First lumbrical latency and amplitude. Control values and findings in carpal tunnel syndrome.

It has been suggested that the nerve fibers to the first and second lumbrical muscles are relatively spared in patients with carpal tunnel syndrome compared with the fibers to the abductor pollicis brevis. Latencies and amplitudes of the first lumbrical and the abductor pollicis brevis (APB) muscles were compared with both wrist stimulation and midpalmar stimulation. The lumbrical latency at 12 cm and the APB at 8 cm were found to be similar at less than 4.3 ms. Amplitudes to the lumbrical were 0.6-8 mV compared with 4-13.6 mV for the APB. Three patients who had an abnormal sensory study, yet normal motor values to the APB, were found to have abnormal lumbrical latencies. This is an additional technique that could be useful for individuals suspected of carpal tunnel syndrome who have normal latencies and amplitudes to the APB.