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Polymyositis vs. Dermatomyositis




General Features


  • Inflammatory myopathies with symmetric proximal weakness
  • Involves muscle fiber destruction and inflammatory infiltration of muscles
  • May cause patients trouble rising from chairs, climbing stairs, and combing hair
  • Myalgias
  • Occur more frequently in women
  • Polymyositis does not involve the skin
  • T cell-mediated muscle injury
  • Dermatomyositis involves humoral immune mechanisms and is associated with characteristic dermatologic features
  • Heliotrope rash: around the eyes, bridge of the nose, and cheeks
  • Shawl sign: rash around neck, shoulders, and upper back
  • Mechanic’s hands: hyperpigmentation and fissuring of the skin at the fingertips
  • Gottron’s sign: erythematous, scaly papules on the knuckles (extensor surfaces of the MCPs, PIPs, and DIPs)
  • Other systems affected include the joints, lungs, heart, and GI tract

Diagnosis


  • Elevated serum CPK
  • Elevated aldolase
  • Positive anti-Jo-1 antibodies
  • EMG will show short, low-amplitude movements
  • Gold standard is muscle biopsy
  • Polymyositis: endomysial involvement
  • Dermatomyositis: perivascular and perifascicular involvement

Treatment


  • High dose steroids are the first-line treatment
  • Consider methotrexate as a second-line agent
  • Refractory cases may also respond to azathioprine or intravenous immunoglobulin (IVIg)