Ulnar nerve conduction study of the first dorsal interosseous muscle.

Ulnar neuropathy at or distal to the wrist is difficult to diagnose. Sensation is normal in the majority of cases, and the interosseous muscles are usually more severely involved than are the hypothenar muscles. A technique for ulnar nerve conduction study of the first dorsal interosseous muscle is described, and normal values based upon 373 studies are presented. Using this technique the upper limit for distal motor latency (DML) to the first dorsal interosseous (FDI) muscle is 4.5 ms. More precisely, DML to the FDI should not exceed DML to the contralateral FDI by more than 1.3 ms, nor should this value exceed the DML to the ipsilateral abductor digiti minimi by more than 2.0 ms. The lower limit for amplitude of the compound muscle action potential recorded over FDI is 6mV. If motor conduction study of the first dorsal interosseous muscle is more routinely performed, earlier and more frequent recognition could be followed by improved surgical remediation of this compression neuropathy.