Entrapment neuropathy of the ulnar nerve at its point of exit from the flexor carpi ulnaris muscle.

The ulnar nerve normally enters the flexor carpi ulnaris (FCU) proximally and anteriorly between the humeral and ulnar heads of the muscle. After an intramuscular course of several centimeters, the nerve exits the FCU distally to lie in a tissue plane between the FCU and the flexor digitorum profundus (FDP). A patient with ulnar neuropathy studied by intraoperative electroneurography demonstrated major focal conduction block at the point where the nerve exited the FCU. A fivefold increase in amplitude and reversal of a dispersed, irregular compound muscle action potential to a more normal configuration occurred with stimulation just distal to the point of exit. There was no evidence by inspection, probing, or electroneurography of compression in the retrocondylar groove or at the cubital tunnel. In a series of 100 cadaver dissections, the intermuscular septum between FCU and FDP was thick and tough in several specimens. This septum may represent a site of ulnar nerve entrapment.