Comparison of SNAP and CMAP before and after carpal tunnel release: a clinical study

ObjectiveChanges of SNAP (sensory nerve action potential) and CMAP (compound muscle action potential) following carpal tunnel release and median nerve decompression were investigated to evaluate the sensitivity of these electrophysiologic parameters and their clinical significance.Methods20 patients with carpal tunnel syndrome were involved in the study. Electrophysiologic examination was carried out to record the following parameters: CMAP of abductor pollicis brevis muscle, SNAP of the median nerve at the wrist when the index and middle fingers were stimulated, and SNAP of the median nerve and ulnar nerve at the wrist when the ring finger was stimulated. These parameters were recorded before the operation (pre op) and 10 minutes following carpal tunnel release and median nerve decompression (immediately post op). Student t test was applied to compare the pre op and post op changes of these parameters.ResultsThere were no statistically significant differences between the latent periods and amplitudes of abductor pollicis brevis CMAP recorded pre operatively and those recorded immediately post operatively. There were, however, significant changes in SNAP recorded by stimulating the index, middle and ring fingers. The latent periods of SNAP were shortened by 5% on average after carpal tunnel release, while the amplitudes were increased by 13%. The differences between the pre operative levels and post operative levels were of statistic significance.ConclusionComparing to CMAP, SNAP is a more sensitive and accurate parameter in evaluating the effectiveness of carpal tunnel release and median nerve decompression.