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- Systemic connective tissue disease characterized by inflammatory and degenerative changes in proximal muscles, sometimes accompanied by characteristic skin rash:
- If skin manifestations (Gottron sign [symmetric, scaly, violaceous, erythematous eruption over the extensor surfaces of the metacarpophalangeal and interphalangeal joints of the fingers]; heliotrope [reddish violaceous eruption on the upper eyelids]) are present, it is designated as dermatomyositis (1).
- Different types of myositis include:
- Idiopathic polymyositis
- Idiopathic dermatomyositis
- Polymyositis/dermatomyositis as an overlap (usually with lupus or systemic sclerosis or as part of mixed connective-tissue disease)
- Myositis associated with malignancy
- HIV-associated myopathy
- Inclusion-body myositis (IBM), a variant with atypical patterns of weakness and biopsy findings (2)
- System(s) affected: Cardiovascular; Musculoskeletal; Pulmonary; Skin/Exocrine
- Synonym(s): Myositis; Inflammatory myopathy; Antisynthetase syndrome (subset with certain antibodies)
- Estimated at 0.5–0.8/100,000 population
- Predominant age: 5–15 years, 40–60 years, peak incidence in mid-40s
- Predominant sex: Female > Male (2:1)
1–2 patients/100,000 population
Elderly patients with myositis or dermatomyositis are at increased risk of neoplasm.
Childhood dermatomyositis is likely a separate entity associated with cutaneous vasculitis and muscle calcifications.
Family history of autoimmune disease (e.g., systemic lupus, myositis) or vasculitis
Mild association with human leukocyte antigen (HLA)–DR3, HLA-DRw52
- Inflammatory process, mediated by T cells and cytokine release, leading to damage to muscle cells (predominantly skeletal muscles)
- In patients with IBM, degenerative mechanisms may be important.
Unknown; potential viral, genetic factors
Commonly Associated Conditions
- Malignancy (in 15–25%)
- Progressive systemic sclerosis
- Systemic lupus erythematosus (SLE)
- Mixed connective-tissue disease