Changes in excitability and impulse transmission following prolonged repetitive activity in normal subjects and patients with a focal nerve lesion

Abstract

The present study was undertaken to document the excitability changes produced by prolonged high-frequency trains of impulses and to determine whether these changes in excitability would impair neural transmission in cutaneous afferents of patients with focal slowing of conduction across the carpal tunnel. A submaximal test stimulus was used to measure the changes in axonal excitability following trains of supermaximal stimuli delivered at 200 Hz for 30 s, 1 min of 2 min. These trains produced a prolonged depression in excitability in normal axons with gradual recovery to control levels over 20-30 min, presumably due to hyperpolarization associated with activation of the electrogenic Na+/K+ pump. The decrease in excitability was demonstrated at nerve segments remote from the site of tetanic stimulation. Based on these findings, the effect on neural transmission were then assessed in normal subjects there was a small activity-dependent decrease in amplitude of the compound sensory action potential (CSAP), associated with a prolongation in its latency In patients with focal slowing of conduction across the carpal tunnel there was a more marked post-tetanic prolongation in latency, but the reduction in amplitude of the maximal CSAP was no greater than in the control subjects. It is concluded that activity-dependant conduction block is not a major cause of symptoms in carpal tunnel syndrome. It is suggested that the conduction slowing seen in patients with mild-moderate carpal tunnel could result from mechanisms other than demyelination.