Idiopathic brachial plexus lesion with conduction block of the ulnar nerve.

A 24-year-old male with idiopathic acute aching of the right wrist, weakness of ulnar innervated muscles of the forearm and hand, and numbness of the ulnar two digits, was examined electrophysiologically. Conduction studies showed almost complete conduction block of motor fibers of the ulnar nerve distally in the brachial plexus. Sensory fibers were less affected. The symptoms and pathophysiological findings improved over a period of 1-2 months and were normal at the final study performed 7 months after presentation. There was mild evidence of chronic partial denervation of the abductor digiti V and the flexor digitorum profundus of the two ulnar digits. There were additional mild EMG changes of the abductor pollicis muscle indicating minor involvement of the median nerve. There was no traumatic or other obvious cause for these findings. It is suggested that the patient had a mild and abortive form of brachial plexus neuropathy presenting as a demyelinating lesion rather than the more typical wallerian degeneration seen in most established cases.