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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.
- Different types of myositis include the following (1):
- 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
- Necrotizing autoimmune myositis (often statin associated) (2)
- Inclusion body myositis (IBM), a variant with atypical patterns of weakness and biopsy findings
- System(s) affected: cardiovascular, musculoskeletal, pulmonary, skin/exocrine
- Synonym(s): myositis; inflammatory myopathy; antisynthetase syndrome (subset with certain antibodies)
- Estimated at 1.2 to 19 per million population per year
- Predominant age: 5 to 15 years, 40 to 60 years, peak incidence in mid-40s
- Predominant sex: female > male (2:1)
2.4 to 33.8 patients per 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.
Etiology and Pathophysiology
- 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
Mild association with human leukocyte antigen (HLA)-DR3, HLA-DRw52
Family history of autoimmune disease (e.g., systemic lupus, myositis) or vasculitis
Commonly Associated Conditions
- Malignancy more common in dermatomyositis
- Progressive systemic sclerosis
- Systemic lupus erythematosus (SLE)
- Mixed connective tissue disease