I. Definitions, pathophysiology, and epidemiology
  1. An idiopathic inflammatory condition of skin and muscles
  2. Involves cell-mediated immunity with involvement of  B and CD4-T lymphocytes, macrophages, plasma cells, etc.
  3. Similar to polymyositis but with a rash
  4. "Amyotrophic dermatomyositis" = a variant involving rash without muscle involvement, though most patients eventually progress to full-blown dermatomyositis with muscle involvement
  5. Affects women more than men
  6. Peak incidence is in childhood and age 50-70

II. Clinical features
  1. Gradual-onset of progressive weakness in proximal muscles with symmetrical distribution
  2. Skin involvement-Various patterns are seen:
    1. Gottron's papules
      1. Erythematous papules, macules, or placques over distal interphalangeal joints, elbows, and/or knees
      2. Pathognomonic
      3. Found in 70-80% of patients
    2. Heliotrope rash
      1. Purplish rash around eyes, sometimes with periorbital edema
    3. "Shawl sign"
      1. aka "V-sign"
      2. Erythematous macules on neck and shoulder in a "shawl" distribution
    4. Periungual changes: Erythema, telangiectasias of proximal nail folds, and thickened, irregular, and/or hyperkeratotic cuticles
    5. "Mechanic's hand"-Fissured, hyperpigmented, scaly hands that suggest prolonged manual labor
  3. Other manifestations
    1. Arthralgias or arthritis
    2. Interstitial lung disease (occurs in up to 50% of patients)
    3. Esophageal involvement with dysphagia (occurs in upt o 50% of patients)
    4. Cardiomyopathy and conduction defects (rare)
  4. Association with occult malignancy
    1. Most commonly ovarian, gastric, lung, and prostate; also lymphoma, Kaposi's sarcoma, and melanoma
    2. HIghest if dermatomyositis diagnosed after age 45
III. Diagnosis
  1. No single universally-accepted set of diagnostic criteria as of 2012
  2. In some cases, clinical diagnosis is made based on triad of typical rash + proximal muscle weakness + elevated serum CPK
  3. EMG/nerve conduction testing-Can help rule out neuropathy as cause for weakness
  4. Muscle biopsy can establish the presence of myositis (should be from an area of weakness)
  5. Skin biopsy can distinguish dermatomyositis rash from rashes from other inflammatory diseases eg. lupus, but is not 100% specific for dermatomyositis
  6. Lab abnormalities that may be seen:
    1. Positive ANA
    2. Elevated rheumatoid factor
    3. Elevated ESR
    4. Elevated "muscle enzymes" e.g. CPK, LDH
    5. Myositis-specific autoantibodies-Eight different ones have been identified as of 2012 (Anti-Jo-1 is one; others include anti-P155 and anti-P140; Seen in up to 80% of patients with dermatomyositis or polymyositis; Not in common usage as of 2012)
IV. Management
  1. Systemic corticosteroids are the primary therapy (e.g. prednisone starting at 1mg/kg/d up to 80mg/d); assess response by muscle strength testing and taper to lowest effective dose after 4-6wks
  2. Other immunosuppressants are 2nd-line (methotrexate, azathioprine) though are sometimes used initially along wiht corticosteroids to minimize needed corticosteroid dose
  3. Hydroxychloroquine can help with skin menifestations but not myositis
  4. Other treatments used: rituximab, intravenous immunoglobulin, cyclosporine, tacrolimus, mycophenolate, cyclophosphamide, and tumor necrosis factor inhibitors
  5. Physical therapy can help address muscle weakness
  6. Careful control of sun exposure since rash often exhibits photosensitivity
  7. Rash may respond to topical corticosteroids

(Sources include Core Content Review of Family Medicine, 2012)