Short-term electrophysiological conduction change in median nerve fibres after carpal tunnel release

Standard electrophysiological techniques and analysis of the stimulus-response relationship (i.e., the input-output (I-O) curves) of the median nerve were performed in 16 patients with carpal tunnel syndrome (CTS) both prior to and 1 and 6 months after surgical decompression at the wrist. One month after carpal tunnel release (CTR), conduction in cutaneous and motor axons (i.e., sensory conduction velocity and distal motor latency) was found to be improved with respect to pre-surgical values, whereas motor action potentials and the motor I-O curve showed a decrease with respect to control values. This suggested reduced efficiency of axon recruitment following CTR. Six months after surgery, all parameters were significantly improved with respect to control values. The sensory and motor I-O curves suggested that the reduced motor fibre recruitment efficiency observed 1 month after CTR was due to changes in current density distributions under the surface stimulating electrode on the median nerve at the wrist. Slight transient compression (such as that due to post-surgical oedema) acting on median fibres located superficially within the nerve cannot be excluded, however. Since electrophysiological studies are an important, objective method of evaluating the outcome of surgical CTR, electrophysiologists must be aware of the possibility of reduced compound motor action potential (CMAP) in the first few months after surgery.