Inflammatory Myopathies

Inflammatory Myopathies

 

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
Distribution of Antibodies in Connective Tissue Disease Types
  Systemic Lupus Erythematosus (SLE) Sjögren Syndrome Mixed Connective Tissue Disease (MCTD) Progressive Systemic Sclerosis (PSS) Scleroderma
dsDNA abs 50-60% 20-30% 20-25% <5%  
Histone abs Idiopathic 18-53%
Drug-induced 80-95%		   <20% <20% <20%
RNP 20-30%   95-100% 15-25% 5-10%
Scl-70       25% 20-60%
SSA ANA positive patients
30-40%		 70-75%   5-10%  
SSB 15-25% 50-60%   5-10%  
Jo-1 abs Found in polymyositis, dermatomyositis, myositis associated with rheumatic disease

See Also