A clinical study of motor evoked potentials using a triple stimulation technique

Amplitudes of motor evoked potentials (MEPs) are usually much smaller than those of motor responses to maximal peripheral nerve stimulation, and show marked variation between normal subjects and from one stimulus to another. Consequently, amplitude measurements have low sensitivity to detect central motor conduction failures due to the broad range of normal values, Since t hese characteristics are mostly due to varying desynchronization of the des cending action potentials, causing different degrees of phase cancellation, we applied the recently developed triple stimulation technique (TST) to st udy corticospinal conduction to 489 abductor digiti minimi muscles of 271 u nselected patients referred for possible corticospinal dysfunction, The TST allows resynchronization of the MEP, and thereby a quantification of the p roportion of motor units activated by the transcranial stimulus. TST result s were compared with those of conventional MEPs, In 212 of 489 sides, abnor mal TST responses suggested conduction failure of various degrees. By contr ast, conventional MEPs detected conduction failures in only 77 of 489 sides , The TST was therefore 2.75 times more sensitive than conventional MEPs in disclosing corticospinal conduction failures. When the results of the TST and conventional MEPs were combined, 225 sides were abnormal: 145 sides sho wed central conduction failure, 13 sides central conduction slowing and 67 sides both conduction failure and slowing, It is concluded that the TST is a valuable addition to the study of MEPs, since it improves detection and g ives quantitative information on central conduction failure, an abnormality which appears to be much more frequent than conduction slowing. This new t echnique will be useful in following the natural course and the benefit of treatments in disorders affecting central motor conduction.