Contributed by: Thomas Hirschauer MD PhD, Felix Chang MD, Viet Nguyen MD, S. Charles Cho MD, Leslie Lee MD, Jaime López MD, Scheherazade Le MD
A 56-year-old Cantonese-speaking woman with nasopharyngeal carcinoma status post chemotherapy and radiation presented with recurrence on surveillance imaging. MRI revealed a 2.4 cm right nasopharyngeal mass extending to the right hypoglossal canal and abutting the proximal carotid space structures without obvious vascular irregularities or stenoses (Figure 1). Her baseline neurologic exam was intact except for decreased hearing in the left ear. She underwent an awake endovascular balloon test occlusion (BTO) of the right internal carotid artery (ICA) to assess whether she would tolerate intraoperative ICA sacrifice during tumor resection.
Baseline 8-channel EEG prior to BTO was normal and symmetric, and median nerve SSEPs were well-formed bilaterally. Serial neurologic exams were monitored with the help of a telephone interpreter. Mean arterial pressure before the BTO was 95 mm Hg, which was dropped to 60-70 mm Hg during the BTO to stress collateral perfusion. Soon after the balloon was inflated in the petrous segment of the right ICA, the EEG showed diffuse high-amplitude 1.5-2 Hz rhythmic delta slowing (Figure 2). The EEG density spectral array showed decreased power in the alpha frequency band. Four minutes after occlusion, the patient complained of dizziness and had difficulty following some commands. The balloon was deflated five minutes after occlusion. The EEG slowing completely resolved within 7 minutes of the balloon being deflated, and the patient returned to her baseline mental status.
The patient exhibited mild encephalopathy during BTO, characterized by difficulty following commands, which was challenging to reliably discern on exam due to the patient’s hearing loss and the need for a telephone interpreter. However, the EEG showed an abrupt onset of diffuse slowing after occlusion, which allowed for prompt and definitive identification of neurologic abnormalities. Because the patient failed the BTO, it was determined that she would not be able to tolerate ICA sacrifice during tumor resection. As there was no evidence of an incomplete circle of Willis or flow-limiting stenosis in the contralateral carotid artery, it was hypothesized that the diffuse EEG slowing was caused by global cerebral hypoperfusion in the setting of a concomitant drop in systemic blood pressure.
Question 1: Which pattern of intraoperative neuromonitoring changes can result from cerebral hypoperfusion during carotid artery occlusion?
Correct answer: D. All of the above.
A carotid balloon test occlusion can be performed preoperatively to assess the adequacy of collateral circulation in patients who may require sacrifice of a carotid artery, such as due to encasement by a tumor1, 2. Cerebral hypoperfusion due to insufficient collateral flow can be identified by changes in EEG and somatosensory evoked potentials (SSEPs), which have been shown to be sensitive modalities for detecting cerebral ischemia2, 3. During carotid artery occlusion, ischemic changes are most often seen in the cerebral hemisphere ipsilateral to occlusion3. An incomplete circle of Willis is a known major risk factor for hemispheric cerebral ischemia4. However, global changes in EEG and SSEPs have also been shown to occur during unilateral carotid occlusion3, 5, 6, especially in the setting of flow-limiting stenosis or occlusion of the contralateral carotid artery7.
Question 2: Which change in EEG spectral components is most associated with hemispheric cerebral ischemia?
Correct answer: C. Decrease in alpha-delta power ratio.
EEG changes during carotid occlusion have been shown to correlate well with cerebral blood flow6. Early ischemic changes are characterized by attenuation of fast frequencies followed by an increase in slower frequencies8, 9. These findings are associated with a decrease in the alpha-delta power ratio on quantitative EEG10. Hemispheric slowing, as can often be seen during carotid occlusion, is associated with an increase in the EEG asymmetry index3, 10. While there may be an increase in slower frequency (i.e. theta or delta) power, the total EEG power is expected to decrease in the setting of ischemia10, 11. A decrease in spectral edge frequency has also been shown to be associated with ischemic changes during carotid occlusion, particularly in patients receiving isoflurane instead of propofol for sedation11, 12.