Pii: S1388-2457(00)00494-6

Objective: We studied sequential waveform changes associated with a progressive conduction block to elucidate the relative vulnerability of slow versus fast conducting ®bers to a focal compression. Methods: In 12 healthy men, orthodromic compound sensory nerve action potentials (SNAPs) of the median nerve were recorded unipolarly at 4 sites over the forearm during a 30 min period of constant, localized compression of the nerve at the wrist. Results: Initial ®ndings at the compression site consisted of nearly immediate reduction in size of the negative component accompanied by progressive enlargement of the initial-positive component. Recording at 2 cm proximal to the compression and at the elbow showed no change in onset latency initially, indicating at least partial preservation of the fast conducting ®bers. Amplitude also remained unchanged for about 20 min, presumably because loss of negative and positive peaks compensated each other until conduction block began to involve a greater number of the fast conducting ®bers. Conclusions: The analysis of waveform changes and their time course suggests that a focal compression initially affects the slow conducting small diameter ®bers. Partial conduction block gives rise to complex waveform changes depending on recording sites. A reduction in one polarity of constituent nerve ®ber action potentials may enhance the other polarity of the SNAP. q 2001 Elsevier Science Ireland Ltd. All rights reserved.