Partial median nerve entrapment in the distal arm because of an accessory bicipital aponeurosis.

We present a newly described entrapment of the median nerve caused by compression in the distal arm because of an accessory bicipital aponeurosis. It is characterized by the paresis or paralysis of muscles innervated by the anterior interosseous branch of the median nerve--the flexor pollicis longus, the flexor digitorum profundus, and the pronator quadratus--as well as other more proximal median nerve innervated muscles, namely, the pronator teres and flexor carpi radialis. Sensibility is intact. The site of the Tinel's sign in the distal arm and the clinical appearance of an accessory bicipital aponeurosis help to localize the lesion. Electrodiagnostic studies are also important in establishing the site of the entrapment. The clinical and surgical findings are correlated with the internal topography of the median nerve at its site of compression. It is important to differentiate this syndrome from the classic anterior interosseous syndrome and other nerve entrapments at the elbow and arm. Surgical exploration is indicated if there is no clinical or electromyographic improvement in three to four months after the onset of symptoms.