Epidemiology

• The AIM are the most common myopathies, but are still rare
  
• Annual incidence: adults: 5–10/million, children: 1–5/million
  
• Prevalence: 50–100 cases/million

Types of Autoimmune-Mediated Myopathies

• Polymyositis (PM)
  
  
  
  • Pathogenesis: cytotoxic CD8+ T cells
    
  • Clinical presentation: weeks to months of progressive proximal muscle weakness
    
  • Unique features: interstitial lung disease (ILD), cardiac disease
    
  • Treatment response: good
  
  
• Dermatomyositis (DM)
  
  
  
  • Pathogenesis: C5b-9 MAC (membrane attack complex), CD4+ T cell perivascular inflammation
    
  • Clinical presentation: weeks to months of proximal muscle weakness
    
  • Unique features: dermatologic manifestations (see physical examination in AIM section)
    
  • Treatment response: good
  
  
• Inclusion body myositis (IBM)
  
  
  
  • Pathogenesis: neurodegenerative
    
  • Clinical presentation: years of progressive muscle weakness
    
  • Unique features: atrophy, asymmetric, distal weakness in addition to proximal weakness
    
  • Treatment response: poor, significant disability
  
  
• Immune-mediated necrotizing myopathy (IMNM)
  
  
  
  • Pathogenesis: myofiber necrosis, degeneration/regeneration, few if any inflammatory cells
    
  • Clinical presentation: not well described
    
  • Unique features: can be drug induced/paraneoplastic
    
  • Treatment response: can respond to immunosuppression

• Myositis associated with connective tissue disease (CTD)
  
  
  
  • Pathogenesis: probably a combination of mechanisms
    
  • Clinical presentation: generally proximal muscle weakness, plus features of CTD
    
  • Typically associated with:
    
    
    
    • Scleroderma (16–93%)
      
    • Lupus
      
    • Mixed connective tissue disease (MCTD)
      
    • Sjögrens
      
    • Rheumatoid arthritis
    
    
  • Symptoms may be mild to severe
  
  
• Juvenile polymositis/dermatomyositis
  
  
  
  • Pathogenesis: similar to adult
    
  • Clinical presentation: similar to adult presentation, however DM is more common
    
  • Unique features: painful subcutaneous calcifications can occur, and tend to be more common in pediatric populations
  
  
• Amyopathic dermatomyositis
  
  
  
  • Pathogenesis: same as DM
    
  • Clinical presentation:
    
    
    
    • Skin manifestations without muscle disease or elevated muscle enzymes
      
    • ILD
      
    • May be associated with pneumothoraces or pneumomediastinum

Classification Criteria

The original criteria were defined by Bohan and Peter in 1975.

• These were devised prior to the discovery of myositis-specific anti­bodies and the recognition of IBM as a separate disease entity
  
• Patients were assessed by the following criteria:
  
  
  
  • Symmetric, proximal muscle weakness
    
  • Rash typical of DM
    
  • Elevated muscle enzymes
    
  • Myopathic changes on electromyography (EMG)
    
  • Characteristic muscle biopsy without signs of other myopathies
  
  
• Based on the pattern of the above, patients were divided into either probable or definite inflammatory myopathy

Physical Examination in AIM

• Neurologic
  
  
  
  • Weakness in proximal muscle groups of upper and lower extremities, neck flexors
    
  • Muscle pain is not a primary feature
    
  • Distal weakness is characteristic of IBM
    
  • Muscle atrophy is characteristic of IBM or late PM/DM
    
  • Note: a patient presenting with weakness over years, asymmetric/distal/facial weakness and scapular winging should prompt evaluation for dystrophy, not myopathy, as a cause of their weakness
  
  
• Musculoskeletal
  
  
  
  • Synovitis can be present in overlap syndromes
  
  
• Dermatologic (primarily seen in DM)
  
  
  
  • Heliotrope rash: violaceous papules around eyes
    
  • Gottron papules: erythematous papules, plaques over MCPs, PIPs, DIPs
    
  • Poikiloderma/photosensitive erythematous rash characterized by location:
    
    
    
    • Shawl sign: upper back
      
    • V sign: sun-exposed areas of upper chest
      
    • Holster sign: lateral thighs
    
    
  • Nail bed changes: periungual telangiectasias, cuticular hypertrophy
    
  • Mechanics’ hands: rough, cracked skin over fingertips and lateral aspect of fingers, most commonly seen in antisynthetase syndrome
    
  • Skin changes are typically very photosensitive
    
  • Painful subcutaneous calcifications can also occur, seen primarily in juvenile DM and CTD overlap syndromes
    
  • If dermatologic signs are seen without evidence of associated muscle disease, these patients are categorized as amyopathic dermatomyositis

Laboratory Findings in AIM

• Muscle enzymes
  
  
  
  • Creatine kinase (CK): most sensitive and specific
    
  • Aldolase: can be elevated in isolation
    
  • Lactate dehydrogenase (LDH)
  
  
• Aspartate transaminase (AST)
  
• Alanine transaminase (ALT)
  
• Normal thyroid studies are needed to exclude thyroid disease as a cause of myopathy