The inflammatory myopathies are a group of autoimmune disorders characterized by an inflammatory infiltrate in skeletal muscle and the presence of autoantibodies.

Subgroups of myositis

• Dermatomyositis (DM)
• Polymyositis (PM)
• Inclusion body myositis (IBM)
• Antisynthetase syndrome
• Overlap syndromes
• Amyopathic dermatomyositis

Epidemiology

• Incidence - 2-8/1,000,000
• Age
  • DM and PM - 40 to 60 years
  • IBM - >50 years
  • Antisynthetase - no specific age distribution
• Gender
  • DM - F:M; 2:1
  • IBM - M:F; 2:1
  • Antisynthetase - almost exclusively females

Pathophysiology

• DM is a microangiopathy affecting skin and muscle with deposition of complement causing lysis of endomysial capillaries and muscle ischemia
• In PM and IBM, T-cells invade muscle fibers leading to necrosis via the perforin pathway

Clinical Presentation

• General features
  • Presentation of progressive (usually symmetrical) muscle weakness
  • Pharyngeal and neck flexion muscles frequently involved, leading to dysphagia
  • May have systemic symptoms such as fever, weight loss
  • Raynaud phenomenon common
• Dermatomyositis
  • Characteristic rash accompanied by muscle weakness
    • Blue-purple rash - symmetrical distribution
    • Discoloration of upper eyelids with edema (Heliotrope rash)
    • Erythema of knuckles with raised violaceous rash (Gottron rash)
  • Dilated capillaries at base of fingernails
  • Cancer-associated myositis
    • Most common with DM
    • Increased risk of malignancy (15%) in the following types:
      • Ovarian
      • Breast
      • Melanoma
      • Non-Hodgkin lymphoma
• Polymyositis
  • Rare in children
  • Usually subacute presentation
  • May be associated with other autoimmune diseases
  • Diagnosis of exclusion - none of the following are present:
    • Rash
    • Family history of
      • Neuromuscular disease
      • Endocrinopathy
      • Muscular dystrophy
      • Known biochemical muscle disorder
      • Drug-induced myopathy
• Inclusion body myositis
  • May be misdiagnosed as PM
  • Associated with other autoimmune diseases 20% of the time
  • Small muscles in hand frequently involved
• Antisynthetase syndrome
  • Almost exclusively in middle-aged women
  • Low grade fevers, mechanic’s hands, pulmonary interstitial disease, Raynaud syndrome
    • Mechanic’s hands - hyperkeratosis of the palmic and lateral aspects of the hands
    • PM and DM skin features
• Amyopathic DM
  • Characteristic cutaneous findings of DM
  • No evidence of muscle disease
    • May have fatigue

Diagnosis

• Laboratory testing
  • Antibodies to autoantigen Jo-1 (histidyl-RNA synthetase) are found in 18-36% of patients satisfying myositis criteria; however these are not useful in confirming diagnosis
  • Nearly all Jo-1 positive patients have lung involvement
  • Relationship between Jo-1 antibody titer and disease activity reported but not confirmed
  • Rarely found in any other disease states
    • DM and mixed connective tissue disease
      • May have specific antibody (such as anti PM-Sc1) directed against nucleolar-protein complex
  • Aminoacyl - RNA synthetase antibody
• Biopsy
  • Muscle biopsy - perivascular and perimysial inflammation in DM and PM
  • PM - negative muscle biopsy for IBM