCASIONAL
  description: >
    A widened anterior fontanel can be present as an occasional cranial
    ossification finding in achondroplasia.
  phenotype_term:
    preferred_term: Wide anterior fontanel
    term:
      id: HP:0000260
      label: Wide anterior fontanel
  evidence:
  - reference: ORPHA:15
    reference_title: Achondroplasia
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "HP:0000260 | Wide anterior fontanel | Occasional (29-5%)"
    explanation: Orphanet classifies wide anterior fontanel as occasional in achondroplasia.
- name: Prominent forehead
  description: >
    Prominent forehead is a characteristic craniofacial feature.
  phenotype_term:
    preferred_term: Prominent forehead
    term:
      id: HP:0011220
      label: Prominent forehead
  evidence:
  - reference: PMID:37072824
    reference_title: "Craniofacial growth and function in achondroplasia: a multimodal 3D study on 15 patients."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Craniofacial phenotype was characterized by maxillo-zygomatic retrusion, deep nasal root, and prominent forehead."
    explanation: Multimodal craniofacial study directly documents prominent forehead in pediatric achondroplasia.
- name: Midface retrusion
  description: >
    Midface retrusion contributes to the characteristic craniofacial appearance.
  phenotype_term:
    preferred_term: Midface retrusion
    term:
      id: HP:0011800
      label: Midface retrusion
  evidence:
  - reference: PMID:37072824
    reference_title: "Craniofacial growth and function in achondroplasia: a multimodal 3D study on 15 patients."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Craniofacial phenotype was characterized by maxillo-zygomatic retrusion, deep nasal root, and prominent forehead."
    explanation: Maxillo-zygomatic retrusion directly supports midface retrusion as a core craniofacial phenotype.
- name: Anteverted nares
  frequency: FREQUENT
  description: >
    Anteverted nares are a frequent nasal manifestation within the characteristic
    midface phenotype of achondroplasia.
  phenotype_term:
    preferred_term: Anteverted nares
    term:
      id: HP:0000463
      label: Anteverted nares
  evidence:
  - reference: ORPHA:15
    reference_title: Achondroplasia
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "HP:0000463 | Anteverted nares | Frequent (79-30%)"
    explanation: Orphanet classifies anteverted nares as frequent in achondroplasia.
- name: Trident hand
  frequency: FREQUENT
  description: >
    Trident hand is a characteristic hand configuration in achondroplasia.
  phenotype_term:
    preferred_term: Trident hand
    term:
      id: HP:0004060
      label: Trident hand
  evidence:
  - reference: PMID:29972438
    reference_title: "Natural history of 39 patients with Achondroplasia."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The most prevalent clinical findings were short stature, high forehead, trident hands, genu varum and macrocephaly."
    explanation: Clinical cohort data identify trident hands as one of the most prevalent physical findings.
  - reference: ORPHA:15
    reference_title: Achondroplasia
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "HP:0004060 | Trident hand | Frequent (79-30%)"
    explanation: Orphanet classifies trident hand as frequent in achondroplasia.
- name: Bowing of the legs
  frequency: VERY_FREQUENT
  description: >
    Lower-limb bowing is a very frequent orthopedic feature in achondroplasia,
    comprising genu varum (outward bowing at the knees, bow-legs) and a complex
    deformity involving lateral bowing, differential tibial and fibular growth,
    internal tibial torsion, and dynamic knee instability. It is typically
    recognized during childhood.
  phenotype_term:
    preferred_term: Bowing of the legs
    term:
      id: HP:0002979
      label: Bowing of the legs
    onset:
      onset_category: CHILDHOOD
  evidence:
  - reference: PMID:29972438
    reference_title: "Natural history of 39 patients with Achondroplasia."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The most prevalent clinical findings were short stature, high forehead, trident hands, genu varum and macrocephaly."
    explanation: Clinical cohort data identify lower-limb deformity (including genu varum) as one of the most prevalent physical findings.
  - reference: ORPHA:15
    reference_title: Achondroplasia
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "HP:0002979 | Bowing of the legs | Very frequent (99-80%)"
    explanation: Orphanet classifies bowing of the legs as very frequent in achondroplasia.
  - reference: PMID:32864841
    reference_title: "Natural history of achondroplasia: A retrospective review of longitudinal clinical data."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "genu varum, elbow contractures, and radial head dislocations were identified during childhood"
    explanation: Chart-review cohort places recognition of genu varum in childhood, preserving the onset nuance of the merged entry.
  - reference: url:https://www.ncbi.nlm.nih.gov/books/NBK1152/
    reference_title: Achondroplasia - GeneReviews® - NCBI Bookshelf
    supports: SUPPORT
    evidence_source: OTHER
    snippet: '"Bowing" is actually a complex deformity arising from a combination of lateral bowing, differential growth between the tibia and fibula, internal tibial torsion, and dynamic instability of the knee'
    explanation: Sources the complex-deformity clause of the description (lateral bowing, differential tibia-fibula growth, internal tibial torsion, dynamic knee instability).
- name: Thoracolumbar kyphosis
  frequency: VERY_FREQUENT
  description: >
    Thoracolumbar kyphosis is common in young children and often improves over time.
  phenotype_term:
    preferred_term: Thoracolumbar kyphosis
    term:
      id: HP:0005619
      label: Thoracolumbar kyphosis
  evidence:
  - reference: PMID:37493935
    reference_title: "Walking status and spinopelvic parameters in young children with achondroplasia: 10-year follow-up."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Thoracolumbar kyphosis (TLK) is common in children with achondroplasia and resolves in 90% by 10 years of age."
    explanation: Supports both childhood onset and the usual tendency toward spontaneous improvement by age 10.
  - reference: PMID:27927547
    reference_title: "Prevalence of Scoliosis and Thoracolumbar Kyphosis in Patients With Achondroplasia."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Thoracolumbar kyphosis was observed in 79%, with 52% exhibiting moderate to severe curvature."
    explanation: Large orthopedic cohort documents thoracolumbar kyphosis as a common spinal phenotype.
  - reference: ORPHA:15
    reference_title: Achondroplasia
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "HP:0005619 | Thoracolumbar kyphosis | Very frequent (99-80%)"
    explanation: Orphanet classifies thoracolumbar kyphosis as very frequent in achondroplasia.
- name: Scoliosis
  description: >
    Scoliosis was reported in 60% of one orthopedic cohort; this
    specialty-clinic estimate is not treated as a population frequency.
  phenotype_term:
    preferred_term: Scoliosis
    term:
      id: HP:0002650
      label: Scoliosis
  evidence:
  - reference: PMID:27927547
    reference_title: "Prevalence of Scoliosis and Thoracolumbar Kyphosis in Patients With Achondroplasia."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Scoliosis was observed in 60%."
    explanation: An orthopedic cohort documents scoliosis in 60% of its selected patient population.
- name: Foramen magnum stenosis
  description: >
    Foramen magnum stenosis is a major infant complication that can cause cervical cord compression.
  phenotype_term:
    preferred_term: Small foramen magnum
    term:
      id: HP:0002677
      label: Small foramen magnum
  evidence:
  - reference: PMID:32883660
    reference_title: "Achondroplasia Foramen Magnum Score: screening infants for stenosis."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Of 36 infants (M:F, 18:18), 2 (5.6%) did not have FMS (AFMS0); 13 (36.1%) had FMS with preservation of the cerebrospinal fluid (CSF) spaces (AFMS1); 3 (8.3%) had FMS with loss of the CSF space but no spinal cord distortion (AFMS2); 13 (36.1%) had FMS with flattening of the cervical cord without signal change (AFMS3); and 5 (13.9%) had FMS resulting in cervical cord signal change (AFMS4)."
    explanation: Infant MRI cohort shows that most screened infants had foramen magnum stenosis, including many with cord flattening or signal change.
  - reference: PMID:38554024
    reference_title: "Clinical outcomes and medical management of achondroplasia in Japanese children: A retrospective medical record review of clinical data."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "All patients were monitored by magnetic resonance imaging; 73.0% had foramen magnum stenosis, while 54.1% had Achondroplasia Foramen Magnum Score 3 or 4."
    explanation: Independent pediatric cohort confirms that foramen magnum stenosis is a frequent early complication.
- name: Spinal canal stenosis
  frequency: FREQUENT
  description: >
    Cervical and lumbar spinal canal stenosis occur across the lifespan and symptomatic lumbar disease is especially important in adults.
  phenotype_term:
    preferred_term: Spinal canal stenosis
    term:
      id: HP:0003416
      label: Spinal canal stenosis
  evidence:
  - reference: PMID:32864841
    reference_title: "Natural history of achondroplasia: A retrospective review of longitudinal clinical data."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Cervical and lumbar stenosis were diagnosed in children and adults while, genu varum, elbow contractures, and radial head dislocations were identified during childhood."
    explanation: Longitudinal chart review supports spinal stenosis as a cross-lifespan complication affecting both cervical and lumbar regions.
  - reference: PMID:32170149
    reference_title: "Lumbar spinal stenosis and disc alterations affect the upper lumbar spine in adults with achondroplasia."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Unlike the general population, spinal stenosis and disc degeneration involve the upper part of the lumbar spine in adults with achondroplasia, associated with thoraco-lumbar kyphosis and loss of lumbar lordosis."
    explanation: Adult imaging study refines the adult lumbar phenotype by showing upper-lumbar predominance with associated degenerative change.
  - reference: ORPHA:15
    reference_title: Achondroplasia
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "HP:0003416 | Spinal canal stenosis | Frequent (79-30%)"
    explanation: Orphanet classifies spinal canal stenosis as frequent in achondroplasia.
- name: Cervical spinal canal stenosis
  frequency: FREQUENT
  description: >
    Cervical canal stenosis is a more specific spinal stenosis manifestation
    that can occur in childhood and contributes to craniocervical risk.
  phenotype_term:
    preferred_term: Cervical spinal canal stenosis
    term:
      id: HP:0008445
      label: Cervical spinal canal stenosis
  evidence:
  - reference: PMID:32864841
    reference_title: "Natural history of achondroplasia: A retrospective review of longitudinal clinical data."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Cervical and lumbar stenosis were diagnosed in children and adults while, genu varum, elbow contractures, and radial head dislocations were identified during childhood."
    explanation: Longitudinal chart review explicitly reports cervical stenosis in affected patients.
  - reference: ORPHA:15
    reference_title: Achondroplasia
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "HP:0008445 | Cervical spinal canal stenosis | Frequent (79-30%)"
    explanation: Orphanet classifies cervical spinal canal stenosis as frequent in achondroplasia.
- name: Obstructive sleep apnea
  frequency: FREQUENT
  description: >
    Obstructive sleep apnea is a major respiratory complication in children and can recur later in life.
    Polysomnography can reveal sleep-disordered breathing even when children have
    no prior symptoms.
  phenotype_term:
    preferred_term: Obstructive sleep apnea
    term:
      id: HP:0002870
      label: Obstructive sleep apnea
  evidence:
  - reference: PMID:40675782
    reference_title: "Sleep-disordered breathing in children with achondroplasia assessed by polysomnography: a retrospective chart review."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Sleep-disordered breathing subtypes included obstructive sleep apnoea in 81% (55/68), central sleep apnoea in 3% (2/68), mixed sleep apnoea in 7% (5/68) and primary snoring in 9% (6/68)."
    explanation: In a pediatric polysomnography cohort, 55 of 80 children had obstructive sleep apnoea and 5 additional children had mixed apnoea with an obstructive component.
  - reference: PMID:40675782
    reference_title: "Sleep-disordered breathing in children with achondroplasia assessed by polysomnography: a retrospective chart review."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "A total of 85% (68/80) had sleep-disordered breathing and 21% reported no prior symptoms."
    explanation: Pediatric polysomnography data show that sleep-disordered breathing can be clinically silent before screening.
  - reference: PMID:32864841
    reference_title: "Natural history of achondroplasia: A retrospective review of longitudinal clinical data."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Central sleep apnea and obstructive sleep apnea were present in children, while the diagnosis of obstructive sleep apnea was shown to recur in adulthood."
    explanation: Longitudinal review supports both childhood occurrence and adult recurrence of obstructive sleep apnea.
  - reference: ORPHA:15
    reference_title: Achondroplasia
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "HP:0002870 | Obstructive sleep apnea | Frequent (79-30%)"
    explanation: Orphanet classifies obstructive sleep apnea as frequent in achondroplasia.
  - reference: PMID:37072824
    reference_title: "Craniofacial growth and function in achondroplasia: a multimodal 3D study on 15 patients."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "We report a significant correlation between the severity of maxillo-mandibular retrusion and obstructive sleep apnea syndrome (p < 0.01)."
    explanation: The correlation supports an anatomical contribution from maxillomandibular retrusion but does not establish sole causation.
- name: Central sleep apnea
  description: >
    Central sleep apnea occurs in some affected children and is part of the sleep-disordered breathing spectrum in achondroplasia.
  phenotype_term:
    preferred_term: Central sleep apnea
    term:
      id: HP:0010536
      label: Central sleep apnea
  evidence:
  - reference: PMID:32864841
    reference_title: "Natural history of achondroplasia: A retrospective review of longitudinal clinical data."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Central sleep apnea and obstructive sleep apnea were present in children, while the diagnosis of obstructive sleep apnea was shown to recur in adulthood."
    explanation: Longitudinal natural-history data explicitly document central sleep apnea in children with achondroplasia.
  - reference: PMID:40675782
    reference_title: "Sleep-disordered breathing in children with achondroplasia assessed by polysomnography: a retrospective chart review."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Sleep-disordered breathing subtypes included obstructive sleep apnoea in 81% (55/68), central sleep apnoea in 3% (2/68), mixed sleep apnoea in 7% (5/68) and primary snoring in 9% (6/68)."
    explanation: Polysomnography cohort confirms central sleep apnoea as a measured pediatric sleep phenotype, although much less common than obstructive disease.
- name: Functional abnormality of the middle ear
  frequency: FREQUENT
  description: >
    Middle-ear dysfunction is a frequent childhood intercurrent problem and
    underlies the more specific otitis-media-with-effusion and conductive
    hearing-loss burden.
  phenotype_term:
    preferred_term: Functional abnormality of the middle ear
    term:
      id: HP:0011452
      label: Functional abnormality of the middle ear
  evidence:
  - reference: PMID:29972438
    reference_title: "Natural history of 39 patients with Achondroplasia."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The most common clinical intercurrences were middle ear dysfunction, sleep apnea, limb pain and obesity from 2 to 9 years of age."
    explanation: Natural-history cohort lists middle-ear dysfunction among the most common childhood clinical intercurrences.
  - reference: ORPHA:15
    reference_title: Achondroplasia
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "HP:0011452 | Functional abnormality of the middle ear | Frequent (79-30%)"
    explanation: Orphanet classifies functional abnormality of the middle ear as frequent in achondroplasia.
- name: Otitis media with effusion
  description: >
    Otitis media with effusion is a common otologic complication and often leads to repeated ventilation-tube placement.
  phenotype_term:
    preferred_term: Otitis media with effusion
    term:
      id: HP:0031353
      label: Otitis media with effusion
  evidence:
  - reference: PMID:39660705
    reference_title: "Otologic Manifestations in Patients with Achondroplasia: A Multicenter Study."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Fifty-one patients (72.9%) had middle ear effusion at least once."
    explanation: Multicenter otology cohort shows otitis media with effusion in most evaluated patients.
- name: Conductive hearing impairment
  description: >
    Hearing loss is common and is usually conductive or mixed rather than purely sensorineural.
  phenotype_term:
    preferred_term: Conductive hearing impairment
    term:
      id: HP:0000405
      label: Conductive hearing impairment
  evidence:
  - reference: PMID:39660705
    reference_title: "Otologic Manifestations in Patients with Achondroplasia: A Multicenter Study."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Among 53 patients who underwent audiometry, 26 showed conductive hearing loss, 2 had mixed-type hearing loss, and 4 had sensorineural hearing loss."
    explanation: Pediatric multicenter cohort shows that hearing loss in achondroplasia is usually conductive or mixed.
  - reference: PMID:34736503
    reference_title: "Hearing loss in Norwegian adults with achondroplasia."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The majority (15/24) had conductive hearing loss, or a combination of conductive and sensorineural hearing loss (8/24)."
    explanation: Adult population-based cohort shows conductive or mixed hearing loss remains common beyond childhood.
  - reference: ORPHA:15
    reference_title: Achondroplasia
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "HP:0000365 | Hearing impairment | Frequent (79-30%)"
    explanation: Orphanet supports frequent hearing impairment generally; the conductive subtype is established by the cohort evidence above.
- name: Obesity
  description: >
    Obesity can begin in early childhood and was common across ages in a classic
    clinical series; a population frequency is not assigned here.
  phenotype_term:
    preferred_term: Obesity
    term:
      id: HP:0001513
      label: Obesity
  evidence:
  - reference: PMID:3228140
    reference_title: "Obesity in achondroplasia."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Obesity is a significant and potentially serious health problem in achondroplasia. Body mass indices, weight-to-square of the height ratio (W/H2), and triceps skinfold measurements show that obesity is common. It begins in early childhood and is prevalent at all ages."
    explanation: Classic cohort study directly supports obesity as an early and persistent complication.
- name: Hydrocephalus
  frequency: VERY_RARE
  description: >
    Hydrocephalus is a rare but clinically important neurosurgical complication in childhood.
    It is typically communicating and attributed to endochondral hypoplasia of the skull base, which
    narrows the jugular foramina and obstructs cerebral venous outflow, raising intracranial venous
    pressure and impairing CSF resorption.
  phenotype_term:
    preferred_term: Hydrocephalus
    term:
      id: HP:0000238
      label: Hydrocephalus
  evidence:
  - reference: PMID:33579320
    reference_title: "Predictors of cervical myelopathy and hydrocephalus in young children with achondroplasia."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Cervical myelopathy and hydrocephalus occasionally occur in young children with achondroplasia."
    explanation: Supports hydrocephalus as a recognized pediatric complication without overstating its frequency.
  - reference: PMID:29972438
    reference_title: "Natural history of 39 patients with Achondroplasia."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "One patient developed hydrocephalus at 10 years old."
    explanation: Natural-history cohort provides direct longitudinal evidence that hydrocephalus can occur during follow-up.
  - reference: ORPHA:15
    reference_title: Achondroplasia
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "HP:0000238 | Hydrocephalus | Very rare (<4-1%)"
    explanation: Orphanet classifies hydrocephalus as very rare in achondroplasia.
  - reference: PMID:33579320
    reference_title: "Predictors of cervical myelopathy and hydrocephalus in young children with achondroplasia."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "the size of the cranial base, which is formed by endochondral ossification, is smaller, and the jugular foramen is also smaller. This condition causes a partial obstruction of venous flow, which increases intracranial venous pressure and causes problems with the venous resorption of cerebrospinal fluid (CSF)"
    explanation: Describes the mechanistic basis of communicating hydrocephalus in achondroplasia—endochondral hypoplasia of the skull base narrows the jugular foramina, causing venous outflow obstruction, intracranial venous hypertension, and impaired CSF resorption.
- name: Delayed gross motor development
  description: >
    Children with achondroplasia show delayed acquisition of gross motor and ambulatory skills.
  phenotype_term:
    preferred_term: Delayed gross motor development
    term:
      id: HP:0002194
      label: Delayed gross motor development
  evidence:
  - reference: PMID:22409389
    reference_title: "Development in children with achondroplasia: a prospective clinical cohort study."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "children with achondroplasia are delayed in development of gross motor and ambulatory skills."
    explanation: Prospective developmental cohort directly supports delayed gross motor development.
  - reference: PMID:29972438
    reference_title: "Natural history of 39 patients with Achondroplasia."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "There was motor developmental delay in 18 patients and speech delay in 16 patients."
    explanation: Independent natural-history cohort corroborates developmental delay affecting motor milestones.
- name: Delayed speech and language development
  description: >
    Speech and language delay is reported in pediatric achondroplasia cohorts.
  phenotype_term:
    preferred_term: Delayed speech and language development
    term:
      id: HP:0000750
      label: Delayed speech and language development
  evidence:
  - reference: PMID:29972438
    reference_title: "Natural history of 39 patients with Achondroplasia."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "There was motor developmental delay in 18 patients and speech delay in 16 patients."
    explanation: Natural-history cohort directly reports speech delay in a substantial subset of patients.
  - reference: PMID:22409389
    reference_title: "Development in children with achondroplasia: a prospective clinical cohort study."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "While delays were seen in development of later communication items, there were fewer delays seen across development of early communication, fine motor, and feeding skills."
    explanation: Prospective developmental study supports delayed later communication skill acquisition.
- name: Elbow contracture
  description: >
    Elbow contractures are recognized childhood orthopedic manifestations.
  phenotype_term:
    preferred_term: Elbow contracture
    term:
      id: HP:0034391
      label: Elbow contracture
  evidence:
  - reference: PMID:32864841
    reference_title: "Natural history of achondroplasia: A retrospective review of longitudinal clinical data."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Cervical and lumbar stenosis were diagnosed in children and adults while, genu varum, elbow contractures, and radial head dislocations were identified during childhood."
    explanation: Longitudinal chart review identifies elbow contractures as a childhood orthopedic phenotype.
- name: Radial head dislocation
  description: >
    Radial head dislocation is an orthopedic manifestation recognized during childhood.
  phenotype_term:
    preferred_term: Radial head dislocation
    term:
      id: HP:0005070
      label: Proximal radial head dislocation
  evidence:
  - reference: PMID:32864841
    reference_title: "Natural history of achondroplasia: A retrospective review of longitudinal clinical data."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Cervical and lumbar stenosis were diagnosed in children and adults while, genu varum, elbow contractures, and radial head dislocations were identified during childhood."
    explanation: Longitudinal chart review identifies radial head dislocation as a childhood orthopedic phenotype.
- name: Lumbar hyperlordosis
  frequency: FREQUENT
  description: >
    Exaggerated lumbar lordosis is a characteristic spinal deformity in achondroplasia.
  phenotype_term:
    preferred_term: Lumbar hyperlordosis
    term:
      id: HP:0002938
      label: Lumbar hyperlordosis
  evidence:
  - reference: ORPHA:15
    reference_title: Achondroplasia
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "HP:0002938 | Lumbar hyperlordosis | Frequent (79-30%)"
    explanation: Orphanet classifies lumbar hyperlordosis as frequent in achondroplasia.
- name: Brachydactyly
  frequency: FREQUENT
  description: >
    Short fingers are a characteristic hand phenotype in achondroplasia.
  phenotype_term:
    preferred_term: Brachydactyly
    term:
      id: HP:0001156
      label: Brachydactyly
  evidence:
  - reference: ORPHA:15
    reference_title: Achondroplasia
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "HP:0001156 | Brachydactyly | Frequent (79-30%)"
    explanation: Orphanet classifies brachydactyly as frequent in achondroplasia.
- name: Short middle phalanx of finger
  frequency: FREQUENT
  description: >
    Shortening of the middle phalanges contributes to the characteristic short
    fingers and trident hand configuration.
  phenotype_term:
    preferred_term: Short middle phalanx of finger
    term:
      id: HP:0005819
      label: Short middle phalanx of finger
  evidence:
  - reference: ORPHA:15
    reference_title: Achondroplasia
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "HP:0005819 | Short middle phalanx of finger | Frequent (79-30%)"
    explanation: Orphanet classifies short middle phalanx of finger as frequent in achondroplasia.
- name: Short proximal phalanx of finger
  frequency: FREQUENT
  description: >
    Shortening of proximal phalanges is another frequent phalangeal component of
    the achondroplasia hand phenotype.
  phenotype_term:
    preferred_term: Short proximal phalanx of finger
    term:
      id: HP:0010241
      label: Short proximal phalanx of finger
  evidence:
  - reference: ORPHA:15
    reference_title: Achondroplasia
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "HP:0010241 | Short proximal phalanx of finger | Frequent (79-30%)"
    explanation: Orphanet classifies short proximal phalanx of finger as frequent in achondroplasia.
- name: Limited elbow extension
  frequency: FREQUENT
  description: >
    Limited elbow extension is a common upper-limb joint restriction in achondroplasia.
  phenotype_term:
    preferred_term: Limited elbow extension
    term:
      id: HP:0001377
      label: Limited elbow extension
  evidence:
  - reference: ORPHA:15
    reference_title: Achondroplasia
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "HP:0001377 | Limited elbow extension | Frequent (79-30%)"
    explanation: Orphanet classifies limited elbow extension as frequent in achondroplasia.
- name: Floppy infant
  frequency: FREQUENT
  description: >
    Infantile hypotonia is frequently observed and contributes to delayed motor milestones.
  phenotype_term:
    preferred_term: Floppy infant
    term:
      id: HP:0008947
      label: Floppy infant
  evidence:
  - reference: ORPHA:15
    reference_title: Achondroplasia
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "HP:0008947 | Floppy infant | Frequent (79-30%)"
    explanation: Orphanet lists floppy infant as frequent in achondroplasia, supporting infantile muscular hypotonia.
- name: Depressed nasal bridge
  frequency: FREQUENT
  description: >
    Depressed nasal bridge is part of the characteristic midface hypoplasia.
  phenotype_term:
    preferred_term: Depressed nasal bridge
    term:
      id: HP:0005280
      label: Depressed nasal bridge
  evidence:
  - reference: ORPHA:15
    reference_title: Achondroplasia
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "HP:0005280 | Depressed nasal bridge | Frequent (79-30%)"
    explanation: Orphanet classifies depressed nasal bridge as frequent in achondroplasia.
- name: Frontal bossing
  frequency: FREQUENT
  description: >
    Frontal bossing is a characteristic craniofacial feature of achondroplasia.
  phenotype_term:
    preferred_term: Frontal bossing
    term:
      id: HP:0002007
      label: Frontal bossing
  evidence:
  - reference: ORPHA:15
    reference_title: Achondroplasia
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "HP:0002007 | Frontal bossing | Frequent (79-30%)"
    explanation: Orphanet classifies frontal bossing as frequent in achondroplasia.
- name: Restrictive ventilatory defect
  frequency: OCCASIONAL
  description: >
    Restrictive lung disease can occur due to thoracic hypoplasia and reduced chest wall compliance.
  phenotype_term:
    preferred_term: Restrictive ventilatory defect
    term:
      id: HP:0002091
      label: Restrictive ventilatory defect
  evidence:
  - reference: ORPHA:15
    reference_title: Achondroplasia
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "HP:0002091 | Restrictive ventilatory defect | Occasional (29-5%)"
    explanation: Orphanet classifies restrictive ventilatory defect as occasional in achondroplasia.
- name: Thoracic hypoplasia
  frequency: OCCASIONAL
  description: >
    Thoracic hypoplasia provides a structural thoracic context for occasional
    restrictive ventilatory physiology in achondroplasia.
  phenotype_term:
    preferred_term: Thoracic hypoplasia
    term:
      id: HP:0005257
      label: Thoracic hypoplasia
  evidence:
  - reference: ORPHA:15
    reference_title: Achondroplasia
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "HP:0005257 | Thoracic hypoplasia | Occasional (29-5%)"
    explanation: Orphanet classifies thoracic hypoplasia as occasional in achondroplasia.
- name: Acanthosis nigricans
  frequency: OCCASIONAL
  description: >
    Acanthosis nigricans is an occasional skin manifestation in achondroplasia.
  phenotype_term:
    preferred_term: Acanthosis nigricans
    term:
      id: HP:0000956
      label: Acanthosis nigricans
  evidence:
  - reference: ORPHA:15
    reference_title: Achondroplasia
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "HP:0000956 | Acanthosis nigricans | Occasional (29-5%)"
    explanation: Orphanet classifies acanthosis nigricans as occasional in achondroplasia.
  - reference: url:https://www.ncbi.nlm.nih.gov/books/NBK1152/
    reference_title: Achondroplasia - GeneReviews® - NCBI Bookshelf
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "may be seen in about 10% of individuals with achondroplasia"
    explanation: The 2026 clinical review supports a peripheral association while not placing it in the core skeletal mechanism graph.
- name: Knee joint hypermobility
  frequency: FREQUENT
  description: >
    Knee joint hypermobility is a frequent finding in achondroplasia.
  phenotype_term:
    preferred_term: Knee joint hypermobility
    term:
      id: HP:0045086
      label: Knee joint hypermobility
  evidence:
  - reference: ORPHA:15
    reference_title: Achondroplasia
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "HP:0045086 | Knee joint hypermobility | Frequent (79-30%)"
    explanation: Orphanet classifies knee joint hypermobility as frequent in achondroplasia.
- name: Hip joint hypermobility
  frequency: FREQUENT
  description: >
    Hip joint hypermobility is a frequent finding in achondroplasia.
  phenotype_term:
    preferred_term: Hip joint hypermobility
    term:
      id: HP:0045087
      label: Hip joint hypermobility
  evidence:
  - reference: ORPHA:15
    reference_title: Achondroplasia
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "HP:0045087 | Hip joint hypermobility | Frequent (79-30%)"
    explanation: Orphanet classifies hip joint hypermobility as frequent in achondroplasia.
- name: Limb undergrowth
  frequency: VERY_FREQUENT
  description: >
    Limb undergrowth is the core skeletal manifestation of achondroplasia, reflecting impaired endochondral ossification of the appendicular skeleton.
  phenotype_term:
    preferred_term: Limb undergrowth
    term:
      id: HP:0009826
      label: Limb undergrowth
  evidence:
  - reference: ORPHA:15
    reference_title: Achondroplasia
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "HP:0009826 | Limb undergrowth | Very frequent (99-80%)"
    explanation: Orphanet classifies limb undergrowth as very frequent in achondroplasia.
- name: Cervical myelopathy
  description: >
    Cervical (high) myelopathy from cervicomedullary compression at the stenotic foramen
    magnum is a major early complication and can present with long-tract signs, central
    apnea, or respiratory arrest.
  phenotype_term:
    preferred_term: Cervical myelopathy
    term:
      id: HP:0002318
      label: Cervical myelopathy
  evidence:
  - reference: PMID:33579320
    reference_title: "Predictors of cervical myelopathy and hydrocephalus in young children with achondroplasia."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Thirteen (20%) of the 65 patients showed T2 HSI on MRI, long tract sign, or respiratory difficulty that corresponded to cervical myelopathy"
    explanation: A selected tertiary pediatric MRI cohort documented cervical myelopathy in 13 of 65 children; this is not used as a population frequency.
- name: Caudal interpedicular narrowing
  description: >
    Caudal narrowing of lumbar interpedicular distance is a characteristic
    radiographic finding in achondroplasia.
  phenotype_term:
    preferred_term: Caudal interpedicular narrowing
    term:
      id: HP:0008457
      label: Caudal interpedicular narrowing
  evidence:
  - reference: PMID:29972438
    reference_title: "Natural history of 39 patients with Achondroplasia."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The most prevalent radiographic findings were rhizomelic shortening of the long bones and narrowing of the interpediculate distance of the caudal spine."
    explanation: Natural-history cohort identifies caudal interpedicular narrowing as one of the most prevalent radiographic findings in achondroplasia.
- name: Limb pain
  description: >
    Limb pain was among the common clinical intercurrences reported from ages
    two to nine years in one natural-history cohort; individual mechanisms vary.
  phenotype_term:
    preferred_term: Limb pain
    term:
      id: HP:0009763
      label: Limb pain
  evidence:
  - reference: PMID:29972438
    reference_title: "Natural history of 39 patients with Achondroplasia."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The most common clinical intercurrences were middle ear dysfunction, sleep apnea, limb pain and obesity from 2 to 9 years of age."
    explanation: Natural-history cohort lists limb pain among the most common clinical intercurrences in childhood.
imaging_findings:
- name: Rhizomelic long-bone shortening on skeletal radiographs
  modality: XRAY
  description: >-
    Skeletal radiographs show disproportionate proximal long-bone shortening.
    This is part of the characteristic diagnostic pattern rather than a
    stand-alone molecular confirmation.
  phenotype_term:
    preferred_term: Rhizomelia
    term:
      id: HP:0008905
      label: Rhizomelia
  diagnostic: false
  evidence:
  - reference: PMID:29972438
    reference_title: Natural history of 39 patients with Achondroplasia.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The most prevalent radiographic findings were rhizomelic shortening of the long bones and narrowing of the interpediculate distance of the caudal spine."
    explanation: A clinical cohort directly documents the radiographic pattern.
- name: Caudal interpedicular narrowing on spinal radiographs
  modality: XRAY
  description: >-
    The interpedicular distance narrows toward the caudal spine, a
    characteristic vertebral radiographic feature.
  phenotype_term:
    preferred_term: Caudal interpedicular narrowing
    term:
      id: HP:0008457
      label: Caudal interpedicular narrowing
  diagnostic: false
  evidence:
  - reference: PMID:29972438
    reference_title: Natural history of 39 patients with Achondroplasia.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The most prevalent radiographic findings were rhizomelic shortening of the long bones and narrowing of the interpediculate distance of the caudal spine."
    explanation: A clinical cohort directly documents caudal narrowing.
- name: Foramen-magnum stenosis and cord compression spectrum on MRI
  modality: MRI
  description: >-
    Craniocervical MRI can show foramen-magnum narrowing, loss of surrounding
    CSF space, cord flattening, and T2 signal change. This evaluates a
    complication and is not required to establish the skeletal diagnosis.
  phenotype_term:
    preferred_term: Small foramen magnum
    term:
      id: HP:0002677
      label: Small foramen magnum
  diagnostic: false
  evidence:
  - reference: PMID:32883660
    reference_title: "Achondroplasia Foramen Magnum Score: screening infants for stenosis."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Of 36 infants (M:F, 18:18), 2 (5.6%) did not have FMS (AFMS0); 13 (36.1%) had FMS with preservation of the cerebrospinal fluid (CSF) spaces (AFMS1); 3 (8.3%) had FMS with loss of the CSF space but no spinal cord distortion (AFMS2); 13 (36.1%) had FMS with flattening of the cervical cord without signal change (AFMS3); and 5 (13.9%) had FMS resulting in cervical cord signal change (AFMS4)."
    explanation: The cohort defines an MRI severity spectrum from no stenosis to cord signal change.
- name: Premature skull-base synchondrosis fusion on CT
  modality: CT
  description: >-
    CT can demonstrate premature fusion of skull-base synchondroses. This
    finding informs craniofacial mechanism and complication assessment but is
    not presented as a universal diagnostic requirement.
  diagnostic: false
  evidence:
  - reference: PMID:37072824
    reference_title: "Craniofacial growth and function in achondroplasia: a multimodal 3D study on 15 patients."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "All patients with available CT-scan had premature fusion of skull base synchondroses."
    explanation: CT in a small clinical cohort directly documents premature fusion.
- name: Upper-lumbar stenosis and disc degeneration on adult MRI
  modality: MRI
  description: >-
    Adult imaging shows preferential upper-lumbar canal narrowing and disc
    degeneration, often with thoracolumbar kyphosis and loss of lordosis.
  diagnostic: false
  evidence:
  - reference: PMID:32170149
    reference_title: Lumbar spinal stenosis and disc alterations affect the upper lumbar spine in adults with achondroplasia.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Unlike the general population, spinal stenosis and disc degeneration involve the upper part of the lumbar spine in adults with achondroplasia, associated with thoraco-lumbar kyphosis and loss of lumbar lordosis."
    explanation: Adult MRI data define the characteristic distribution.

genetic:
- name: FGFR3
  gene_term:
    preferred_term: FGFR3
    term:
      id: hgnc:3690
      label: FGFR3
  association: Causative
  relationship_type: CAUSATIVE
  variant_origin: GERMLINE
  notes: >-
    Achondroplasia is genetically homogeneous. Two substitutions at nucleotide
    1138 produce p.Gly380Arg in the FGFR3 transmembrane domain. Experimental
    evidence supports gain of function with impaired receptor down-regulation;
    strict ligand-independent activation is not required by this record.
  variants:
  - name: c.1138G>A (p.Gly380Arg)
    description: >-
      The predominant recurrent allele; 150 of 153 p.Gly380Arg-positive cases
      in one 154-person series carried c.1138G>A.
    evidence:
    - reference: PMID:7847369
      reference_title: Achondroplasia is defined by recurrent G380R mutations of FGFR3.
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "All but one, an atypical case, were found to have a glycine-to-arginine substitution at codon 380. Of these, 150 had a G-to-A transition at nt 1138, and 3 had a G-to-C transversion at this same position."
      explanation: The large series directly supplies the allele counts.
  - name: c.1138G>C (p.Gly380Arg)
    description: >-
      A less common nucleotide substitution producing the same p.Gly380Arg
      protein change; three cases were observed in the same series.
    evidence:
    - reference: PMID:7847369
      reference_title: Achondroplasia is defined by recurrent G380R mutations of FGFR3.
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "All but one, an atypical case, were found to have a glycine-to-arginine substitution at codon 380. Of these, 150 had a G-to-A transition at nt 1138, and 3 had a G-to-C transversion at this same position."
      explanation: The series directly documents the c.1138G>C minority allele.
  evidence:
  - reference: CGGV:assertion_0adafa1b-e609-4b99-8e90-f73473b70771-2023-09-28T040000.000Z
    reference_title: FGFR3 / achondroplasia (Definitive)
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "FGFR3 | HGNC:3690 | achondroplasia | MONDO:0007037 | AD | Definitive"
    explanation: ClinGen classifies the FGFR3-achondroplasia relationship as definitive.
  - reference: ORPHA:15
    reference_title: Achondroplasia
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "FGFR3 | fibroblast growth factor receptor 3 | hgnc:3690 | Disease-causing germline mutation(s) (gain of function) in"
    explanation: Orphanet identifies germline FGFR3 gain of function.

treatments:
- name: Vosoritide (Voxzogo)
  action_category: THERAPEUTIC
  description: >-
    Daily CNP analogue indicated in the United States to increase linear growth
    in pediatric patients with achondroplasia and open epiphyses. Accelerated
    approval is based on annualized growth velocity; final adult height and
    prevention of non-growth complications are not established by the approval.
    A three-year external-control comparison estimated 5.75 cm additional
    height, explicitly not a three-year randomized comparison.
  treatment_term:
    preferred_term: Pharmacotherapy
    term:
      id: NCIT:C15986
      label: Pharmacotherapy
    therapeutic_agent:
    - preferred_term: vosoritide
      term:
        id: NCIT:C152918
        label: Vosoritide
  target_phenotypes:
  - preferred_term: Limb undergrowth
    term:
      id: HP:0009826
      label: Limb undergrowth
  target_mechanisms:
  - target: CNP-NPR2 counter-regulation of FGFR3-MAPK signaling
    treatment_effect: ACTIVATES
    description: Vosoritide activates the CNP-NPR2 counter-regulatory branch.
    evidence:
    - reference: PMID:31269546
      reference_title: C-Type Natriuretic Peptide Analogue Therapy in Children with Achondroplasia.
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "Vosoritide is a biologic analogue of C-type natriuretic peptide, a potent stimulator of endochondral ossification."
      explanation: The clinical trial identifies vosoritide as a CNP analogue.
    - reference: PMID:14702637
      reference_title: Overexpression of CNP in chondrocytes rescues achondroplasia through a MAPK-dependent pathway.
      supports: SUPPORT
      evidence_source: MODEL_ORGANISM
      snippet: "These results demonstrate that activation of the CNP-GC-B system in endochondral bone formation constitutes a new therapeutic strategy for human achondroplasia."
      explanation: Preclinical data support the CNP-GC-B/NPR2 mechanism.
  evidence:
  - reference: url:https://www.accessdata.fda.gov/drugsatfda_docs/label/2024/214938s004lbl.pdf
    reference_title: https://www.accessdata.fda.gov/drugsatfda_docs/label/2024/214938s004lbl.pdf
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "VOXZOGO is a C type natriuretic peptide (CNP) analog indicated to increase linear growth in pediatric patients with achondroplasia with open epiphyses."
    explanation: The current US label has no obsolete five-year minimum-age clause.
  - reference: url:https://www.accessdata.fda.gov/drugsatfda_docs/label/2024/214938s004lbl.pdf
    reference_title: https://www.accessdata.fda.gov/drugsatfda_docs/label/2024/214938s004lbl.pdf
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "This indication is approved under accelerated approval based on an improvement in annualized growth velocity."
    explanation: The label identifies the surrogate endpoint and accelerated-approval basis.
  - reference: PMID:39740666
    reference_title: Sustained growth-promoting effects of vosoritide in children with achondroplasia from an ongoing phase 3 extension study.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Data from the CLARITY achondroplasia study provided an external untreated control population and reference data."
    explanation: This specifies that the comparator was external and untreated.
  - reference: PMID:39740666
    reference_title: Sustained growth-promoting effects of vosoritide in children with achondroplasia from an ongoing phase 3 extension study.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Three-year comparisons of treated versus untreated children demonstrated an additional height gain of 5.75 cm"
    explanation: The extension analysis reports the calibrated external-control height estimate.

- name: Navepegritide (Yuviwel)
  action_category: THERAPEUTIC
  description: >-
    Once-weekly CNP analogue indicated in the United States to increase linear
    growth in patients aged two years and older with achondroplasia and open
    epiphyses. The 2026 accelerated approval is based on annualized growth
    velocity; continued approval may be contingent on verification and
    description of clinical benefit in confirmatory trials.
  treatment_term:
    preferred_term: Pharmacotherapy
    term:
      id: NCIT:C15986
      label: Pharmacotherapy
    therapeutic_agent:
    - preferred_term: navepegritide
  target_phenotypes:
  - preferred_term: Limb undergrowth
    term:
      id: HP:0009826
      label: Limb undergrowth
  target_mechanisms:
  - target: CNP-NPR2 counter-regulation of FGFR3-MAPK signaling
    treatment_effect: ACTIVATES
    description: CNP released from navepegritide binds NPR-B and inhibits the MAPK component of FGFR3 signaling.
    evidence:
    - reference: url:https://www.accessdata.fda.gov/drugsatfda_docs/label/2026/219164Orig1s000lbl.pdf
      reference_title: https://www.accessdata.fda.gov/drugsatfda_docs/label/2026/219164Orig1s000lbl.pdf
      supports: SUPPORT
      evidence_source: OTHER
      snippet: "CNP released from navepegritide has the same receptor binding affinity and activity as endogenous CNP. CNP binds to natriuretic peptide receptor-B (NPR-B), which inhibits the mitogen activated protein kinase signaling (MAPK) pathway."
      explanation: The current label directly supports NPR-B activation and MAPK counter-regulation.
  evidence:
  - reference: url:https://www.accessdata.fda.gov/drugsatfda_docs/label/2026/219164Orig1s000lbl.pdf
    reference_title: https://www.accessdata.fda.gov/drugsatfda_docs/label/2026/219164Orig1s000lbl.pdf
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "YUVIWEL is a C-type natriuretic peptide (CNP) analog indicated to increase linear growth in pediatric patients 2 years of age and older with achondroplasia with open epiphyses."
    explanation: The 2026 US label supplies the indication and eligible population.
  - reference: url:https://www.accessdata.fda.gov/drugsatfda_docs/label/2026/219164Orig1s000lbl.pdf
    reference_title: https://www.accessdata.fda.gov/drugsatfda_docs/label/2026/219164Orig1s000lbl.pdf
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "This indication is approved under accelerated approval based on an improvement in annualized growth velocity."
    explanation: The label identifies the accelerated-approval endpoint.
  - reference: url:https://www.accessdata.fda.gov/drugsatfda_docs/label/2026/219164Orig1s000lbl.pdf
    reference_title: https://www.accessdata.fda.gov/drugsatfda_docs/label/2026/219164Orig1s000lbl.pdf
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "Continued approval for this indication may be contingent upon verification and description of clinical benefit in confirmatory trial(s)."
    explanation: The label qualifies the confirmatory-evidence condition for continued approval.
  - reference: PMID:41247754
    reference_title: "Once-Weekly Navepegritide in Children With Achondroplasia: The APPROACH Randomized Clinical Trial."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The trial met its primary end point, demonstrating superiority of navepegritide in annualized growth velocity at week 52 vs placebo (least-squares mean treatment difference of 1.49 cm/y; 95% CI, 1.05 to 1.93; P < .001)."
    explanation: The randomized trial quantifies the placebo-adjusted growth-velocity result.

- name: Infigratinib (oral FGFR1-3 tyrosine kinase inhibitor)
  action_category: THERAPEUTIC
  therapeutic_modality: SMALL_MOLECULE
  description: >-
    Once-daily oral ATP-competitive FGFR1-3 tyrosine kinase inhibitor, repurposed
    from oncology at roughly a hundredth of the oncology dose (0.25 mg/kg/day). It
    is mechanistically distinct from the approved CNP analogues: vosoritide and
    navepegritide act on a parallel counter-regulatory arm (NPR-B/cGMP braking
    MAPK downstream of the receptor), whereas infigratinib inhibits the mutant
    receptor's kinase output directly — the first drug in achondroplasia to do so.
    In the 52-week phase 3 PROPEL 3 trial (NCT06164951) it increased annualized
    height velocity by 1.74 cm/year over placebo, and it is oral rather than
    injected. Two caveats belong with the result: the upper-to-lower segment
    ratio, the measure of *disproportion* as opposed to height, did not change,
    and the drug is a pan-FGFR1-3 rather than FGFR3-selective inhibitor, so
    long-term off-target skeletal effects are not yet characterized in children.
    Not approved at time of curation.
  treatment_term:
    preferred_term: Pharmacotherapy
    term:
      id: NCIT:C15986
      label: Pharmacotherapy
    therapeutic_agent:
    - preferred_term: infigratinib
      term:
        id: CHEBI:63451
        label: BGJ-398
  target_phenotypes:
  - preferred_term: Limb undergrowth
    term:
      id: HP:0009826
      label: Limb undergrowth
  target_mechanisms:
  - target: Prolonged FGFR3 signaling in growth-plate chondrocytes
    treatment_effect: INHIBITS
    description: >-
      Infigratinib inhibits the FGFR kinase domain directly, reducing the
      sustained downstream signal from the constitutively active p.Gly380Arg
      receptor rather than opposing it from a parallel pathway.
    evidence:
    - reference: PMID:42370681
      reference_title: "Phase 3 Trial of Oral Infigratinib in Children with Achondroplasia."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "Infigratinib, an oral FGFR1-3 tyrosine kinase inhibitor, down-regulates key pathways in the pathogenesis of achondroplasia."
      explanation: >-
        States the receptor-directed mechanism that distinguishes infigratinib
        from the CNP-analogue class.
  evidence:
  - reference: PMID:42370681
    reference_title: "Phase 3 Trial of Oral Infigratinib in Children with Achondroplasia."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "In children with achondroplasia, treatment with once-daily oral infigratinib for 52 weeks resulted in a significantly greater increase from baseline in the annualized height velocity than placebo."
    explanation: The phase 3 primary result.
  - reference: PMID:42370681
    reference_title: "Phase 3 Trial of Oral Infigratinib in Children with Achondroplasia."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The difference between infigratinib and placebo in the least-squares mean change from baseline to week 52 was 1.74 cm per year (95% confidence interval [CI], 1.31 to 2.17; P<0.001) for the annualized height velocity, 0.32 (96% CI, 0.23 to 0.41; P<0.001) for the height z score, and -0.02 (96% CI, -0.06 to 0.01) for the upper-to-lower body segment ratio."
    explanation: >-
      Quantifies the effect and records that the upper-to-lower segment ratio,
      the disproportion measure, was not improved.
  - reference: PMID:42370681
    reference_title: "Phase 3 Trial of Oral Infigratinib in Children with Achondroplasia."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Adverse events occurred in 71 of 74 patients (96%) in the infigratinib group and in 37 of 39 patients (95%) in the placebo group; serious adverse events occurred in 4 of 74 patients (5%) and 1 of 39 patients (3%), respectively."
    explanation: The 52-week safety comparison; longer-term skeletal safety is untested.

- name: Surgical limb lengthening
  therapeutic_modality: SURGERY
  action_category: THERAPEUTIC
  description: >-
    Extended limb lengthening is an elective option for selected individuals.
    Decisions require specialized multidisciplinary counseling about goals,
    prolonged treatment, complications, and patient preferences; the evidence
    cited here supports use, not a universal recommendation.
  treatment_term:
    preferred_term: Limb lengthening surgery
    term:
      id: NCIT:C15329
      label: Surgical Procedure
  evidence:
  - reference: url:https://www.ncbi.nlm.nih.gov/books/NBK1152/
    reference_title: Achondroplasia - GeneReviews® - NCBI Bookshelf
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "Additional treatment for short stature may include extended limb lengthening;"
    explanation: The current clinical review lists limb lengthening as an optional intervention.

- name: Foramen magnum decompression
  action_category: THERAPEUTIC
  description: >-
    Experienced neurosurgical teams may perform suboccipital or
    cervicomedullary decompression when clinical, respiratory, and imaging
    findings indicate significant craniocervical compression. The procedure is
    not equated with lumbar laminectomy.
  treatment_term:
    preferred_term: Foramen magnum decompression
    term:
      id: NCIT:C15329
      label: Surgical Procedure
  target_phenotypes:
  - preferred_term: Small foramen magnum
    term:
      id: HP:0002677
      label: Small foramen magnum
  evidence:
  - reference: url:https://www.ncbi.nlm.nih.gov/books/NBK1152/
    reference_title: Achondroplasia - GeneReviews® - NCBI Bookshelf
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "suboccipital decompression as indicated for signs and symptoms of craniocervical junction compression;"
    explanation: The current clinical review supports symptom- and sign-based surgical selection.

- name: Surgery for symptomatic spinal stenosis
  action_category: THERAPEUTIC
  description: >-
    Adults with symptomatic spinal stenosis may require decompressive spinal
    surgery. Selection depends on neurologic symptoms and specialist assessment.
  treatment_term:
    preferred_term: spinal decompression surgery
    term:
      id: NCIT:C15329
      label: Surgical Procedure
  target_phenotypes:
  - preferred_term: Spinal canal stenosis
    term:
      id: HP:0003416
      label: Spinal canal stenosis
  evidence:
  - reference: url:https://www.ncbi.nlm.nih.gov/books/NBK1152/
    reference_title: Achondroplasia - GeneReviews® - NCBI Bookshelf
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "surgery to correct spinal stenosis in symptomatic adults;"
    explanation: The current clinical review supports surgery for selected symptomatic adults.

- name: Lifelong multidisciplinary surveillance
  action_category: MONITORING
  description: >-
    Age-specific follow-up continues across the lifespan, with visits at least
    every six to twelve months in infancy and childhood and every three to five
    years in adulthood.
  treatment_term:
    preferred_term: clinical monitoring
    term:
      id: NCIT:C124351
      label: Clinical Evaluation
  evidence:
  - reference: url:https://www.ncbi.nlm.nih.gov/books/NBK1152/
    reference_title: Achondroplasia - GeneReviews® - NCBI Bookshelf
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "Infants and children should be seen at a minimum of every six to 12 months."
    explanation: The review specifies regular pediatric surveillance.
  - reference: PMID:32457214
    reference_title: Health Supervision for People With Achondroplasia.
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "This document is intended to provide guidance for health care providers to help identify individual patients at high risk of developing serious sequelae and to enable intervention before complications develop."
    explanation: AAP guidance supports anticipatory health supervision.
  - reference: url:https://www.ncbi.nlm.nih.gov/books/NBK1152/
    reference_title: Achondroplasia - GeneReviews® - NCBI Bookshelf
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "Adults should be seen every three to five years."
    explanation: The current review specifies continued adult follow-up.

- name: Middle-ear and hearing management
  action_category: THERAPEUTIC
  description: >-
    Recurrent effusion and hearing impairment require hearing surveillance and
    individualized ENT management, including pressure-equalizing tubes when
    indicated.
  treatment_term:
    preferred_term: supportive care
    term:
      id: NCIT:C15747
      label: Supportive Care
  target_phenotypes:
  - preferred_term: Otitis media with effusion
    term:
      id: HP:0031353
      label: Otitis media with effusion
  - preferred_term: Conductive hearing impairment
    term:
      id: HP:0000405
      label: Conductive hearing impairment
  evidence:
  - reference: url:https://www.ncbi.nlm.nih.gov/books/NBK1152/
    reference_title: Achondroplasia - GeneReviews® - NCBI Bookshelf
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "aggressive management of frequent otitis media including long-lasting pressure-equalizing tubes for middle ear dysfunction;"
    explanation: The current review explicitly recommends active middle-ear management.

- name: Obstructive sleep apnea management
  action_category: THERAPEUTIC
  description: >-
    Treatment is individualized and may include adenotonsillar surgery, positive
    airway pressure, or rarely tracheostomy after appropriate sleep and airway
    assessment.
  treatment_term:
    preferred_term: supportive care
    term:
      id: NCIT:C15747
      label: Supportive Care
  target_phenotypes:
  - preferred_term: Obstructive sleep apnea
    term:
      id: HP:0002870
      label: Obstructive sleep apnea
  evidence:
  - reference: url:https://www.ncbi.nlm.nih.gov/books/NBK1152/
    reference_title: Achondroplasia - GeneReviews® - NCBI Bookshelf
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "adenotonsillectomy, positive airway pressure, and, rarely, tracheostomy to correct obstructive sleep apnea;"
    explanation: The current review lists the principal individualized modalities.

- name: Genetic counseling
  action_category: COUNSELING_INFORMATIONAL
  description: >-
    Counseling addresses the 50% transmission risk from an affected
    heterozygous parent with an average-stature partner.
  treatment_term:
    preferred_term: genetic counseling
    term:
      id: NCIT:C15240
      label: Genetic Counseling
  evidence:
  - reference: url:https://www.ncbi.nlm.nih.gov/books/NBK1152/
    reference_title: Achondroplasia - GeneReviews® - NCBI Bookshelf
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "If an individual with achondroplasia has a reproductive partner of average stature, each child has a 50% chance of having achondroplasia."
    explanation: The current review directly states the recurrence risk.

diagnosis:
- name: Characteristic physical examination
  description: >-
    Examination identifies disproportionate short-limb stature with rhizomelic
    shortening, macrocephaly, frontal bossing and midface retrusion,
    brachydactyly, and trident hands.
  diagnosis_term:
    preferred_term: physical examination
    term:
      id: NCIT:C20989
      label: Physical Examination
  results: Characteristic disproportionate skeletal and craniofacial pattern.
  evidence:
  - reference: url:https://www.ncbi.nlm.nih.gov/books/NBK1152/
    reference_title: Achondroplasia - GeneReviews® - NCBI Bookshelf
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "Individuals with achondroplasia have short stature with rhizomelic shortening of the limbs, macrocephaly, characteristic facies with frontal bossing and midface retrusion, exaggerated lumbar lordosis, limitation of elbow extension and rotation, genu varum, brachydactyly, and trident appearance of the hands."
    explanation: The current review defines the characteristic examination pattern.
- name: Skeletal radiographic evaluation
  description: >-
    Characteristic clinical and radiographic findings establish the diagnosis
    in most individuals; radiographs define long-bone, hand, pelvic, and spinal
    morphology.
  diagnosis_term:
    preferred_term: radiograph imaging procedure
    term:
      id: NCIT:C38101
      label: X-Ray Imaging
  results: Rhizomelic long-bone shortening and characteristic spinal and pelvic pattern.
  evidence:
  - reference: url:https://www.ncbi.nlm.nih.gov/books/NBK1152/
    reference_title: Achondroplasia - GeneReviews® - NCBI Bookshelf
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "Achondroplasia can be diagnosed by characteristic clinical and radiographic findings in most affected individuals."
    explanation: The current review explicitly supports clinical-radiographic diagnosis.
- name: FGFR3 molecular confirmation
  description: >-
    Targeted testing for c.1138G>A and c.1138G>C is highly efficient because
    both produce p.Gly380Arg and account for nearly all typical cases.
  diagnosis_term:
    preferred_term: molecular genetic testing
    term:
      id: NCIT:C19770
      label: Molecular Analysis
  results: Heterozygous pathogenic FGFR3 variant, usually c.1138G>A or c.1138G>C (p.Gly380Arg).
  evidence:
  - reference: PMID:9718331
    reference_title: Mutations in fibroblast growth-factor receptor 3 in sporadic cases of achondroplasia occur exclusively on the paternally derived chromosome.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "More than 97% of achondroplasia cases are caused by one of two mutations (G1138A and G1138C) in the fibroblast growth factor receptor 3 (FGFR3) gene, which results in a specific amino acid substitution, G380R."
    explanation: The study supports targeted testing for the two recurrent nucleotide substitutions.
  - reference: CGGV:assertion_0adafa1b-e609-4b99-8e90-f73473b70771-2023-09-28T040000.000Z
    reference_title: FGFR3 / achondroplasia (Definitive)
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "FGFR3 | HGNC:3690 | achondroplasia | MONDO:0007037 | AD | Definitive"
    explanation: ClinGen supports molecular confirmation through the definitive gene-disease relationship.

differential_diagnoses:
- name: Hypochondroplasia
  disease_term:
    preferred_term: hypochondroplasia
    term:
      id: MONDO:0007793
      label: hypochondroplasia
  description: >-
    A milder FGFR3 skeletal dysplasia that can overlap mild achondroplasia.
  distinguishing_features:
  - Skeletal and medical manifestations tend to be milder than in achondroplasia.
  evidence:
  - reference: url:https://www.ncbi.nlm.nih.gov/books/NBK1152/
    reference_title: Achondroplasia - GeneReviews® - NCBI Bookshelf
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "The skeletal features are very similar to those seen in achondroplasia but tend to be milder."
    explanation: The current review states the principal clinical distinction.
- name: Pseudoachondroplasia
  disease_term:
    preferred_term: pseudoachondroplasia
    term:
      id: MONDO:0008322
      label: pseudoachondroplasia
  description: >-
    A clinically and genetically distinct skeletal dysplasia whose similar name
    can cause diagnostic confusion.
  distinguishing_features:
  - It is genetically distinct from FGFR3 p.Gly380Arg achondroplasia.
  - The radiographic and craniofacial pattern should be assessed rather than relying on nomenclature.
  evidence:
  - reference: url:https://www.ncbi.nlm.nih.gov/books/NBK1152/
    reference_title: Achondroplasia - GeneReviews® - NCBI Bookshelf
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "is a clinically and genetically distinct skeletal dysplasia but the similar nomenclature may cause confusion with achondroplasia."
    explanation: GeneReviews explicitly warns that the similar name masks a distinct disorder.
- name: Thanatophoric dysplasia type 2
  disease_term:
    preferred_term: thanatophoric dysplasia type 2
    term:
      id: MONDO:0008547
      label: thanatophoric dysplasia type 2
  description: >-
    A much more severe, usually life-limiting FGFR3 short-limb skeletal dysplasia.
  distinguishing_features:
  - Severe, usually lethal short-limb skeletal dysplasia favors thanatophoric dysplasia.
  evidence:
  - reference: PMID:25800480
    reference_title: "A case of thanatophoric dysplasia type 2: a novel mutation."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Thanatophoric dysplasia (TD) is a lethal form of skeletal dysplasia with short-limb dwarfism."
    explanation: The report supports the severe life-limiting short-limb phenotype.
- name: SADDAN
  disease_term:
    preferred_term: severe achondroplasia-developmental delay-acanthosis nigricans syndrome
    term:
      id: MONDO:0014658
      label: severe achondroplasia-developmental delay-acanthosis nigricans syndrome
  description: >-
    SADDAN is a severe FGFR3 allelic disorder in which developmental delay,
    structural brain abnormalities, hearing loss, and acanthosis nigricans
    accompany the skeletal dysplasia.
  distinguishing_features:
  - Marked developmental and neurologic abnormalities with acanthosis nigricans favor SADDAN.
  - SADDAN is associated with FGFR3 p.Lys650Met rather than p.Gly380Arg.
  evidence:
  - reference: PMID:24352917
    reference_title: Suppression of severe achondroplasia with developmental delay and acanthosis nigricans by the p.Thr651Pro mutation.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Severe achondroplasia with developmental delay and acanthosis nigricans (SADDAN) is an extremely rare severe skeletal dysplasia characterized by significant developmental delay, brain structural abnormalities, hearing loss, and acanthosis nigricans."
    explanation: The report directly states the distinguishing multisystem phenotype.
  - reference: PMID:24352917
    reference_title: Suppression of severe achondroplasia with developmental delay and acanthosis nigricans by the p.Thr651Pro mutation.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The disorder is the result of a single missense mutation at codon 650 (p.Lys650Met) in the fibroblast growth factor receptor 3 gene (FGFR3)."
    explanation: The report identifies the distinct SADDAN allele.

clinical_trials:
- name: NCT06164951
  phase: PHASE_III
  status: ACTIVE_NOT_RECRUITING
  description: >-
    PROPEL 3 — the pivotal 52-week randomized, double-blind, placebo-controlled
    trial of once-daily oral infigratinib in children aged 3 to <18 years with
    achondroplasia. It is the first phase 3 result for a drug acting on the FGFR3
    receptor kinase itself rather than on the CNP counter-regulatory arm.
  target_phenotypes:
  - preferred_term: Limb undergrowth
    term:
      id: HP:0009826
      label: Limb undergrowth
  - preferred_term: Short stature
    term:
      id: HP:0004322
      label: Short stature
  evidence:
  - reference: clinicaltrials:NCT06164951
    reference_title: "A Phase 3, Multicenter, Double-Blind, Randomized, Placebo-Controlled Study to Evaluate the Efficacy and Safety of Infigratinib in Children 3 to <18 Years of Age With Achondroplasia: PROPEL 3"
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "This is a Phase 3, multicenter, double-blind, randomized, placebo-controlled study to evaluate the efficacy and safety of infigratinib in children and adolescents with achondroplasia (ACH)"
    explanation: The registry record establishes the trial's design and population.
  - reference: PMID:42370681
    reference_title: "Phase 3 Trial of Oral Infigratinib in Children with Achondroplasia."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "In this phase 3, multicenter, double-blind, placebo-controlled trial, we randomly assigned children with achondroplasia (3 to 17 years of age) in a 2:1 ratio to receive infigratinib (at a dose of 0.25 mg per kilogram of body weight) or placebo once daily for 52 weeks."
    explanation: The publication reporting this trial's results.

animal_models:
- species: Mouse
  genotype: Human FGFR3 G380R transgene under mouse Fgfr3 transcriptional control
  description: >-
    Mutant-receptor immunoreactivity extends across a larger growth-plate area,
    supporting impaired in vivo receptor down-regulation. This record does not
    claim an unsupported full phenotypic inventory.
  evidence:
  - reference: PMID:10611230
    reference_title: The transmembrane mutation G380R in fibroblast growth factor receptor 3 uncouples ligand-mediated receptor activation from down-regulation.
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: "transgenic mice expressing the human G380R mutant receptor under the mouse receptor transcriptional control demonstrated a markedly expanded area of FGFR3 immunoreactivity within their epiphyseal growth plates, compatible with an in vivo defect in receptor down-regulation."
    explanation: The experiment directly supports the modeled receptor-persistence phenotype.
- species: Mouse
  genotype: Activated Fgfr3 achondroplasia model with chondrocyte CNP overexpression
  description: >-
    Targeted CNP overexpression counteracted dwarfism and corrected reduced
    matrix synthesis through MAPK inhibition, providing proof of concept for CNP
    analogues.
  associated_phenotypes:
  - Rescue of bone shortening
  - Restored extracellular matrix synthesis
  evidence:
  - reference: PMID:14702637
    reference_title: Overexpression of CNP in chondrocytes rescues achondroplasia through a MAPK-dependent pathway.
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: "targeted overexpression of CNP in chondrocytes counteracts dwarfism in a mouse model of achondroplasia with activated fibroblast growth factor receptor 3 (FGFR-3) in the cartilage."
    explanation: The intervention directly demonstrates rescue of dwarfism.
  - reference: PMID:14702637
    reference_title: Overexpression of CNP in chondrocytes rescues achondroplasia through a MAPK-dependent pathway.
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: "CNP prevented the shortening of achondroplastic bones by correcting the decreased extracellular matrix synthesis in the growth plate through inhibition of the MAPK pathway of FGF signaling."
    explanation: The experiment supplies the matrix and MAPK mechanism.
- species: Mouse
  genotype: Constitutively active MEK1 driven by Col2a1 in chondrocytes
  description: >-
    Chondrocyte MEK1 activation produces achondroplasia-like dwarfism and
    inhibits skeletal overgrowth in Fgfr3-deficient mice, supporting MAPK as a
    downstream effector.
  associated_phenotypes:
  - Achondroplasia-like dwarfism
  evidence:
  - reference: PMID:14871928
    reference_title: Constitutive activation of MEK1 in chondrocytes causes Stat1-independent achondroplasia-like dwarfism and rescues the Fgfr3-deficient mouse phenotype.
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: "We generated transgenic mice that express a constitutively active mutant of MEK1 in chondrocytes. These mice showed a dwarf phenotype similar to achondroplasia, the most common human dwarfism, caused by activating mutations in FGFR3."
    explanation: Directly demonstrates sufficiency of chondrocyte MAPK activation for an achondroplasia-like phenotype.
  - reference: PMID:14871928
    reference_title: Constitutive activation of MEK1 in chondrocytes causes Stat1-independent achondroplasia-like dwarfism and rescues the Fgfr3-deficient mouse phenotype.
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: "Expression of a constitutively active mutant of MEK1 in chondrocytes of Fgfr3-deficient mice inhibited skeletal overgrowth, strongly suggesting that regulation of bone growth by FGFR3 is mediated at least in part by the MAPK pathway."
    explanation: The epistasis experiment positions MAPK downstream of FGFR3.

notes: >-
  Intelligence is usually near normal, while motor and sometimes speech milestones
  can be delayed for physical, hearing, or airway-related reasons. Clinical care is
  lifelong and multidisciplinary. Current CNP analogues have demonstrated improved
  growth velocity; claims about final adult height and prevention of neurologic,
  respiratory, otologic, or orthopedic complications require separate evidence.
datasets: []
references:
- reference: CGGV:assertion_0adafa1b-e609-4b99-8e90-f73473b70771-2023-09-28T040000.000Z
  title: "FGFR3 / achondroplasia (Definitive)"
  findings: []
- reference: DOI:10.1038/s41574-021-00595-x
  title: "International Consensus Statement on the diagnosis, multidisciplinary management and lifelong care of individuals with achondroplasia"
  findings: []
- reference: ORPHA:15
  title: "Achondroplasia"
  findings: []
- reference: PMID:10611230
  title: "The transmembrane mutation G380R in fibroblast growth factor receptor 3 uncouples ligand-mediated receptor activation from down-regulation."
  findings: []
- reference: PMID:14702637
  title: "Overexpression of CNP in chondrocytes rescues achondroplasia through a MAPK-dependent pathway."
  findings: []
- reference: PMID:14751560
  title: "Overexpression of FGFR3, Stat1, Stat5 and p21Cip1 correlates with phenotypic severity and defective chondrocyte differentiation in FGFR3-related chondrodysplasias."
  findings: []
- reference: PMID:14871928
  title: "Constitutive activation of MEK1 in chondrocytes causes Stat1-independent achondroplasia-like dwarfism and rescues the Fgfr3-deficient mouse phenotype."
  findings: []
- reference: PMID:18923003
  title: "FGFR3 promotes synchondrosis closure and fusion of ossification centers through the MAPK pathway."
  findings: []
- reference: PMID:22409389
  title: "Development in children with achondroplasia: a prospective clinical cohort study."
  findings: []
- reference: PMID:24352917
  title: "Suppression of severe achondroplasia with developmental delay and acanthosis nigricans by the p.Thr651Pro mutation."
  findings: []
- reference: PMID:25800480
  title: "A case of thanatophoric dysplasia type 2: a novel mutation."
  findings: []
- reference: PMID:27927547
  title: "Prevalence of Scoliosis and Thoracolumbar Kyphosis in Patients With Achondroplasia."
  findings: []
- reference: PMID:29972438
  title: "Natural history of 39 patients with Achondroplasia."
  findings: []
- reference: PMID:31269546
  title: "C-Type Natriuretic Peptide Analogue Therapy in Children with Achondroplasia."
  findings: []
- reference: PMID:31294928
  title: "Epidemiology of achondroplasia: A population-based study in Europe."
  findings: []
- reference: PMID:32170149
  title: "Lumbar spinal stenosis and disc alterations affect the upper lumbar spine in adults with achondroplasia."
  findings: []
- reference: PMID:3228140
  title: "Obesity in achondroplasia."
  findings: []
- reference: PMID:32457214
  title: "Health Supervision for People With Achondroplasia."
  findings: []
- reference: PMID:32803853
  title: "Birth prevalence of achondroplasia: A systematic literature review and meta-analysis."
  findings: []
- reference: PMID:32864841
  title: "Natural history of achondroplasia: A retrospective review of longitudinal clinical data."
  findings: []
- reference: PMID:32883660
  title: "Achondroplasia Foramen Magnum Score: screening infants for stenosis."
  findings: []
- reference: PMID:33579320
  title: "Predictors of cervical myelopathy and hydrocephalus in young children with achondroplasia."
  findings: []
- reference: PMID:34736503
  title: "Hearing loss in Norwegian adults with achondroplasia."
  findings: []
- reference: PMID:37072824
  title: "Craniofacial growth and function in achondroplasia: a multimodal 3D study on 15 patients."
  findings: []
- reference: PMID:37493935
  title: "Walking status and spinopelvic parameters in young children with achondroplasia: 10-year follow-up."
  findings: []
- reference: PMID:38554024
  title: "Clinical outcomes and medical management of achondroplasia in Japanese children: A retrospective medical record review of clinical data."
  findings: []
- reference: PMID:39660705
  title: "Otologic Manifestations in Patients with Achondroplasia: A Multicenter Study."
  findings: []
- reference: PMID:39740666
  title: "Sustained growth-promoting effects of vosoritide in children with achondroplasia from an ongoing phase 3 extension study."
  findings: []
- reference: PMID:40675782
  title: "Sleep-disordered breathing in children with achondroplasia assessed by polysomnography: a retrospective chart review."
  findings: []
- reference: PMID:41247754
  title: "Once-Weekly Navepegritide in Children With Achondroplasia: The APPROACH Randomized Clinical Trial."
  findings: []
- reference: PMID:7847369
  title: "Achondroplasia is defined by recurrent G380R mutations of FGFR3."
  findings: []
- reference: PMID:8078586
  title: "Mutations in the gene encoding fibroblast growth factor receptor-3 in achondroplasia."
  findings: []
- reference: PMID:9718331
  title: "Mutations in fibroblast growth-factor receptor 3 in sporadic cases of achondroplasia occur exclusively on the paternally derived chromosome."
  findings: []
- reference: url:https://www.accessdata.fda.gov/drugsatfda_docs/label/2024/214938s004lbl.pdf
  title: "https://www.accessdata.fda.gov/drugsatfda_docs/label/2024/214938s004lbl.pdf"
  findings: []
- reference: url:https://www.accessdata.fda.gov/drugsatfda_docs/label/2026/219164Orig1s000lbl.pdf
  title: "https://www.accessdata.fda.gov/drugsatfda_docs/label/2026/219164Orig1s000lbl.pdf"
  findings: []
- reference: url:https://www.ncbi.nlm.nih.gov/books/NBK1152/
  title: "Achondroplasia - GeneReviews® - NCBI Bookshelf"
  findings: []

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