Arthritis due to synovial involvement by extramedullary haematopoiesis in myelofibrosis with myeloid metaplasia

A 60-year-old man presented with polyarthralgias, a psoriasiform rash, and severe elbow pain. Peripheral blood smear and bone marrow biopsy established a diagnosis of myelofibrosis with myeloid metaplasia. Biopsy of the skin lesions revealed a nonspecific dermatitis. The clinical presentation was inconsistent with psoriatic arthritis, and there was no evidence for associated gout or collagen-vascular disease. Histological examination of tissue taken at the time of synovectomy indicated elbow arthritis to be due to myeloid metaplasia involving the synovial membrane.