Measurement of central motor conduction in multiple sclerosis by magnetic brain stimulation.

Central motor conduction time (CMCT) to abductor digiti minimi was measured in 18 healthy subjects and in 15 patients with multiple sclerosis. A novel percutaneous magnetic stimulator was used to stimulate the motor cortex painlessly and CMCT was calculated by subtracting the onset latency of muscle responses obtained by electrical stimulation over the C7/T1 interspace from that obtained from stimulation over the scalp. In healthy subjects CMCT was 6.0 +/- 0.76 ms. In multiple sclerosis patients with pyramidal signs in the arms, CMCT was almost always prolonged, the longest conduction time being 39 ms. In 10 patients CMCT was prolonged despite normal strength in the muscle.