Intraoperative Motor Evoked Potentials

.Motor impairment is among the most feared and devastating outcomes following major surgery. Traditionally, the “wake-up test” has been used for intraoperative evaluation of motor function, but is associated with a number of limitations such as its general crudeness, common anesthetic risks such as extubation and recall, inadequacy as a continuous neuromonitoring technique, and lack of clinical utility with a neurologically uncooperative patient. Various neurophysiological monitoring techniques have been championed as alternatives. Although intraoperative neuromonitoring of dorsal column function using somatosensory evoked potentials (SSEPs) has been well established and routinely used for over 20 years during spinal surgeries, selective injury to the anterior spinal cord and its motor pathways can go undetected. Because of the distinct differences in blood supply to the dorsal and ventral aspects of the spinal cord and since somatosensory evoked potentials do not directly monitor the motor pathways, a number of other neurophysiological techniques have been recently developed for intraoperative monitoring of motor pathway integrity. These techniques have involved either direct or indirect stimulation of the spinal cord rostral to the site of surgery, and transcranial magnetic or electrical stimulation of the motor cortex. Distal recording sites have included the spinal cord, cauda equina, peripheral nerves, and muscles of the upper and lower extremities. For most of the spinal cord stimulation techniques, considerable controversy exists over the neural composition of the waveform that is recorded and referred to as a “motor evoked potential (MEP).” A review of these protocols is presented: highlighting the historical background, critically comparing methods, and summarizing optimal intraoperative neuromonitoring strategies.