Comparison of second lumbrical and interosseus latencies with standard measures of median nerve function across the carnal tunnel: a prospective study of 450 hands

The second lumbrical-interosseus distal motor latency (2LI-DML) was compare d prospectively in 450 hands. Median nerve function was assessed by standar d motor and sensory electrophysiological tests. In a control group of 100 h ands the upper limit of normal for the 2LI-DML was 0.5 ms. In all hands stu died the correlation coefficients of 2LI-DML were higher with sensory nerve tests than with motor studies. Carpel tunnel syndrome (CTS) was diagnosed clinically in 276 hands, and 174 showed no clinical signs of CTS. The 2LI-D ML was prolonged in 269 of the 276 hands, with clinical signs of CTS and no rmal in 170 of 174 non-CTS hands. Thus the 2LI-DML resulted in a sensitivit y of 97.5%. On the other hand, combining the standard tests yielded a sensi tivity of 98.5%. In 31 of 36 additional hands a lumbrical response was reco rded, although motor and sensory responses form standard median nerve condu ction studies were absent, and the 2LI-DML was substantially prolonged. The 2LI-DML therefore represents a highly sensitive, fast, easy-to-perform, an d cost-efficient method to study median nerve function across the wrist and may help to localize the lesion in cases in which standard electrophysiolo gical methods fail.