Inflammatory Myopathies

Inflammatory Myopathies – Greg Jackson

Background

  • Inflammation and damage of skeletal muscle primarily leading to weakness
  • Subtypes: Dermatomyositis, polymyositis, statin-induced myositis, inclusion body myositis, metabolic (hypothyroid, electrolyte) myositis, viral/infection myositis, diabetic myonecrosis
  • Important point: this differs from PMR in that myopathy presentation is usually painless weakness whereas PMR will have myalgias with preserved strength
  • Hypomyopathic (or amyopathic) variants: skin and/or lungs are affected >> muscles

 

Presentation

  • Proximal, symmetric muscle weakness in upper and lower extremities in all but inclusion body myositis which is typically more distal and asymmetric
  • Dermatomyositis: skin involvement can include heliotrope rash of upper eyelids, Gottron’s papules (on MCP, PIP), “mechanic’s hands,” and shawl sign
  • Raynaud’s and arthralgias can also be present

 

Evaluation

  • BMP & TSH ; CK level
  • Anti Jo-1 most common specific antibody
  • EMG: findings indicate myopathy
  • MRI extremity/affected muscle group: muscle edema
  • Skin biopsy in dermatomyositis: “interface dermatitis.”
  • Muscle biopsy (do not do biopsy in same muscle as EMG done)
    • Polymyositis, inclusion body myositis: T cell mediated muscle injury
    • Dermatomyositis: immune complex/B cell mediated muscle injury


Management

  • Prednisone 1 mg/kg/day for 4-6 weeks with subsequent taper
  • Consult Rheumatology
  • MTX or AZA used if refractory to steroids, may need IVIG, rituximab
  • If statin induced: stop statin or switch to lower intensity
  • If dermatomyositis: evaluate for underlying malignancy