Anomalous muscles of the upper extremity are common, but symptomatic anomalies are rare, usually masquerading as soft tissue tumors, ganglia, or peripheral neuropathy of undertermined etiology. Tingling, numbness, and weakness may arise from nerve compression, or may be produced by the muscle itself. EDBM (Extensor digitorum brevis manus) is a small accessory extensor of the fingers and is found on the dorsum of the hand in 1∼3% of the population, and occasionally it is present bilaterally. It arises form the dorsal carpus and may have tendons to the index, middle, ring, and rarely to the fifth fingers-but nerve to the thumb.

The evolutionary origin of the EDBM is uncertain. Some authors consider it to be derived from the dorsal interosseous musculature. Others believe that it is an atavistic muscle, which may be homologous with the extensor brevis in the foot, produced by delamination of forearm extensors. Electrophysiologic documentation of the innervation of the EDBM by the posterior interosseous nerve favors the latter suggestion.

Anomalous hand muscles consistent with EDBM muscles were noted in a 31 year-old man. Well-defined CMAP (compound muscle action potentials) were ellicited in these muscles with stimulation of the posterior interosseous nerve. Needle electromyography showed poor recruitment and large motor unit potentials only in the anomalous muscles.

We report a case of EDBM with electrophysiologic documentation that this accessory muscle is innervated by the posterior interosseous nerve.