INTERHEMISPHERIC INHIBITION IN PATIENTS WITH MULTIPLE-SCLEROSIS

Objectives: A single focal magnetic stimulus applied to the motor cort ex of normal subjects can suppress ongoing voluntary electromyographic activity in ipsilateral small hand muscles. This inhibition is mediat ed from one motor cortex to the contralateral side via a transcallosal pathway. Methods: We have investigated transcallosal inhibition in 24 patients with definite multiple sclerosis (MS) and in 24 healthy volu nteers. A focal magnetic stimulus was applied to the hand area of the motor cortex and the onset latency of the inhibition of the ongoing EM G activity of the ipsilateral first dorsal interosseus muscle was eval uated. Cortico-motor conduction time to the same muscle was revealed, using a magnetic stimulus over the contralateral motor cortex. The dif ference between these values was calculated as transcallosal conductio n time. Cerebral magnetic resonance imaging (MRI) scans including sagi ttal T-2-weighted images were performed in 18 patients. Results: The d epth of inhibition (maximal inhibition as percentage of the baseline E MG) in the MS patients was comparable to normal values, but the transc allosal conduction time was significantly delayed (patients 17.2 +/- 6 .4 ms; normal subjects 12.2 +/- 2.6 ms; P < 0.001). The duration of th e inhibition was significantly prolonged in MS patients (patients 47.9 +/- 20.9 ms; normal subjects 38.9 +/- 10.1 ms; P = 0.02). Transcallos al conduction time was delayed in 11 (46%) of 24 patients, compared wi th normal subjects. It exceeded the normal range (mean +/- 2.5 SD) in one normal subject (specifity 96%). No correlation could be found betw een the size or extent of the lesions obtained from the MRI scan and t he onset latency or the depth of the inhibition. Conclusions: We concl ude that conduction over transcallosal connections is significantly sl ower in patients with MS. (C) 1998 Elsevier Science Ireland Ltd. All r ights reserved.