Transcranial stimulation of the motor cortex to produce motor-evoked potentials.

Monitoring of the nervous system using evoked potentials is a developing tool. A new evoked potential, the motor-evoked potential (MEP), based on a traditional test, uses brain stimulation to monitor the motor system. The MEP complements the existing modalities, which are wholly sensory. The MEP can be prompted by direct stimulation of the motor cortex, but, in a more general way, by transcranial stimulation. Electric or magnetic means can be used. One electric system involves placement of an electrode on the scalp over the motor cortex, paired with a cathodal plate on the roof of the hard palate. Recording electrodes are placed over the spinal cord, peripheral nerves, and muscles. Signals are recorded with a standard, evoked-potential, signal-averaging computer. Animal studies indicate that the electric stimulus activates primarily the pyramidal system to produce a descending evoked potential in the ventral and dorsolateral spinal cord. It is more sensitive than the sensory-evoked potential to spinal cord injury produced by the weight-drop method in cats. The peripheral nerve responses, much more sensitive to injury than the cord responses, can be altered by metabolic abnormalities. To date, the MEP has been an accurate indicator of ambulation in chronic spinal cord injury in animals. Parallel clinical development in the operating room has shown that the test is valuable; and it monitors brainstem or cortex manipulation, as well as cord manipulation. Safety studies are encouraging. The MEP is developmentally and technically demanding, but it has produced high quality signals.(ABSTRACT TRUNCATED AT 250 WORDS)