Idiopathic inflammatory myopathies (myositis spectrum)

name: Idiopathic Inflammatory Myopathies
display_name: Idiopathic inflammatory myopathies (myositis spectrum)
creation_date: "2026-08-28T00:00:00Z"
description: >-
  The idiopathic inflammatory myopathies (IIM) are the acquired,
  immune-mediated diseases of skeletal muscle: dermatomyositis, polymyositis,
  antisynthetase syndrome, and sporadic inclusion body myositis. All present
  with muscle weakness and myofiber injury with mononuclear cell infiltration,
  and none is explained by a dystrophy gene, infection, toxin, or endocrine
  cause - "idiopathic" marks the exclusionary step that defines the category
  in practice. Modern classification splits the spectrum by autoantibody and
  histopathology rather than by the older polymyositis-versus-dermatomyositis
  dichotomy.
grouping_basis:
- SHARED_PHENOTYPE
- CLINICAL_CONVENTION
grouping_rationale: >-
  Grouped on the shared clinical phenotype (acquired immune-mediated myositis
  with weakness and elevated muscle enzymes) and on a deep clinical
  convention: the IIM are classified together from Bohan and Peter through
  the ENMC and EULAR/ACR criteria, share a diagnostic workup (autoantibody
  panel, EMG, MRI, muscle biopsy), and are managed by the same specialty with
  broadly overlapping immunosuppressive regimens. The members are
  deliberately kept as separate Disease entries because their effector
  mechanisms differ fundamentally: dermatomyositis is an interferon-driven
  microvasculopathy with perifascicular pathology and skin disease;
  polymyositis - now a shrinking diagnosis of exclusion - is a CD8
  cytotoxic-T-cell attack on non-necrotic fibers; antisynthetase syndrome is
  defined by anti-aminoacyl-tRNA-synthetase autoantibodies and couples
  myositis to interstitial lung disease that often dominates outcome; and
  inclusion body myositis combines inflammation with a degenerative
  rimmed-vacuole/protein-aggregate component, follows a distinctive finger
  flexor and quadriceps pattern, and - uniquely in the group - does not
  respond to immunosuppression. A merged entry would average away exactly
  the mechanism-treatment correlations the spectrum is studied for.
mappings:
  mondo_mappings:
  - term:
      id: MONDO:0020122
      label: acquired idiopathic inflammatory myopathy
    mapping_predicate: skos:closeMatch
    mapping_source: MONDO
    mapping_justification: >-
      closeMatch rather than exactMatch, in both directions. The MONDO class
      omits inclusion body myositis, which the clinical IIM convention
      (ENMC, EULAR/ACR) includes and which is curated as a member here; and
      it includes eosinophilic fasciitis and focal myositis, which the
      convention excludes (a fasciitis and a benign pseudotumoral lesion,
      respectively, not systemic autoimmune myopathies). Descendants of the
      MONDO class that the convention does include - immune-mediated
      necrotizing myopathy and overlap myositis - are curation gaps rather
      than deliberate exclusions.
membership_criteria:
- description: >-
    A disease belongs to the idiopathic inflammatory myopathies if it is an
    acquired, immune-mediated inflammatory disease of skeletal muscle - not
    attributable to a dystrophy gene, infection, drug, toxin, or endocrine
    disease - presenting with muscle weakness.
  criteria_semantics: NECESSARY
  logic:
    operator: AND
    operands:
    - criterion_predicate: OTHER
      description: >-
        Acquired and immune-mediated, with recognized causes of secondary
        myopathy excluded - the exclusionary step is clinical and is not
        machine-checkable against entry annotations.
    - criterion_predicate: HAS_PHENOTYPE
      description: Muscle weakness (evaluated over HP closure).
      phenotype_term:
        preferred_term: Muscle weakness
        term:
          id: HP:0001324
          label: Muscle weakness
members:
- member: Dermatomyositis
  member_type: DISEASE
  differentiating_mechanisms:
  - description: >-
      Type I interferon-driven disease with complement-mediated
      microvasculopathy: capillary dropout and perifascicular atrophy on
      biopsy, pathognomonic skin disease (heliotrope rash, Gottron papules),
      and autoantibody-defined subphenotypes (anti-MDA5, anti-TIF1,
      anti-NXP2, anti-Mi2) that stratify lung disease and cancer risk.
      Juvenile dermatomyositis is the predominant juvenile IIM (juvenile
      polymyositis and juvenile overlap myositis exist but are far rarer).
  - description: >-
      Distinguishing phenotype axis - the immunopathology attacks vessels
      and the perifascicular region rather than invading individual
      non-necrotic fibers.
- member: Polymyositis
  member_type: DISEASE
  differentiating_mechanisms:
  - description: >-
      CD8+ cytotoxic T cells invade non-necrotic, MHC-I-upregulated fibers
      without rash, rimmed vacuoles, or a defining autoantibody. Under
      modern autoantibody- and histology-based classification it is a
      shrinking diagnosis of exclusion, with many historical cases
      reclassified as antisynthetase syndrome, immune-mediated necrotizing
      myopathy, or inclusion body myositis - a reclassification history the
      member entry itself documents.
- member: Antisynthetase Syndrome
  member_type: DISEASE
  differentiating_mechanisms:
  - description: >-
      Defined by autoantibodies against aminoacyl-tRNA synthetases (anti-Jo-1
      and others) and by a systemic triad beyond muscle: interstitial lung
      disease - frequently the presenting and prognosis-defining feature -
      with mechanic's hands, arthritis, Raynaud phenomenon, and fever.
      Myositis can be mild or absent at onset, which is why the syndrome is
      classified by its serology rather than its biopsy.
- member: Inclusion Body Myositis
  member_type: DISEASE
  differentiating_mechanisms:
  - description: >-
      The hybrid member: endomysial CD8 inflammation coexists with a
      degenerative component - rimmed vacuoles, p62/TDP-43-positive protein
      aggregates, and mitochondrial abnormalities. Onset after 50, slowly
      progressive asymmetric finger flexor and quadriceps weakness with
      dysphagia, and refractoriness to glucocorticoids and conventional
      immunosuppression separate it from every other member and make its
      recognition the single most consequential differential call in the
      group.
notes: >-
  Created from the stub-queue lump/split review: the seeded stub for
  MONDO:0020122 (acquired idiopathic inflammatory myopathy) resolved as
  entry_type GROUPING because the concept is a classification category over
  distinct diseases, four of which are already curated as separate entries.
  Immune-mediated necrotizing myopathy and overlap myositis are the
  remaining conventional members without entries; inflammatory myopathy
  with abundant macrophages and idiopathic eosinophilic myositis, also
  descendants of the mapped MONDO class, likewise sit inside the spectrum
  and are curation gaps rather than exclusions. All should be nominated as
  ordinary disease stubs and added to members: when curated. The OTHER
  leaf makes the necessary criterion unsatisfiable by the evaluator, so
  members report as UNKNOWN when the weakness leaf alone is satisfied -
  expected three-valued behavior, not a violation.