The assessment of MEP patterns can help detect and localize lesions of the central and peripheral motor system. This technique has value in preoperative assessment, intraoperative monitoring, and postoperative follow-up. MEPs may be obtained using several types of stimulation and recording methods. MEPs are most commonly recorded from peripheral nerves or limb muscles, but may also be recorded from muscles supplied by cranial nerves, muscles of the trunk, the diaphragm, the external anal sphincter, the spinal cord and fibers of the cauda equina. MEPs have been used to define motor abnormalities in lumbar root damage, cervical spine disorders, motor neuron disease and multiple sclerosis.

Central Motor Conduction Time

The concept of central motor conduction time for the upper limb involves an evaluation of the latency of electromyographic (EMG) responses in the hand following stimulation of the head overlying the cerebral cortical motor area for the hand, and the latency following stimulation of the cervical cord over the seventh cervical vertebra. The difference in EMG latency of the responses obtained from these two stimulus sites represents the time taken for conduction in descending motor pathways to the cervical cord level, about 5.0 ms, and is referred to as central motor conduction time (CMCT) or central motor latency (CML-M).

CML-M is the conduction time from the motor cortex to the intervertebral foramen and includes the conduction time over the motor roots, which in the lumbar spinal canal can measure 15 to 20 cm and therefore contribute considerably to CML-M. The true CMCT can be calculated by using M-wave and F-wave recordings; subtraction of the peripheral latency from the cortical latency will provide a value designated CML-F, because the peripheral latency measures conduction time from the anterior horn cell to the muscle.

Estimation of CMCT may be performed with the transcranial magnetic stimulation technique, which is noninvasive and nonpainful, or by transcranial electrical stimulation, which is also noninvasive, but is associated with some discomfort when performed in a conscious, unanesthetized subject. An increase in CMCT may result from demyelination, degeneration of the corticospinal tracts caused by motor neuron disease or a hereditary disorder, cerebral vascular disease, cerebral glioma, or spondylotic compression of the cervical cord or nerve roots.

Normative values for CMCT in the upper limb obtained with magnetic stimulation have been published for the biceps brachii, abductor pollicis brevis, and abductor digiti minimi muscles. Increases in normal CMCT to muscles in the upper limb have been described in multiple sclerosis and in various disorders of the cervical spine. Delays in CMCT were found in 72 percent of patients with degenerative changes of the cervical spine, 67 percent of patients with rheumatoid arthritis. and 57 percent of patients with trauma of the cervical spine.

Normative values for CMCT in the lower limb have been documented for the quadriceps, anterior tibial, and extensor digitorum brevis muscles. CMCT to muscles in the lower limbs was found to be delayed significantly in 65 percent of patients with spinal stenosis and in 50 percent of patients with nerve root compression syndromes.

The use of MEPs for studying the motor innervation of the pelvic floor has been described and may be useful in the evaluation of patients with fecal incontinence. The MEP technique would allow a differentiation between central and peripheral components of the motor innervation of the external anal sphincter, which would, in turn, provide more precise localization of dysfunction in these patients. CMCT values in normal subjects for muscles innervating the external anal sphincter have been described.

Intraoperative Monitoring

The intraoperative monitoring of MEPs is especially important in attempting to preserve motor function during procedures in which surgically induced damage may be specific to the motor system. MEPs have been monitored in operations involving the surgical correction of spinal deformities, the resection of tumors of the spinal cord, the clipping of cerebral aneurysms and the resection of intracranial tumors and arteriovenous malformations.

Three methods of monitoring MEPs are in current use. In the first, transcranial electrical stimulation is used and the responses in the spinal cord and peripheral nerves are recorded. A second method involves stimulating the spinal cord electrically and recording from peripheral nerves. The third method involves transcranial magnetic stimulation and recording from either peripheral nerves or muscles.

An appropriate anesthesia protocol and controlled levels of muscle relaxants, if EMG potentials are to be used, are essential for the use of MEPs in intraoperative monitoring. An evaluation of different anesthetic agents with both transcranial electrical and magnetic stimulation showed that the amplitude depression after etomidate was less pronounced and of shorter duration than with propofol as an induction agent.

Transcranial Electrical Stimulation

MEPs recorded from the spinal cord and peripheral nerves have been produced by electrical stimulation with electrodes placed directly in the vicinity of the motor cortex or by transcranial stimulation involving a scalp anode and a cathode placed on the hard palate. Later authors used this technique to monitor 20 consecutive patients during upper cervical spine surgery; they reported a loss of MEPs in one patient, which was associated with quadriplegia, and transient decreases in MEPs in five patients, which were associated with no neurological deficits.

Boyd et al. used electrical stimulation of the scalp with a voltage condenser discharge and recorded the MEPs from the epidural space during the surgical correction of scoliosis; a nitrous oxide­narcotic-halothane anesthesia technique was used, and reproducible MEPs were obtained. Further studies using this stimulation technique, but recording from muscle, resulted in successful MEP recordings in approximately 87 percent of patients undergoing spinal surgery. Correlation of MEP changes and postoperative status was found in 76.2 percent of patients monitored with upper limb MEPs and in 81.4 percent of patients monitored with lower limb MEPs, with false-positive results in 23.8 percent and 18.6 percent, respectively. No false-negative results were found.

Another study of patients undergoing spinal surgery demonstrated reproducible MEPs in all patients and further showed that isoflurane caused marked attenuation in MEP amplitudes. Total intravenous anesthesia with propofol, although causing a reduction in the amplitude of the MEPs (up to 7 percent of the baseline values obtained in conscious relaxed subjects) has been used to provide reliable MEP monitoring in 88.5 percent of patients undergoing lumbar discectomy and in 87 percent of patients undergoing surgery for spinal tumors and spinal arteriovenous malformations. Intraoperative MEP changes in this study correlated well with postoperative clinical findings. It was found that amplitude decreases exceeding 50 percent and latency changes exceeding 3 ms compared to baseline values were significant. Others found that nitrous oxide, when used with propofol, produced reductions in MEP amplitude and increases in latency; however, when nitrous oxide concentrations were kept below 50 percent, reliable MEP monitoring was achieved.

Transcranial Magnetic Stimulation

Intraoperative MEP monitoring with transcranial magnetic stimulation is highly dependent on the anesthesia protocol. The use of nitrous oxide-narcotic anesthesia with 75 to 95 percent muscle relaxation resulted in reproducible MEP latencies in 9 of 11 patients undergoing spinal instrumentation surgery for scoliosis. A second study in scoliosis patients with MEP monitoring documented a case in which the loss of MEPs was associated with inability to move during repeated wake-up tests, which was corrected by adjustment of instrumentation until symmetric motor responses were seen in both legs; the patient had no postoperative deficits. This latter study also reported four patients with spinal cord tumors and three patients with cord compression who had low-amplitude MEPs preoperatively and failed to show any MEPs intraoperatively, which indicates that even minor compromise of descending motor tracts may interfere with MEPs when these are evaluated under anesthesia.

Direct Spinal Cord Stimulation

MEPs obtained by direct stimulation of the spinal cord was initially investigated by epidural stimulation while recording from peripheral nerves and muscles. This technique required either a laminectomy or the use of a Tuohy needle for electrode placement, and the electrode had to be placed and secured in the midline to achieve consistent activation of both limbs. With this technique there is a potential of causing spinal cord damage by electrode manipulation in the epidural space and burning of the cord if the stimulus intensity is not kept to a minimum.

A second method of direct spinal cord stimulation, termed the neurogenic-MEP (NMEP), involves translaminar electrical stimulation of the spinal cord, by placement of needle electrodes in two adjacent spinous processes, while recording from peripheral nerves. The anesthesia protocol used with this monitoring technique, was balanced narcotics or ketamine. In the assessment of 300 patients with this technique, eight true positives were identified in which the loss of NMEPs correlated with postoperative motor deficits and none of these patients demonstrated a loss of sensory function or change in somatosensory evoked potentials. Two patients demonstrated loss of somatosensory evoked potentials that was associated with sensory deficits following surgery. The efficacy of this technique was also shown in a case report in which an intraoperative loss of NMEPs was described that was associated with the patient's inability to move either lower extremity during a wakeup test and resulted in significant loss of motor function in the lower extremities.

Summary

The use of MEPs in preoperative patient evaluations provides a valuable assessment of the functional status of descending motor tracts and may also suggest whether MEPs can be used in intraoperative monitoring. Compromise of descending motor tracts may not allow recording of MEPs of significant magnitude for use in intraoperative monitoring. The use of MEPs in intraoperative monitoring is gaining popularity and undergoing further improvement, with the use of appropriate anesthesia protocols and well-trained neurophysiology personnel, MEPs provide an effective real-time assessment of the status of descending motor tracts and have value in predicting postoperative motor deficits.