Isolated Focal Myositis of the Flexor Pollicis Longus Muscle: A Case Report.

Focal myositis is an uncommon inflammatory myopathy that often mimics other pathologies such as soft tissue tumors, infections, or compartment syndromes. Upper limb involvement, especially limited to a single muscle like the flexor pollicis longus (FPL), is rare and diagnostically challenging. We report a case of a 23-year-old woman who presented with progressive atraumatic swelling of the volar forearm. Ultrasound revealed hypertrophy and heterogeneity of the FPL muscle with intermuscular edema. Laboratory markers were within normal limits. A provisional diagnosis of focal myositis was made. The patient gave a favorable response to conservative treatment with non-steroidal anti-inflammatory drugs (NSAIDs) and wrist splinting, along with activity modification. This case underscores the importance of clinical suspicion and radiologic correlation in avoiding misdiagnosis and unnecessary invasive intervention.