The usefulness of terminal latency index of median nerve and f-wave difference between median and ulnar nerves in assessing the severity of carpal tunnel syndrome.

The calculated electrophysiological parameters, such as terminal latency index (TLI), residual latency, modified F ratio, and F-wave inversion, have been investigated as a diagnostic tool for detection of early stage of carpal tunnel syndrome (CTS) in the literature. However, the correlation of these calculated electrophysiological parameters with the clinical severity of CTS has not been reported. The aim of this study was to determine the correlation of the calculated electrophysiological parameters and clinical severity in patients with CTS. A retrospective study was performed with 212 hands of 106 CTS patients. The CTS hands were classified as asymptomatic, mild, moderate, and severe according to the clinical severity. The distal motor latency and distal motor conduction velocity of median nerve, minimal F-wave latency of median and ulnar nerves, and sensory nerve conduction velocity in the finger-wrist and palm-wrist segment of median nerve (SNCV f-w and SNCV p-w) were obtained in a conventional nerve conduction study. The TLI, residual latency, and modified F ratio of the median nerve and the difference of minimal F-wave latencies between the median and ulnar nerves (F-diff M-U) were calculated. The distal motor latency, residual latency, and F-diff M-U were significantly increased according to the clinical severity of CTS. The motor conduction velocity, SNCV p-w, SNCV f-w, TLI, and modified F ratio were significantly decreased according to the clinical severity of CTS. In analyses of variance and Kruskal-Wallis test, we used the Scheffe test as a post-hoc comparison analysis. The TLI, F-diff M-U, and SNCV f-w showed a significant difference among all groups of each CTS severity. The sensitivity, specificity, and cut-off value of TLI, F-diff M-U, and SNCV f-w between asymptomatic and mild, mild and moderate, and moderate and severe CTS groups were calculated by using receiver operating characteristic curve analysis. The cut-off values of TLI, F-diff M-U, and SNCV f-w between the asymptomatic and mild CTS groups were, respectively, 0.33 millisecond, 0.3 millisecond, and 40 cm/second. The cut-off values of TLI, F-diff M-U, and SNCV f-w between mild and moderate were, respectively, 0.27 millisecond, 2.3 milliseconds, and 34.8 cm/second. The cut-off values of TLI, F-diff M-U, and SNCV f-w between moderate and severe CTS groups were, respectively, 0.20 millisecond, 4.2 milliseconds, and 26.4 cm/second. We found that calculated electrophysiological parameters of conventional nerve conduction study could be a good indicator to determine the severity of CTS.