📘

Definitions

1

EULAR/ACR 2025 classification criteria for paediatric chronic nonbacterial osteomyelitis

Jointly developed EULAR/ACR classification criteria for paediatric CNO. The structure mirrors the disease's diagnostic problem: an obligatory entry requirement of typical radiographic or MRI findings plus bone pain, then explicit exclusion of malignancy, infection, vitamin C deficiency and hypophosphatasia, and only then additive weighted scoring across five clinical domains (site and pattern of bone lesions, age at onset, coexisting conditions, fever) and four pathology/laboratory domains (bone biopsy findings if performed, anaemia, C-reactive protein, erythrocyte sedimentation rate). A total score of at least 55 classifies as CNO. These are classification criteria, developed for research cohort definition, not diagnostic criteria for individual patient care - a distinction the entry preserves because they are not interchangeable.

PHENOTYPE_ALGORITHM

Show evidence (2 references)

PMID:39966038 SUPPORT Human Clinical

"The new EULAR/ACR classification criteria for CNO require typical radiographic or magnetic resonance imaging findings and bone pain as an obligatory entry criterion and exclusion criteria of malignancy, infection, vitamin C deficiency, and hypophosphatasia, followed by additive weighted criteria..."

The full structure of the criteria, including entry, exclusion and scoring domains.

PMID:39966038 SUPPORT Human Clinical

"refinement of weights and threshold score in a development cohort of 441 patients and validation in another cohort of 514 patients"

Documents the development and validation cohorts underlying the criteria.

?

Discussions and Knowledge Gaps

1

What is the molecular defect in sporadic CNO/CRMO, and why does no single cytokine-blocking strategy define the disease the way IL-1 blockade defines its monogenic mimics?

KNOWLEDGE GAP gap_crmo_molecular_pathophysiology_unresolved

Attached to

pathophysiology#Innate Immune Signalling Dysregulation

Both cited reviews state that the pathophysiology is not understood - one calls it "somewhat elusive", the other "not fully understood" - and neither identifies a causal gene for sporadic disease. What is established is downstream and cellular: monocytes show inflammasome activation and a pro- versus anti-inflammatory cytokine imbalance. The contrast with the monogenic mimics is the informative part. Majeed syndrome has a single defined cytokine dependency - it responds to IL-1-beta blockade and not to TNF blockade - and DIRA resolves on recombinant IL-1 receptor antagonist. Sporadic CNO has no such signature: NSAIDs, corticosteroids, anti-TNF agents and bisphosphonates are all reported effective in some patients and insufficient in others, and every report is retrospective with small numbers. That heterogeneity of response is what a mechanistic account has to explain, and it is equally consistent with one poorly characterised pathway and with sporadic CNO being several diseases under one name.

Proposed experiments

Stratify CNO monocyte signalling defects against treatment response

exp_cno_monocyte_pathway_stratification

Profile TLR4/MAPK/inflammasome signalling and cytokine output in monocytes from a prospectively followed CNO cohort, and score each patient's profile against their documented response to NSAIDs, corticosteroids, TNF blockade and bisphosphonates. The discriminating result is whether response tracks a signalling subgroup: if it does, sporadic CNO is more than one disease and should eventually be curated as such, and the retrospective series reporting partial efficacy for every agent are averaging over that structure. A prospective design is the point - the existing evidence base is explicitly retrospective and small.

⚙

Pathophysiology

4

Innate Immune Signalling Dysregulation

The upstream lesion, and the one the literature is least certain about. Variable defects in the TLR4/MAPK/inflammasome signalling cascade are implicated. No single causal gene is established for sporadic CNO/CRMO - unlike its monogenic mimics - so this node describes a class of defects rather than one.

Downstream

Monocyte Pro-Inflammatory Cytokine Imbalance

Show evidence (1 reference)

PMID:29080202 SUPPORT Human Clinical

"it appears likely that variable defects in the TLR4/MAPK/inflammasome signaling cascade result in an imbalance between pro- and anti-inflammatory cytokine expressions in monocytes from CNO/CRMO patients"

Names the implicated signalling cascade and its immediate consequence, with the authors' own hedging ("appears likely") preserved in the node description.

Monocyte Pro-Inflammatory Cytokine Imbalance

Monocytes from patients show an imbalance between pro- and anti-inflammatory cytokine expression, with increased inflammasome activation. The imbalance is bidirectional: it is not only that inflammatory cytokines rise, but that the regulatory arm is impaired - which is why the disease behaves as a failure of restraint rather than as a response to a stimulus.

monocyte CL:0000576

interleukin-1 beta production GO:0032611 ↑ INCREASED interleukin-10 production GO:0032613 ↓ DECREASED

Downstream

Sterile Osteoinflammation

Show evidence (1 reference)

PMID:33870159 SUPPORT Human Clinical

"imbalanced cytokine expression and increased inflammasome activation in monocytes from CNO patients contribute to a pro-inflammatory phenotype that contributes to bone inflammation"

Establishes the monocyte cytokine imbalance and inflammasome activation as the cellular mechanism linking innate signalling to bone inflammation.

Sterile Osteoinflammation

Inflammatory bone lesions form without an infectious agent. This is the node that defines the disease and the one that cannot be demonstrated positively - it is established by failing to find an organism, and by lesions that neither respond to antibiotics nor behave like infection over time.

Downstream

Multifocal Relapsing Bone Disease

Show evidence (1 reference)

PMID:33870159 SUPPORT Human Clinical

"Chronic nonbacterial osteomyelitis (CNO) is an inflammatory bone disorder that most frequently affects children and adolescents."

Establishes the disease as an inflammatory - explicitly nonbacterial - bone disorder of childhood.

Multifocal Relapsing Bone Disease

The clinical endpoint. Lesions are typically symmetrical and multifocal, and the course waxes and wanes rather than progressing steadily or resolving. Chronic pain can be severe enough to affect quality of life and psychosocial development, which in a paediatric population is a substantial part of the disease burden rather than a footnote to it.

Show evidence (2 references)

PMID:33870159 SUPPORT Human Clinical

"Chronic recurrent multifocal osteomyelitis (CRMO) is a severe form of CNO, usually characterized by symmetrical inflammatory bone lesions and its waxing and waning character."

Establishes the multifocal, symmetrical and relapsing character that names the disease.

PMID:33870159 SUPPORT Human Clinical

"Sometimes severe and chronic pain can significantly affect the quality of life and psychosocial development of individuals affected."

Documents the burden of chronic pain in this paediatric population.

⬡

Pathograph

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

●

Phenotypes

5

Immune 2

Osteomyelitis HP:0002754

Show evidence (1 reference)

PMID:33870159 SUPPORT Human Clinical

"Chronic recurrent multifocal osteomyelitis (CRMO) is a severe form of CNO, usually characterized by symmetrical inflammatory bone lesions and its waxing and waning character."

Establishes the inflammatory bone lesions that constitute the phenotype.

Severe Acne OCCASIONAL HP:0001061

Severity: SEVERE

Show evidence (1 reference)

PMID:29080202 SUPPORT Human Clinical

"severe acne (10%) and inflammatory bowel disease (10%)"

Gives an explicit 10% figure, which maps to the OCCASIONAL band (5-29%).

Musculoskeletal 1

Vertebral Compression Fracture HP:0002953

Show evidence (1 reference)

PMID:33870159 SUPPORT Human Clinical

"some patients will develop sequalae which can include vertebral compression fractures"

Names vertebral compression fracture as a documented sequela. No denominator is given, so no frequency band is asserted.

Constitutional 1

Bone Pain HP:0002653

Temporal: CHRONIC

Show evidence (1 reference)

PMID:39966038 SUPPORT Human Clinical

"The new EULAR/ACR classification criteria for CNO require typical radiographic or magnetic resonance imaging findings and bone pain as an obligatory entry criterion"

Establishes bone pain as obligatory for classification, i.e. present in essentially all cases.

Other 1

Inflammatory Bowel Disease OCCASIONAL Inflammation of the large intestine HP:0002037

Show evidence (1 reference)

PMID:29080202 SUPPORT Human Clinical

"severe acne (10%) and inflammatory bowel disease (10%)"

Gives an explicit 10% figure, which maps to the OCCASIONAL band (5-29%).

🧬

Genetic Associations

2

FBLIM1 (The only susceptibility gene reported for sporadic CNO, and the reason the entry does not claim the disease has no genetic component - it has no established Mendelian cause, which is a different statement. Two biallelic hits in the filamin-binding domain were found by exome sequencing in unrelated South Asian patients. FBLIM1 is proposed to act as an anti-inflammatory brake on bone remodelling through regulation of RANKL activation, which places it on the same osteoclast arm the bisphosphonates target.)

Gene: FBLIM1 hgnc:24686 relationship_type: SUSCEPTIBILITY

Show evidence (1 reference)

PMID:29080202 SUPPORT Human Clinical

"in the filamin-binding domain of the FBLIM1 gene were detected in unrelated CNO patients from South Asia"

Establishes FBLIM1 as a reported CNO susceptibility gene. Curated as SUSCEPTIBILITY rather than causal: two unrelated probands is not an established Mendelian cause, and the review itself frames sporadic CNO as having no causal gene.

IL10 (Promoter haplotypes at rs1800896/rs1800871/rs1800872 set IL-10 expression capacity, and IL-10 drives STAT3, which in turn regulates FBLIM1 - so the two genetic findings in this disease sit on one axis rather than two. The direction of the cohort finding is counterintuitive and is curated as reported: high-expression (GCC) haplotypes were the ones over-represented in CRMO, not low-expression ones.)

Gene: IL10 hgnc:5962 relationship_type: MODIFIER

Show evidence (1 reference)

PMID:29080202 SUPPORT Human Clinical

"IL-10 expression is predetermined by genetic variants within the IL10 proximal promoter region."

Establishes the IL10 promoter haplotypes as determinants of IL-10 expression.

💊

Medical Actions

3

Nonsteroidal Anti-Inflammatory Therapy

Action: Pharmacotherapy NCIT:C15986

First-line, and effective at least partially in most patients. Note the framing in the cited review: no medication is licensed for CNO at all, so every treatment listed here is used off-label. That is a property of the disease's evidence base rather than a gap in this entry.

Mechanism Target:

INHIBITS Sterile Osteoinflammation — Anti-inflammatory therapy suppresses the sterile bone inflammation without addressing the upstream innate signalling defect.

Show evidence (1 reference)

PMID:33870159 SUPPORT Human Clinical

"Most patients show at least some response to nonsteroidal anti-inflammatory drugs"

Documents the first-line response that grounds this treatment link.

Show evidence (1 reference)

PMID:33870159 SUPPORT Human Clinical

"Currently, no medications are licensed for the use in CNO."

Establishes that all pharmacotherapy in this disease is off-label, which is the context for every treatment curated here.

Escalation Therapy

Action: Pharmacotherapy NCIT:C15986

For patients who do not respond adequately to NSAIDs: corticosteroids, disease-modifying anti-rheumatic drugs and cytokine-blocking agents. Corticosteroids control inflammatory activity quickly but rarely induce long-term remission, so they function as a bridge rather than as maintenance. Anti-TNF agents are reported to be highly effective in a significant percentage of patients, though every report is retrospective and small, which is why the evidence base - not the drug class - is the limiting factor here.

Show evidence (3 references)

PMID:33870159 SUPPORT Human Clinical

"others require more aggressive treatment that can include corticosteroids, cytokine-blocking agents and/or bisphosphonates"

Documents the escalation options used when NSAIDs are insufficient.

PMID:29080202 SUPPORT Human Clinical

"In NSAID-refractory cases, treatment can be escalated with corticosteroids."

Establishes corticosteroids as the first escalation step after NSAID failure.

PMID:29080202 SUPPORT Human Clinical

"treatment with pamidronate or anti-TNF agents has been reported to be highly effective in a significant percentage of CNO/CRMO patients, inducing long-lasting remission in a large subset of patients"

Documents reported anti-TNF efficacy in sporadic CNO/CRMO. Recorded explicitly because the same review reports the opposite for the monogenic mimic Majeed syndrome, and the two must not be conflated.

Bisphosphonate Therapy

Action: Pharmacotherapy NCIT:C15986

Agent: pamidronate CHEBI:7903

Curated as its own node rather than folded into generic escalation because its target differs from the rest of the escalation ladder: bisphosphonates act on the osteoclast-mediated bone resorption arm rather than on cytokine signalling. It is prioritised where vertebral bodies are involved, which is also where the disease does its irreversible structural damage. As with every treatment here the evidence is retrospective and small - the cited review states plainly that large prospective trials to determine the best medication are lacking.

Mechanism Target:

INHIBITS Sterile Osteoinflammation — Bisphosphonates suppress the osteoclastic bone resorption driven by the pro-inflammatory cytokine milieu, without correcting the upstream innate signalling defect.

Show evidence (1 reference)

PMID:29080202 SUPPORT Human Clinical

"treatment with pamidronate or anti-TNF agents has been reported to be highly effective in a significant percentage of CNO/CRMO patients, inducing long-lasting remission in a large subset of patients"

Grounds the therapeutic link between bisphosphonate exposure and remission.

Show evidence (1 reference)

PMID:29080202 SUPPORT Human Clinical

"In patients with vertebral body involvement and structural damage, aggressive treatment should be discussed initially, e.g., with bisphosphonates"

Establishes vertebral involvement as the setting in which bisphosphonates are prioritised over stepwise escalation.

🔬

Diagnosis

2

Diagnosis of Exclusion

Until the 2025 classification criteria there were no prospectively tested, widely accepted diagnostic criteria and no disease biomarker, so CNO was and largely remains a diagnosis of exclusion. Infection and malignancy are the two exclusions that matter most - a solitary lesion in particular can be indistinguishable from bacterial osteomyelitis or from a bone tumour without biopsy. The absence of a biomarker is not a gap in this entry's sourcing; it is a stated property of the disease.

diagnosis of exclusion

Show evidence (1 reference)

PMID:33870159 SUPPORT Human Clinical

"In the absence of prospectively tested and widely accepted diagnostic criteria or disease biomarkers, CNO remains a diagnosis of exclusion, and infections, malignancy and other differentials require consideration"

States directly that CNO is a diagnosis of exclusion and names the differentials that must be considered.

Radiographic and MRI Assessment

Typical radiographic or MRI findings are an obligatory entry criterion for classification. Whole-body MRI additionally reveals clinically silent lesions, which is what establishes multifocality in a patient with one painful site.

Magnetic Resonance Imaging NCIT:C16809

Show evidence (1 reference)

PMID:39966038 SUPPORT Human Clinical

"The new EULAR/ACR classification criteria for CNO require typical radiographic or magnetic resonance imaging findings and bone pain as an obligatory entry criterion"

Establishes imaging as obligatory for classification.

🔀

Differential Diagnoses

3

Conditions with similar clinical presentations that must be differentiated from Chronic Recurrent Multifocal Osteomyelitis:

Bacterial osteomyelitis

Overlapping Features The differential the disease is named against. Imaging and histology can look identical, and it is an explicit exclusion criterion in the 2025 classification criteria. Distinguishing them is what the entire diagnostic workup exists to do, and getting it wrong in either direction is harmful - antibiotics for CNO, or immunosuppression for an untreated infection.

Distinguishing Features

No causative organism is isolated in CNO
CNO lesions do not respond to antibiotics
CNO is typically multifocal and symmetrical with a waxing and waning course

Show evidence (1 reference)

PMID:39966038 SUPPORT Human Clinical

"exclusion criteria of malignancy, infection, vitamin C deficiency, and hypophosphatasia"

Establishes infection as a formal exclusion criterion for classification.

Malignancy involving bone

Overlapping Features Leukaemia and primary bone tumours can present with bone pain and lytic lesions in the same age group, and are the exclusion with the highest cost of error. Also a formal exclusion criterion.

Distinguishing Features

Formal exclusion criterion in the EULAR/ACR classification criteria
Biopsy is included as a scoring domain when performed, largely to address this differential

Show evidence (1 reference)

PMID:33870159 SUPPORT Human Clinical

"CNO remains a diagnosis of exclusion, and infections, malignancy and other differentials require consideration"

Names malignancy among the differentials requiring exclusion.

Majeed syndrome

Overlapping Features A monogenic autoinflammatory syndrome (LPIN2) whose features include CRMO alongside congenital dyserythropoietic anaemia and, in some patients, a neutrophilic dermatosis. It is the reason a child with CRMO plus unexplained anaemia should be considered for genetic evaluation rather than treated as sporadic disease.

Distinguishing Features

Majeed syndrome pairs CRMO with congenital dyserythropoietic anaemia from early infancy
Monogenic and inherited, versus sporadic CNO with no established causal gene
Onset is characteristically early with a lifelong course
Treatment response separates the two: Majeed is an IL-1-beta-mediated disease that responds to IL-1 blockade while TNF-alpha blockers have almost no effect, whereas in sporadic CNO/CRMO anti-TNF agents are reported to be highly effective in a significant percentage of patients

Show evidence (2 references)

PMID:29080202 SUPPORT Human Clinical

"TNFα blockers have almost no effect"

Curated here, on the Majeed differential, because that is the disease this sentence is about - it sits inside the review's monogenic-syndromes section, several pages above the "Sporadic" CNO/CRMO section, which reports the opposite. The TNF-responsiveness asymmetry is a discriminating feature between the two, not a property of sporadic disease.

PMID:20301735 SUPPORT Human Clinical

"Majeed syndrome is characterized by: Chronic recurrent multifocal osteomyelitis (CRMO) that is of early onset with a lifelong course; and Congenital dyserythropoietic anemia (CDA)"

Establishes the CRMO-plus-anaemia combination that distinguishes Majeed syndrome. Note this GeneReviews chapter is formally retired - see notes - so it is cited only for this stable definitional statement.

{ }

Source YAML

click to show

name: Chronic Recurrent Multifocal Osteomyelitis
creation_date: "2026-08-21T00:00:00Z"
category: Complex
description: >-
  Chronic nonbacterial osteomyelitis (CNO), in its multifocal relapsing form chronic
  recurrent multifocal osteomyelitis (CRMO), is an autoinflammatory bone disease of
  children and adolescents. It produces multiple painful inflammatory bone lesions,
  typically in the metaphyses of long bones but also in pelvis, shoulder girdle and
  spine, with a characteristic waxing and waning course.

  The defining feature is a negative one: the osteomyelitis is sterile. Imaging and
  histology look infectious, and the name says osteomyelitis, but no organism is
  responsible and antibiotics do not treat it. That makes the central clinical problem
  exclusion rather than detection - CNO has been a diagnosis of exclusion, requiring
  infection, malignancy and metabolic bone disease to be ruled out first, and
  2025 EULAR/ACR classification criteria formalise exactly that structure.

  Mechanistically it is an innate immune disorder rather than an adaptive one: variable
  defects in TLR4/MAPK/inflammasome signalling shift monocytes toward a
  pro-inflammatory cytokine profile. The molecular pathophysiology is explicitly not
  settled, and this entry curates it as unsettled.
disease_term:
  preferred_term: chronic recurrent multifocal osteomyelitis
  term:
    id: MONDO:0009813
    label: chronic recurrent multifocal osteomyelitis

pathophysiology:
- name: Innate Immune Signalling Dysregulation
  biological_scale: MOLECULAR
  description: >-
    The upstream lesion, and the one the literature is least certain about. Variable
    defects in the TLR4/MAPK/inflammasome signalling cascade are implicated. No single
    causal gene is established for sporadic CNO/CRMO - unlike its monogenic mimics - so
    this node describes a class of defects rather than one.
  downstream:
  - target: Monocyte Pro-Inflammatory Cytokine Imbalance
    causal_link_type: DIRECT
  evidence:
  - reference: PMID:29080202
    reference_title: "Chronic Recurrent Multifocal Osteomyelitis (CRMO): Presentation, Pathogenesis, and Treatment."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "it appears likely that variable defects in the TLR4/MAPK/inflammasome
      signaling cascade result in an imbalance between pro- and anti-inflammatory cytokine
      expressions in monocytes from CNO/CRMO patients"
    explanation: >-
      Names the implicated signalling cascade and its immediate consequence, with the
      authors' own hedging ("appears likely") preserved in the node description.

- name: Monocyte Pro-Inflammatory Cytokine Imbalance
  biological_scale: CELLULAR
  description: >-
    Monocytes from patients show an imbalance between pro- and anti-inflammatory cytokine
    expression, with increased inflammasome activation. The imbalance is bidirectional:
    it is not only that inflammatory cytokines rise, but that the regulatory arm is
    impaired - which is why the disease behaves as a failure of restraint rather than as
    a response to a stimulus.
  cell_types:
  - preferred_term: monocyte
    term:
      id: CL:0000576
      label: monocyte
  biological_processes:
  - preferred_term: interleukin-1 beta production
    term:
      id: GO:0032611
      label: interleukin-1 beta production
    modifier: INCREASED
  - preferred_term: interleukin-10 production
    term:
      id: GO:0032613
      label: interleukin-10 production
    modifier: DECREASED
  downstream:
  - target: Sterile Osteoinflammation
    causal_link_type: DIRECT
  evidence:
  - reference: PMID:33870159
    reference_title: "Chronic nonbacterial osteomyelitis (CNO) and chronic recurrent multifocal osteomyelitis (CRMO)."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "imbalanced cytokine expression and increased inflammasome activation in
      monocytes from CNO patients contribute to a pro-inflammatory phenotype that
      contributes to bone inflammation"
    explanation: >-
      Establishes the monocyte cytokine imbalance and inflammasome activation as the
      cellular mechanism linking innate signalling to bone inflammation.

- name: Sterile Osteoinflammation
  biological_scale: TISSUE
  description: >-
    Inflammatory bone lesions form without an infectious agent. This is the node that
    defines the disease and the one that cannot be demonstrated positively - it is
    established by failing to find an organism, and by lesions that neither respond to
    antibiotics nor behave like infection over time.
  downstream:
  - target: Multifocal Relapsing Bone Disease
    causal_link_type: DIRECT
  evidence:
  - reference: PMID:33870159
    reference_title: "Chronic nonbacterial osteomyelitis (CNO) and chronic recurrent multifocal osteomyelitis (CRMO)."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Chronic nonbacterial osteomyelitis (CNO) is an inflammatory bone disorder
      that most frequently affects children and adolescents."
    explanation: >-
      Establishes the disease as an inflammatory - explicitly nonbacterial - bone disorder
      of childhood.

- name: Multifocal Relapsing Bone Disease
  biological_scale: ORGANISM
  description: >-
    The clinical endpoint. Lesions are typically symmetrical and multifocal, and the
    course waxes and wanes rather than progressing steadily or resolving. Chronic pain can
    be severe enough to affect quality of life and psychosocial development, which in a
    paediatric population is a substantial part of the disease burden rather than a
    footnote to it.
  evidence:
  - reference: PMID:33870159
    reference_title: "Chronic nonbacterial osteomyelitis (CNO) and chronic recurrent multifocal osteomyelitis (CRMO)."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Chronic recurrent multifocal osteomyelitis (CRMO) is a severe form of CNO,
      usually characterized by symmetrical inflammatory bone lesions and its waxing and
      waning character."
    explanation: Establishes the multifocal, symmetrical and relapsing character that names the disease.
  - reference: PMID:33870159
    reference_title: "Chronic nonbacterial osteomyelitis (CNO) and chronic recurrent multifocal osteomyelitis (CRMO)."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Sometimes severe and chronic pain can significantly affect the quality of
      life and psychosocial development of individuals affected."
    explanation: Documents the burden of chronic pain in this paediatric population.

phenotypes:
- category: Musculoskeletal
  name: Bone Pain
  description: >-
    Painful swelling of affected bones, the cardinal symptom and an obligatory entry
    criterion in the 2025 EULAR/ACR classification criteria.
  phenotype_term:
    preferred_term: Bone pain
    term:
      id: HP:0002653
      label: Bone pain
    temporality: CHRONIC
  evidence:
  - reference: PMID:39966038
    reference_title: "EULAR/ACR classification criteria for paediatric chronic nonbacterial osteomyelitis (CNO)."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The new EULAR/ACR classification criteria for CNO require typical
      radiographic or magnetic resonance imaging findings and bone pain as an obligatory
      entry criterion"
    explanation: Establishes bone pain as obligatory for classification, i.e. present in essentially all cases.

- category: Musculoskeletal
  name: Osteomyelitis
  description: >-
    Inflammatory bone lesions with the imaging appearance of osteomyelitis but no
    causative organism. Curated with the HPO osteomyelitis term because that is what the
    lesion is, while the entry's descriptions and differentials carry the sterility that
    distinguishes it from infectious osteomyelitis.
  phenotype_term:
    preferred_term: Osteomyelitis
    term:
      id: HP:0002754
      label: Osteomyelitis
  evidence:
  - reference: PMID:33870159
    reference_title: "Chronic nonbacterial osteomyelitis (CNO) and chronic recurrent multifocal osteomyelitis (CRMO)."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Chronic recurrent multifocal osteomyelitis (CRMO) is a severe form of CNO,
      usually characterized by symmetrical inflammatory bone lesions and its waxing and
      waning character."
    explanation: Establishes the inflammatory bone lesions that constitute the phenotype.

- category: Musculoskeletal
  name: Vertebral Compression Fracture
  description: >-
    The sequela that makes vertebral involvement the highest-stakes distribution in this
    disease. Curated as a phenotype rather than only as a treatment indication because it
    is structural and irreversible, and it occurs despite the otherwise good prognosis
    under experienced care.
  phenotype_term:
    preferred_term: Vertebral compression fracture
    term:
      id: HP:0002953
      label: Vertebral compression fracture
  evidence:
  - reference: PMID:33870159
    reference_title: "Chronic nonbacterial osteomyelitis (CNO) and chronic recurrent multifocal osteomyelitis (CRMO)."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "some patients will develop sequalae which can include vertebral compression
      fractures"
    explanation: >-
      Names vertebral compression fracture as a documented sequela. No denominator is
      given, so no frequency band is asserted.

- category: Gastrointestinal
  name: Inflammatory Bowel Disease
  description: >-
    An extraosseous association rather than a complication of the bone lesions. It is one
    of the comorbid inflammatory conditions cited as evidence that sporadic CNO has a
    genetic predisposition, and it is the reason a CNO diagnosis is not purely a skeletal
    one.
  frequency: OCCASIONAL
  phenotype_term:
    preferred_term: Inflammatory bowel disease
    term:
      id: HP:0002037
      label: Inflammation of the large intestine
  evidence:
  - reference: PMID:29080202
    reference_title: "Chronic Recurrent Multifocal Osteomyelitis (CRMO): Presentation, Pathogenesis, and Treatment."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "severe acne (10%) and inflammatory bowel disease (10%)"
    explanation: >-
      Gives an explicit 10% figure, which maps to the OCCASIONAL band (5-29%).

- category: Dermatologic
  name: Severe Acne
  description: >-
    The second of the two cited extraosseous associations, and part of the same
    skin-and-gut inflammatory pattern that links CNO to microbiome-associated disease.
  frequency: OCCASIONAL
  phenotype_term:
    preferred_term: Severe acne
    term:
      id: HP:0001061
      label: Acne
    severity: SEVERE
  evidence:
  - reference: PMID:29080202
    reference_title: "Chronic Recurrent Multifocal Osteomyelitis (CRMO): Presentation, Pathogenesis, and Treatment."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "severe acne (10%) and inflammatory bowel disease (10%)"
    explanation: >-
      Gives an explicit 10% figure, which maps to the OCCASIONAL band (5-29%).

genetic:
- name: FBLIM1
  relationship_type: SUSCEPTIBILITY
  gene_term:
    preferred_term: FBLIM1
    term:
      id: hgnc:24686
      label: FBLIM1
  association: >-
    The only susceptibility gene reported for sporadic CNO, and the reason the entry does
    not claim the disease has no genetic component - it has no established Mendelian
    cause, which is a different statement. Two biallelic hits in the filamin-binding
    domain were found by exome sequencing in unrelated South Asian patients. FBLIM1 is
    proposed to act as an anti-inflammatory brake on bone remodelling through regulation
    of RANKL activation, which places it on the same osteoclast arm the bisphosphonates
    target.
  evidence:
  - reference: PMID:29080202
    reference_title: "Chronic Recurrent Multifocal Osteomyelitis (CRMO): Presentation, Pathogenesis, and Treatment."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "in the filamin-binding domain of the FBLIM1 gene were detected in unrelated
      CNO patients from South Asia"
    explanation: >-
      Establishes FBLIM1 as a reported CNO susceptibility gene. Curated as SUSCEPTIBILITY
      rather than causal: two unrelated probands is not an established Mendelian cause,
      and the review itself frames sporadic CNO as having no causal gene.

- name: IL10
  relationship_type: MODIFIER
  gene_term:
    preferred_term: IL10
    term:
      id: hgnc:5962
      label: IL10
  association: >-
    Promoter haplotypes at rs1800896/rs1800871/rs1800872 set IL-10 expression capacity,
    and IL-10 drives STAT3, which in turn regulates FBLIM1 - so the two genetic findings
    in this disease sit on one axis rather than two. The direction of the cohort finding
    is counterintuitive and is curated as reported: high-expression (GCC) haplotypes were
    the ones over-represented in CRMO, not low-expression ones.
  evidence:
  - reference: PMID:29080202
    reference_title: "Chronic Recurrent Multifocal Osteomyelitis (CRMO): Presentation, Pathogenesis, and Treatment."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "IL-10 expression is predetermined by genetic variants within the IL10
      proximal promoter region."
    explanation: Establishes the IL10 promoter haplotypes as determinants of IL-10 expression.
  notes: >-
    Two tensions are left visible rather than resolved. First, the entry curates IL-10
    production as DECREASED at the cellular level while this cohort reports
    over-representation of high-expression germline haplotypes; these are different
    measurements - monocyte output in disease versus promoter capacity - but the tension
    is real. Second, the review's own reconciliation, that low-expression carriers may
    present as other inflammatory conditions and so never be diagnosed as CNO, is
    explicitly flagged there as an unconfirmed hypothesis, so it is recorded as such and
    not curated as mechanism.

definitions:
- name: EULAR/ACR 2025 classification criteria for paediatric chronic nonbacterial osteomyelitis
  definition_type: PHENOTYPE_ALGORITHM
  derivation_basis: ESTABLISHED_CRITERIA
  description: >-
    Jointly developed EULAR/ACR classification criteria for paediatric CNO. The structure
    mirrors the disease's diagnostic problem: an obligatory entry requirement of typical
    radiographic or MRI findings plus bone pain, then explicit exclusion of malignancy,
    infection, vitamin C deficiency and hypophosphatasia, and only then additive weighted
    scoring across five clinical domains (site and pattern of bone lesions, age at onset,
    coexisting conditions, fever) and four pathology/laboratory domains (bone biopsy
    findings if performed, anaemia, C-reactive protein, erythrocyte sedimentation rate).
    A total score of at least 55 classifies as CNO.

    These are classification criteria, developed for research cohort definition, not
    diagnostic criteria for individual patient care - a distinction the entry preserves
    because they are not interchangeable.
  validation_status:
    status: VALIDATED_AGAINST_GOLD_STANDARD
    rationale: >-
      Weights and threshold were refined in a development cohort of 441 patients and
      tested in a separate validation cohort of 514 patients, achieving 82% sensitivity
      and 98% specificity. The high specificity relative to sensitivity is consistent with
      criteria built around exclusion.
    evidence:
    - reference: PMID:39966038
      reference_title: "EULAR/ACR classification criteria for paediatric chronic nonbacterial osteomyelitis (CNO)."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "The new criteria had a sensitivity of 82% and specificity of 98% in the
        validation cohort."
      explanation: Reports the validation performance in an independent cohort.
  evidence:
  - reference: PMID:39966038
    reference_title: "EULAR/ACR classification criteria for paediatric chronic nonbacterial osteomyelitis (CNO)."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The new EULAR/ACR classification criteria for CNO require typical
      radiographic or magnetic resonance imaging findings and bone pain as an obligatory
      entry criterion and exclusion criteria of malignancy, infection, vitamin C
      deficiency, and hypophosphatasia, followed by additive weighted criteria in 5
      clinical (site of bone lesions, pattern of bone lesions, age at onset, coexisting
      conditions, fever) and 4 pathology/laboratory domains (bone biopsy findings if done,
      anaemia, C-reactive protein level, and erythrocyte sedimentation rate)."
    explanation: The full structure of the criteria, including entry, exclusion and scoring domains.
  - reference: PMID:39966038
    reference_title: "EULAR/ACR classification criteria for paediatric chronic nonbacterial osteomyelitis (CNO)."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "refinement of weights and threshold score in a development cohort of 441
      patients and validation in another cohort of 514 patients"
    explanation: Documents the development and validation cohorts underlying the criteria.

diagnosis:
- name: Diagnosis of Exclusion
  description: >-
    Until the 2025 classification criteria there were no prospectively tested, widely
    accepted diagnostic criteria and no disease biomarker, so CNO was and largely remains
    a diagnosis of exclusion. Infection and malignancy are the two exclusions that matter
    most - a solitary lesion in particular can be indistinguishable from bacterial
    osteomyelitis or from a bone tumour without biopsy. The absence of a biomarker is not
    a gap in this entry's sourcing; it is a stated property of the disease.
  diagnosis_term:
    preferred_term: diagnosis of exclusion
  evidence:
  - reference: PMID:33870159
    reference_title: "Chronic nonbacterial osteomyelitis (CNO) and chronic recurrent multifocal osteomyelitis (CRMO)."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "In the absence of prospectively tested and widely accepted diagnostic
      criteria or disease biomarkers, CNO remains a diagnosis of exclusion, and infections,
      malignancy and other differentials require consideration"
    explanation: >-
      States directly that CNO is a diagnosis of exclusion and names the differentials that
      must be considered.

- name: Radiographic and MRI Assessment
  description: >-
    Typical radiographic or MRI findings are an obligatory entry criterion for
    classification. Whole-body MRI additionally reveals clinically silent lesions, which
    is what establishes multifocality in a patient with one painful site.
  diagnosis_term:
    preferred_term: Magnetic Resonance Imaging
    term:
      id: NCIT:C16809
      label: Magnetic Resonance Imaging
  evidence:
  - reference: PMID:39966038
    reference_title: "EULAR/ACR classification criteria for paediatric chronic nonbacterial osteomyelitis (CNO)."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The new EULAR/ACR classification criteria for CNO require typical
      radiographic or magnetic resonance imaging findings and bone pain as an obligatory
      entry criterion"
    explanation: Establishes imaging as obligatory for classification.

treatments:
- name: Nonsteroidal Anti-Inflammatory Therapy
  description: >-
    First-line, and effective at least partially in most patients. Note the framing in the
    cited review: no medication is licensed for CNO at all, so every treatment listed here
    is used off-label. That is a property of the disease's evidence base rather than a
    gap in this entry.
  therapeutic_modality: SMALL_MOLECULE
  treatment_term:
    preferred_term: Pharmacotherapy
    term:
      id: NCIT:C15986
      label: Pharmacotherapy
  target_mechanisms:
  - target: Sterile Osteoinflammation
    treatment_effect: INHIBITS
    description: >-
      Anti-inflammatory therapy suppresses the sterile bone inflammation without
      addressing the upstream innate signalling defect.
    evidence:
    - reference: PMID:33870159
      reference_title: "Chronic nonbacterial osteomyelitis (CNO) and chronic recurrent multifocal osteomyelitis (CRMO)."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "Most patients show at least some response to nonsteroidal anti-inflammatory
        drugs"
      explanation: Documents the first-line response that grounds this treatment link.
  evidence:
  - reference: PMID:33870159
    reference_title: "Chronic nonbacterial osteomyelitis (CNO) and chronic recurrent multifocal osteomyelitis (CRMO)."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Currently, no medications are licensed for the use in CNO."
    explanation: >-
      Establishes that all pharmacotherapy in this disease is off-label, which is the
      context for every treatment curated here.

- name: Escalation Therapy
  description: >-
    For patients who do not respond adequately to NSAIDs: corticosteroids,
    disease-modifying anti-rheumatic drugs and cytokine-blocking agents. Corticosteroids
    control inflammatory activity quickly but rarely induce long-term remission, so they
    function as a bridge rather than as maintenance. Anti-TNF agents are reported to be
    highly effective in a significant percentage of patients, though every report is
    retrospective and small, which is why the evidence base - not the drug class - is the
    limiting factor here.
  therapeutic_modality: SMALL_MOLECULE
  treatment_term:
    preferred_term: Pharmacotherapy
    term:
      id: NCIT:C15986
      label: Pharmacotherapy
  evidence:
  - reference: PMID:33870159
    reference_title: "Chronic nonbacterial osteomyelitis (CNO) and chronic recurrent multifocal osteomyelitis (CRMO)."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "others require more aggressive treatment that can include corticosteroids,
      cytokine-blocking agents and/or bisphosphonates"
    explanation: Documents the escalation options used when NSAIDs are insufficient.
  - reference: PMID:29080202
    reference_title: "Chronic Recurrent Multifocal Osteomyelitis (CRMO): Presentation, Pathogenesis, and Treatment."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "In NSAID-refractory cases, treatment can be escalated with corticosteroids."
    explanation: >-
      Establishes corticosteroids as the first escalation step after NSAID failure.
  - reference: PMID:29080202
    reference_title: "Chronic Recurrent Multifocal Osteomyelitis (CRMO): Presentation, Pathogenesis, and Treatment."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "treatment with pamidronate or anti-TNF agents has been reported to be highly
      effective in a significant percentage of CNO/CRMO patients, inducing long-lasting
      remission in a large subset of patients"
    explanation: >-
      Documents reported anti-TNF efficacy in sporadic CNO/CRMO. Recorded explicitly
      because the same review reports the opposite for the monogenic mimic Majeed
      syndrome, and the two must not be conflated.

- name: Bisphosphonate Therapy
  description: >-
    Curated as its own node rather than folded into generic escalation because its target
    differs from the rest of the escalation ladder: bisphosphonates act on the
    osteoclast-mediated bone resorption arm rather than on cytokine signalling. It is
    prioritised where vertebral bodies are involved, which is also where the disease does
    its irreversible structural damage. As with every treatment here the evidence is
    retrospective and small - the cited review states plainly that large prospective
    trials to determine the best medication are lacking.
  therapeutic_modality: SMALL_MOLECULE
  treatment_term:
    preferred_term: Pharmacotherapy
    term:
      id: NCIT:C15986
      label: Pharmacotherapy
    therapeutic_agent:
    - preferred_term: pamidronate
      term:
        id: CHEBI:7903
        label: pamidronate
  target_mechanisms:
  - target: Sterile Osteoinflammation
    treatment_effect: INHIBITS
    description: >-
      Bisphosphonates suppress the osteoclastic bone resorption driven by the
      pro-inflammatory cytokine milieu, without correcting the upstream innate signalling
      defect.
    evidence:
    - reference: PMID:29080202
      reference_title: "Chronic Recurrent Multifocal Osteomyelitis (CRMO): Presentation, Pathogenesis, and Treatment."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "treatment with pamidronate or anti-TNF agents has been reported to be highly
        effective in a significant percentage of CNO/CRMO patients, inducing long-lasting
        remission in a large subset of patients"
      explanation: Grounds the therapeutic link between bisphosphonate exposure and remission.
  evidence:
  - reference: PMID:29080202
    reference_title: "Chronic Recurrent Multifocal Osteomyelitis (CRMO): Presentation, Pathogenesis, and Treatment."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "In patients with vertebral body involvement and structural damage,
      aggressive treatment should be discussed initially, e.g., with bisphosphonates"
    explanation: >-
      Establishes vertebral involvement as the setting in which bisphosphonates are
      prioritised over stepwise escalation.

differential_diagnoses:
- name: Bacterial osteomyelitis
  description: >-
    The differential the disease is named against. Imaging and histology can look
    identical, and it is an explicit exclusion criterion in the 2025 classification
    criteria. Distinguishing them is what the entire diagnostic workup exists to do, and
    getting it wrong in either direction is harmful - antibiotics for CNO, or
    immunosuppression for an untreated infection.
  distinguishing_features:
  - No causative organism is isolated in CNO
  - CNO lesions do not respond to antibiotics
  - CNO is typically multifocal and symmetrical with a waxing and waning course
  evidence:
  - reference: PMID:39966038
    reference_title: "EULAR/ACR classification criteria for paediatric chronic nonbacterial osteomyelitis (CNO)."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "exclusion criteria of malignancy, infection, vitamin C deficiency, and
      hypophosphatasia"
    explanation: Establishes infection as a formal exclusion criterion for classification.

- name: Malignancy involving bone
  description: >-
    Leukaemia and primary bone tumours can present with bone pain and lytic lesions in the
    same age group, and are the exclusion with the highest cost of error. Also a formal
    exclusion criterion.
  distinguishing_features:
  - Formal exclusion criterion in the EULAR/ACR classification criteria
  - Biopsy is included as a scoring domain when performed, largely to address this differential
  evidence:
  - reference: PMID:33870159
    reference_title: "Chronic nonbacterial osteomyelitis (CNO) and chronic recurrent multifocal osteomyelitis (CRMO)."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "CNO remains a diagnosis of exclusion, and infections, malignancy and other
      differentials require consideration"
    explanation: Names malignancy among the differentials requiring exclusion.

- name: Majeed syndrome
  description: >-
    A monogenic autoinflammatory syndrome (LPIN2) whose features include CRMO alongside
    congenital dyserythropoietic anaemia and, in some patients, a neutrophilic dermatosis.
    It is the reason a child with CRMO plus unexplained anaemia should be considered for
    genetic evaluation rather than treated as sporadic disease.
  distinguishing_features:
  - Majeed syndrome pairs CRMO with congenital dyserythropoietic anaemia from early infancy
  - Monogenic and inherited, versus sporadic CNO with no established causal gene
  - Onset is characteristically early with a lifelong course
  - >-
    Treatment response separates the two: Majeed is an IL-1-beta-mediated disease that
    responds to IL-1 blockade while TNF-alpha blockers have almost no effect, whereas in
    sporadic CNO/CRMO anti-TNF agents are reported to be highly effective in a significant
    percentage of patients
  evidence:
  - reference: PMID:29080202
    reference_title: "Chronic Recurrent Multifocal Osteomyelitis (CRMO): Presentation, Pathogenesis, and Treatment."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "TNFα blockers have almost no effect"
    explanation: >-
      Curated here, on the Majeed differential, because that is the disease this sentence
      is about - it sits inside the review's monogenic-syndromes section, several pages
      above the "Sporadic" CNO/CRMO section, which reports the opposite. The
      TNF-responsiveness asymmetry is a discriminating feature between the two, not a
      property of sporadic disease.
  - reference: PMID:20301735
    reference_title: "Majeed Syndrome – RETIRED CHAPTER, FOR HISTORICAL REFERENCE ONLY."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Majeed syndrome is characterized by: Chronic recurrent multifocal
      osteomyelitis (CRMO) that is of early onset with a lifelong course; and Congenital
      dyserythropoietic anemia (CDA)"
    explanation: >-
      Establishes the CRMO-plus-anaemia combination that distinguishes Majeed syndrome.
      Note this GeneReviews chapter is formally retired - see notes - so it is cited only
      for this stable definitional statement.

discussions:
- discussion_id: gap_crmo_molecular_pathophysiology_unresolved
  kind: KNOWLEDGE_GAP
  attaches_to:
  - pathophysiology#Innate Immune Signalling Dysregulation
  prompt: >-
    What is the molecular defect in sporadic CNO/CRMO, and why does no single
    cytokine-blocking strategy define the disease the way IL-1 blockade defines its
    monogenic mimics?
  rationale: >-
    Both cited reviews state that the pathophysiology is not understood - one calls it
    "somewhat elusive", the other "not fully understood" - and neither identifies a causal
    gene for sporadic disease. What is established is downstream and cellular: monocytes
    show inflammasome activation and a pro- versus anti-inflammatory cytokine imbalance.
    The contrast with the monogenic mimics is the informative part. Majeed syndrome has a
    single defined cytokine dependency - it responds to IL-1-beta blockade and not to TNF
    blockade - and DIRA resolves on recombinant IL-1 receptor antagonist. Sporadic CNO has
    no such signature: NSAIDs, corticosteroids, anti-TNF agents and bisphosphonates are all
    reported effective in some patients and insufficient in others, and every report is
    retrospective with small numbers. That heterogeneity of response is what a mechanistic
    account has to explain, and it is equally consistent with one poorly characterised
    pathway and with sporadic CNO being several diseases under one name.
  proposed_experiments:
  - experiment_id: exp_cno_monocyte_pathway_stratification
    name: Stratify CNO monocyte signalling defects against treatment response
    description: >-
      Profile TLR4/MAPK/inflammasome signalling and cytokine output in monocytes from a
      prospectively followed CNO cohort, and score each patient's profile against their
      documented response to NSAIDs, corticosteroids, TNF blockade and bisphosphonates. The
      discriminating result is whether response tracks a signalling subgroup: if it does,
      sporadic CNO is more than one disease and should eventually be curated as such, and
      the retrospective series reporting partial efficacy for every agent are averaging
      over that structure. A prospective design is the point - the existing evidence base
      is explicitly retrospective and small.

notes: >-
  Dual identity. MONDO:0009813 carries xref HP:0002754, so this concept has both a
  disease and a phenotype identifier - the pattern the knowledge base handles as a
  "disease-like phenotype". It is curated here as a Disease, and HP:0002754 is used
  separately as the phenotype term for the bone lesions themselves. Both usages are
  deliberate rather than a conflation.

  No GeneReviews baseline applies. A GeneReviews search for chronic recurrent multifocal
  osteomyelitis returns exactly one chapter, PMID:20301735, which was fetched and read: it
  is "Majeed Syndrome" - a different, monogenic disease in which CRMO is one feature - and
  it is additionally marked "RETIRED CHAPTER, FOR HISTORICAL REFERENCE ONLY". It is
  therefore not used as a phenotype baseline for sporadic CNO/CRMO. It is cited once, in
  the Majeed differential, for a definitional statement about that syndrome, with its
  retired status recorded there and here.

  Frequency bands are assigned only where the source gives a number. Inflammatory bowel
  disease and severe acne are each cited at 10%, which maps to OCCASIONAL. Bone pain
  carries no band because it is obligatory for classification rather than reported as a
  proportion; osteomyelitis and vertebral compression fracture carry none because the
  reviews describe them qualitatively.

  Category is Complex rather than Mendelian: no causal gene is established for sporadic
  CNO/CRMO, and the monogenic diseases in which CRMO appears - Majeed syndrome, IL1RN
  deficiency - are curated as differentials rather than as subtypes of this entry.

references:
- reference: PMID:29080202
  title: "Chronic Recurrent Multifocal Osteomyelitis (CRMO): Presentation, Pathogenesis,
    and Treatment."
- reference: PMID:33870159
  title: Chronic nonbacterial osteomyelitis (CNO) and chronic recurrent multifocal
    osteomyelitis (CRMO).
- reference: PMID:39966038
  title: EULAR/ACR classification criteria for paediatric chronic nonbacterial
    osteomyelitis (CNO).

📚

References & Deep Research

References

3

PMID:29080202

Chronic Recurrent Multifocal Osteomyelitis (CRMO): Presentation, Pathogenesis, and Treatment.

No top-level findings curated for this source.

PMID:33870159

Chronic nonbacterial osteomyelitis (CNO) and chronic recurrent multifocal osteomyelitis (CRMO).

No top-level findings curated for this source.

PMID:39966038

EULAR/ACR classification criteria for paediatric chronic nonbacterial osteomyelitis (CNO).

No top-level findings curated for this source.

Deep Research

1

Claude Code ▸

Chronic Recurrent Multifocal Osteomyelitis (CRMO) / Chronic Nonbacterial Osteomyelitis (CNO): Comprehensive Research Report

claude-haiku-4-5-20251001, claude-sonnet-5 41 citations 2026-08-21T16:29:53.960451

Chronic Recurrent Multifocal Osteomyelitis (CRMO) / Chronic Nonbacterial Osteomyelitis (CNO): Comprehensive Research Report

1. Disease Information

Overview. Chronic recurrent multifocal osteomyelitis (CRMO) — increasingly referred to in the broader literature as chronic nonbacterial osteomyelitis (CNO), of which CRMO represents the recurrent/multifocal end of the spectrum — is a rare, non-infectious autoinflammatory bone disease that causes sterile inflammatory bone lesions, predominantly in children and adolescents. It mimics infectious (bacterial) osteomyelitis clinically and radiographically but yields negative cultures and does not respond to antibiotics. CRMO is classified as an autoinflammatory disorder of the innate immune system rather than an autoimmune disease, since it lacks high-titer autoantibodies or clear autoreactive lymphocyte involvement (PMC10483435).

Key identifiers: - ICD-10-CM: M86.3 (Chronic multifocal osteomyelitis), M86.30 (AAPC) - Orphanet: ORPHA:324964 (also historically ORPHA:169147 for the broader syndrome group) - Disease Ontology: DOID:0060645 - MONDO: MONDO:0958177 (non-syndromic CRMO); syndromic monogenic forms carry separate MONDO/OMIM entries - OMIM (monogenic subtypes): - CRMO2 (with periostitis and pustulosis) — OMIM #612852 - CRMO3 — OMIM #259680 (autosomal dominant form) - Majeed syndrome (CRMO + congenital dyserythropoietic anemia) — OMIM #609628, caused by LPIN2 - DIRA (deficiency of IL-1 receptor antagonist), a related monogenic autoinflammatory bone disease — caused by IL1RN

Synonyms: Chronic nonbacterial osteomyelitis (CNO), chronic nonbacterial osteitis, nonbacterial osteitis (NBO), sterile multifocal osteomyelitis; the adult-spectrum manifestation overlaps with SAPHO syndrome (Synovitis, Acne, Pustulosis, Hyperostosis, Osteitis), which is now widely regarded as part of the same disease continuum in adults (Oxford Rheumatology Advances in Practice).

Evidence base: Because CRMO is rare, most disease-level knowledge derives from aggregated resources — multicenter retrospective cohorts, international registries (e.g., Eurofever), and national prospective surveillance studies (e.g., the UK/Republic of Ireland British Paediatric Surveillance Unit [BPSU] study) — rather than single-patient EHR mining, supplemented by murine genetic models.

2. Etiology

Disease Causal Factors

CRMO is understood as resulting from dysregulated innate immune/monocyte signaling causing an imbalance between pro- and anti-inflammatory cytokines, culminating in NLRP3 inflammasome hyperactivation, IL-1β-driven inflammation, and pathological osteoclastogenesis. Three converging mechanisms have been proposed: (1) imbalanced cytokine expression, (2) increased inflammasome activation, and (3) enhanced osteoclast differentiation (PMC10483435; PMC5705736).

Genetic Risk Factors

Monogenic syndromic forms (Mendelian, high-penetrance): - Majeed syndrome — autosomal recessive, caused by loss-of-function mutations in LPIN2 (encoding lipin-2, a phosphatidic acid phosphatase); presents with CRMO, congenital dyserythropoietic anemia, and often neutrophilic dermatosis. Only ~24 individuals from 10 families with genetically confirmed disease reported to date (PMC8252456; OMIM #609628). Novel LPIN2 mutations have been shown to link bone inflammation to inflammatory M2 macrophages and accelerated osteoclastogenesis (PMID:33314777). - DIRA (Deficiency of IL-1 Receptor Antagonist) — autosomal recessive, caused by loss-of-function mutations in IL1RN, producing unopposed IL-1 signaling; presents with neonatal-onset pustulosis, marked inflammatory markers, sterile multifocal osteitis, and periostitis. - PAPA syndrome — caused by mutations in PSTPIP1, driving pyrin inflammasome activation and elevated IL-1β; shares mechanistic overlap with CRMO. - CRMO3 (OMIM #259680) — autosomal dominant form, early childhood-onset bone pain/arthritis from sterile osteomyelitis. - CRMO2 (OMIM #612852) — CRMO with periostitis and pustulosis.

Non-syndromic (sporadic) CRMO susceptibility genes: - FBLIM1 (filamin-binding LIM protein 1) — recessive coding and regulatory mutations identified via whole-exome sequencing in a consanguineous family (South Asian ancestry); Fblim1 is the most differentially expressed gene (>20-fold downregulated) in bone marrow macrophages of the murine cmo (chronic multifocal osteomyelitis) model, implicating impaired RANKL regulation and osteoclast differentiation (PMID:28301468; PLOS ONE). - P2RX7 — a 2023–2024 targeted/exome sequencing study (n=190 CNO patients vs. 1,873 controls) found rare, presumably damaging P2RX7 variants in 5.8% of CNO patients vs. 1.9% of controls, and rare lower-impact variants in 32.4% vs. 4.4% of controls — the gene was "more than 10-fold more variable" among patients. P2X7 is an ATP-gated cation channel that mediates potassium efflux, a potent NLRP3 inflammasome trigger; CNO-associated variants alter inflammasome assembly and reduce pyroptosis, potentially prolonging monocyte survival and cytokine output. Patients with damaging P2RX7 variants had more GI symptoms and lymphadenopathy but less spinal/joint/skin (psoriasis) involvement (ScienceDirect 2024; Liverpool repository). - FGR (Src-family kinase) — expressed in mast cells/neutrophils, contributes to aseptic bone inflammation independently of NLRP3 (PMID:31138708). - IL-10 promoter haplotypes (GCC/ACC/ATA) modulate transcriptional output; most sporadic CRMO patients show enrichment for the lower-expressing haplotype pattern relative to controls, consistent with the impaired-IL-10 mechanistic model. - HLA-B27 positivity has been noted as a susceptibility marker in some cohorts, particularly overlapping spondyloarthropathy phenotypes.

Environmental/demographic risk factors: Female sex predominance in classic pediatric CRMO (though a "severe" phenotype subgroup skews male — see Phenotypes below); White/Caucasian ancestry is over-represented in published cohorts, though this likely reflects ascertainment bias rather than true differential susceptibility; family history of autoinflammatory/autoimmune disease (psoriasis, IBD, spondyloarthritis) increases risk.

Protective Factors

No well-established genetic or environmental protective factors are documented in the literature; this remains an evidence gap.

Gene–Environment Interactions

Not well characterized for CRMO specifically; the leading hypothesis is that in genetically susceptible individuals (e.g., carrying P2RX7 or IL-10 promoter risk variants), an as-yet-unidentified triggering stimulus (possibly microbial/microbiome-related, per murine data) precipitates monocyte/macrophage TLR4-MAPK signaling defects and downstream inflammasome activation.

3. Phenotypes

Core symptoms and signs

Bone pain — insidious onset, localized, often worse at night, with a waxing-and-waning course (suggest HP:0002653 Bone pain / HP:0006414)
Local swelling, tenderness, warmth over affected bone (suggest HP:0025378 or general inflammatory swelling terms)
Arthritis/synovitis adjacent to lesions
Bone overgrowth/hyperostosis, particularly clavicular and mandibular (suggest HP:0004422 Bone hyperostosis)
Vertebral compression fractures, kyphosis, leg-length discrepancy in spinal disease
Generally afebrile or low-grade fever, distinguishing from infectious osteomyelitis in most cases

Laboratory abnormalities

Normal-to-mildly-elevated ESR/CRP (a substantial subset show no systemic inflammatory marker elevation)
Anemia of chronic disease (common)
Elevated serum cytokines: IL-6, TNF-α (see Mechanism section for full cytokine panel)

Phenotype characteristics

Age of onset: Peak incidence 7–12 years, though reported across all pediatric age groups and increasingly recognized in adults (where it overlaps with SAPHO syndrome). Median age of symptom onset in one UK cohort was 12 years.
Severity/progression: Variable — a relapsing-remitting or progressive course is now recognized as more typical than the historically assumed "self-limited" course. Diagnosis delay averages ~15 months (range 0–92 months) due to low disease awareness (PMID:27576444).
Distinct phenotype clusters (2022 medRxiv study, "Two phenotypes of CRMO"):
Severe phenotype: predominantly male, multifocal, rare clavicular involvement, prominent inflammatory syndrome
Mild phenotype: predominantly female, unifocal, common clavicular involvement, minimal systemic inflammation, rare extraosseous lesions
Intermediate phenotype: predominantly female, multifocal, common inflammatory syndrome, some with family history and extraosseous disease

Skeletal distribution

Most commonly affected sites: metaphyses/epiphyses of long bones (femur, tibia, humerus), pelvis, clavicle, vertebrae, mandible
The clavicle and mandible are distinctively involved in CRMO but rare in bacterial osteomyelitis — involvement at these sites should raise diagnostic suspicion for CRMO specifically (Radsource; PathologyOutlines).
Periosteal reaction reported in ~33% of symptomatic sites; radiographic pattern: 50% lytic, 53% sclerotic lesions in one cohort of 36 patients (Bristol study, PMID:27576444).

Associated (extra-osseous) manifestations / comorbidities

Inflammatory bowel disease (Crohn disease, ulcerative colitis): ~10% of patients
Psoriasis / palmoplantar pustulosis: ~8–21% (estimates vary by cohort)
Severe acne: ~10%
Ankylosing spondylitis / spondyloarthropathy features: up to ~25% in some series
Pulmonary involvement: 3–8% (German cohort)

Quality of life impact

Chronic pain and activity restriction (especially with spinal involvement, given vertebral fracture risk) substantially affect quality of life; amplified musculoskeletal pain syndromes can persist even when active inflammation resolves, per longitudinal UK cohort follow-up data.

Suggested HPO terms: HP:0002653 (Bone pain), HP:0100774 (Osteomyelitis), HP:0004422 (Hyperostosis), HP:0002758 (Osteoarthritis-adjacent joint involvement), HP:0001369 (Arthritis), HP:0100255 (Long bone bowing — for structural sequelae), HP:0002650 (Scoliosis/kyphosis — vertebral involvement), HP:0004616 (Vertebral wedging/compression), HP:0100785 (Recurrent fractures).

4. Genetic/Molecular Information

Causal genes (monogenic forms)

Gene	HGNC	Disorder	Inheritance	Mechanism
LPIN2	phosphatidic acid phosphatase	Majeed syndrome	AR	Loss of lipin-2 function → dysregulated lipid metabolism, macrophage polarization toward inflammatory M2 phenotype, accelerated osteoclastogenesis
IL1RN	IL-1 receptor antagonist	DIRA	AR	Loss of IL-1Ra function → unopposed IL-1α/β signaling
PSTPIP1	proline-serine-threonine phosphatase-interacting protein 1	PAPA syndrome	AD	Pyrin inflammasome dysregulation, elevated IL-1β
FBLIM1	filamin-binding LIM protein 1	Non-syndromic CRMO (rare)	AR (reported family)	Impaired RANKL regulation, altered osteoclast differentiation

Variant classification and population data

Most reported pathogenic variants in LPIN2 and IL1RN are classified pathogenic/likely pathogenic per ACMG/AMP criteria in ClinVar given clear loss-of-function consequences and segregation in consanguineous pedigrees. P2RX7 CNO-associated variants are predominantly rare, missense, presumed-damaging variants rather than clear loss-of-function alleles; population frequency data (gnomAD-derived control cohorts) show these variants at low but non-zero frequency (~1.9–4.4% carrying rare variants in healthy controls vs. 5.8–32.4% in CNO cohorts), consistent with a susceptibility/risk-modifier rather than fully penetrant Mendelian model.

Functional consequences

LPIN2 loss-of-function → gain of inflammatory signaling (M2 macrophage skewing, enhanced osteoclastogenesis)
IL1RN loss-of-function → gain of IL-1 pathway activity (classic loss-of-inhibitor mechanism)
P2RX7 risk variants → gain-of-function-like effect on inflammasome assembly coupled with reduced pyroptotic cell death, prolonging pro-inflammatory monocyte/macrophage activity
FBLIM1 loss-of-function → dysregulated RANKL-driven osteoclast differentiation

Modifier genes

IL-10 promoter haplotype (GCC/ACC/ATA) modulates baseline anti-inflammatory tone and may modify severity/expressivity in sporadic disease.

Epigenetic information

CRMO monocytes show reduced DNA methylation at loci controlling NLRP3 inflammasome components (NLRP3, ASC/PYCARD) and IL-1β, and reduced histone H3 serine-10 phosphorylation at the IL-10 promoter — both consistent with an epigenetically reinforced pro-inflammatory monocyte state (PMC10483435).

Chromosomal abnormalities

No recurrent aneuploidies, translocations, or copy-number syndromes are established causes of CRMO; it is not currently modeled as a chromosomal disorder.

Suggested gene/ontology annotations: HGNC gene symbols LPIN2, IL1RN, PSTPIP1, FBLIM1, P2RX7, FGR (all lowercase hgnc: CURIEs per local convention); GO:0061702 (canonical inflammasome complex), GO:0043123 (positive regulation of NF-kB signaling), GO:0045453 (bone resorption), GO:0030316 (osteoclast differentiation).

5. Environmental Information

Environmental factors: No specific toxin, chemical, or occupational exposure has been robustly linked to CRMO onset; the disease is pediatric-onset in the great majority of cases.

Lifestyle factors: Not established as causal; physical activity/trauma to affected long bones can precipitate symptomatic flares once disease is established (mechanical loading is a recognized aggravator, not an initiator).

Infectious agents: CRMO is explicitly non-infectious and cultures are sterile by definition. However, murine model data (Pstpip2-deficient mice) and some human hypotheses implicate the microbiome as a possible disease-modifying or triggering factor via innate immune signaling (TLR activation), rather than a specific causal pathogen — this remains an active but unresolved research area ("New discoveries in CRMO: IL-1β, the neutrophil, and the microbiome implicated in disease pathogenesis in Pstpip2-deficient mice," Seminars in Immunopathology).

6. Mechanism / Pathophysiology

Causal chain overview

Trigger (unknown in sporadic disease; monogenic loss-of-function in syndromic forms) →
Defective TLR4/MAPK/ERK1-2 signaling in monocytes → failure to phosphorylate ERK1/2 → failure to activate Sp-1 transcription factor →
Impaired expression of anti-inflammatory cytokines IL-10 and IL-19 (chromatin/epigenetically mediated) →
Disinhibited NLRP3 inflammasome assembly (elevated NLRP3, ASC, caspase-1) →
Increased IL-1β and IL-18 secretion →
RANKL upregulation on osteoblasts/bone marrow macrophages, OPG downregulation →
Enhanced osteoclast differentiation and activation →
Bone resorption, sterile inflammatory bone lesions, and secondary reparative hyperostosis/sclerosis

Molecular pathways

TLR4 → MAPK/ERK → Sp-1 → IL-10/IL-19 transcription (defective in CRMO monocytes; PMID:22940633)
NF-κB and MAPK signaling downstream of IL-1β, TNF-α, IL-6, IL-8, IL-18 — all converge on osteoclastogenic and pro-inflammatory transcriptional programs (KEGG hsa04621 NOD-like receptor signaling; KEGG hsa04064 NF-kB signaling)
JAK-STAT3 signaling downstream of IL-6, driving Th17 differentiation and IL-17 production
RANK/RANKL/OPG axis — central convergence point for bone resorption

Cytokine profile (imbalance model)

Elevated (pro-inflammatory)	Reduced (anti-inflammatory)
IL-1β, TNF-α, IL-6, IL-8, IL-18, IL-17, IL-23, IL-20	IL-10, IL-19, IL-9

Elevated serum IL-6 (>17 pg/mL) and eotaxin (>110 pg/mL) have been proposed as a minimal diagnostic biomarker panel with 93% sensitivity and 97% specificity in preliminary ROC analyses, alongside candidate markers S100A8, collagen Iα, RANTES, and soluble IL-2 receptor (JBMR 2024 review); these require validation in larger independent cohorts.

Cellular processes

Monocyte/macrophage dysfunction is the central cellular lesion — attenuated TLR4/MAPK signaling, chromatin-level failure of IL-10/IL-19 induction
Osteoclast hyperactivation — RANKL-driven differentiation from monocyte/macrophage precursors
Mast cell involvement — a preclinical study ("Mast Cells Enhance Sterile Inflammation in Chronic Nonbacterial Osteomyelitis," PMC6737947) demonstrated mast cells amplify sterile bone inflammation in the murine model
Th17 cell differentiation — chronic pro-inflammatory cytokine release (IL-6, IL-23) may drive IL-17-expressing effector T cells, further amplifying osteoclastogenesis (adaptive immune contribution to a primarily innate disease)
Reduced pyroptosis in P2RX7 variant carriers — paradoxically prolongs pro-inflammatory monocyte survival rather than clearing them via cell death

Tissue damage mechanisms

Cycles of osteolysis (active inflammatory phase) followed by reparative sclerosis/hyperostosis (chronic phase) — this destruction-repair cycle underlies the mixed lytic/sclerotic radiographic appearance and progressive bone overgrowth seen at sites like the clavicle and mandible.

Immune system involvement

Predominantly innate immune dysregulation (monocyte/macrophage/neutrophil axis, inflammasome), with secondary adaptive immune (Th17) amplification; not classically autoimmune (no dominant autoantibody or autoreactive T-cell clone identified).

Molecular profiling data

Genomics: Whole-exome sequencing has identified FBLIM1 (familial) and P2RX7 (cohort-level risk variant) associations
Transcriptomics: Single-cell RNA-sequencing in the Pstpip2⁻/⁻ mouse model identified co-expression of the lncRNA Morrbid with Pstpip2 in mature myeloid cells (neutrophils, eosinophils, classical monocytes); Morrbid knockout significantly inhibited CRMO initiation and progression in this model by reducing inflammatory myeloid cell lifespan and cytokine output (2025, Disease Models & Mechanisms, PMID:40503910) — a promising mechanistic and therapeutic lead published very recently.
Proteomics/serum biomarker panels: As above (IL-6, eotaxin, S100A8, RANTES).

Advanced technologies

Single-cell RNA-seq has been applied in the murine Pstpip2-deficient model (Morrbid study) to define myeloid-lineage-specific contributions; equivalent human single-cell/spatial transcriptomic data in bone biopsy tissue remains an area for future research — not yet reported at scale for CRMO.

Suggested GO terms: GO:0032640 (TNF production), GO:0032611 (IL-1β production), GO:0032693 (negative regulation of IL-10 production — CRMO shows failure of this), GO:0045453 (bone resorption), GO:0030316 (osteoclast differentiation), GO:0002534 (cytokine production involved in inflammatory response), GO:0061702 (inflammasome complex). Suggested CL terms: CL:0000576 (monocyte), CL:0000235 (macrophage), CL:0000775 (neutrophil), CL:0000097 (mast cell), CL:0000092 (osteoclast), CL:0001051 (CD4-positive, IL-17-secreting Th17 cell).

7. Anatomical Structures Affected

Organ level

Primary: Skeletal system — long bone metaphyses (femur, tibia, humerus), clavicle, mandible, pelvis, vertebrae
Secondary: Joints (adjacent arthritis/synovitis), skin (psoriasis, palmoplantar pustulosis, acne), gastrointestinal tract (IBD), lungs (rare, 3-8%)
Body systems involved: Musculoskeletal (primary), integumentary, gastrointestinal, and — in SAPHO overlap — the axial skeleton/sacroiliac joints

Tissue and cell level

Bone tissue: metaphyseal/epiphyseal trabecular and cortical bone; periosteum (periosteal reaction/new bone formation)
Cell populations: monocytes, macrophages, osteoclasts, neutrophils, mast cells, and — in chronic lesions — lymphocytes, plasma cells, histiocytes

Subcellular level

Implicated: inflammasome assembly at the cytoplasmic level (NLRP3/ASC/caspase-1 complex — GO Cellular Component: inflammasome complex), plasma membrane P2X7 channel activity.

Localization

UBERON terms (suggested): UBERON:0002481 (clavicle), UBERON:0002397 (mandible), UBERON:0001474 (bone element), UBERON:0002228 (vertebral column), UBERON:0004538 (long bone metaphysis is not a distinct UBERON term but metaphysis-adjacent structures apply)
Lateralization: Typically asymmetric/multifocal, non-contiguous lesions distributed across multiple bones simultaneously — a hallmark distinguishing feature from unifocal bacterial osteomyelitis.

8. Temporal Development

Onset

Typical age: Peak 7–12 years; can present at any pediatric age and, less commonly, in adults (where overlap with SAPHO is emphasized)
Pattern: Insidious, gradually worsening bone pain/swelling rather than acute high-fever presentation typical of bacterial osteomyelitis

Progression

Disease course pattern: Historically viewed as self-limited, but increasingly recognized as chronic with a relapsing-remitting OR progressive course — a key revision in current understanding (JBMR 2024 review; UK longitudinal cohort data)
Stages: acute/active inflammatory (lytic) phase → reparative/chronic (sclerotic, hyperostotic) phase, often cycling at the same or different sites over time
Duration: Chronic, potentially lifelong disease activity in a subset; median follow-up studies show many but not all patients eventually reach durable remission

Patterns

Remission: Can be spontaneous or treatment-induced (NSAID, bisphosphonate, or biologic-associated); >50% of NSAID responders experience a flare at a median of ~29 months despite initial response
Relapse: A UK tertiary center cohort (17 patients, 1999–2015) demonstrated ongoing relapse risk into adolescence/young adulthood requiring long-term rheumatology follow-up
Critical periods: Early diagnosis (median delay currently ~15 months) is emphasized as a window to prevent structural bone damage (vertebral fracture, growth disturbance) before it occurs.

9. Inheritance and Population

Epidemiology

Incidence (most authoritative recent estimate): 0.65 per 100,000 person-years in children <16 in the UK/Republic of Ireland (2024 BPSU national prospective surveillance study, October 2020–November 2022; 288 patients reported, 165 confirmed + 20 probable cases analyzed) (PMC11962910; Rheumatology Oxford 2025).
Other national estimates: 0.4 per 100,000 children in Germany; 2.3 per 100,000 children in one large catchment area of the northwestern United States; a pooled pediatric estimate of ~0.605 CNO cases per 100,000 person-years has also been cited (JBMR 2024).
Historic literature-review estimate: as low as 4 per million children, though rising with increased clinical recognition — likely reflecting under-ascertainment rather than a true rising incidence.

For genetic etiology

Inheritance pattern: Sporadic/non-syndromic CRMO is generally considered complex/multifactorial (polygenic risk-variant model, e.g., P2RX7); the monogenic syndromic forms (Majeed, DIRA, PAPA) follow classic autosomal recessive (Majeed, DIRA) or autosomal dominant (PAPA, CRMO3) Mendelian inheritance.
Penetrance: Complete/high for monogenic LPIN2/IL1RN loss-of-function alleles; incomplete/variable for P2RX7 and IL-10 haplotype risk variants (risk-modifier model, not deterministic).
Consanguinity: A recognized risk factor for the recessive monogenic forms — the original FBLIM1 and several LPIN2 pedigrees were identified in consanguineous families.
Carrier frequency: Not systematically established for LPIN2/IL1RN given extreme rarity (~24 genetically confirmed Majeed syndrome individuals worldwide reported to date).

Population demographics

Sex ratio: Overall female predominance in classic pediatric CRMO cohorts, though the "severe/multifocal" phenotype subgroup skews male (see Phenotypes section) — UK longitudinal cohort: 10 female : 7 male.
Ethnic/geographic distribution: White/Caucasian populations most frequently reported in the literature, though this likely reflects study-site bias; global epidemiologic data remain limited. FBLIM1-associated CRMO was identified in a South Asian consanguineous family, indicating the disease is not confined to any single ancestry.
Age distribution: Concentrated in the pediatric/adolescent range (peak 7–12 years), with adult-onset presentations increasingly reported (overlapping SAPHO nomenclature).

10. Diagnostics

Clinical/laboratory tests

Blood tests: ESR, CRP (often normal-to-mildly elevated — a distinguishing feature versus bacterial osteomyelitis), CBC (anemia of chronic disease)
Biomarkers (investigational): Serum IL-6 + eotaxin panel (93% sensitivity/97% specificity in preliminary studies), S100A8, RANTES, soluble IL-2 receptor — not yet standard of care
Bone biopsy/histopathology: Traditionally required to exclude infection/malignancy; early lesions show neutrophils, lymphocyte clusters, occasional eosinophils; chronic lesions show lymphocytes, plasma cells, histiocytes; established lesions show necrotic bone fragments, fibrosis, increased osteoblasts, and dilated vessels. Cultures are sterile.

Imaging

Plain radiographs: May be normal early; later show lytic and/or sclerotic lesions with periosteal reaction (Bristol cohort, n=36: 50% lytic, 53% sclerotic, 33% periosteal reaction)
Whole-body MRI (WB-MRI): the diagnostic "gold standard." Detects bone marrow edema on fat-saturated T2/STIR sequences before structural change is visible, and — critically — reveals clinically silent, radiographically occult multifocal lesions, establishing the multifocal distribution pattern central to diagnosis and helping exclude alternative diagnoses (Insights into Imaging 2022; PMID:36114435).
Bone biopsy may be avoided when WB-MRI shows the characteristic multifocal pattern at typical sites (clavicle, mandible, metaphyses) with no systemic infectious signs.

Diagnostic criteria

Two named clinical criteria sets exist, neither prospectively validated at scale: - Jansson criteria — found to be more sensitive than Bristol criteria (OR 3.94, P<0.001) - Bristol criteria — use by an experienced clinician may obviate the need for biopsy in some patients - ACR/EULAR candidate classification criteria, derived from ~450 international cases, are in development/pending final dissemination as of the 2024 JBMR review — representing an important near-term advance in standardized diagnosis.

Genetic testing

Not routine for sporadic CRMO; recommended when syndromic features suggest Majeed syndrome (congenital dyserythropoietic anemia, neutrophilic dermatosis — test LPIN2), DIRA (neonatal pustulosis, extreme inflammatory markers — test IL1RN), or PAPA syndrome (pyoderma gangrenosum, cystic acne — test PSTPIP1).

Differential diagnosis

Bacterial/fungal/mycobacterial osteomyelitis, malignancy (Ewing sarcoma, Langerhans cell histiocytosis, leukemia/lymphoma bone involvement), benign bone tumors (osteoid osteoma, bone cysts), other monogenic autoinflammatory disorders (PAPA, DIRA, Majeed syndrome), metabolic bone disease (hypophosphatasia), osteonecrosis, osteopetrosis, juvenile idiopathic arthritis.

11. Outcome/Prognosis

Mortality: CRMO is not associated with excess mortality; it is a morbidity-driving, not life-threatening, condition.
Disease course: The historically assumed "self-limiting" natural history is now understood to be an oversimplification — CRMO frequently follows a relapsing-remitting or progressive course, with a substantial proportion of patients experiencing relapses even after apparent remission.
Complications: Vertebral compression fractures, kyphosis/scoliosis, leg-length discrepancy (from growth-plate involvement), progressive bone overgrowth/hyperostosis (clavicle, mandible), and — independent of active inflammation — amplified musculoskeletal pain syndromes that can persist and are treatment-refractory.
Prognostic factors: Multifocal/severe phenotype (male predominant subgroup), spinal involvement, and delayed diagnosis (median 15 months) are associated with greater risk of structural damage.
Quality of life: Chronic pain and activity restriction substantially affect daily functioning; long-term rheumatology follow-up into adulthood is recommended given persistent relapse risk documented in UK tertiary-center cohorts.

12. Treatment

First-line

NSAIDs (typically naproxen) — inhibit cyclooxygenase, reducing prostaglandin E–mediated osteoclast activation. Effective in >60% of patients over 12–18 months, but >50% experience a flare at a median of ~29 months. (Suggested NCIT: NCIT:C15986 Pharmacotherapy; therapeutic_agent CHEBI-bound NSAID)

Second-line / escalation

Corticosteroids: Short courses (e.g., prednisone-equivalent 2 mg/kg/day for 5–10 days, or bridging 0.1–0.2 mg/kg/day); mechanism via phospholipase A2 and NF-κB-regulated cytokine (IL-1, IL-6, TNF-α) inhibition; long-term use limited by adverse-effect burden.
Bisphosphonates (pamidronate, zoledronic acid):
Pamidronate: 1 mg/kg/dose (max 60 mg) monthly, or 3 consecutive days every 3 months, for 9–12 months
Zoledronic acid: 0.0125–0.025 mg/kg/dose (max 4 mg) every 6–12 months
Mechanism: osteoclast apoptosis + reduction of pro-inflammatory cytokine expression
Particularly effective for vertebral involvement; mean MRI resolution of inflammation ~6 months (range 2–12 months)
The only published RCT in this disease is pamidronate vs. placebo in adults, underscoring the overall paucity of controlled trial evidence
(NCIT:C15986 Pharmacotherapy; therapeutic_agent CHEBI-bound pamidronate/zoledronic acid)
Conventional DMARDs (methotrexate, sulfasalazine, leflunomide): increasingly used but with sparse supporting evidence

Biologics / emerging therapies

TNF inhibitors (adalimumab, infliximab, etanercept): 40–50% remission rates in European registry data; beneficial for vertebral lesions refractory to pamidronate; paradoxical psoriasis is a recognized adverse effect, arguably more common than with other indications; not currently licensed for CNO/CRMO. A comparative international multicenter retrospective study (n=91: pamidronate=47, TNFi=22, both sequentially=22) found both therapies associated with clinical remission at 6 months and MRI lesion reduction at 12 months; pamidronate trended toward faster MRI resolution (not statistically significant), while TNF inhibitors were associated with fewer flares (PMID:35460903).
IL-1 blockade (anakinra, canakinumab, rilonacept): beneficial for osteitis/arthritis with variable mucocutaneous response; canakinumab produced rapid response in a refractory CRMO-with-pyoderma-gangrenosum case, though efficacy for bone disease waned over time in some reports (PMID:36004431). Majeed syndrome and DIRA patients respond well to IL-1 blockade, supporting mechanistic centrality of this pathway; canakinumab has produced long-lasting remission in Majeed syndrome.
IL-17/IL-23 inhibition: Secukinumab (anti-IL-17A) successfully used in SAPHO patients in some reports but led to recurrence of osteomyelitis despite psoriasis non-response in at least one case — efficacy is inconsistent. Ustekinumab (anti-IL-12/23) has shown case-report success for CNO, including resolution of back pain.
JAK inhibitors: Deucravacitinib (TYK2 inhibitor) showed effectiveness in a 2024/2025 case report for CRMO with concomitant psoriasis (PMC12138201); tofacitinib shows promise in SAPHO with limited pediatric CNO data.
IL-6 blockade (tocilizumab): mixed results despite IL-6 being a consistent serum biomarker.
RANKL inhibition (denosumab): theoretical potential given RANK/RANKL centrality to mechanism, but no published CNO reports to date.
Investigational/preclinical targets: P2X7 antagonism, NLRP3 inflammasome small-molecule inhibitors (e.g., MCC950), ASC inhibitors (e.g., MM01), IL-18-selective inhibitors, and — from the newest (2025) murine mechanistic work — targeting the lncRNA Morrbid to shorten inflammatory myeloid cell lifespan.

Surgical/other

Surgery is generally reserved for structural complications (e.g., severe vertebral deformity) rather than as primary disease-modifying treatment.

Treatment strategy / trial design gaps

No treatments are currently FDA/EMA-licensed specifically for CNO/CRMO. Expert consensus prioritizes IL-1 and IL-17 blockade as the leading candidate interventions for future controlled trials, with pamidronate as an active comparator. ACR/EULAR classification criteria and OMERACT core outcome measures are anticipated as near-term advances that should enable better-powered trials (JBMR 2024 review).

Suggested NCIT terms: NCIT:C15986 (Pharmacotherapy), NCIT:C15632 (Chemotherapy — n/a here), NCIT:C49236 (Therapeutic Procedure), NCIT:C15329 (Surgical Procedure — for structural complications), NCIT:C20401 (Monoclonal Antibody — for biologics), NCIT:C2986 (drug class terms per specific agent).

13. Prevention

Primary prevention: None established — no known modifiable risk factor or vaccination strategy exists, since etiology in sporadic disease remains incompletely defined.
Secondary prevention (early detection): The chief actionable lever is reducing diagnostic delay (currently median ~15 months) through greater clinical awareness and appropriate use of whole-body MRI, since earlier treatment initiation is presumed (though not RCT-proven) to reduce risk of structural damage (vertebral fracture, growth disturbance, permanent hyperostosis).
Genetic counseling: Relevant for families with confirmed monogenic disease (Majeed syndrome, DIRA, PAPA) given autosomal recessive/dominant inheritance patterns and availability of single-gene testing; carrier screening/prenatal diagnosis can be discussed in consanguineous families with a confirmed proband mutation.
Tertiary prevention: Activity modification (particularly avoiding high-impact loading with active spinal lesions) to reduce fracture risk; routine surveillance imaging and bloodwork during treatment to catch flares early.

14. Other Species / Natural Disease

No naturally occurring veterinary counterpart of CRMO in companion animals or wildlife is well documented in the literature surveyed (unlike many Mendelian diseases with OMIA veterinary entries); the disease's key animal correlates are engineered/spontaneous mouse mutants (below) rather than natural disease in other species.

15. Model Organisms

Murine genetic models (the field's principal disease models)

cmo (chronic multifocal osteomyelitis) mice — spontaneous homozygous missense mutation in Pstpip2 (PMID:16122996); develop sterile osteomyelitis with severe systemic cytokine/chemokine dysregulation, extramedullary hematopoiesis, and inflammatory skin lesions.
lupo mice — chemically induced (ENU) homozygous Pstpip2 mutation with a very similar phenotype to cmo mice.
Mechanistic insight from these models: Bone inflammation in Pstpip2-deficient mice is IL-1β-mediated but NLRP3-inflammasome- and caspase-1-independent — notably different from most other IL-1-mediated autoinflammatory diseases, and an important nuance for interpreting IL-1 pathway targeting (PNAS; PMC3903222).
Mast cell contribution: Mast cells enhance sterile inflammation in this model (PMC6737947/biorxiv preprint), implicating a non-myeloid innate immune cell type beyond monocytes/macrophages.
Morrbid lncRNA (2025): Single-cell RNA-seq in Pstpip2⁻/⁻ mice identified co-expression of the myeloid-restricted long non-coding RNA Morrbid with Pstpip2; genetic disruption of Morrbid significantly inhibited CRMO initiation and progression, mitigating myeloid cell activation and excessive cytokine release — proposed as a novel therapeutic strategy of shortening inflammatory myeloid cell lifespan (Disease Models & Mechanisms, PMID:40503910; PMC12309896).
FBLIM1-related insight originated from microarray analysis of the cmo mouse bone marrow macrophage transcriptome, which nominated Fblim1 before the human familial mutation was identified — a model-to-human translational discovery pathway.

Phenotype recapitulation and limitations

The Pstpip2-mutant models recapitulate multifocal sterile osteomyelitis, systemic inflammatory cytokine dysregulation, and skin inflammation, closely mirroring human CRMO/Majeed-spectrum disease. However, they model the syndromic/PSTPIP-pathway disease axis specifically; they do not directly model the P2RX7-variant or IL-10-haplotype-driven sporadic human CRMO subtype, and — as with most mouse inflammatory models — species differences in innate immune receptor biology and inflammasome regulation limit direct translational certainty (a candidate HUMAN_MODEL_MISMATCH consideration for any KB entry: the NLRP3-independence of murine IL-1β-driven disease contrasts with the NLRP3-centric human mechanistic literature).

Other model systems

Zebrafish: No CRMO-specific zebrafish model was identified in the literature searched, but zebrafish are an established platform for skeletal disorder and innate-immune/notochord-infection modeling generally, and represent a plausible future avenue (real-time imaging of innate immune cells, ease of genetic manipulation) — not yet applied to CRMO specifically per available sources.
iPSC/organoid models: No CRMO-specific iPSC-derived or organoid models were identified in the current literature — a research gap.

Resources

MGI (Mouse Genome Informatics) for Pstpip2 allele records (cmo, lupo); no dedicated CRMO patient-derived cell biobank was identified in this search.

Summary of Key Evidence Gaps (for curation prioritization)

No licensed treatment exists for CNO/CRMO; only one RCT (pamidronate vs. placebo, adults) has been conducted.
ACR/EULAR classification criteria are in development but not yet finalized/published in final form.
Sporadic (non-syndromic) CRMO genetics remain largely unresolved — P2RX7 explains only a minority of cases.
The NLRP3-independence of the leading murine model (Pstpip2-deficient mice) versus the NLRP3-centric human mechanistic literature is a notable model-translatability nuance.
Biomarker panels (IL-6/eotaxin) require independent validation before clinical adoption.
Long-term natural history / outcome data beyond adolescence remain sparse — most cohorts are pediatric-center-based with limited adult follow-up.

Sources

Reference Validation

Checked with linkml-reference-validator 0.2.1.

Outcome	Count
References checked	31
Resolved	31
Unresolved (possible confabulation)	0
Unverifiable	0
References weighed for topical relevance	31
On topic	26
Off topic	0

All extracted references resolved successfully.

Ask OpenScientist

OpenScientist Report

Definitions

Discussions and Knowledge Gaps

Pathophysiology

Pathograph

Phenotypes

Genetic Associations

Medical Actions

Diagnosis

Differential Diagnoses

Source YAML

References & Deep Research

References

Deep Research

Chronic Recurrent Multifocal Osteomyelitis (CRMO) / Chronic Nonbacterial Osteomyelitis (CNO): Comprehensive Research Report

1. Disease Information

2. Etiology

Disease Causal Factors

Genetic Risk Factors

Protective Factors

Gene–Environment Interactions

3. Phenotypes

Core symptoms and signs

Laboratory abnormalities

Phenotype characteristics

Skeletal distribution

Associated (extra-osseous) manifestations / comorbidities

Quality of life impact

4. Genetic/Molecular Information

Causal genes (monogenic forms)

Variant classification and population data

Functional consequences

Modifier genes

Epigenetic information

Chromosomal abnormalities

5. Environmental Information

6. Mechanism / Pathophysiology

Causal chain overview

Molecular pathways

Cytokine profile (imbalance model)

Cellular processes

Tissue damage mechanisms

Immune system involvement

Molecular profiling data

Advanced technologies

7. Anatomical Structures Affected

Organ level

Tissue and cell level

Subcellular level

Localization

8. Temporal Development

Onset

Progression

Patterns

9. Inheritance and Population

Epidemiology

For genetic etiology

Population demographics

10. Diagnostics

Clinical/laboratory tests

Imaging

Diagnostic criteria

Genetic testing

Differential diagnosis

11. Outcome/Prognosis

12. Treatment

First-line

Second-line / escalation

Biologics / emerging therapies

Surgical/other

Treatment strategy / trial design gaps

13. Prevention

14. Other Species / Natural Disease

15. Model Organisms

Murine genetic models (the field's principal disease models)

Phenotype recapitulation and limitations

Other model systems

Resources

Summary of Key Evidence Gaps (for curation prioritization)