Magnetic transcranial brain stimulation: painless evaluation of central motor pathways. Normal values and clinical...

Painless transcranial magnetic stimulation has been performed on 53 healthy adults in order to obtain normal values of central motor pathway conduction times to cervical segments innervating upper limb muscles. Central motor latency was 5.1 msec when recording from the biceps brachii and 5.2 msec when recording from the abductor pollicis brevis or abductor digiti minimi muscles. The plexus motor latency as the conduction time between the cervical motor root and the proximal nerve trunks in the axillary region was 2.2 msec and 2.6 msec when recorded from the abductor pollicis brevis and abductor digiti minimi muscles, respectively. Conduction times higher than mean latency + 2 standard deviation may indicate a pathologic conduction slowing. Magnetic stimulation of the motor system is a new painless neurophysiologic technique enabling examination of the central motor pathways in awake subjects. With respect to spinal cord diagnostics, this method is useful in conditions such as compression of the spinal cord or of the motor root in the intervertebral foramen or canal.