Carpal tunnel syndrome, a median neuropathy at the wrist is the most common entrapment neuropathy. There are many methods to diagnose carpal tunnel syndrome, but each study has some limitations.

In carpal tunnel syndrome, the median motor distal latency is normal from 35% to 50% of patients. This lack of sensitivity of distal motor latency may be due either to spairing of motor as compaired to sensory fibers or to inability of standard median motor studies to detect abnormality.

We performed the conventional nerve conduction studies 50 controls and in 20 patients with clinically suspected carpal tunnel syndrome and compared with Preston and Logigian's method that used the same active electrode lateral to the third metacarpal bone, to record the second lumbrical or the deeper interossei responses by stimulating at median and ulnar nerves, respectively.

In control group, the mean distal motor latency of median nerve on lumbrical recording was 3.09±0.21 msec, and that of ulnar on interossei recording was 3.00±0.21 msec and the mean lumbrical-interossei difference was 0.10±0.15 msec. In carpal tunnel syndrome group, the mean distal motor latency of median nerve on lumbrical recording was 4.11±1.24 msec, and that of ulnar on interossei recording was 3.08±0.42 msec and the mean lumbrical-interossei difference was 0.91±0.70 msec, The abnormal lumbrical-interossei latency difference was set at greater than 0.4 msec as the mean±2SD. there was no significant correlation between age and lumbrical-interossei latency difference (r=-0.09). the sensitivity of lumbrical-interossei latency difference was 90.9%.

The lumbrical-interossei latency difference is a more sensitive test than conventional median motor nerve conduction study recording at abductor pollicis brevis in patient with carpal tunnel syndrome.