🔑 Key Learning

  • Dermatomyositis = Inflammatory myopathy + skin rash (photosensitive).
    • Heliotrope rash, Gottron's papules
    • Antibodies: Anti-Mi-2
  • Polymyositis = Inflammatory myopathy without skin involvement.
    • Antibodies:  Anti-Jo-1
  • Dermatomyositis is a paraneoplastic syndrome in ~25%: associated with ovarian, breast, lung cancers. Consider screening for cancer. 

🧬 Pathophysiology

  • Autoimmune inflammation of striated muscle fibres ± skin.
  • CD8+ T-cell-mediated muscle fibre damage in polymyositis.
  • Complement-mediated microangiopathy in dermatomyositis, particularly affecting capillaries in skin and muscle.

👀 Clinical Features

Dermatomyositis

🧑‍🦰 Skin

  • Heliotrope rash: violaceous discolouration of eyelids
  • Gottron’s papules: red/purple papules over knuckles, PIP, DIP
  • Gottron’s sign: erythematous rash over extensor surfaces (e.g. knees, elbows)
  • Photosensitive red macular rash over shoulders and back (‘shawl sign’)
  • Mechanic’s hands: dry, cracked skin on hands

💪 Muscle

  • Symmetrical proximal muscle weakness
    • Exam MCQs may describe difficulty climbing stairs, rising from chair, lifting objects, brushing hair

⚠️ Systemic

  • Weight loss, fatigue, night sweats
  • ILD: dry cough, SOB
  • Raynaud’s phenomenon
  • Malignancy risk: screen for ovarian, breast, lung, GI cancers
Figure 223: Heliotrope rash in dermatomyositis; violaceous to erythematous discrete or confluent macules confined to the upper eyelids. Elizabeth M. Dugan, Adam M. Huber, Frederick W. Miller, Lisa G. Rider, CC BY-SA 3.0.
Figure 224: Gottron's papules in a patient with juvenile dermatomyositis. Elizabeth M. Dugan, Adam M. Huber, Frederick W. Miller, Lisa G. Rider, CC BY-SA 3.0
Figure 224: Gottron's sign. Confluent macular erythema with scale confined to the skin overlying the patellae in a girl with juvenile dermatomyositis. Elizabeth M. Dugan, Adam M. Huber, Frederick W. Miller, Lisa G. Rider, CC BY-SA 3.0.

    

Polymyositis

  • Similar proximal muscle weakness as dermatomyositis, but no skin features
  • Can occur in association with connective tissue diseases (SLE, systemic sclerosis)

🧪 Investigations

Bloods

  • Raised CK and other muscle enzymes (LDH, ALT/AST)
  • Consider muscle biopsy
  • ANA positive (~80%)
  • Autoantibodies:
    • Anti-Mi-2 → Dermatomyositis
    • Anti-Jo-1 → Polymyositis 

💊 Management

  • 1st Line: High-dose corticosteroids (e.g. prednisolone)
  • Immunosuppressants: Methotrexate or azathioprine if inadequate response
  • Screen for malignancy in dermatomyositis (especially if age > 40 or rapid onset)

📝 Exam Clues & Clinchers

  • Heliotrope rash + proximal weakness + Shawl sign = dermatomyositis
  • Elevated CK + anti-Jo-1 + ILD = polymyositis
  • Associated cancers: ovarian, breast, lung, GI
  • Gottron’s papules = red, scaly bumps over knuckles
  • Anti-Mi-2 = Dermatomyositis
  • Anti-Jo-1 = Polymyositis