Current Techniques

Currently available techniques for intraoperative neurophysiologic monitoring of the spinal cord include somatosensory-evoked potential (SSEP), motor-evoked potential (MEP), and electromyography (EMG). In addition, some institutions use other methodologies, including neurogenic MEP, also known as spinally elicited MEP and dermatomal SSEP. Continuous neurophysiologic monitoring is an important tool that may reduce intraoperative neurologic morbidity. The principles, advantages, and disadvantages are discussed.

Although objective evidence for efficacy of electrophysiologic monitoring techniques is sparse, improvements in monitoring and advances in technique have made their use routine. Reliance on electrodiagnostic techniques remains solely at the discretion of the surgeon, and there are some instances where external monitoring is either not necessary or not helpful. Alternative monitoring techniques, such as nonparalytic monitoring techniques, may provide an alternative approach in these cases. This chapter discusses such a strategy.

Wake-Up Test

A number of methods have been developed for the intraoperative monitoring of spinal cord function. The earliest documented systematic approach to the intraoperative monitoring of spinal cord function was the wake-up test.¹ Commonly referred to as the Stagnara wake-up test, this technique often provides an unequivocal demonstration of the integrity of long motor tract function. However, it provides this information for one point in time and, in fact, is observed after any potentially harmful surgical manipulation has already taken place. The ability to monitor evoked potentials accurately has made the need for this test less common.^1–4

In 1973, Vauzelle et al.⁵ described the intraoperative wake-up test, which is still considered the gold standard by some centers. It enables the surgeon to perform a brief neurologic examination to the sedated but arousable patient and to directly evaluate the functional integrity of the neuronal structures. The patient is awoken from general anesthesia prior to wound closure, allowing the surgeon to assess for any neurologic deficits that may have resulted from surgical manipulation or instrumentation. If there is a neurologic deficit, the deformity correction and/or instrumentation can be revised immediately.

The requirements of the intraoperative wake-up test are simple. The patient must be told that the test may be performed. If the test is anticipated, a full explanation must ensue, describing the procedure and expected conditions on awakening (i.e., intubation, confusion, levels of discomfort).⁶

The anesthetic technique is critical for managing patient wakefulness and limiting the degree of discomfort. Introduction of local anesthetic to the pharynx and larynx, either by transcutaneous injection or spray, ensures patient comfort on awakening. An initial loading dose of a narcotic with continuous infusion is the preferred anesthetic regimen, because concomitant electrophysiologic monitoring may be affected by halogenated anesthesia. Narcotics and halogenated anesthesia, typically fentanyl and isoflurane, are discontinued 30 minutes prior to awakening the patient. Approximately 10 minutes prior to awakening, paralytic agents are reversed and nitrous oxide is discontinued. This regimen should provide a wakeful patient who is able to follow commands in approximately 5 more minutes.

Once awakened, the patient is asked to appropriately move the arms and legs (e.g., “squeeze my hand,” “wiggle your toes”). Discovered deficits require reevaluation of the patient’s position, the instrumentation, and consideration of vascular compromise. Therefore, both the surgeon and anesthesia team must be prepared for reawakening following their initial assessment. The use of the intraoperative wake-up test is limited by patient tolerance, additional operative time, and a limited assessment of the neurologic status during the test only. Modern neurophysiologic monitoring provides the benefit of patient comfort with the indirect continuous assessment of neurologic status during surgery.

Although electrophysiologic monitoring may obviate the need for the intraoperative wake-up test in most patients, technical difficulties or potentially misleading results of the neurophysiologic monitoring may warrant a wake-up test. It is particularly useful when electrophysiologic changes suggest a progressive deterioration with no apparent cause—the test can either confirm or refute the concern that a clinically relevant problem exists. For instance, in a case of scoliosis correction, 2 hours after a ventral correction with limited vessel ligation and no related change in monitoring at that time, and 30 minutes after completion of dorsal instrumentation and curve correction, a change in amplitudes and latencies was noted on neural monitoring. The implants were checked, and no apparent compression at any level could be seen. Because the monitoring had been inconsistent during the case, a Stagnara wake-up test was performed. The test confirmed a dense paresis of the lower extremities in the face of normal upper extremity function. Medical management was optimized and steroids given. Hemodynamic support was optimized to increase cord perfusion, and imaging was performed to rule out a compressive lesion along the extent of the constructs. Reduction was relaxed to a modest degree. The patient recovered normal function within 1 hour of the observation and recovered completely. The ability to confirm the suspected deficit supported the use of all medical means to increase cord perfusion in this case.

There are two fundamental disadvantages associated with the Stagnara wake-up test: (1) it provides only momentary information about the integrity of the nervous system, and (2) the information pertains to voluntary movements only. However, this test should remain a tool in the surgeon’s armamentarium.

Other Monitoring Methods

Some surgeons believe that the data acquired from intraoperative evoked potential monitoring most often are less helpful than the immediate surgical feedback provided by observation of unparalyzed muscles. However, the ability to perform monitoring in the face of partial paralysis allows surgeons to follow both electrodiagnostic input and direct motor responses to intraoperative spinal cord or nerve root manipulation. The spine surgeon, therefore, must determine the relative benefits and risks associated with these alternative techniques of intraoperative monitoring.

Deficiencies of Electrophysiologic Monitoring

In the past, authors have argued that electrophysiologic monitoring was flawed and unreliable, by virtue of its nature, because the observation of an electrophysiologic response becomes manifest after the adverse event occurs. There is a real-time lag associated with monitoring, either due to the time required for signal averaging or the need to stop for triggered motor stimulation. This time lag could mean that injury might not be recognized or minimized for some time after it occurs in the paralyzed patient.

SSEPs monitor sensory function that is transmitted through the posterior columns of the spinal cord,^2,7,8 whereas the majority of pathologic compressive lesions are ventrally located. The posterior columns monitored by SSEPs are, therefore, positioned farthest from the operative site of all spinal tracts. In the days before this vulnerability was recognized, and before MEPs were widely available, failure to detect injury was reported.^4,9,10

SSEPs monitor the dorsal column function within the spinal cord via stimulation of peripheral nerves in the arms and in the legs. False-negative and false-positive recordings have been reported. Although reported rates vary, a large series of 50,000 patients demonstrated a 0.067% false-negative rate.¹¹ On average, the false-negative rate is reported to be less than 2%, and the false-positive rate is less than 3%.^12–16

One must be aware of the technical limitations of SSEP to avoid misinterpretation. Securing both peripheral and central recording sites for the afferent volley helps differentiate technical failure from neurologic injury. A technical failure affects changes in the latencies or amplitude in both the peripheral and central recorded responses; neurologic injury affects only central responses. Moreover, following significant spinal cord injury, changes in SSEP signals may be delayed for up to 30 minutes.¹⁴

An undetermined fraction of monitoring errors results from unrecorded injury to the ventral spinal cord.^17,18 Deformation of the ventral spinal cord may have a limited effect on dorsal column function, thereby accounting for the false-negative rate of SSEP. Monitoring with MEP enables the assessment of the ventral spinal cord and is associated with a shorter delay between injury and changes.

Transcranial electrical stimulation must overcome the resistance of the skull. As such, current requirements are higher and associated with a significant degree of discomfort in the awake state.^19,20 Preinduction baseline recording of MEPs is therefore not possible.

MEPs can be elicited without averaging, unlike SSEP, which offers a significant advantage in terms of assessing spinal cord function in real time. In addition, it enables direct assessment of the motor function. But the interpreter must select the time to trigger and record the MEPs and must sometimes interrupt the procedure to obtain a reading.

EMG has been increasingly used during lumbar pedicle screw placement, when free-run EMG is continuously monitored for mechanical responses. The presence of neurotonic discharges or continuously discharging EMG activity may indicate nerve root irritation. The data can be confusing at times, especially with a fractured pedicle, because the clinical impact of a fractured pedicle is unknown, and the changes in capacitance associated with cortical fracture do not correlate with actual nerve injury or irritation. Because the patient is not fully paralyzed during this form of monitoring, the direct observation of nerve root stimulation and muscle contraction is still possible.

Rationale for Nonparalytic Intraoperative Monitoring

The principal argument in favor of nonparalytic intraoperative monitoring revolves around the use of intraoperative observation of motor responses evoked by surgical manipulation as a monitoring technique. The point is made that the majority of surgical procedures during which electrodiagnostic monitoring might be beneficial involve neural decompression.²¹ The surgeon is able to directly observe the neural elements during the critical components of laminectomy and decompression, and this “direct observation” type of monitoring is, therefore, the most useful to the surgeon. For example, the observation of the dural sac and its relationship to surrounding and confining bony and soft tissue structures affords the skilled and alert surgeon the opportunity to monitor the extent of neural distortion and compression in real time. Some might argue that this is not so much monitoring as just careful surgical technique.

Direct observation can play little or no role in scoliosis surgery, however. In scoliosis surgery, the spinal cord is threatened indirectly by ischemia associated with stretching or tethering, brought on by curve correction or ligation of segmental vessels. There is no way to predict the degree of cord stress by observing the magnitude of curve correction achieved during instrumentation. Similarly, in situations where the cord has been chronically stressed, as in thoracic kyphosis and chronic stenosis, there is no way to predict the effect of additional stresses imparted during surgical exposure or fluctuations in perfusion pressure. In these circumstances, electrophysiologic monitoring clearly provides insight that cannot be gained otherwise.

A purported advantage of immediate intraoperative feedback provided by a motor response “felt” by the surgeon during a particular maneuver is that there is no delay; the patient’s movement is indeed so frightening that one stops reflexively whatever action evoked the response. Even if minuscule, the delays inherent in electrophysiologic monitoring of the fully paralyzed patient are deemed unacceptable when compared with the immediacy of clinical monitoring of the unparalyzed patient. Proponents of observational monitoring note that motor responses triggered by nerve root stimulation have been noted frequently with bipolar or unipolar coagulation on the lateral vertebral body and the posterior longitudinal ligament. Patients experiencing surgical maneuvers that have caused a more global, total body movement, demonstrate a spinal cord response to inadvertent percussion or pressure. Unfortunately, even real-time feedback may not prevent permanent injury in this kind of event. Any device or instrument that triggers this sort of response should be removed from the canal immediately.

Using current electrodiagnostic monitoring methodologies, full paralysis is unnecessary, and almost any case can be carried out with some residual capacity for neural response. In these cases, inadvertent stimulation of a nerve root with a probe or cautery triggers a motor response that the surgeon will recognize immediately. Intrusion into the spinal canal or shift in spinal structures that impinge the cord itself still results in a sudden, profound total body “jump.” The surgical effort is reflexively stopped even before the monitoring personnel can sound a warning. With monitoring in place, however, the surgeon and staff can assess the clinical relevance of that “jump” and the possibility of injury and weigh the need for medical or further surgical treatment based on data as opposed to clinical suspicions (Table 179-1).

Summary

If inadvertent neural trauma occurs from heating, compression, distraction, or trauma, a motor response in affected nonparalyzed muscles usually occurs. This, in a sense, is the equivalent of a real-time MEP. Immediate surgical feedback is provided by observation of the motor response to a specific intraoperative neural manipulation. Immediate feedback to the surgeon makes this an effective method of preventing or limiting neurologic injury.

Technologic advances, such as electrophysiologic monitoring, cannot be used to replace intelligent surgeon input into the intraoperative decision-making process. However, common sense suggests that electrophysiologic monitoring should be used as an intraoperative decision-making tool alongside classic observational techniques of nonparalytic monitoring.