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 0.2ms between 1.0 and 5ms. 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 (Fisherʹs 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) Conclusion: 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.