IMPROVEMENT OF AMPLITUDE VARIABILITY OF MOTOR EVOKED-POTENTIALS IN MULTIPLE-SCLEROSIS PATIENTS AND IN HEALTHY-SUBJECTS

Variability of the amplitude of motor evoked potentials was studied in 33 multiple sclerosis (MS) patients and 21 healthy subjects. Normal p robability plots revealed skewness of the distribution of amplitudes o btained from the abductor pollicis brevis (APE) muscle during muscle r elaxation, muscle contraction, and by paired magnetic stimulation. Nat ural logarithmic transformation of amplitude data resulted in normal d istribution. Negative first-order autocorrelations were established fo r consecutive recordings independent of the repetition rate tested (5 s, 8 s, 12 s, and 20 s). A given measurement had a tendency to be syst ematically followed by a measurement of a lower value, indicating that magnetic stimulations induce a long-lasting inhibitory effect on the excitability of the motor pathways. A significant decrease in amplitud e variability was achieved by a controlled muscle contraction of the t arget muscle compared with an uncontrolled muscle contraction in MS pa tients. Paired magnetic stimulation halved the coefficient of variatio n of amplitude data compared with amplitude obtained during muscle rel axation, and is introduced to reduce amplitude variability in conditio ns where no collaboration is possible.