Hereditary methemoglobinemia

Mendelian MONDO:0018963 Pathograph 9 Methemoglobinemia Inherited red cell disorder

Hereditary methemoglobinemia is an inherited red-cell disorder in which deficient NADH-cytochrome b5 reductase activity, usually from biallelic CYB5R3 pathogenic variants, impairs reduction of ferric methemoglobin back to ferrous oxygen-binding hemoglobin. The resulting chronic methemoglobin accumulation causes cyanosis from birth or infancy, lip and nail discoloration, exertional dyspnea, and in the generalized type II form severe neurodevelopmental impairment, movement disorder, progressive microcephaly, and brain volume loss.

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1
Mappings
1
Definitions
1
Inheritance
5
Pathophys.
13
Phenotypes
9
Pathograph
1
Genes
3
Treatments
2
Subtypes
9
References
1
Deep Research
🔗

Mappings

MONDO
MONDO:0018963 hereditary methemoglobinemia
skos:exactMatch Orphanet ORPHA:621
Orphanet ORPHA:621 lists MONDO:0018963 as an exact cross-reference for autosomal recessive methemoglobinemia.
📘

Definitions

1
Orphanet autosomal recessive methemoglobinemia definition
A rare red-cell disorder divided principally into autosomal recessive congenital or hereditary methemoglobinemia types I and II.
OTHER
Show evidence (1 reference)
ORPHA:621 SUPPORT Other
"A rare red cell disorder classified principally into two clinical phenotypes: autosomal recessive congenital (or hereditary) methemoglobinemia types I and II (RCM/RHM type 1; RCM/RHM type 2)."
Orphanet defines the disease and its type I/type II clinical subdivision.
👪

Inheritance

1
Autosomal recessive inheritance HP:0000007
Hereditary methemoglobinemia due to CYB5R3 deficiency is inherited in an autosomal recessive pattern.
Autosomal recessive inheritance
Show evidence (2 references)
ORPHA:621 SUPPORT Other
"Autosomal recessive"
Orphanet records autosomal recessive inheritance.
PMID:31898843 SUPPORT Human Clinical
"occurs worldwide in autosomal recessive inheritance."
The mutation update explicitly states autosomal recessive inheritance for CYB5R3-related RCM.
◆

Subtypes

2
Type I (erythrocyte-restricted recessive congenital methemoglobinemia)
Type I disease is characterized by CYB5R3 deficiency that is functionally restricted to erythrocytes, causing cyanosis without the severe generalized neurologic disease typical of type II.
Show evidence (2 references)
ORPHA:621 SUPPORT Other
"autosomal recessive congenital (or hereditary) methemoglobinemia types I and II"
Orphanet identifies type I and type II as the principal clinical phenotypes.
PMID:19480335 SUPPORT Human Clinical
"Type I presents with mild cyanosis due to a significant deficiency of cytb5r in erythrocytes only."
This clinical report distinguishes the erythrocyte-restricted type I phenotype.
Type II (generalized recessive congenital methemoglobinemia)
Type II disease reflects more generalized CYB5R3 deficiency and combines cyanosis with severe neurologic impairment, developmental delay, movement disorder, microcephaly, and shortened life expectancy.
Show evidence (2 references)
PMID:18318771 SUPPORT Human Clinical
"In type II, the cyanosis is accompanied by neurological impairment and reduced life expectancy."
The review directly distinguishes the severe neurologic type II phenotype.
PMID:19480335 SUPPORT Human Clinical
"In type II, the deficiency occurs in all tissues and causes growth and mental retardation and other neurological impairments."
This clinical report supports generalized tissue involvement and neurologic disease in type II.
⚙

Pathophysiology

5
CYB5R3 cytochrome b5 reductase deficiency
Biallelic CYB5R3 pathogenic variants reduce NADH-cytochrome b5 reductase activity, the erythrocyte enzyme system that normally transfers electrons needed to reduce ferric methemoglobin.
erythrocyte link
CYB5R3 link
cytochrome-b5 reductase activity, acting on NAD(P)H link ↓ DECREASED
Downstream
Impaired methemoglobin reduction Loss of CYB5R3 activity impairs reduction of ferric methemoglobin back to oxygen-binding hemoglobin.
Type II generalized neurologic involvement Generalized CYB5R3 deficiency across all tissues in type II disease causes neurological involvement beyond the erythrocyte-restricted form.
Show evidence (2 references)
PMID:18318771 SUPPORT Human Clinical
"This disorder, now known as recessive congenital methaemoglobinaemia (RCM), is caused by NADH-cytochrome b5 reductase (cb(5)r) deficiency."
The review directly supports CYB5R3/cytochrome b5 reductase deficiency as the primary defect.
PMID:31898843 SUPPORT Human Clinical
"NADH-cytochrome b5 reductase 3 deficiency is an important genetic cause of recessive congenital methemoglobinemia (RCM)"
The mutation update supports the enzyme-deficiency mechanism.
Impaired methemoglobin reduction
Methaemoglobin is continuously formed as heme iron becomes ferric; CYB5R3 deficiency prevents efficient reduction back to the ferrous state.
heme oxidation-reduction balance link ⚠ ABNORMAL
Downstream
Methemoglobin accumulation Inefficient reduction causes chronic ferric methemoglobin accumulation.
Show evidence (2 references)
PMID:3752898 SUPPORT Human Clinical
"the rate at which ferric heme is formed and the rate at which it is reduced back to the ferrous state."
The case report review describes normal methemoglobin redox balance.
PMID:3752898 SUPPORT Human Clinical
"the methaemoglobin formed at physiological rate is not efficiently reduced."
The report directly supports impaired methemoglobin reduction in congenital enzymopenic disease.
Methemoglobin accumulation
Inefficient reduction of ferric methemoglobin leads to chronically elevated methemoglobin levels in blood.
Downstream
Reduced oxygen transport and cyanosis Ferric methemoglobin cannot bind oxygen efficiently, causing cyanosis and exertional symptoms.
Show evidence (2 references)
PMID:3752898 SUPPORT Human Clinical
"A chronically elevated level of oxidized haemoglobin results."
The report supports chronic accumulation of oxidized hemoglobin.
PMID:27879543 SUPPORT Human Clinical
"The median methemoglobin level was 13.5% (ranging between 4.2% and 33.9%)"
Pediatric registry data quantify elevated methemoglobin in affected children.
Reduced oxygen transport and cyanosis
Methemoglobin is unable to bind oxygen, so chronic methemoglobin elevation reduces effective oxygen transport and produces cyanosis, hypoxia, and dyspnea.
oxygen transport link ↓ DECREASED
Show evidence (2 references)
PMID:3752898 SUPPORT Human Clinical
"Methaemoglobin is incapable of binding oxygen."
The report supports the impaired oxygen-binding mechanism.
PMID:22024786 SUPPORT Other
"The diagnosis of methemoglobinemia should be considered in patients presenting with cyanosis and hypoxia."
The management review links methemoglobinemia with cyanosis and hypoxia.
Type II generalized neurologic involvement
Severe type II disease reflects more generalized CYB5R3 dysfunction, especially variants affecting FAD-binding or active-site function, and is associated with neurodevelopmental impairment and movement disorders.
CYB5R3 link
Show evidence (2 references)
PMID:31898843 SUPPORT Human Clinical
"The majority of the mutations associated with the severe form with a neurological disorder (RCM Type 2) were associated with the FAD-binding domain"
The mutation update supports domain-level association with severe neurologic type II disease.
PMID:30614390 SUPPORT Human Clinical
"mild neonatal cyanosis, early onset severe progressive developmental delay, movement disorders, and progressive microcephaly."
The sibling report supports the neurologic manifestation pattern in type II disease.
⬡

Pathograph

Use the checkboxes to hide or show graph categories. Hover nodes for evidence and cross-linked metadata.
Pathograph: causal mechanism network for Hereditary methemoglobinemia Interactive directed graph showing how pathophysiology mechanisms, phenotypes, genetic factors and variants, experimental models, environmental triggers, and treatments relate through causal and linked edges.
●

Phenotypes

13
Head and Neck 1
Microcephaly OCCASIONAL Microcephaly (HP:0000252)
Show evidence (2 references)
ORPHA:621 SUPPORT Other
"HP:0000252 | Microcephaly | Occasional (29-5%)"
Orphanet lists microcephaly as occasional.
PMID:30614390 SUPPORT Human Clinical
"movement disorders, and progressive microcephaly."
Type II sibling cases support progressive microcephaly.
Integument 2
Cyanosis VERY_FREQUENT Cyanosis (HP:0000961)
Show evidence (3 references)
ORPHA:621 SUPPORT Other
"HP:0000961 | Cyanosis | Very frequent (99-80%)"
Orphanet lists cyanosis as very frequent.
PMID:18318771 SUPPORT Human Clinical
"both characterized by cyanosis from birth."
The review supports congenital cyanosis in both type I and type II disease.
PMID:27879543 SUPPORT Human Clinical
"physical examination revealed cyanosis of the skin and mucus membranes."
Pediatric registry examination supports cyanosis as a clinical finding.
Abnormal nail morphology FREQUENT Abnormal nail morphology (HP:0001597)
Show evidence (1 reference)
ORPHA:621 SUPPORT Other
"HP:0001597 | Abnormality of the nail | Frequent (79-30%)"
Orphanet lists nail abnormality as frequent.
Musculoskeletal 1
Spasticity OCCASIONAL Spasticity (HP:0001257)
Show evidence (1 reference)
ORPHA:621 SUPPORT Other
"HP:0001257 | Spasticity | Occasional (29-5%)"
Orphanet lists spasticity as occasional.
Nervous System 3
Neurologic involvement in type II FREQUENT Abnormality of the nervous system (HP:0000707)
Show evidence (2 references)
ORPHA:621 SUPPORT Other
"HP:0000707 | Abnormality of the nervous system | Frequent (79-30%)"
Orphanet lists nervous system abnormalities as frequent.
PMID:18318771 SUPPORT Human Clinical
"the cyanosis is accompanied by neurological impairment and reduced life expectancy."
The review supports neurologic involvement in type II disease.
Severe global developmental delay OCCASIONAL Severe global developmental delay (HP:0011344)
Show evidence (2 references)
ORPHA:621 SUPPORT Other
"HP:0011344 | Severe global developmental delay | Occasional (29-5%)"
Orphanet lists severe global developmental delay as occasional.
PMID:30614390 SUPPORT Human Clinical
"early onset severe progressive developmental delay, movement disorders, and progressive microcephaly."
Type II sibling cases support severe progressive developmental delay.
Severe intellectual disability OCCASIONAL Severe intellectual disability (HP:0010864)
Show evidence (2 references)
ORPHA:621 SUPPORT Other
"HP:0010864 | Intellectual disability, severe | Occasional (29-5%)"
Orphanet lists severe intellectual disability as occasional.
PMID:19480335 SUPPORT Human Clinical
"growth and mental retardation and other neurological impairments."
Type II clinical report supports severe neurodevelopmental impairment.
Other 6
Methemoglobinemia VERY_FREQUENT Methemoglobinemia (HP:0012119)
Show evidence (2 references)
ORPHA:621 SUPPORT Other
"HP:0012119 | Methemoglobinemia | Very frequent (99-80%)"
Orphanet lists methemoglobinemia as very frequent.
PMID:27879543 SUPPORT Human Clinical
"The median methemoglobin level was 13.5% (ranging between 4.2% and 33.9%)"
The clinical registry demonstrates elevated methemoglobin levels in affected children.
Lip discoloration FREQUENT Lip discoloration (HP:0025118)
Show evidence (1 reference)
ORPHA:621 SUPPORT Other
"HP:0025118 | Lip discoloration | Frequent (79-30%)"
Orphanet lists lip discoloration as frequent.
Exertional dyspnea FREQUENT Exertional dyspnea (HP:0002875)
Show evidence (1 reference)
ORPHA:621 SUPPORT Other
"HP:0002875 | Exertional dyspnea | Frequent (79-30%)"
Orphanet lists exertional dyspnea as frequent.
Athetosis OCCASIONAL Athetosis (HP:0002305)
Show evidence (1 reference)
ORPHA:621 SUPPORT Other
"HP:0002305 | Athetosis | Occasional (29-5%)"
Orphanet lists athetosis as occasional.
Limb dystonia OCCASIONAL Limb dystonia (HP:0002451)
Show evidence (1 reference)
ORPHA:621 SUPPORT Other
"HP:0002451 | Limb dystonia | Occasional (29-5%)"
Orphanet lists limb dystonia as occasional.
Small basal ganglia VERY_RARE Small basal ganglia (HP:0012697)
Show evidence (2 references)
ORPHA:621 SUPPORT Other
"HP:0012697 | Small basal ganglia | Very rare (<4-1%)"
Orphanet lists small basal ganglia as very rare.
PMID:30614390 SUPPORT Human Clinical
"basal ganglia hypoplasia is an interesting clue to the very rare and frequently unsuspected diagnosis"
Type II sibling MRI report supports basal ganglia hypoplasia as a diagnostic clue.
🧬

Genetic Associations

1
Biallelic CYB5R3 pathogenic variants (Causative)
Autosomal recessive inheritance
Show evidence (3 references)
ORPHA:621 SUPPORT Other
"CYB5R3 | cytochrome b5 reductase 3 | hgnc:2873 | Disease-causing germline mutation(s) in"
Orphanet asserts disease-causing germline CYB5R3 mutations.
PMID:18318771 SUPPORT Human Clinical
"It is encoded by the CYB5R3 (previously known as DIA1) gene and more than 40 mutations have been described"
The review identifies CYB5R3 as the gene encoding cytochrome b5 reductase and summarizes known mutations.
PMID:31898843 SUPPORT Human Clinical
"over 78 different variants have been described for RCM globally."
The mutation update supports broad CYB5R3 allelic heterogeneity.
💊

Treatments

3
Methylene blue pharmacotherapy
Action: Pharmacotherapy NCIT:C15986
Agent: methylene blue ↗
Methylene blue can lower methemoglobin levels and is generally reserved for symptomatic or significantly elevated methemoglobinemia; evidence in type II congenital disease is limited to case-level symptomatic benefit.
Mechanism Target:
MODULATES Methemoglobin accumulation — Methylene blue treatment aims to reduce elevated methemoglobin levels.
Show evidence (1 reference)
PMID:26574897 PARTIAL Human Clinical
"Since the MB treatment regimen has commenced, his methemoglobin level has been significantly lower."
Single-case type II treatment evidence supports methemoglobin lowering, but not broad disease modification.
Show evidence (2 references)
PMID:22024786 SUPPORT Other
"treatment with methylene blue is reserved for patients with significantly elevated methemoglobin levels."
The management review supports methylene blue use for significant methemoglobinemia.
PMID:26574897 PARTIAL Human Clinical
"modest behavioral improvements (as assessed on the Achenbach behavior report scales)."
Case-level type II evidence suggests limited symptomatic improvement.
Ascorbic acid pharmacotherapy
Action: Pharmacotherapy NCIT:C15986
Agent: ascorbic acid ↗
Ascorbic acid has been reported as symptomatic treatment for cyanosis in recessive congenital methemoglobinemia.
Target Phenotypes: Cyanosis ↗
Show evidence (1 reference)
PMID:19480335 PARTIAL Human Clinical
"Cyanosis can be well treated by 200-500 mg of ascorbic acid daily"
Clinical report text supports ascorbic acid as symptomatic therapy for cyanosis.
Genetic counseling
Action: genetic counseling MAXO:0000079
Genetic counseling is appropriate because CYB5R3-related hereditary methemoglobinemia is autosomal recessive and molecular diagnosis clarifies carrier and recurrence risks for relatives.
Show evidence (1 reference)
PMID:19480335 SUPPORT Human Clinical
"Prenatal diagnosis is possible."
The report supports familial molecular risk assessment and reproductive counseling.
🔬

Biochemical Markers

2
Elevated methemoglobin (Elevated)
Context: Blood methemoglobin measurement
Show evidence (1 reference)
PMID:27879543 SUPPORT Human Clinical
"The median methemoglobin level was 13.5% (ranging between 4.2% and 33.9%)"
Registry data support elevated blood methemoglobin.
Reduced methemoglobin reductase activity (Reduced)
Context: Erythrocyte enzyme deficiency
Show evidence (1 reference)
PMID:3752898 SUPPORT Human Clinical
"Congenital methaemoglobinaemia caused by a deficiency or absence of methaemoglobin reductase"
The report supports reduced methemoglobin reductase activity as the biochemical defect.
{ }

Source YAML

click to show 
name: Hereditary methemoglobinemia
category: Mendelian
creation_date: "2026-05-09T13:01:56Z"
updated_date: "2026-05-09T13:01:56Z"
synonyms:
- Autosomal recessive congenital methemoglobinemia
- Recessive congenital methemoglobinemia
- Congenital enzymopenic methemoglobinemia
description: >
  Hereditary methemoglobinemia is an inherited red-cell disorder in which
  deficient NADH-cytochrome b5 reductase activity, usually from biallelic
  CYB5R3 pathogenic variants, impairs reduction of ferric methemoglobin back to
  ferrous oxygen-binding hemoglobin. The resulting chronic methemoglobin
  accumulation causes cyanosis from birth or infancy, lip and nail
  discoloration, exertional dyspnea, and in the generalized type II form severe
  neurodevelopmental impairment, movement disorder, progressive microcephaly,
  and brain volume loss.
disease_term:
  preferred_term: hereditary methemoglobinemia
  term:
    id: MONDO:0018963
    label: hereditary methemoglobinemia
parents:
- Methemoglobinemia
- Inherited red cell disorder
mappings:
  mondo_mappings:
  - term:
      id: MONDO:0018963
      label: hereditary methemoglobinemia
    mapping_predicate: skos:exactMatch
    mapping_source: Orphanet ORPHA:621
    mapping_justification: >
      Orphanet ORPHA:621 lists MONDO:0018963 as an exact cross-reference for
      autosomal recessive methemoglobinemia.
external_assertions:
- name: Orphanet autosomal recessive methemoglobinemia record
  source: Orphanet
  assertion_type: structured_disease_record
  external_id: ORPHA:621
  url: http://www.orpha.net/consor/cgi-bin/OC_Exp.php?lng=en&Expert=621
  description: >
    Orphanet's ORPHA:621 structured record for autosomal recessive
    methemoglobinemia includes the exact MONDO cross-reference, ICD-10 and OMIM
    cross-references, autosomal recessive inheritance, CYB5R3 disease-gene
    assertion, definition, epidemiology, and HPO phenotype rows used in this
    entry.
  evidence:
  - reference: ORPHA:621
    reference_title: Autosomal recessive methemoglobinemia (Orphanet structured-database record)
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "MONDO:0018963 | Exact"
    explanation: Orphanet maps ORPHA:621 exactly to the MONDO identifier used by this entry.
  - reference: ORPHA:621
    reference_title: Autosomal recessive methemoglobinemia (Orphanet structured-database record)
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "ICD-10:D74.0 | Narrower"
    explanation: Orphanet lists congenital methemoglobinemia as an ICD-10 cross-reference.
definitions:
- name: Orphanet autosomal recessive methemoglobinemia definition
  definition_type: OTHER
  description: >
    A rare red-cell disorder divided principally into autosomal recessive
    congenital or hereditary methemoglobinemia types I and II.
  evidence:
  - reference: ORPHA:621
    reference_title: Autosomal recessive methemoglobinemia (Orphanet structured-database record)
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "A rare red cell disorder classified principally into two clinical phenotypes: autosomal recessive congenital (or hereditary) methemoglobinemia types I and II (RCM/RHM type 1; RCM/RHM type 2)."
    explanation: Orphanet defines the disease and its type I/type II clinical subdivision.
inheritance:
- name: Autosomal recessive inheritance
  description: Hereditary methemoglobinemia due to CYB5R3 deficiency is inherited in an autosomal recessive pattern.
  inheritance_term:
    preferred_term: Autosomal recessive inheritance
    term:
      id: HP:0000007
      label: Autosomal recessive inheritance
  evidence:
  - reference: ORPHA:621
    reference_title: Autosomal recessive methemoglobinemia (Orphanet structured-database record)
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "Autosomal recessive"
    explanation: Orphanet records autosomal recessive inheritance.
  - reference: PMID:31898843
    reference_title: "Mutation update: Variants of the CYB5R3 gene in recessive congenital methemoglobinemia."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "occurs worldwide in autosomal recessive inheritance."
    explanation: The mutation update explicitly states autosomal recessive inheritance for CYB5R3-related RCM.
has_subtypes:
- name: Type I
  display_name: Type I (erythrocyte-restricted recessive congenital methemoglobinemia)
  description: >
    Type I disease is characterized by CYB5R3 deficiency that is functionally
    restricted to erythrocytes, causing cyanosis without the severe generalized
    neurologic disease typical of type II.
  evidence:
  - reference: ORPHA:621
    reference_title: Autosomal recessive methemoglobinemia (Orphanet structured-database record)
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "autosomal recessive congenital (or hereditary) methemoglobinemia types I and II"
    explanation: Orphanet identifies type I and type II as the principal clinical phenotypes.
  - reference: PMID:19480335
    reference_title: "A rare cause of mental motor retardation: recessive congenital methemoglobinemia type II."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Type I presents with mild cyanosis due to a significant deficiency of cytb5r in erythrocytes only."
    explanation: This clinical report distinguishes the erythrocyte-restricted type I phenotype.
- name: Type II
  display_name: Type II (generalized recessive congenital methemoglobinemia)
  description: >
    Type II disease reflects more generalized CYB5R3 deficiency and combines
    cyanosis with severe neurologic impairment, developmental delay, movement
    disorder, microcephaly, and shortened life expectancy.
  evidence:
  - reference: PMID:18318771
    reference_title: "Recessive congenital methaemoglobinaemia: cytochrome b(5) reductase deficiency."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "In type II, the cyanosis is accompanied by neurological impairment and reduced life expectancy."
    explanation: The review directly distinguishes the severe neurologic type II phenotype.
  - reference: PMID:19480335
    reference_title: "A rare cause of mental motor retardation: recessive congenital methemoglobinemia type II."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "In type II, the deficiency occurs in all tissues and causes growth and mental retardation and other neurological impairments."
    explanation: This clinical report supports generalized tissue involvement and neurologic disease in type II.
prevalence:
- population: Worldwide
  percentage: Unknown
  notes: >
    Worldwide point prevalence is unknown in Orphanet. Regional founder effects
    can produce substantially higher local prevalence, including reported
    pediatric prevalence ranges in Yakutia.
  evidence:
  - reference: ORPHA:621
    reference_title: Autosomal recessive methemoglobinemia (Orphanet structured-database record)
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "Unknown | Worldwide | Point prevalence | ORPHANET"
    explanation: Orphanet records unknown worldwide point prevalence.
  - reference: PMID:27879543
    reference_title: Enzymopenic Congenital Methemoglobinemia in Children of the Republic of Sakha (Yakutia).
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The prevalence per 100,000 children ranged from 12.7 to 47.0 in 3 geographic regions of Yakutia."
    explanation: The Yakutia pediatric registry provides a regional high-prevalence estimate.
progression:
- phase: Neonatal or infancy onset
  age_range: Neonatal period to infancy
  notes: >
    Cyanosis typically begins at birth or in infancy, with type I often
    remaining otherwise mild and type II developing progressive neurologic
    manifestations.
  evidence:
  - reference: ORPHA:621
    reference_title: Autosomal recessive methemoglobinemia (Orphanet structured-database record)
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "Age of onset: Neonatal"
    explanation: Orphanet records neonatal onset.
  - reference: PMID:18318771
    reference_title: "Recessive congenital methaemoglobinaemia: cytochrome b(5) reductase deficiency."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "both characterized by cyanosis from birth."
    explanation: The review supports congenital cyanosis as an early manifestation.
- phase: Type II progressive neurodevelopmental disease
  age_range: Infancy to childhood
  notes: >
    Type II disease adds early, severe, progressive developmental and movement
    disorder manifestations, with progressive microcephaly and structural brain
    volume loss described in affected children.
  evidence:
  - reference: PMID:30614390
    reference_title: "Recessive congenital methemoglobinemia type II: Hypoplastic basal ganglia in two siblings with a novel mutation of the cytochrome b5 reductase gene."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "early onset severe progressive developmental delay, movement disorders, and progressive microcephaly."
    explanation: Type II case evidence supports progressive neurodevelopmental disease.
genetic:
- name: Biallelic CYB5R3 pathogenic variants
  gene_term:
    preferred_term: CYB5R3
    term:
      id: hgnc:2873
      label: CYB5R3
  association: Causative
  relationship_type: CAUSATIVE
  variant_origin: GERMLINE
  features: >
    CYB5R3 encodes cytochrome b5 reductase 3. Biallelic pathogenic variants
    reduce NADH-cytochrome b5 reductase function; variants affecting FAD-binding
    or active-site function are enriched in the severe neurologic type II form.
  inheritance:
  - name: Autosomal recessive inheritance
  evidence:
  - reference: ORPHA:621
    reference_title: Autosomal recessive methemoglobinemia (Orphanet structured-database record)
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "CYB5R3 | cytochrome b5 reductase 3 | hgnc:2873 | Disease-causing germline mutation(s) in"
    explanation: Orphanet asserts disease-causing germline CYB5R3 mutations.
  - reference: PMID:18318771
    reference_title: "Recessive congenital methaemoglobinaemia: cytochrome b(5) reductase deficiency."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "It is encoded by the CYB5R3 (previously known as DIA1) gene and more than 40 mutations have been described"
    explanation: The review identifies CYB5R3 as the gene encoding cytochrome b5 reductase and summarizes known mutations.
  - reference: PMID:31898843
    reference_title: "Mutation update: Variants of the CYB5R3 gene in recessive congenital methemoglobinemia."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "over 78 different variants have been described for RCM globally."
    explanation: The mutation update supports broad CYB5R3 allelic heterogeneity.
pathophysiology:
- name: CYB5R3 cytochrome b5 reductase deficiency
  description: >
    Biallelic CYB5R3 pathogenic variants reduce NADH-cytochrome b5 reductase
    activity, the erythrocyte enzyme system that normally transfers electrons
    needed to reduce ferric methemoglobin.
  genes:
  - preferred_term: CYB5R3
    term:
      id: hgnc:2873
      label: CYB5R3
  cell_types:
  - preferred_term: erythrocyte
    term:
      id: CL:0000232
      label: erythrocyte
  molecular_functions:
  - preferred_term: cytochrome-b5 reductase activity, acting on NAD(P)H
    modifier: DECREASED
    term:
      id: GO:0004128
      label: cytochrome-b5 reductase activity, acting on NAD(P)H
  evidence:
  - reference: PMID:18318771
    reference_title: "Recessive congenital methaemoglobinaemia: cytochrome b(5) reductase deficiency."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "This disorder, now known as recessive congenital methaemoglobinaemia (RCM), is caused by NADH-cytochrome b5 reductase (cb(5)r) deficiency."
    explanation: The review directly supports CYB5R3/cytochrome b5 reductase deficiency as the primary defect.
  - reference: PMID:31898843
    reference_title: "Mutation update: Variants of the CYB5R3 gene in recessive congenital methemoglobinemia."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "NADH-cytochrome b5 reductase 3 deficiency is an important genetic cause of recessive congenital methemoglobinemia (RCM)"
    explanation: The mutation update supports the enzyme-deficiency mechanism.
  downstream:
  - target: Impaired methemoglobin reduction
    description: Loss of CYB5R3 activity impairs reduction of ferric methemoglobin back to oxygen-binding hemoglobin.
    causal_link_type: DIRECT
  - target: Type II generalized neurologic involvement
    description: Generalized CYB5R3 deficiency across all tissues in type II disease causes neurological involvement beyond the erythrocyte-restricted form.
    causal_link_type: DIRECT
- name: Impaired methemoglobin reduction
  description: >
    Methaemoglobin is continuously formed as heme iron becomes ferric; CYB5R3
    deficiency prevents efficient reduction back to the ferrous state.
  biological_processes:
  - preferred_term: heme oxidation-reduction balance
    modifier: ABNORMAL
    term:
      id: GO:0006788
      label: heme oxidation
  evidence:
  - reference: PMID:3752898
    reference_title: Congenital enzymopenic methaemoglobinaemia.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "the rate at which ferric heme is formed and the rate at which it is reduced back to the ferrous state."
    explanation: The case report review describes normal methemoglobin redox balance.
  - reference: PMID:3752898
    reference_title: Congenital enzymopenic methaemoglobinaemia.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "the methaemoglobin formed at physiological rate is not efficiently reduced."
    explanation: The report directly supports impaired methemoglobin reduction in congenital enzymopenic disease.
  downstream:
  - target: Methemoglobin accumulation
    description: Inefficient reduction causes chronic ferric methemoglobin accumulation.
    causal_link_type: DIRECT
- name: Methemoglobin accumulation
  description: >
    Inefficient reduction of ferric methemoglobin leads to chronically elevated
    methemoglobin levels in blood.
  chemical_entities:
  - preferred_term: methemoglobin
    modifier: INCREASED
    term:
      id: CHEBI:17423
      label: methemoglobin
  evidence:
  - reference: PMID:3752898
    reference_title: Congenital enzymopenic methaemoglobinaemia.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "A chronically elevated level of oxidized haemoglobin results."
    explanation: The report supports chronic accumulation of oxidized hemoglobin.
  - reference: PMID:27879543
    reference_title: Enzymopenic Congenital Methemoglobinemia in Children of the Republic of Sakha (Yakutia).
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The median methemoglobin level was 13.5% (ranging between 4.2% and 33.9%)"
    explanation: Pediatric registry data quantify elevated methemoglobin in affected children.
  downstream:
  - target: Reduced oxygen transport and cyanosis
    description: Ferric methemoglobin cannot bind oxygen efficiently, causing cyanosis and exertional symptoms.
    causal_link_type: DIRECT
- name: Reduced oxygen transport and cyanosis
  description: >
    Methemoglobin is unable to bind oxygen, so chronic methemoglobin elevation
    reduces effective oxygen transport and produces cyanosis, hypoxia, and
    dyspnea.
  biological_processes:
  - preferred_term: oxygen transport
    modifier: DECREASED
    term:
      id: GO:0015671
      label: oxygen transport
  evidence:
  - reference: PMID:3752898
    reference_title: Congenital enzymopenic methaemoglobinaemia.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Methaemoglobin is incapable of binding oxygen."
    explanation: The report supports the impaired oxygen-binding mechanism.
  - reference: PMID:22024786
    reference_title: "Methemoglobinemia: pathogenesis, diagnosis, and management."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "The diagnosis of methemoglobinemia should be considered in patients presenting with cyanosis and hypoxia."
    explanation: The management review links methemoglobinemia with cyanosis and hypoxia.
- name: Type II generalized neurologic involvement
  description: >
    Severe type II disease reflects more generalized CYB5R3 dysfunction,
    especially variants affecting FAD-binding or active-site function, and is
    associated with neurodevelopmental impairment and movement disorders.
  genes:
  - preferred_term: CYB5R3
    term:
      id: hgnc:2873
      label: CYB5R3
  evidence:
  - reference: PMID:31898843
    reference_title: "Mutation update: Variants of the CYB5R3 gene in recessive congenital methemoglobinemia."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The majority of the mutations associated with the severe form with a neurological disorder (RCM Type 2) were associated with the FAD-binding domain"
    explanation: The mutation update supports domain-level association with severe neurologic type II disease.
  - reference: PMID:30614390
    reference_title: "Recessive congenital methemoglobinemia type II: Hypoplastic basal ganglia in two siblings with a novel mutation of the cytochrome b5 reductase gene."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "mild neonatal cyanosis, early onset severe progressive developmental delay, movement disorders, and progressive microcephaly."
    explanation: The sibling report supports the neurologic manifestation pattern in type II disease.
phenotypes:
- name: Methemoglobinemia
  frequency: VERY_FREQUENT
  diagnostic: true
  description: Elevated blood methemoglobin is the defining biochemical and clinical phenotype.
  phenotype_term:
    preferred_term: Methemoglobinemia
    term:
      id: HP:0012119
      label: Methemoglobinemia
  evidence:
  - reference: ORPHA:621
    reference_title: Autosomal recessive methemoglobinemia (Orphanet structured-database record)
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "HP:0012119 | Methemoglobinemia | Very frequent (99-80%)"
    explanation: Orphanet lists methemoglobinemia as very frequent.
  - reference: PMID:27879543
    reference_title: Enzymopenic Congenital Methemoglobinemia in Children of the Republic of Sakha (Yakutia).
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The median methemoglobin level was 13.5% (ranging between 4.2% and 33.9%)"
    explanation: The clinical registry demonstrates elevated methemoglobin levels in affected children.
- name: Cyanosis
  frequency: VERY_FREQUENT
  diagnostic: true
  description: Cyanosis is typically present from birth or infancy.
  phenotype_term:
    preferred_term: Cyanosis
    term:
      id: HP:0000961
      label: Cyanosis
  evidence:
  - reference: ORPHA:621
    reference_title: Autosomal recessive methemoglobinemia (Orphanet structured-database record)
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "HP:0000961 | Cyanosis | Very frequent (99-80%)"
    explanation: Orphanet lists cyanosis as very frequent.
  - reference: PMID:18318771
    reference_title: "Recessive congenital methaemoglobinaemia: cytochrome b(5) reductase deficiency."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "both characterized by cyanosis from birth."
    explanation: The review supports congenital cyanosis in both type I and type II disease.
  - reference: PMID:27879543
    reference_title: Enzymopenic Congenital Methemoglobinemia in Children of the Republic of Sakha (Yakutia).
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "physical examination revealed cyanosis of the skin and mucus membranes."
    explanation: Pediatric registry examination supports cyanosis as a clinical finding.
- name: Lip discoloration
  frequency: FREQUENT
  description: Bluish or dark discoloration of the lips reflects chronic cyanosis.
  phenotype_term:
    preferred_term: Lip discoloration
    term:
      id: HP:0025118
      label: Lip discoloration
  evidence:
  - reference: ORPHA:621
    reference_title: Autosomal recessive methemoglobinemia (Orphanet structured-database record)
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "HP:0025118 | Lip discoloration | Frequent (79-30%)"
    explanation: Orphanet lists lip discoloration as frequent.
- name: Abnormal nail morphology
  frequency: FREQUENT
  description: Orphanet lists nail abnormality as a frequent phenotypic feature.
  phenotype_term:
    preferred_term: Abnormal nail morphology
    term:
      id: HP:0001597
      label: Abnormal nail morphology
  evidence:
  - reference: ORPHA:621
    reference_title: Autosomal recessive methemoglobinemia (Orphanet structured-database record)
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "HP:0001597 | Abnormality of the nail | Frequent (79-30%)"
    explanation: Orphanet lists nail abnormality as frequent.
- name: Exertional dyspnea
  frequency: FREQUENT
  description: Reduced oxygen carrying capacity can produce exertional breathlessness.
  phenotype_term:
    preferred_term: Exertional dyspnea
    term:
      id: HP:0002875
      label: Exertional dyspnea
  evidence:
  - reference: ORPHA:621
    reference_title: Autosomal recessive methemoglobinemia (Orphanet structured-database record)
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "HP:0002875 | Exertional dyspnea | Frequent (79-30%)"
    explanation: Orphanet lists exertional dyspnea as frequent.
- name: Neurologic involvement in type II
  subtype: Type II
  frequency: FREQUENT
  description: Type II disease includes broad neurologic abnormalities, including developmental and movement disorder manifestations.
  phenotype_term:
    preferred_term: Abnormality of the nervous system
    term:
      id: HP:0000707
      label: Abnormality of the nervous system
  evidence:
  - reference: ORPHA:621
    reference_title: Autosomal recessive methemoglobinemia (Orphanet structured-database record)
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "HP:0000707 | Abnormality of the nervous system | Frequent (79-30%)"
    explanation: Orphanet lists nervous system abnormalities as frequent.
  - reference: PMID:18318771
    reference_title: "Recessive congenital methaemoglobinaemia: cytochrome b(5) reductase deficiency."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "the cyanosis is accompanied by neurological impairment and reduced life expectancy."
    explanation: The review supports neurologic involvement in type II disease.
- name: Severe global developmental delay
  subtype: Type II
  frequency: OCCASIONAL
  description: Severe developmental delay is a type II neurodevelopmental manifestation.
  phenotype_term:
    preferred_term: Severe global developmental delay
    term:
      id: HP:0011344
      label: Severe global developmental delay
  evidence:
  - reference: ORPHA:621
    reference_title: Autosomal recessive methemoglobinemia (Orphanet structured-database record)
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "HP:0011344 | Severe global developmental delay | Occasional (29-5%)"
    explanation: Orphanet lists severe global developmental delay as occasional.
  - reference: PMID:30614390
    reference_title: "Recessive congenital methemoglobinemia type II: Hypoplastic basal ganglia in two siblings with a novel mutation of the cytochrome b5 reductase gene."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "early onset severe progressive developmental delay, movement disorders, and progressive microcephaly."
    explanation: Type II sibling cases support severe progressive developmental delay.
- name: Severe intellectual disability
  subtype: Type II
  frequency: OCCASIONAL
  description: Severe intellectual disability is part of the type II neurologic phenotype.
  phenotype_term:
    preferred_term: Severe intellectual disability
    term:
      id: HP:0010864
      label: Severe intellectual disability
  evidence:
  - reference: ORPHA:621
    reference_title: Autosomal recessive methemoglobinemia (Orphanet structured-database record)
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "HP:0010864 | Intellectual disability, severe | Occasional (29-5%)"
    explanation: Orphanet lists severe intellectual disability as occasional.
  - reference: PMID:19480335
    reference_title: "A rare cause of mental motor retardation: recessive congenital methemoglobinemia type II."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "growth and mental retardation and other neurological impairments."
    explanation: Type II clinical report supports severe neurodevelopmental impairment.
- name: Microcephaly
  subtype: Type II
  frequency: OCCASIONAL
  description: Progressive microcephaly is described in type II disease.
  phenotype_term:
    preferred_term: Microcephaly
    term:
      id: HP:0000252
      label: Microcephaly
  evidence:
  - reference: ORPHA:621
    reference_title: Autosomal recessive methemoglobinemia (Orphanet structured-database record)
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "HP:0000252 | Microcephaly | Occasional (29-5%)"
    explanation: Orphanet lists microcephaly as occasional.
  - reference: PMID:30614390
    reference_title: "Recessive congenital methemoglobinemia type II: Hypoplastic basal ganglia in two siblings with a novel mutation of the cytochrome b5 reductase gene."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "movement disorders, and progressive microcephaly."
    explanation: Type II sibling cases support progressive microcephaly.
- name: Athetosis
  subtype: Type II
  frequency: OCCASIONAL
  description: Athetosis is one of the movement disorder manifestations reported in Orphanet.
  phenotype_term:
    preferred_term: Athetosis
    term:
      id: HP:0002305
      label: Athetosis
  evidence:
  - reference: ORPHA:621
    reference_title: Autosomal recessive methemoglobinemia (Orphanet structured-database record)
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "HP:0002305 | Athetosis | Occasional (29-5%)"
    explanation: Orphanet lists athetosis as occasional.
- name: Limb dystonia
  subtype: Type II
  frequency: OCCASIONAL
  description: Limb dystonia is listed among neurologic motor manifestations.
  phenotype_term:
    preferred_term: Limb dystonia
    term:
      id: HP:0002451
      label: Limb dystonia
  evidence:
  - reference: ORPHA:621
    reference_title: Autosomal recessive methemoglobinemia (Orphanet structured-database record)
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "HP:0002451 | Limb dystonia | Occasional (29-5%)"
    explanation: Orphanet lists limb dystonia as occasional.
- name: Spasticity
  subtype: Type II
  frequency: OCCASIONAL
  description: Pyramidal motor involvement can produce spasticity.
  phenotype_term:
    preferred_term: Spasticity
    term:
      id: HP:0001257
      label: Spasticity
  evidence:
  - reference: ORPHA:621
    reference_title: Autosomal recessive methemoglobinemia (Orphanet structured-database record)
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "HP:0001257 | Spasticity | Occasional (29-5%)"
    explanation: Orphanet lists spasticity as occasional.
- name: Small basal ganglia
  subtype: Type II
  frequency: VERY_RARE
  diagnostic: true
  description: Basal ganglia hypoplasia can be a diagnostic imaging clue in type II disease.
  phenotype_term:
    preferred_term: Small basal ganglia
    term:
      id: HP:0012697
      label: Small basal ganglia
  evidence:
  - reference: ORPHA:621
    reference_title: Autosomal recessive methemoglobinemia (Orphanet structured-database record)
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "HP:0012697 | Small basal ganglia | Very rare (<4-1%)"
    explanation: Orphanet lists small basal ganglia as very rare.
  - reference: PMID:30614390
    reference_title: "Recessive congenital methemoglobinemia type II: Hypoplastic basal ganglia in two siblings with a novel mutation of the cytochrome b5 reductase gene."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "basal ganglia hypoplasia is an interesting clue to the very rare and frequently unsuspected diagnosis"
    explanation: Type II sibling MRI report supports basal ganglia hypoplasia as a diagnostic clue.
biochemical:
- name: Elevated methemoglobin
  presence: Elevated
  context: Blood methemoglobin measurement
  notes: >
    Affected individuals have elevated blood methemoglobin; levels in a Yakutia
    pediatric registry ranged from mild to markedly elevated values.
  evidence:
  - reference: PMID:27879543
    reference_title: Enzymopenic Congenital Methemoglobinemia in Children of the Republic of Sakha (Yakutia).
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The median methemoglobin level was 13.5% (ranging between 4.2% and 33.9%)"
    explanation: Registry data support elevated blood methemoglobin.
- name: Reduced methemoglobin reductase activity
  presence: Reduced
  context: Erythrocyte enzyme deficiency
  notes: >
    Congenital enzymopenic disease reflects deficiency or absence of the
    methemoglobin reductase system that normally reduces ferric heme.
  evidence:
  - reference: PMID:3752898
    reference_title: Congenital enzymopenic methaemoglobinaemia.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Congenital methaemoglobinaemia caused by a deficiency or absence of methaemoglobin reductase"
    explanation: The report supports reduced methemoglobin reductase activity as the biochemical defect.
diagnosis:
- name: Blood methemoglobin and co-oximetry assessment
  diagnosis_term:
    preferred_term: clinical assessment
    term:
      id: MAXO:0000487
      label: clinical assessment
  description: >
    Clinical suspicion is raised by cyanosis, hypoxia, saturation gap, and
    chocolate-brown blood; confirmation requires measuring elevated blood
    methemoglobin.
  results: Elevated methemoglobin confirms methemoglobinemia in the appropriate clinical context.
  evidence:
  - reference: PMID:22024786
    reference_title: "Methemoglobinemia: pathogenesis, diagnosis, and management."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "Unique features, such as a saturation gap and chocolate-brown-colored blood, can raise suspicion for methemoglobinemia."
    explanation: The review supports the saturation-gap and chocolate-brown-blood diagnostic clues.
  - reference: PMID:27879543
    reference_title: Enzymopenic Congenital Methemoglobinemia in Children of the Republic of Sakha (Yakutia).
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The median methemoglobin level was 13.5% (ranging between 4.2% and 33.9%)"
    explanation: Registry data demonstrate diagnostic methemoglobin elevation in congenital disease.
- name: CYB5R3 molecular genetic testing
  diagnosis_term:
    preferred_term: genetic testing
    term:
      id: MAXO:0000127
      label: genetic testing
  description: >
    Molecular testing for biallelic CYB5R3 pathogenic variants confirms the
    autosomal recessive enzymopenic form and supports subtype and family-risk
    assessment.
  results: Biallelic pathogenic CYB5R3 variants support diagnosis of CYB5R3-related hereditary methemoglobinemia.
  evidence:
  - reference: PMID:31898843
    reference_title: "Mutation update: Variants of the CYB5R3 gene in recessive congenital methemoglobinemia."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "review of all the pathogenic mutations and their molecular pathology in RCM"
    explanation: The mutation update supports molecular characterization of CYB5R3 variants in RCM.
- name: Brain MRI basal ganglia hypoplasia assessment
  diagnosis_term:
    preferred_term: diagnostic procedure
    term:
      id: MAXO:0000003
      label: diagnostic procedure
  description: >
    Brain MRI showing small lentiform and caudate nuclei with reduced
    frontotemporal volume can support recognition of type II disease in a child
    with congenital cyanosis and neurodevelopmental impairment.
  results: Basal ganglia hypoplasia supports type II disease in the appropriate phenotype.
  evidence:
  - reference: PMID:30614390
    reference_title: "Recessive congenital methemoglobinemia type II: Hypoplastic basal ganglia in two siblings with a novel mutation of the cytochrome b5 reductase gene."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "bilateral small size of the lentiform and caudate nuclei and reduced frontotemporal brain volume."
    explanation: Type II sibling MRI findings support basal ganglia hypoplasia as a diagnostic clue.
treatments:
- name: Methylene blue pharmacotherapy
  description: >
    Methylene blue can lower methemoglobin levels and is generally reserved for
    symptomatic or significantly elevated methemoglobinemia; evidence in type II
    congenital disease is limited to case-level symptomatic benefit.
  treatment_term:
    preferred_term: Pharmacotherapy
    term:
      id: NCIT:C15986
      label: Pharmacotherapy
    therapeutic_agent:
    - preferred_term: methylene blue
      term:
        id: CHEBI:6872
        label: methylene blue
  target_mechanisms:
  - target: Methemoglobin accumulation
    treatment_effect: MODULATES
    description: Methylene blue treatment aims to reduce elevated methemoglobin levels.
    evidence:
    - reference: PMID:26574897
      reference_title: Congenital Methemoglobinemia Type II-Clinical Improvement with Short-Term Methylene Blue Treatment.
      supports: PARTIAL
      evidence_source: HUMAN_CLINICAL
      snippet: "Since the MB treatment regimen has commenced, his methemoglobin level has been significantly lower."
      explanation: Single-case type II treatment evidence supports methemoglobin lowering, but not broad disease modification.
  evidence:
  - reference: PMID:22024786
    reference_title: "Methemoglobinemia: pathogenesis, diagnosis, and management."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "treatment with methylene blue is reserved for patients with significantly elevated methemoglobin levels."
    explanation: The management review supports methylene blue use for significant methemoglobinemia.
  - reference: PMID:26574897
    reference_title: Congenital Methemoglobinemia Type II-Clinical Improvement with Short-Term Methylene Blue Treatment.
    supports: PARTIAL
    evidence_source: HUMAN_CLINICAL
    snippet: "modest behavioral improvements (as assessed on the Achenbach behavior report scales)."
    explanation: Case-level type II evidence suggests limited symptomatic improvement.
- name: Ascorbic acid pharmacotherapy
  description: >
    Ascorbic acid has been reported as symptomatic treatment for cyanosis in
    recessive congenital methemoglobinemia.
  treatment_term:
    preferred_term: Pharmacotherapy
    term:
      id: NCIT:C15986
      label: Pharmacotherapy
    therapeutic_agent:
    - preferred_term: ascorbic acid
      term:
        id: CHEBI:22652
        label: ascorbic acid
  target_phenotypes:
  - preferred_term: Cyanosis
    term:
      id: HP:0000961
      label: Cyanosis
  evidence:
  - reference: PMID:19480335
    reference_title: "A rare cause of mental motor retardation: recessive congenital methemoglobinemia type II."
    supports: PARTIAL
    evidence_source: HUMAN_CLINICAL
    snippet: "Cyanosis can be well treated by 200-500 mg of ascorbic acid daily"
    explanation: Clinical report text supports ascorbic acid as symptomatic therapy for cyanosis.
- name: Genetic counseling
  description: >
    Genetic counseling is appropriate because CYB5R3-related hereditary
    methemoglobinemia is autosomal recessive and molecular diagnosis clarifies
    carrier and recurrence risks for relatives.
  treatment_term:
    preferred_term: genetic counseling
    term:
      id: MAXO:0000079
      label: genetic counseling
  evidence:
  - reference: PMID:19480335
    reference_title: "A rare cause of mental motor retardation: recessive congenital methemoglobinemia type II."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Prenatal diagnosis is possible."
    explanation: The report supports familial molecular risk assessment and reproductive counseling.
notes: >
  ORPHA:621 is scoped to autosomal recessive congenital/hereditary
  methemoglobinemia types I and II. The Orphanet gene table also lists CYB5A as
  a candidate-tested gene, but this entry focuses on the disease-causing
  CYB5R3 reductase-deficiency axis supported by the cited clinical literature.
references:
- reference: ORPHA:621
  title: Autosomal recessive methemoglobinemia
  found_in:
  - Hereditary_Methemoglobinemia-deep-research-fallback.md
- reference: PMID:18318771
  title: "Recessive congenital methaemoglobinaemia: cytochrome b(5) reductase deficiency."
  found_in:
  - Hereditary_Methemoglobinemia-deep-research-fallback.md
- reference: PMID:19480335
  title: "A rare cause of mental motor retardation: recessive congenital methemoglobinemia type II."
  found_in:
  - Hereditary_Methemoglobinemia-deep-research-fallback.md
- reference: PMID:22024786
  title: "Methemoglobinemia: pathogenesis, diagnosis, and management."
  found_in:
  - Hereditary_Methemoglobinemia-deep-research-fallback.md
- reference: PMID:26574897
  title: Congenital Methemoglobinemia Type II-Clinical Improvement with Short-Term Methylene Blue Treatment.
  found_in:
  - Hereditary_Methemoglobinemia-deep-research-fallback.md
- reference: PMID:27879543
  title: Enzymopenic Congenital Methemoglobinemia in Children of the Republic of Sakha (Yakutia).
  found_in:
  - Hereditary_Methemoglobinemia-deep-research-fallback.md
- reference: PMID:30614390
  title: "Recessive congenital methemoglobinemia type II: Hypoplastic basal ganglia in two siblings with a novel mutation of the cytochrome b5 reductase gene."
  found_in:
  - Hereditary_Methemoglobinemia-deep-research-fallback.md
- reference: PMID:31898843
  title: "Mutation update: Variants of the CYB5R3 gene in recessive congenital methemoglobinemia."
  found_in:
  - Hereditary_Methemoglobinemia-deep-research-fallback.md
- reference: PMID:3752898
  title: Congenital enzymopenic methaemoglobinaemia.
  found_in:
  - Hereditary_Methemoglobinemia-deep-research-fallback.md
📚

References & Deep Research

References

9
ORPHA:621
Autosomal recessive methemoglobinemia
No top-level findings curated for this source.
PMID:18318771
Recessive congenital methaemoglobinaemia: cytochrome b(5) reductase deficiency.
No top-level findings curated for this source.
PMID:19480335
A rare cause of mental motor retardation: recessive congenital methemoglobinemia type II.
No top-level findings curated for this source.
PMID:22024786
Methemoglobinemia: pathogenesis, diagnosis, and management.
No top-level findings curated for this source.
PMID:26574897
Congenital Methemoglobinemia Type II-Clinical Improvement with Short-Term Methylene Blue Treatment.
No top-level findings curated for this source.
PMID:27879543
Enzymopenic Congenital Methemoglobinemia in Children of the Republic of Sakha (Yakutia).
No top-level findings curated for this source.
PMID:30614390
Recessive congenital methemoglobinemia type II: Hypoplastic basal ganglia in two siblings with a novel mutation of the cytochrome b5 reductase gene.
No top-level findings curated for this source.
PMID:31898843
Mutation update: Variants of the CYB5R3 gene in recessive congenital methemoglobinemia.
No top-level findings curated for this source.
PMID:3752898
Congenital enzymopenic methaemoglobinaemia.
No top-level findings curated for this source.

Deep Research

1
Hereditary Methemoglobinemia Deep Research Fallback ▸

Hereditary Methemoglobinemia Deep Research Fallback

Provider Attempts

  • 2026-05-09T13:04Z: timeout 240s just research-disorder falcon Hereditary_Methemoglobinemia started successfully but produced no usable research artifact during the bounded wait. The timeout terminated the provider process with SIGTERM.
  • 2026-05-09T13:08Z: timeout 240s just research-disorder openai Hereditary_Methemoglobinemia also started successfully but produced no usable research artifact during the bounded wait. The timeout terminated the provider process with SIGTERM.

No provider-generated deep-research narrative was available within the bounded runtime. Curation therefore proceeded from generated structured Orphanet evidence and fetched PubMed caches, without hand-editing any references_cache/*.md files.

Evidence Scope Used For Curation

  • ORPHA:621 provides the direct disease mapping to MONDO:0018963, ICD-10:D74.0 and OMIM cross-references, autosomal recessive inheritance, neonatal/infancy onset, unknown worldwide point prevalence, CYB5R3 disease-causing gene row, CYB5A candidate-tested gene row, and the structured HPO phenotype table.
  • PMID:18318771 supports recessive congenital methemoglobinemia as NADH-cytochrome b5 reductase deficiency, distinguishes type I and type II, identifies CYB5R3 as the encoding gene, and links type II to neurologic impairment and reduced life expectancy.
  • PMID:31898843 supports global CYB5R3 allelic heterogeneity, autosomal recessive inheritance, molecular pathology, and the association of FAD-domain variants with the severe neurologic type II form.
  • PMID:3752898 supports the biochemical redox mechanism: ferric methemoglobin is normally reduced back to ferrous hemoglobin, but congenital reductase deficiency prevents efficient reduction and produces chronically elevated oxidized hemoglobin, cyanosis, and impaired oxygen binding.
  • PMID:27879543 supports pediatric registry evidence for type I congenital methemoglobinemia in Yakutia, including median methemoglobin levels, cyanosis on examination, hypoxic-response laboratory correlations, and regional prevalence estimates.
  • PMID:30614390 supports type II clinical progression with neonatal cyanosis, severe progressive developmental delay, movement disorders, progressive microcephaly, CYB5R3 variants, small lentiform/caudate nuclei, reduced frontotemporal volume, and basal ganglia hypoplasia as a diagnostic clue.
  • PMID:19480335 supports the type I erythrocyte-restricted versus type II generalized tissue-deficiency distinction, type II neurologic features, prenatal diagnosis, and symptomatic ascorbic acid treatment of cyanosis.
  • PMID:22024786 supports clinical diagnostic clues and management framing for methemoglobinemia, including cyanosis, hypoxia, saturation gap, chocolate-brown blood, symptom correlation with methemoglobin level, and methylene blue for significantly elevated methemoglobin.
  • PMID:26574897 supports case-level type II congenital methemoglobinemia treatment with methylene blue, including CYB5R3 compound heterozygosity, cytochrome-b5 reductase deficiency, lowered methemoglobin level, and modest behavioral improvement.

Curation Conclusions

The curated YAML represents the CYB5R3-related autosomal recessive mechanism: germline CYB5R3 variants reduce NADH-cytochrome b5 reductase activity in erythrocytes and, in type II, more broadly across tissues. The enzyme defect impairs reduction of ferric methemoglobin back to ferrous oxygen-binding hemoglobin, causing chronic methemoglobin accumulation, reduced oxygen transport, cyanosis, lip and nail discoloration, and exertional dyspnea. Type II adds generalized neurologic involvement, with developmental delay, severe intellectual disability, movement disorder, progressive microcephaly, small basal ganglia, and frontotemporal brain volume loss. Treatment evidence is symptomatic: methylene blue can lower methemoglobin, while ascorbic acid is reported for cyanosis.