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.
