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 (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)

Epidemiology

• 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)

Prevalence
1–2 patients/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.

Risk Factors

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

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

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.

Etiology

Unknown; potential viral, genetic factors

Commonly Associated Conditions

• Malignancy (in 15–25%)
• Progressive systemic sclerosis
• Vasculitis
• Systemic lupus erythematosus (SLE)
• Mixed connective-tissue disease