The Stagnara wake-up test was first described by Vauzelle et al. (5) in 1973 as the only technique available to assess spinal cord function before the availability of electrophysiologic monitoring. The wake-up test allows the surgeon to evaluate the functional integrity of the neuronal structures directly, as opposed to indirectly by electrophysiologic techniques.

In the wake-up test, the patient is awoken from anesthesia to undergo a neurologic examination. This can be done at any point in surgery but is usually performed after the completion of spinal instrumentation and/or manipulation, including patient positioning. When the patient reaches a lightened anesthetic state, they are first asked to move their hands. Next, lower extremity function is verified by asking the patient to move their feet. If the patient is unable to the move the lower extremities, the test is repeated. A second negative response is then addressed by examining possible causes of a neurologic insult, including reevaluation of head position, instrumentation, or possible vascular compromise.

Before exposing a patient to an intraoperative wake-up test, he or she should first undergo preoperative screening to assess their capacity for cooperation and tolerance of the technique. This assessment is unfortunately not a completely reliable predictor of the patient’s ability to tolerate intraoperative awakening. Nevertheless, a full explanation must ensue, including a description of the procedure and conditions upon awakening such as confusion, intubation, and discomfort. Language barriers may be overcome by prerecording commands for examination (6). Anesthetic
technique is critical to a successful wake-up test, and therefore, the anesthesiologist must take part in the preoperative screening process. Although specific anesthetic regimens vary, there are general principles fundamental to the technique, such as the use of local anesthetic to the pharynx and larynx to help with patient comfort upon awakening and the careful monitoring by muscle twitches of any paralytic agents administered during induction. Moreover, communication is imperative between the surgeon and the anesthesiologist regarding surgical timing to wakefulness so that all medications can be carefully titrated.

The principal advantages of the wake-up test include the lack of expensive equipment or the need for trained personnel. Another benefit is that it directly assesses motor function but unfortunately has limited evaluation of sensory function. Unfortunately, the motor evaluation reflects gross motor function and not fine motor control. There have been reports of false-negative monitoring during which no disturbance was noted, and the patient awoke with a motor deficit (7,8).

Potential complications of the wake-up test include pain, air embolism, hardware dislocation, extubation, bronchospasm, dislodgment of head fixation, dislodgment of intravenous lines and catheters, and recall of intraoperative events. A premise of the test is that the function being monitored has been assessed at baseline function for intraoperative comparison. One problem with the use of the wake-up test in cervical spine surgery is that the surgical site involves neurologic tracts to the upper extremities as well as lower extremities. As originally described, the upper extremities served as controls for the test. In cervical spine surgery, the test can demonstrate that the motor tracts are intact if a patient moves his or her hands. If the converse situation occurs (negative test) with no demonstrable motor function elicited, it would be difficult to assess whether the loss is due to the anesthetic state, patient compliance, or a spinal cord injury. The wake-up test therefore only provides beneficial information if the patient demonstrates upper extremity motor function.

Another disadvantage of the wake-up test is that a lag time is required in reducing the state of anesthesia before the test can be administered. Additional lag time implies that repeated testing can add considerable operative time and exposure to anesthesia. One disadvantage of the wake-up test compared to electrophysiologic techniques is that electrophysiologic techniques allow for continuous monitoring of sensory and/or motor tracts and with better patient comfort, thus allowing for earlier detection of an abnormality and the ability to immediately address any surgical insults (9). Although some believe that electrophysiologic monitoring techniques may obviate the need for the wake-up test, technical difficulties or misinterpretation may remain a confounding variable in the indirect assessment. The wake-up test should thus remain a tool in the surgeon’s armamentarium.

The ankle clonus test is predicated on the different stages of recovery from general anesthesia. This test is performed by producing a rapid forced dorsiflexion of the foot and then holding tension on the foot in the dorsiflexed position. A positive result, indicating intact motor tracts in
the spinal cord, is demonstrated by the rhythmic contraction of the gastrocnemius-soleus muscle. The clonus reflex is not found in a normal, awake patient. During anesthetic emergence, lower motor neuron function returns before the inhibitory (cortical) upper motor neuron impulses. Therefore, there is an excitatory state in the early stages of anesthetic reversal when the patient is still unresponsive to verbal stimuli. Ankle clonus is present until inhibitory impulses from the brain override the excitatory impulses. If there is injury to the spinal cord, there is decreased lower-motor function with a loss in the ability to produce ankle clonus (10). One advantage to the ankle clonus test over the wake-up test is that it does not require the patient to be awakened fully, therefore reducing the lag time to examination and possible sterile field contamination.

Intraoperative recordings of SEPs were among the earliest uses of electrophysiologic techniques for monitoring the spinal cord. SEP monitoring was first introduced in the 1940s (11) but was not used in clinical practice until the 1970s (12). SEPs provide continuous, real-time recording of the dorsal column-medial lemniscus pathway that mediates tactile discrimination, vibration sensation, form recognition, and joint/muscle sensation (conscious proprioception) (13, 14 and 15). Testing of these particular sensory modalities is recommended prior to surgery in order to document any deficits that may limit intraoperative monitoring (16).

SEPs are elicited by electrical stimulation of a peripheral nerve, which produces a signal that is conducted through the afferent somatosensory pathway and can be measured either at the level of the cerebral cortex (cortical) or spinal cord (subcortical) (Fig. 18.2). For cervical
spine surgery, median nerve stimulation is appropriate for upper- to midcervical levels (C6, C7, C8, and T1), whereas ulnar nerve stimulation is instituted for lower cervical levels (C8 and T1). The radial nerve contributes to C5, C6, C7, C8, and somewhat to T1 but is rarely stimulated for evoking SEP. In severe peripheral neuropathy, the posterior tibial nerve (L4, L5, S1, and S2) or peroneal nerve (L4, L5, and S1) can also be stimulated. Although there are no universally accepted criteria for interpreting the significance of intraoperative changes in SEP monitoring, a greater than 50% reduction in amplitude or a greater than 10% increase in latency from baseline remains a rough guideline for potential neurologic injury (17). Because of the shortened volley, latency changes become less important in subcortical recording. Moreover, although changes in velocity may be a sensitive indicator of spinal cord injury, quantification has proven cumbersome and therefore does not play a significant role in SEP monitoring.

Major limitations in SEPs include the lack of definitive correlation with anterior column function and the time delay between injury and manifestation. Only a fraction of the SEP signal is conducted through the ventral tracts, and therefore, motor tracts may be damaged without a change in SEP (7,18, 19, 20 and 21). With SEP monitoring alone, there remains a small but definite risk for a false-negative finding, particularly when monitoring patients with obesity, peripheral neuropathy, or preexisting spinal cord compromise, such as those with cervical myelopathy, spinal cord tumors, or acute spinal cord injury. Although reported rates vary, the largest series of monitored spinal surgery patients concluded a 0.067% false-negative rate with an overall sensitivity of 92% and specificity of 98.9% in the overall ability of SEP to detect new intraoperative neurologic deficits (22). On average, the false-negative rate is less than 2%, whereas the false-positive rate is less than 3% (18,23, 24, 25 and 26). The false-positive rate ranges from 9% to 28%, depending on the location and number of channels and recording sites (27,28).

To avoid misinterpretation, it is considered common practice to secure two separate proximal and distal sights for production of the afferent volley. This aids in the determination of technical failure or neurologic injury. A technical failure commonly produces alterations in the latencies or amplitude in one of two volleys, whereas neurologic injury would affect both volleys. Moreover, one must realize that changes in SEP signals may be delayed for up to 30 minutes following significant surgical manipulation that may affect the spinal cord (24). For this reason, it is imperative to continue recording for at least 30 minutes after maneuvers considered substantial risk for neurologic injury.

Despite the high rate of success, there are times when SEPs are either not recordable or simply unreliable because of small amplitudes and poor waveform morphology. This is true even when careful attention is paid to controlling the anesthetic regimen so as not to compromise response amplitude (29). Environmental and physiologic factors can play a significant role in signal recording, including noise from other operating equipment (electric drills, warming blankets, headlights, etc.), hypocarbia, hypothermia, hyperthermia, hypotension, patient age, and limb length (30, 31 and 32). Anesthetic factors may also cause changes to SEP amplitude and latency, including halogenated anesthetics, nitrous oxide, narcotics, barbiturates, and benzodiazepines (32). Factors that play a role in the sensitivity of SEP to anesthetic depression are predominantly twofold. First, recording from lower-order neurons (less synapses) reduces the likelihood of anesthetic depression (subcortical). Second, previous spinal cord injury has been shown to increase the sensitivity of neurons to anesthetic depression (33).