11th August 2024

Dermatomyositis and polymyositis (DM, PM)

Dermatomyositis and polymyositis both present with symmetrical proximal muscle weakness and evidence of inflammation on muscle biopsy (fibre necrosis, degeneration and regeneration, and inflammatory cell infiltrates) which is the gold standard for diagnosis.

Dermatomyositis has additional dermatologic features which may include: Gottron’s papules (scaly purple/red knuckles), periorbital heliotrope rash, shawl-sign (an erythematous rash over the neck and chest), nailfold erythema and periungual telangiectasia, subcutaneous calcification (calcinosis cutis), and ‘mechanic’s hands’ (rough, cracked, darkened areas). Interstitial lung disease occurs in 30-40% of patients.

Both are treated with immunosuppression, usually initially with prednisolone.

Both PM and dermatomyositis may be associated with malignancy via a paraneoplastic mechanism. Cervical, lung, ovarian, pancreatic, bladder and gastric adenocarcinomas are most strongly linked.