Infectious myositis involving the piriformis in a patient with rheumatoid arthritis.

A 49-year-old Japanese woman treated with oral corticosteroids, methotrexate, and infliximab for malignant rheumatoid arthritis was admitted because of fever and low back pain. The white blood cell count and C-reactive protein concentration were elevated. Lumbar and pelvic computed tomography showed enlargement of the piriformis muscle including a hypodense area consistent with gas formation. The patient was treated successfully for infectious myositis with intravenous antibiotics.