Metastatic melanoma presenting with paraneoplastic dermatomyositis mimicking polymyalgia rheumatica

A man in his late 70s presented with 6-week bilateral shoulder and hip girdle pain, morning stiffness and significant weight loss. Although polymyalgia rheumatica was initially suspected, red flag weight loss prompted referral through the 2 week-wait cancer pathway with parallel laboratory and imaging assessment.

Markedly elevated creatine kinase (>5000 U/L), proximal weakness and subtle cutaneous features raised suspicion of dermatomyositis. A CT scan then demonstrated extensive intra-abdominal and right inguinal lymphadenopathy, initially suggesting lymphoma and prompting haematology referral. Ultrasound-guided lymph node biopsy unexpectedly confirmed metastatic melanoma.

Subsequent skin and nail examination identified a pigmented right great toe lesion, previously attributed to trauma, consistent with the primary acral melanoma. The overall picture was paraneoplastic dermatomyositis presenting as a polymyalgia-like syndrome. Corticosteroids and B-Raf proto-oncogene, serine/threonine kinase (BRAF)/mitogen-activated protein kinase (MEK) targeted therapy were followed by resolution of muscle symptoms and radiological response.