Motor evoked potentials to magnetic stimulation: technical considerations and normative data from 50 subjects

Magnetic stimulation of the brain and cervical and lumbar spinal roots was performed on 50 healthy volunteers. Compound muscle action potentials (CMAPs) were recorded from biceps brachii, abductor digiti minimi (ADM), rectus femoris and tibialis anterior (TA). We assessed central conduction times by subtraction of peripheral from central latencies and compared results using either spinal root stimulation or the F-wave method. Side-to-side differences of total conduction time, peripheral conduction time and central conduction time (CCT) were measured and the effect of clockwise vs counterclockwise stimulations on latencies and sizes of CMAPs is emphasized. Amplitudes and areas of CMAPs were expressed as a percentage of the peripheral M response for ADM and TA. There was a positive correlation between CCT to the lumbosacral region and height, but not between the cervical region and height. No correlation was observed between genders and central conduction times, amplitudes or areas of CMAPs.