The value of nerve conduction studies in the assessment of carpal tunnel syndrome (CTS) has been established. There are many criterias for early diagnosis, but most of them use sensory abnormalities of median nerve. There is some limitation to diagnose the carpal tunnel syndrome in the cases of polyneuropathy such as diabetic polyneuropathy.

This study was done to assess the diagnostic value of median-ulnar motor difference recorded on lumbrical and interosseous in carpal tunnel syndrome, especially in the cases associated with polyneuropathy. And we also compared the sensitivity and the specificity of the new cirteria with those of other diagnostic criterias.

Fifty hands with clinical symptoms and signs suggestive of CTS and forty healthy volunteer's hands as control were studied.

In control group, the mean distal motor latency of median nerve on lumbrical recording was 2.67±0.29 msec, and that of ulnar on interosseous recording was 2.66±0.30 msec and the mean lumbrical-interosseous difference was 0.09±0.11 msec. The mean amplitude of the interosseous was higher as 7.05±2.12 mV than that of the lumbrical as 3.18±1.53 mV (p<0.01). In CTS group, the mean value of median motor distal latency on lumbrical recording was 4.30±1.67 msec, and that of ulnar on interosseous was 2.19±0.41 msec. The mean lumbrical-interosseous latency difference was 1.50±1.24 msec, which was statistically different from control group (p<0.01). The amplitude on lumbrical and interosseous recording were 1.76±0.91 mV, 5.97±1.56 mV, respectively. The abnormal lumbrical-interosseous difference criteria was set at greater than 0.42 msec as the mean±3 S.D. of lumbrical-interosseous latency difference. The sensitivity and specificity of the lumbrical-interosseous difference were 84%, 100%, respectively, and were comparable to those of other previously used sensory critierias.

The lumbrical-interosseous latency difference is a new and sensitive measurement in the eletrophysiologic diagnosis of carpal tunnel syndrome and a helpful criteria in patients with coexistent polyneuropathy.