10.3 Intraoperative EEG & Visual Evoked Potentials

10.3 Intraoperative EEG & Visual Evoked Potentials

Quick Answer: Intraoperative EEG watches cortical activity for ischemia (especially during carotid/vascular cases) and for anesthetic depth, including burst suppression. Visual evoked potentials (VEPs) can target the optic pathway (CN II), but flash VEP in the anesthetized OR is often unstable—know the limitations as well as the indications.

Intraoperative EEG: Two Jobs

EEG in the OR serves overlapping purposes:

1. Detect cortical ischemia early enough for the surgeon/anesthesiologist to intervene (shunt, raise MAP, adjust retraction, reverse a vascular step).
2. Monitor anesthetic depth / electrocerebral effect, including titration to burst suppression when the team wants profound metabolic suppression (for example, during aneurysm temporary clipping in some protocols).

These jobs can conflict: deep burst suppression makes subtle ischemic change harder to see. During carotid endarterectomy (CEA), unintended burst suppression should be recognized and addressed so ischemia monitoring remains possible.

Practical OR EEG Setup

Full outpatient epilepsy montages (32+ channels) are impractical under drapes. Intraoperative EEG commonly uses about 8–16 channels with bilateral coverage from the International 10–20 system—for example frontal, central, parietal, and temporal electrodes (F3/F4, C3/C4, P3/P4, T3/T4 and additions as needed). Goals:

• Enough spatial resolution to see lateralized change (CEA clamp side vs contralateral)
• Stable impedances despite head positioning and sterile field constraints
• Continuous raw EEG display; many teams also use quantitative tools (spectral displays, asymmetry indices, burst-suppression ratio)

Baseline EEG under stable anesthesia is your reference—just as with evoked potentials.

Burst Suppression

Burst suppression is an EEG pattern of high-amplitude bursts alternating with flat or nearly flat suppression periods. It reflects profound cortical depression from deep anesthesia (commonly propofol or barbiturates at sufficient dose) or severe pathology.

Processed indices (BIS and similar) may correlate with depth but do not replace watching raw waveforms for focal ischemia.

Ischemia Patterns on EEG

Cerebral ischemia tends to evolve along a severity spectrum:

1. Mild — increased slow activity (theta/delta)
2. Moderate — attenuation of faster frequencies (alpha/beta)
3. Severe — marked suppression or loss of activity in the affected territory

Carotid clamp classic

During CEA, after cross-clamping the carotid, watch for unilateral change ipsilateral to the clamp: loss of faster rhythms and increased slowing on that hemisphere within tens of seconds if collateral flow is inadequate. That pattern suggests ischemia in the clamped carotid territory and may prompt selective shunting or blood-pressure augmentation per surgical plan.

Anesthetic deepening usually produces bilateral, symmetric slowing or suppression. Lateralized change time-locked to clamp, embolization, or vessel sacrifice is surgical/vascular until proven otherwise.

Other vascular / cranial uses

EEG (often with SSEPs) appears in:

• CEA and selected carotid/vascular reconstructions
• Some intracranial vascular cases where cortical perfusion is at risk
• Procedures where seizure activity or afterdischarges matter (mapping contexts—coordinate with the monitoring plan)

EEG does not monitor spinal cord integrity; do not substitute EEG for SSEP/MEP in spine ischemia questions.

Communicating EEG Changes

Effective alerts state:

• Laterality and channels involved
• Nature of change (slowing, fast-frequency loss, suppression, burst suppression)
• Timing relative to clamp, MAP change, or anesthetic bolus
• Whether the pattern improved after intervention

Correlate with SSEP cortical amplitudes when both modalities are running—concordant ipsilateral EEG and cortical SSEP change strengthens the ischemia story.

Visual Evoked Potentials (VEP) in the OR

What VEP is trying to monitor

VEPs assess the visual pathway from retina through optic nerve (CN II), chiasm, tract, and to occipital cortex. In theory, they support surgery near the optic nerves, chiasm, or occipital pathways (selected sellar/parasellar, orbitozygomatic, and occipital approaches).

Flash VEP vs pattern VEP

Awake diagnostic labs often use pattern-reversal VEPs for reliable, quantifiable responses. Anesthetized patients cannot fixate on a checkerboard, so the OR typically attempts flash VEP (light-emitting goggles or similar).

Why intraoperative VEP is limited

CNIM candidates should know the limitations as exam content, not as cynicism:

1. Anesthesia sensitivity — cortical visual responses are easily suppressed by volatiles and deep anesthesia; unstable baselines are common.
2. Stimulus delivery — eyelid goggles shift, ambient OR light leaks, globe compression from retractors, and pupil/ocular media issues alter retinal illumination unpredictably.
3. Response variability — flash VEP morphology and latency can fluctuate without clear surgical correlation, raising false-positive and false-negative risk.
4. Surgical access conflicts — devices near the eyes may interfere with the sterile field or positioning.
5. Interpretation ceiling — even a present flash VEP does not guarantee useful postoperative vision; absence may reflect technical/anesthetic factors rather than irreversible optic injury.

Because of these constraints, many centers use VEP selectively or not at all, and positive surgical decisions based solely on flash VEP require caution and multimodality context.

When VEP still appears in the plan

If the team elects flash VEP:

• Optimize total intravenous anesthesia when possible and keep depth stable
• Verify goggle seating and stimulus output before critical dissection
• Record occipital channels (for example O1/O2/Oz referenced appropriately per protocol)
• Treat large changes as prompts to check technical factors and surgical context—not as automatic proof of transection
• Combine with surgical judgment, pupil checks when available, and other indicated modalities

EEG vs VEP vs BAEP: Keep the Modalities Straight

Exam-Style Integration

Expect questions that mix these ideas:

• Unilateral EEG slowing after carotid clamp → ipsilateral ischemia, consider shunt
• Bilateral burst suppression after propofol bolus → anesthetic depth, not NM blockade
• BAEP Wave V change during CPA retraction with stable EEG → focal auditory pathway/brainstem issue, not hemispheric EEG ischemia
• Flash VEP disappearance after volatile increase with unchanged surgical field → suspect anesthetic/technical limitation before declaring optic nerve transection

Mastering when each modality is trustworthy—and when it is not—is as important as memorizing generator names.