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.