Proximal slowing in carpal tunnel syndrome resulting from either conduction block or retrograde degeneration.

In the carpal tunnel syndrome (CTS), decreased conduction velocity (CV) of the median nerve in the forearm segment has been ascribed to an electrodiagnostic artefact rather than pathophysiological changes. Standard CV of the forearm segment is calculated by subtracting the distal latency, which may not represent an exact assessment of CV in the proximal median nerve. A new technique modified from the method of Stoehr et al. and Pease et al. can exactly measure CV over the forearm. Using this new technique, the forearm nerve action potentials (FNAP) amplitude and forearm nerve conduction velocity (FNCV) proximal to the wrist can be directly determined. Normal subjects and patients with CTS were studied by both the standard and the new FNAP methods. Patients were divided into subgroups according to the severity derived from standard electro-diagnostic findings. By comparing the normal control and patient subgroups, the results show that there was a significant decrease in FNAP amplitudes proportional to severity, but FNCV was reduced to a lesser extent. In addition, the standard forearm median motor CV (MMCV) correlated well with severity, but the reduced MMCV did not correlate with the decreased FNCV. These findings suggest that retrograde degeneration of the median nerve does exist in CTS; however, retrograde degeneration contributes little to the reduced forearm MMCV which substantially results from the block of faster conduction fibres at the wrist. Therefore, technique artefact plays a major role in causing the proximal slowing in the standard electrodiagnosis.