[The anterior interosseous nerve syndrome. A rare lesion of a median nerve branch in the forearm. A case].

Apparently spontaneous and isolated lesions of the anterior interosseous nerve--a purely motor branch of the median nerve in the forearm--are extremely rare. Their clinical manifestations are fairly stereotyped, consisting of paralysis restricted to the long flexor muscle of the thumb, the deep flexor muscle of the index finger and the quadrate pronator muscle, without sensory disorders. The diagnosis is confirmed by electromyography. The syndrome may caused by traumas of the forearm or may develop spontaneously, although underlying anatomical abnormalities which may act as predisposing factors are not infrequently found in this case. The natural outcome may be favourable, but surgery may be required to release the nerve.