Polymyositis/Dermatomyositis

Inflammatory myopathies represent a group of diseases of unknown cause in which muscle injury results from inflammation.

Polymyositis and dermatomyositis are the characteristic diseases in this group.

They affect individuals of all ages. The annual incidence of polymyositis and dermatomyositis ranges from 5–10 new cases per million. The average female-to-male ratio exceeds 2:1, with women predominating in this disease that occurs characteristically between ages 15 and 44 years.

The idiopathic inflammatory myopathies are immune-mediated processes, believed to be triggered by environmental factors in genetically susceptible individuals. The specific causes or triggering events remain unknown, but viruses have been strongly implicated.

The criteria for the diagnosis of polymyositis and dermatomyositis include:

Polymyositis

  • Symmetrical weakness
  • Muscle biopsy evidence
  • Elevation of muscle enzymes
  • Electromyographic evidence
  • Dermatologic features

Dermatomyositis

  • Usually progressive, of the limb-girdle muscles
  • Necrosis of type I and II muscle fibers; phagocytosis; degeneration and regeneration of myofibers with variation in myofiber size; endomysial, perimysial, perivascular or interstitial mononuclear cells
  • Specifically, creatine phosphokinase, aldolase, lactate dehydrogenase, transaminases (ALT and AST)
  • Short, small, low-amplitude polyphasic motor unit potentials; fibrillation potentials, and bizarre high-frequency repetitive changes
  • Heliotrope rash (lilac discoloration of the eyelids and periorbital area), Gottron’s papules (scaly erythematous eruptions over the metacarpophalangeal and interphalangeal joints or over other extensor surfaces), gottron’s sign (erythema in the area of Gottron’s papules but without papules

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