Motor Evoked Potentials

Monitoring of the integrity of the motor tracts within the spinal cord is a technique with great potential benefit, and even during the relative short history of MEP monitoring, there are reported cases of loss of MEPs with preservation of the SSEP.^{[175] [176] [177] [178] [179] [180]} This technique has potential applications in spinal surgery, in which transmission across the operative field can be assessed, and in aortic surgery with the potential for impairment of the blood supply to the vulnerable anterior spinal cord. Relative to SER monitoring, MEP monitoring is quite invasive, and until recently, some equipment associated with MEP testing was not approved by the U.S. Food and Drug Administration (FDA) for use in MEP monitoring in human subjects. These aspects of MEP monitoring have, until recently, substantially limited the applications of MEP monitoring.

MEP monitoring was developed specifically to assess the function of motor pathways and overcome one of the limitations of SEP monitoring. The first of several variants of MEP monitoring involves electrical or magnetic transcranial stimulation. During transcranial electrical MEP monitoring, stimulating electrodes are placed on the scalp overlying the motor cortex. During magnetic stimulation, a powerful magnetic stimulator is placed on the scalp over the motor cortex. Brief repetitive applications of electrical current or a strong magnetic field induce current in the motor cortex and produce an MEP. These transcranial stimulating methods may also activate surrounding cortical structures and subcortical white matter pathways (i.e., sensory and motor). Distal antidromic propagation of the transcranially applied stimulus is blocked by synapses in all of the ascending sensory pathways. The stimulus is propagated easily orthodromically through descending motor pathways. The evoked responses may be recorded over the spinal cord, the peripheral nerve, and the muscle itself. To enhance the MEP, these responses may be averaged in the same manner as SERs, but averaging is often unnecessary. A third method of producing the MEP involves electrical stimulation of the spinal cord itself above the area of the cord at risk during surgery. Responses may be recorded over the distal spinal cord, peripheral nerve, and muscle.

MEP monitoring, although promising in some aspects, has some problems associated with it that still need to be resolved. The exact anatomic pathways and generators involved with MEP production have not been completely determined, and intraoperative experience with MEPs remains relatively limited. Multiple anecdotal reports suggest that MEP monitoring during surgery on the spine or its blood supply may be very useful, but whether this monitor can be used to guide management and predict postoperative neurologic function in a large series of patients is unclear. ^{[181] [182] [183] [184]} For example, it was hoped that MEPs would be able to better predict postoperative motor function than SERs after occlusion of the blood supply to the spinal cord during operations on the thoracic aorta. Two studies suggest that MEPs may not be as effective as hoped. The first study ^[182] recorded MEPs from the lumbar spinal cord in dogs produced by transcranial electrical stimulation. Elmore and coworkers^[185] found that these spinally recorded potentials did not accurately predict postoperative motor function. A second study^[186] recorded MEPs at the spinal cord and peripheral nerve level in dogs produced by transcranial electrical stimulation. Reuter and associates^[186] also found that the spinally recorded responses were inaccurate in predicting motor function postoperatively. The peripheral nerve responses disappeared in all animals and were not present 24 hours later, regardless of whether the animal could move its lower extremities. These studies suggest that the spinally recorded MEP probably represents a response generated by the descending corticospinal tract. This white matter pathway is relatively resistant to ischemia compared with the more metabolically active anterior horn cells (i.e., gray matter). Recovery of this white matter-generated MEP response may occur after reperfusion of the cord, but the gray matter may not recover. Responses recorded from the peripheral nerve can reflect postsynaptic anterior horn cell function, but lower extremity ischemia occurring after aortic cross-clamping often precludes recording this or the response from muscles during surgery.

Limited clinical work has shown much greater success with MEP monitoring during aortic vascular surgery in correctly detecting inadequate spinal cord blood flow and in improving operative outcome. The technique has proved useful, particularly when using operative strategies such as reimplantation of critical intercostal vessels based on results of MEP monitoring.^{[187] [188] [189] [190] [191] [192]} Although there is promise for this monitoring technique in aortic surgery, much more experimental and clinical work is needed before MEP monitoring during aortic surgery becomes widely accepted and used.

The greatest experience with MEP responses has been obtained using electrical stimulation of the spinal cord above the area at risk surgically.^{[193] [194] [195]} Responses are recorded distally, usually over the peripheral nerve, and profound surgical muscle relaxation is used to prevent gross movement during surgery. This type of MEP is

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Except in the case of the NMEP, effects of anesthetics are surprisingly profound, particularly on MEP recordings from muscle produced by single-pulse transcranial electrical or by magnetic stimulation (see Table 38-9 ).^{[171] [197] [198] [199] [200] [201]} Anesthetic techniques typically used by most anesthesiologists for spinal surgery produce prohibitive depression of the MEP.^{[202] [203]} Investigators demonstrated in several studies that intravenous agents produce significantly less depression, and techniques using any combination of ketamine, opiates, etomidate, and propofol have been described. ^{[204] [205] [206] [207] [208] [209] [210]} Anesthetic effects on MEP responses recorded at spinal levels appear to be less serious. When responses are recorded from muscle, neuromuscular blocking agents should be monitored quantitatively, maintaining T1 twitch height at about 30% of control values to prevent excessive movement during the operation.^{[183] [197]} When responses are not recorded from muscle, profound relaxation is desirable because gross muscle movement produced by MEP stimulation is thereby eliminated, facilitating the surgical procedure. Fortunately for the practicing anesthesiologist, studies of transcranial electrical and magnetic stimulus techniques using rapid trains of stimuli have produced responses that are more resistant to the effects of anesthetic agents, and more traditional techniques using inhaled agents and narcotics may be used.^{[211] [212] [213]} Precise control of the anesthetic and avoidance of boluses during critical monitoring periods appears to be even more important than for SSEPs. Although some centers have had consistent success using transcranial MEP monitoring, its use is not widespread. Early data are promising, and the clinician should expect to see increased use over the next decade. FDA approval of transcranial electrical stimulation should increase the intraoperative use of this monitoring modality, especially in spinal column surgery, during which modest movement would not be problematic, and provide considerably more data on the efficacy of its use.