Changes in motor evoked potentials to short-interval paired transcranial magnetic stimuli in multiple sclerosis.
OBJECTIVE
Paired transcranial magnetic stimuli (TMS) were applied in 8 multiple sclerosis (MS) patients with asymmetrical clinical signs and in 8 healthy controls to test the hypothesis that the circuits responsible for the generation and transmission of I-waves are abnormal in the former group


METHODS
A figure-of-8 coil discharging through a Magstim 200/Bistim configuration delivered identical stimuli at an intensity 10% above the motor threshold of the relaxed first dorsal interosseous muscle. The interstimulus intervals (ISIs) used were varied in a pseudo-randomized fashion in steps of 0.2 ms between 1.0 and 5 ms.


RESULTS
In 9 of 12 unilateral studies in the control group, a pattern of 3 peaks of increased motor evoked potential size was found at ISIs of 1.2-1.6 ms, 2.4-3.2 ms and 4.4-5.0 ms. A similar pattern was present in only 5 of 12 studies in the patients (Fisher's exact test, P = 0.1), while it was absent in all the 4 studies of the side with greater clinical involvement in patients (P = 0.01)


CONCLUSIONS
Our results suggest that I-wave generation is more likely to be defective in MS than in normal subjects, that this defect resides in the cortex, and that it correlates with severity of physical signs.
Ho
K H
KH
University Department of Clinical Neurology, The Radcliffe Infirmary, Oxford, UK.
Lee
M
M
Nithi
K
K
Palace
J
J
Mills
K
K
D000328:Adult; D019054:Evoked Potentials, Motor / Q000502:physiology*; D005260:Female; D006801:Humans; D008280:Magnetics; D008297:Male; D008875:Middle Aged; D009103:Multiple Sclerosis / Q000503:physiopathology*; D013997:Time Factors