Nonuniform Motor Dysfunction in Carpal Tunnel Syndrome: Implications From APB and Second Lumbrical CMAPs

OBJECTIVE: In severe carpal tunnel syndrome, compound muscle action potentials (CMAPs) from the abductor pollicis brevis (APB) are often unrecordable, limiting motor nerve assessment. We investigated whether second lumbrical (2L) CMAP reflects motor dysfunction similarly to APB CMAP. METHODS: We retrospectively analyzed 123 patients with carpal tunnel syndrome. Compound muscle action potential amplitudes of APB and 2L were recorded following wrist stimulation. Associations with electrophysiological severity (Bland classification) were evaluated. In surgical cases (n = 47), preoperative and postoperative changes were assessed. Recordability was analyzed on a hand-by-hand basis. Multivariate logistic regression was performed to identify factors associated with APB CMAP absence. RESULTS: 2L CMAP amplitude decreased with increasing electrophysiological severity. Both APB and 2L CMAP amplitudes improved postoperatively, but improvement was significantly greater in APB (P < 0.001). Among 18 hands with absent APB CMAP, 2L CMAP was recordable in 17 (94.4%). Recordability was significantly higher for 2L than APB (99.4% vs. 88.7%, P < 0.001). In multivariate analysis, log-transformed 2L CMAP amplitude was independently associated with APB CMAP absence (odds ratio 0.086, 95% confidence interval 0.028-0.259, P < 0.001). CONCLUSIONS: Motor dysfunction in carpal tunnel syndrome may not be uniformly distributed across distal median nerve branches. Abductor pollicis brevis and 2L CMAPs may reflect different functional aspects of motor impairment. SIGNIFICANCE: These findings provide additional insight into motor nerve conduction studies and may improve diagnostic evaluation, particularly in severe carpal tunnel syndrome where conventional motor assessment is limited.