Electrodiagnosis of retrograde changes in carpal tunnel syndrome.

We measured evoked mixed nerve action potentials (EMNAP) in the forearm segment of 108 hands as an indicator of retrograde degeneration in the carpal tunnel syndrome (CTS). This was made by recording at the elbow and stimulating the median nerve 4 cm proximal to the wrist crease. The amplitude of this EMNAP decreased in proportion to the prolongation of the motor terminal latency (MTL) or the decrease in the amplitude of the compound muscle action potential (CMAP) from the abductor pollicis brevis (APB) muscle. This suggests that retrograde degeneration progresses as the severity of CTS increases. This decrease was also in proportion to the decrease in the motor nerve conduction velocity (MNCV) in the forearm. Both MNCV in the forearm and EMNAP can indicate retrograde degeneration in CTS, but the MNCV cannot be measured when muscle atrophy is severe and the CMAP cannot be elicited. Therefore, we believe that EMNAP is a more useful measure of the retrograde changes in CTS.