FREQUENCY-DEPENDENT CONDUCTION DELAY OF MOTOR-EVOKED POTENTIALS IN MULTIPLE-SCLEROSIS

Paired transcranial magnetic stimulation was applied in 33 multiple sc lerosis (MS) patients and in 21 healthy controls. A major abnormality was found in latency of the second motor-evoked potential in MS patien ts. At interstimulus intervals of 75, 100, and 150 ms the central moto r conduction time (CMCT) was significantly prolonged in MS patients to 139%, 150%, and 125% of the CMCT of a single magnetic stimulation (P = 0.02, P = 0.004, P = 0.03), respectively. Voluntary contraction of t he target muscle abolished the difference in latency independent of th e degree of contraction. Stimulation intensity influenced the length o f the interstimulus interval during which the maximal conduction delay was obtained. In MS patients there was no correlation between prolong ed CMCT to a single magnetic stimulus and the frequency-dependent cond uction delay to paired magnetic stimuli, It is hypothesized that the c onduction delay of the conditioned response of paired magnetic stimuli in MS is of cortical origin and induced by abnormalities the ascendin g volley to the neocortex. (C) 1997 John Wiley & Sons, Inc.