Efficacy of Optimal Recording Electrode Placement for Median-Lumbrical and Ulnar-Interossei/Lumbrical Distal Latency in the Diagnosis of Carpal Tunnel Syndrome

PURPOSE This study is to determine the diagnostic utility of optimal recording electrode placement for distal latency comparison of median-second lumbrical and ulnar-interossei/third lumbrical (M2L-UI3L) in carpal tunnel syndrome. METHODS Sixty-five hands of control and 75 hands of 62 clinically suspected carpal tunnel syndromes were used for the M2L-UI3L and standard conduction studies. To obtain optimal M2L-UI3L, the recording active electrode (E1) was placed at the midpalm over the third metacarpal bone, whereas the reference electrode (E2) was attached to the palmar digital crease area. Then, median and ulnar nerves were stimulated on the wrist each at 8 cm proximal to E1. M2L-UI3L and standard nerve conduction studies were performed. Sensitivity and specificity of M2L-UI3L were measured in the diagnosis of mild carpal tunnel syndrome. RESULTS For statistical analysis, the receiver operating characteristics and Student t-test were used. The area under the receiver operating characteristic curve of M2L-UI3L was 0.993. Diagnostic cutoff value of M2L-UI3L greater than 0.6 milliseconds yields sensitivity of 93% and specificity of 97%. The distal median motor latency to the second lumbrical alone showed the area under the curve of 0.998, and the diagnostic cutoff value greater than 3.4 milliseconds yields sensitivity of 96% and specificity of 100%. CONCLUSIONS This technique for M2L-UI3L shows high sensitivity and specificity compared with the previous reports on the diagnosis of carpal tunnel syndrome. Furthermore, the values of median-second lumbrical motor latency alone have higher sensitivity and specificity, comparable with the median sensory conduction study across the wrist segment.