Selective activation of small nerve fibers for assessing carpal tunnel syndrome.

Carpal tunnel syndrome (CTS) diagnosis could be improved by selectively activating the different types of nerve fibers traversing the carpal tunnel. The objective of this study is to assess the potential of an anodal blocking technique using tripolar surface electrodes for achieving selective fiber activation and evaluating the severity of CTS. Simulations were performed using McNeal's model to determine the stimulating and blocking thresholds for different diameter/conduction velocity groups of nerve fibers. At 9.36 mA stimulus amplitude all nerve fibers from 9 mum to 20 mum were activated. When the current amplitude was increased further, large nerve fibers started getting blocked while small nerve fibers remained active. By gradually increasing the current amplitude small nerve fibers can be selectively activated without the activation of large fibers. The severity of CTS generally progresses from large to small nerve fibers, hence by comparing normal values of amplitude and peak latency of different nerve fiber diameter groups to the CTS affected median nerve recordings of the patient, severity of this syndrome could be detected.