Duration, configuration and amplitude of the motor response evoked by magnetic brain stimulation in patients with multiple sclerosis.

Analysis of motor evoked potentials (MEP) after magnetic brain stimulation is often restricted to measurement of latency and amplitude. In the present study, which was conducted in healthy controls and in patients with multiple sclerosis (MS), the additional value of MEP duration and configuration was evaluated. Recordings were made from distal muscles of the upper and lower extremity. In the MS group, mean MEP duration was found to be prolonged, while amplitudes were reduced. MEP of abnormally prolonged duration, as compared to normative data derived from the control group, were found even in presence of normal central motor conduction times. These findings are compatible with increased temporal dispersion of the impulses arriving at the spinal motoneurone pool. Thus, analysis of MEP duration may provide additional information about impulse propagation along the descending motor tracts, indicating disturbed conduction properties even in the presence of normal central motor conduction time. In contrast, MEP configuration was of no value for differentiating between the two groups.