Description

• 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)
    • 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)

Epidemiology

Incidence

• 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)

Prevalence
2.4 to 33.8 patients per 100,000 population

Geriatric Considerations
Elderly patients with myositis or dermatomyositis are at increased risk of neoplasm.

Pediatric Considerations
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

Genetics
Mild association with human leukocyte antigen (HLA)-DR3, HLA-DRw52

Risk Factors

Family history of autoimmune disease (e.g., systemic lupus erythematosus (SLE), myositis) or vasculitis

Commonly Associated Conditions

• Malignancy more common in dermatomyositis subtype, older patients
• Progressive systemic sclerosis
• Vasculitis
• SLE
• Mixed connective tissue disease