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Case Report

Correlation Between Processed Electroencephalogram and Clinical Findings During Wake-Up Test in Prone Position for Scheduled Posterior Cervical Spine Surgery: A Case Report

Ma, Kan MD; Coutin, Mark MD; Kim, Thomas MD; Koht, Antoun MD

Author Information
A & A Practice: April 2020 - Volume 14 - Issue 6 - p e01170
doi: 10.1213/XAA.0000000000001170
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The Stagnara wake-up test has historically been used to confirm the functional integrity of spinal cord during corrective spine surgeries.1 However, with the advance in neuromonitoring, the wake-up test has since been sparsely used.2,3 Nonetheless, it remains a valuable adjunct in the setting of intraoperative evoked potential (EP) changes.4,5 The wake-up test, exacerbated by the lack of experience, can be a challenging undertaking. It has potential for complications such as accidental extubation if the test is conducted in an uncontrolled fashion.6–8 We present a case of intraoperative wake-up test during spine surgery with concurrent processed electroencephalogram (EEG), documenting changes from general anesthesia (GA), to emergence, to wake-up test, and eventually back to GA. By providing real-time information on the brain state, processed EEG can facilitate clinicians in conducting controlled wake-up tests.

Written Health Insurance Portability and Accountability Act (HIPAA) authorization consent was obtained from patient.


A 65-year-old man weighing 118 kg with cervical myelopathy—manifested as gait imbalance and upper extremity weakness—presented for C2 to T2 posterior cervical decompression and fusion. He had a previous T10 to pelvis fusion, hypertension, chronic kidney disease, iron-deficiency anemia, and type II diabetes mellitus.

Standard American Society of Anesthesiologists (ASA) monitors and Sedline EEG sensor (Masimo, Irvine, CA) were placed preinduction. Patient was induced with sufentanil 10 µg, lidocaine 100 mg, propofol 200 mg, and rocuronium 40 mg. Fiberoptic intubation was uneventful. GA was maintained with propofol 50 µg/kg/min, sufentanil 0.5 µg/kg/h, lidocaine 20 µg/kg/min, and ≤0.5 minimum alveolar concentration (MAC) sevoflurane. Arterial line was subsequently placed. Phenylephrine infusion was titrated to mean arterial pressure ≥85 mm Hg. As per surgical team, no baseline EPs were required before positioning. After placement of Mayfield pins, patient was placed in prone position onto a Jackson frame. Baseline EPs were then obtained, showing normal upper extremity EPs but absent lower extremity sensory and motor EPs. Neither the conversion to total-intravenous-anesthetic (TIVA), or the use of subdermal EP needles, or an increase of blood pressure improved lower extremity EPs. The presence of quantitative train-of-four ratio >0.9 assured us that muscle relaxants were not the culprit and thus we did not reverse neuromuscular blockade. All physiological parameters were within normal limits, and no technical causes were identified. Processed EEG density spectral array (DSA) bands power exhibited strong δ (0.5–4 Hz), moderate θ (4–8 Hz) and α (8–12 Hz), while exhibiting low β (12–25 Hz) and γ (>25 Hz) activities (Figures 1 and 2). This was a typical pattern of volatile and propofol-based anesthetic, with increased α and δ oscillations in the frontal lobes known as “anteriorization.” Because surgery had not started, the possibility of positioning as the cause of absent lower extremity EPs was raised. To avoid multiple repositioning, we performed a wake-up test with patient in prone position. If patient could move his legs, then the team would proceed with surgery.

Figure 1.
Figure 1.:
DSA using Sedline EEG sensor during GA. The DSA display contains left and right spectrograms representing the power of the processed EEG on both sides of the brain. The frequency scale shown on the left vertical axis ranges between 0 and 40 Hz. The power scale is displayed along the right vertical axis. Red color represents high power, green color represents moderate power, and blue color represents low power. The upper and lower DSA marked by “L” and “R” represent processed EEG activities from left and right frontal regions, respectively. The time is displayed in the center between the upper and lower DSA.9 Under GA, initial processed EEG exhibited strong δ, moderate θ and α associated with low β and γ activities. Sedline EEG sensor was disconnected briefly during prone position; thus, a thick black line was shown. Periods of artifact are displayed as vertical white lines. Periods of burst suppression are displayed as vertical black lines with blue bases. DSA indicates density spectral array; EEG, electroencephalogram; GA, general anesthesia; MAC, minimum alveolar concentration.
Figure 2.
Figure 2.:
DSA using Sedline EEG sensor during baseline EPs testing and conversion to TIVA. Propofol, sufentanil, and lidocaine infusion along with <0.5 MACs of sevoflurane were given until 09:15, at which point conversion to TIVA was initiated using propofol and sufentanil infusion only. At 09:41, the decision was made to discontinue TIVA and prepare for the wake-up test. Throughout this time, the patient remained in a state of general anesthesia, as indicated by the strong δ, moderate θ and α with low β and γ activities. DSA indicates density spectral array; EEG, electroencephalogram; EP, evoked potential; MAC, minimum alveolar concentration; MEP, motor evoked potential; SEP, somatosensory evoked potential; TIVA, total intravenous anesthesia.

The possibility of wake-up test, although used rarely, would often be discussed by our preoperative anesthesia team. In our case, after discontinuing TIVA, the attending anesthesiologist kneeled by the bed, placing one hand on patient’s right hand to test for movements and another over patient’s head to ensure airway security. He repeatedly reassured the patient while observing for patient’s facial expressions. We ensured a quiet room and enough assistance on both sides of the patient to monitor for extremity movements. Eye tapes were removed, with the soft bite-block in place to prevent tongue lacerations during wake-up. Mechanical ventilation was continued throughout. Fifteen minutes after discontinuation of TIVA, patient remained unresponsive to commands. Naloxone 40 µg was then given and 5 minutes afterward, processed EEG DSA showed a gradual and subsequently an abrupt increase of β and γ activities bilaterally (Figure 3A)—a typical pattern of emergence. These changes were associated with the patient opening his eyes, first spontaneously and then on-command. At this point, mechanical ventilation was discontinued. Blood pressure and heart rate both increased slightly during awakening. Patient was reassured of his safety and was asked to squeeze with his hands. Patient responded by squeezing both hands, confirming the return of consciousness. This coincided with an increase in β and γ activities bilaterally, suggesting bilateral cortical awakening.

Figure 3.
Figure 3.:
DSA using Sedline EEG sensor during the wake-up test and subsequent reinduction. Twenty minutes after TIVA discontinuation and naloxone administration, processed EEG showed a gradual and subsequently an abrupt increase of β and γ activities bilaterally. At 09:59, patient opened his eyes, and squeezed with both hands on-command (A). A minute later, he squeezed with his right hand only but not with his left hand, coinciding with decreased β and γ activities in the right hemisphere while the β and γ activities remained dominant in the left hemisphere (B, black arrows). Five minutes later, processed EEG showed bilateral increased β and γ activities suggesting bilateral cortical awakening, and it was associated with the patient being able to move all 4 limbs on command (C). At 10:05, after the conclusion of the wake-up test, the patient was induced with midazolam and propofol. The induction of general anesthesia resulted in burst suppressions, displayed as vertical black lines with small blue marks at the bases. DSA indicates density spectral array; EEG, electroencephalogram; MAC, minimum alveolar concentration; TIVA, total intravenous anesthesia.

At this point, the patient had yet to move his legs on-command. We waited patiently, knowing that the lower extremities responses may lag behind those of the upper extremities. A minute later, he squeezed with his right hand but not with his left. This asymmetric response coincided with decreased β and γ activities in the right hemisphere (corresponding to left hand weakness) and dominant β and γ activities in the left hemisphere (corresponding to right hand movement) (Figure 3B).

Five minutes later, processed EEG once again showed increased β and γ activities bilaterally suggesting bilateral cortical awakening, and this time patient moved all extremities on-command (Figure 3C). After demonstrating spinal cord integrity, the patient was reinduced with midazolam and propofol, and was maintained with propofol and sufentanil infusions and ≤0.5 MAC sevoflurane. After a short period of burst suppression, processed EEG again demonstrated strong δ, moderate θ and α with minimal β and γ activities. Remainder of the surgery was uneventful, with absent lower extremity EP throughout. Postoperatively, patient awakened with no neurological deficit or intraoperative recalls.


Intraoperative EP changes can be attributed to 1 of 5 factors: physiological, pharmacological, technical, surgical, and positional.10,11 In special physiological conditions, a localized change may be related to localized ischemia from tourniquet use, vasospasm, or other causes of partial vessel occlusion. Thorough examination of each factor is important to help identify the true cause. Both physiological and pharmacological factors tend to have global effects on EPs, while technical, positional, special physiological, and surgical factors tend to have more localized effects on EPs.

Taking a stepwise approach, we first ensured that patient was within normal physiological parameters and further increased blood pressure to improve spinal cord perfusion. Then, we converted to TIVA to remove any potential EP-suppressing effect of sevoflurane. Technical causes were also excluded. Adhesive surface electrodes were changed to subdermal needles to ensure proper stimulation. Because surgery has not started, the potential cause for the absent EP signals would be either positional, special physiological, or unprecedented technical factor.

We were faced with the dilemma of performing the wake-up test in either supine or prone position. If the patient could move all extremities while supine, we would then position him prone and could potentially encounter absent EP signals yet again in the prone position. This would not rule out prone positioning as a culprit. Therefore, we performed the wake-up test in the prone position.

The Stagnara wake-up test is gradually becoming a lost art. Yet, as evident in this case, the ability to conduct a safe wake-up test remains a critical skill for neuroanesthesiologists. Repeated verbal reassurance, adequate personnel to safeguard patient, and sequential testing of eye-opening, upper extremities, and last lower extremities are essential in performing a smooth wake-up test.10 The eye-opening and the bilateral hand movements were associated with bilateral increase in β and γ activities. The subsequent decrease in β and γ activities in the right hemisphere was associated with absent movements of the left upper extremity. The eventual return of bilateral β and γ activities was associated with movements in all extremities. The cause of EEG oscillation and motor function may be related to fluctuation in anesthetic levels at the brain centers. The β and γ activities seen during the wake-up test may also be, in part, a reflection of electromyography (EMG) activities, which would also be under the influence of brain function.

The patient’s ability to move lower extremities during the wake-up testing provided the surgeon with the comfort to proceed with surgery. Proceeding with the case after a normal wake-up test and a normal postoperative neurological function enable us to characterize the cause for the absent lower extremities’ EP to either undetermined technical cause or to an added limitation for EP monitoring.

To our knowledge, this is the first report detailing the processed EEG changes during a wake-up test. The asymmetric β and γ band oscillations between the 2 hemispheres and how it coincided with asymmetric neurological examination are an important observation. This observation of processed EEG monitoring may be used as a useful adjunct when faced with asymmetric neurological examination after neurosurgical procedures under GA, such as carotid endarterectomy and spinal procedures. In such cases, the unilateral appearance of drug signatures on the processed EEG DSA panels may help explain the cause of weakness on the contralateral limbs. This is reassuring to the surgical team while waiting for full clearance of anesthetic effects.

In conclusion, this case illustrates how processed EEG DSA bands can help during wake-up test. The appearance of high-power β and γ activities is an excellent indicator of timing during wake-up testing. The appearance of such high-power bands can serve as an early warning of the imminent awakening and the need to further secure patient’s movement and prevent accidental dislodge of the endotracheal tube.


Name: Kan Ma, MD.

Contribution: This author helped analyze the data, review the literature, and draft and revise the manuscript.

Name: Mark Coutin, MD.

Contribution: This author helped analyze the data, review the literature, and draft and revise the manuscript.

Name: Thomas Kim, MD.

Contribution: This author helped analyze the data, review the literature, and draft and revise the manuscript.

Name: Antoun Koht, MD.

Contribution: This author helped analyze the data, review the literature, draft and revise the manuscript, and direct the patient care.

This manuscript was handled by: Mark C. Phillips, MD.


ASA = = American Society of Anesthesiologists

DSA = = density spectral array

EEG = = electroencephalogram

EMG = = electromyography

EP = = evoked potential

GA = = general anesthesia

HIPAA = = Health Insurance Portability and Accountability Act

MAC = = minimum alveolar concentration

MEP = = motor evoked potential

SEP = = somatosensory evoked potential

TIVA = = total intravenous anesthetics


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