The Usefulness of Motor Conduction Studies at Abductor Pollicis Brevis and Lumbricalis in the Diagnosis of Carpal Tunnel Syndrome

Abstract

To find out the sensitivity of motor conduction tests and the change of distal latencies, we examined 60 hands of 30 patients referred with clinically diagnosed carpal tunnel syndrome (CTS) and 30 hands of 15 healthy women. Such tests included abductor pollicis brevis distal latency (APB DL), lumbricalis distal latency (LUMB DL), interosseous distal latency (INT DL), lumbricalis-interosseous distal latency difference (LUMB-INT), as well as the difference between APB DL and LUMB DL (APB DL-LUMB DL). The sensitivity of the tests were calculated in accordance with published recommendations (American Association of Electrodiagnostic Medicine 1993). The sensitivities of APB DL, LUMB DL, the combination of the two tests were respectively 88.4, 69.8, 100%. The combination of APB DL and LUMB DL seemed to be useful for diagnosis of mild CTS as well as sensory distal latency of the ring finger. The APB DL-LUMB DL was much prolonged in clinical CTS patients with the APB DL over 6.0ms. The APB DL tended to be more delayed than the LUMB DL, because median motor fibers innervating thenar muscles are more vulnerable to compression than those innervating lumbricalis. It is recommended to record the APB DL and LUMB DL and to calculate the APB DL-LUMB DL in order to diagnose as mild CTS and evaluate the pathology of CTS.