Diagnostic Utility of 2L–IO Latency Difference in Carpal Tunnel Syndrome: An Electrophysiologic Perspective

Purpose: This study aimed to evaluate the diagnostic value of the second lumbrical–interosseous distal motor latency (2L–IO distal motor latency [DML]) difference in the diagnosis of carpal tunnel syndrome, and to assess its utility across different stages of the disease. Methods: A total of 145 carpal tunnel syndrome-affected hands from 89 patients and 52 hands from 31 healthy volunteers were evaluated between January and August 2023. In addition to standard electrophysiologic tests, the DML difference between the 2L and interosseous muscles was recorded. Carpal tunnel syndrome severity was classified according to the Bland scale. The diagnostic sensitivity and specificity of the 2L–IO DML difference were analyzed using receiver operating characteristic analysis. Results: The 2L–IO DML difference was significantly prolonged in the carpal tunnel syndrome group compared with healthy controls (1.85 ± 1.41 ms vs. 0.25 ± 0.28 ms; P < 0.001). With a cutoff value of 0.45 ms, the sensitivity and specificity of the method were calculated as 98.6 and 98.1%, respectively. Although the diagnostic accuracy of conventional tests was 84%, it increased to 98% with the 2L–IO DML difference. Diagnosis was achieved in 85% of early stage (Grade 0–1) cases and in 90% of advanced cases in which the abductor pollicis brevis response was absent. Conclusions: The 2L–IO DML difference offers high diagnostic value in carpal tunnel syndrome, serving as a complementary test in early stages and a decisive tool in advanced stages. Preferentially recording from the 2L muscle instead of the abductor pollicis brevis may facilitate electrophysiologic evaluation, especially in challenging cases.