Focal myopathy mimicking posterior interosseous nerve syndrome.

A 25-year-old man developed weakness of extension of the right index, middle, and fourth fingers at the metacarpophalangeal joints, over 2 years. No sensory deficit was present. Nerve conduction studies, including the right radial nerve, were within normal limits. Needle electromyographic (EMG) examination showed myopathic changes that were limited to the right extensor digitorum communis and extensor indicis proprius muscles. An intravenous edrophonium chloride test had no effect on weakness and repetitive stimulation showed no significant decremental response. An EMG-guided open biopsy of the extensor digitorum communis muscle revealed severe myopathic changes. Evaluation for the cause of myopathic involvement was negative. After 13 months, clinical examination and electrophysiological studies showed no significant progression. This case exemplifies the fact that a focal myopathy may mimic an entrapment neuropathy.