Polymyositis: idiopathic inflammatory myopathy that causes symmetrical, proximal muscle weakness

Dermatomyositis: clinically similar to polymyositis but also has typical cutaneous manifestations

Aetiology

• Idiopathic, autoimmune
• Higher prevalence in females (2:1)
• Peak incidence age 40-50

Pathophysiology

• T cell mediated process against (unidentified) muscle antigens
• CD8+ T cells and macrophages surround, invade and destroy healthy, non-necrotic muscle fibres
• An autoimmune response to nuclear and cytoplasmic autoantigens is detected in about 60-80% of patients with polymyositis and dermatomyositis

Clinical features

Symptoms

• Symmetrical, proximal muscle weakness in the upper and lower extremities
  • Insidious onset, worsening over months
  • Often specific problems e.g. difficulty brushing hair, climbing stairs
• Myalgia in 25-50% (usually mild)

Signs

• Muscle wasting
• Confrontational testing - direct testing of power
• Isotonic testing - 30 second sit to stand

Skin involvement in dermatomyositis