Grid Monitoring and Intraoperative Electroencephalography

Number: 0289

Table Of Contents

Policy
Applicable CPT / HCPCS / ICD-10 Codes
Background
References

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Policy

Scope of Policy

This Clinical Policy Bulletin addresses grid monitoring and intraoperative electroencephalography.

1. Medical Necessity

   1. Intraoperative Electroencephalography (EEG)

      Aetna considers intraoperative scalp EEG medically necessary for the following indications:

      1. Monitoring cerebral function during carotid artery surgery; or
      2. Monitoring cerebral function during intracranial vascular surgical procedures; or
      3. Monitoring cerebral function during parietal tumor resection or resection of lesion near the eloquent cortex.

      Note: The use of intraoperative EEG to monitor brain function for anesthetic drug administration in order to determine depth of anesthesia is considered integral to the anesthesia and not separately reimbursed.  In addition, this use of intraoperative EEG is considered experimental, investigational, or unproven.

   2. Grid Monitoring (Electrocorticography, ECoG)

      Aetna considers grid monitoring to determine the location of the epileptogenic focus for possible surgical resection medically necessary for members with intractable seizures when any of the following conditions is met:

      1. Seizures arise from functionally important brain areas; or
      2. Surface (scalp) electroencephalogrphy (EEG) recording did not adequately localize the epileptogenic area, or
      3. There is a discordance between electrophysiological localization and that provided by other neurodiagnostic studies suggesting an abnormality in more than one region of the brain.

   3. Electroencephalography (EEG) monitoring During WADA Testing

      Aetna considers electroencephalography (EEG) monitoring medically necessary during WADA testing.

2. Experimental, Investigational, or Unproven

   1. Aetna considers intraoperative EEG experimental, investigational, or unproven for open-heart surgery and for all other indications not listed above (e.g., prediction of post-operative delirium) because its clinical value has not been established.
   2. Aetna considers grid monitoring experimental, investigational, or unproven for all other indications not listed above because its clinical value for these indications has not been established.

3. Policy Limitations and Exclusions

   Note: Grid monitoring is considered appropriate only when used by centers that have expertise and experience, especially with younger persons.

4. Related Policies

   • CPB 0322 - Electroencephalography (EEG) Video Monitoring
   • CPB 0394 - Epilepsy Surgery
   • CPB 0425 - Ambulatory Electroencephalography
   • CPB 0697 - Intraoperative Neurophysiological Monitoring

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Table:

CPT Codes / HCPCS Codes / ICD-10 Codes

Code	Code Description
Information in the [brackets] below has been added for clarification purposes.   Codes requiring a 7th character are represented by "+":
Intra-operative electroencephalographic (EEG) monitoring of cerebral function during intracranial vascular surgical procedures:
CPT codes covered if selection criteria are met:
95812	Electroencephalogram (EEG) extended monitoring; 41-60 minutes
95813	greater than 1 hour
95822	Electroencephalogram (EEG); recording in coma or sleep only)
95940	Continuous intraoperative neurophysiology monitoring in the operating room, one on one monitoring requiring personal attendance, each 15 minutes (List separately in addition to code for primary procedure)
95941	Continuous intraoperative neurophysiology monitoring, from outside the operating room (remote or nearby) or for monitoring of more than one case while in the operation room, per hour (List separately in addition to code for primary procedure)
Other CPT codes related to the CPB:
31200 - 31230, 61000 - 61253, 61304 - 61576, 61590 - 61619, 61623 - 61645, 61680 - 61711, 61720 - 61791, 61850 - 61888, 62000 - 62148, 62160 - 62165, 64716, 67570, 69501 69530, 69601 - 69605, 69635 - 69646, 69666 - 69667, 69720 - 69745, 69805 - 69806, 69910 - 69915, 69950 - 69955	Intracranial vascular surgical procedures
HCPCS codes covered if selection criteria are met:
G0453	Continuous intraoperative neurophysiology monitoring, from outside the operating room (remote or nearby), per patient, (attention directed exclusively to one patient) each 15 minutes (list in addition to primary procedure)
ICD-10 codes covered if selection criteria are met:
C71.3	Malignant neoplasm of parietal lobe [parietal tumor]
C79.31	Secondary malignant neoplasm of brain [parietal tumor]
D33.0	Benign neoplasm of brain, supratentorial [parietal tumor]
D43.0	Neoplasm of uncertain behavior of brain, supratentorial [parietal tumor]
D49.6	Neoplasm of unspecified behavior of brain [parietal tumor]
G93.89	Other specified disorders of brain [lesion near the eloquent cortex]
Intra-operative electroencephalographic (EEG) monitoring of cerebral function during carotid artery surgery:
CPT codes covered if selection criteria are met:
95955	Electroencephalogram (EEG) during non-intracranial surery (eg, carotid)
Other CPT codes related to this CPB:
37236 - 37237, 37242, 33510, 33889, 33891, 34001, 34151, 35001 - 35002, 35121 - 35122, 35301, 35341, 35390, 35501, 35506, 35508 - 35512, 35515 - 35516, 35518, 35521 - 35523, 35525 - 35526, 35531, 35601, 35606, 35642, 35691, 35694 - 35695, 35701, 36100, 36221 - 36224, 36227 -36228, 36595, 37215 - 37218, 37600, 37605 -37606, 60600, 60605, 61590 - 61592, 61596, 61611, 61710	Carotid artery surgery
ICD-10 codes not covered for indications listed in the CPB :
F05	Delirium due to known physiological condition [post-operative delirium]
Grid Monitoring (Electrocorticography, ECoG) :
CPT codes covered if selection criteria are met:
95829	Electrocorticogram at surgery (separate procedure)
Other CPT codes related to this CPB:
61531	Subdural implantation of strip electrodes through one or more burr or trephine hole(s) for long term seizure monitoring
61533	Craniotomy with elevation of bone flap; for subdural implantation of an electrode array, for long-term seizure monitoring
61535	for removal of epidural or subdural electrode array, without excision of cerebral tissue (separate procedure)
61760	Stereotactic implantation of depth electrodes into the cerebrum for long term seizure monitoring
95812 - 95830	Electroencephalography (EEG)
95954 - 95967	Special EEG Tests
95961	Functional cortical and subcortical mapping by stimulation and/or recording of electrodes on brain surface, or of depth electrodes, to provoke seizures or identify vital brain structures; initial hour of attendance by a physician or other qualified health care professional
+95962	each additional hour of attendance by a physician or other qualified health care professional (List separately in addition to code for primary procedure)
Other HCPCS codes related to the CPB:
S8040	Topographic brain mapping
ICD-10 codes covered if selection criteria are met:
G40.001 - G40.919	Epilepsy and recurrent seizures
R56.1	Post traumatic seizures
R56.9	Unspecified convulsions
Electroencephalography (EEG) monitoring during WADA testing:
CPT codes covered if selection criteria are met:
95812 – 95822	Electroencephalogram (EEG)
Other CPT codes related to this CPB:
95958	Wada activation test for hemispheric function, including electroencephalographic (EEG) monitoring

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Background

Grid-based electrocorticography and intraoperative electroencephalography represent complementary neurophysiologic modalities that provide real‑time electrophysiologic assessment during neurosurgical procedures. Grid monitoring (electrocorticography) utilizes subdural electrode arrays placed directly on the cortical surface to capture high‑resolution recordings of spontaneous and stimulus‑evoked cortical activity, enabling precise characterization of functional and electrophysiologic properties of the exposed cortex. Intraoperative EEG, typically obtained from scalp electrodes and occasionally supplemented by intracranial contacts, serves as a broader measure of cerebral function, allowing detection of dynamic physiologic changes such as ischemic patterns or anesthetic effects during surgery. It is applied in high-risk vascular (e.g., carotid endarterectomy) and neurosurgical settings.

Standard scalp electroencephalography (EEG) is a technique that measures and records the brain's electrical activity by placing electrodes on the scalp. It is most commonly employed by physicians to determine the presence of seizure disorders. For patients experiencing intractable seizures, achieving optimal surgical outcomes requires accurate localization of the seizure focus. In some cases, scalp EEG monitoring may not provide sufficient information, leading to the use of invasive subdural EEG monitoring, which involves the placement of subdural grid electrodes. These subdural electrodes offer coverage of large areas of the neocortex, making them particularly effective for evaluating children with intractable epilepsy and for functionally mapping critical areas of the cortex.

Multi-contact depth electrodes may be implanted into the brain to record electrical activity from deep or superficial cortical structure. Strips or rectangular grid arrays (subdural electrodes) can be placed under the dura to record activity in this region.

Subdural electrodes, including strip and grid arrays, are placed to record and stimulate neural tissue in order to identify underlying functions such as language areas, sensation, or motor function. These electrodes typically remain implanted for several days—sometimes up to one to two weeks—to record seizure activity and map the brain. They are removed before epilepsy surgery is performed if the findings support intervention. In cases where the initial phase of invasive monitoring does not adequately localize the seizure focus, repeat evaluation with additional subdural electrode coverage may help identify the epileptogenic region and facilitate effective surgical treatment.

Invasive EEG monitoring with subdural grid electrodes is associated with significant complications; however, most of them are transient. Higher complication rates are related to an increased number of electrode contacts, increased length of the monitoring period, placement of burr holes in addition to the craniotomy, and multiple cable exit sites.

An American Academy of Neurology Technology Assessment (Nuwer et al., 1990) stated that electrocorticography (ECoG) from surgically exposed cortex can help to define the optimal limits of a surgical resection, identifying regions of greatest impairment. Regions of attenuated or absent EEG, or those with relatively increased slow activity, decrease in fast activity, or abnormal spike discharges, help to define regions of cortex that are impaired or abnormal. When used together with long-term EEG/video monitoring, ECoG can help to define the limits of resection for surgery for epilepsy.

An American Academy of Neurology Technology Assessment (Nuwer et al., 1990) stated that intraoperative scalp EEG monitoring has long been carried out in an effort to safeguard the brain during carotid endarterectomy. The assessment stated that this technique has been shown to be safe and efficacious for such use and for other similar situations in which cerebral blood flow is at high risk. For this purpose, monitoring should be carried out at least at the anterior and posterior regions over each hemisphere. The AAN technology assessment stated that sixteen channels are preferable to identify occasional embolic complications.

A Medicare National Coverage Determination (CMS, 2006) on EEG monitoring during surgical procedures involving the cerebral vasculature states that EEG monitoring may be covered routinely in carotid endarterectomies and in other neurological procedures where cerebral perfusion could be reduced. Such other procedures might include aneurysm surgery where hypotensive anesthesia is used or other cerebral vascular procedures where cerebral blood flow may be interrupted. A Medicare National Coverage Determination on EEG monitoring for open-heart surgery stated that the value of EEG monitoring during open-heart surgery and in the immediate postoperative period is debatable because there is little published data based on well-designed studies regarding its clinical effectiveness. The NCD states that the procedure is not frequently used for this indication and does not enjoy widespread acceptance of benefit.

One or two-channel intraoperative EEG analysis modules have been used by anesthesiologists to gauge the depth of anesthesia, such as the Bi-Spectral device (BIS). Such use of limited-channel intraoperative EEG for monitoring depth of anesthesia (and level of consciousness) is considered integral to the anesthesia service and not separately reimbursable. In addition, a one or two-channel EEG device does not meet the minimal technical requirements for EEG testing as set forth by the American Clinical Neurophysiology Society.

Intraoperative Electroencephalography During Parietal Tumor Resection

Mueller et al. (1996) examined the usefulness of functional magnetic resonance imaging (fMRI) to map cerebral functions in patients with frontal or parietal tumors. Charts and images of patients with cerebral tumors or vascular malformations who underwent fMRI with an echoplanar technique were reviewed. The fMRI maps of motor (11 patients), tactile sensory (12 patients), and language tasks (4 patients) were obtained. The location of the fMRI activation and the positive responses to intraoperative cortical stimulation were compared. The reliability of the paradigms for mapping the rolandic cortex was evaluated. The rolandic cortex was activated by tactile tasks in all 12 patients and by motor tasks in 10 of 11 patients. Language tasks elicited activation in each of the 4 patients. Activation was obtained within edematous brain and adjacent to tumors. fMRI in 3 cases with intraoperative electrocortical mapping results showed activation for a language, tactile, or motor task within the same gyrus in which stimulation elicited a related motor, sensory, or language function. In patients with greater than 2 cm between the margin of the tumor, as revealed by MRI, and the activation, no decline in motor function occurred from surgical resection. The authors concluded that fMRI of tactile, motor, and language tasks was feasible in patients with cerebral tumors; fMRI showed promise as a means of determining the risk of a postoperative motor deficit from surgical resection of frontal or parietal tumors.

Karatas et al. (2004) noted that cases with intractable epilepsy may present with multiple lesions in their brains. Ictal electroencephalography (EEG) carries great value in the identification of the primary epileptogenic source. On the other hand, removal of low-grade tumors located around the eloquent cortex may be risky with conventional techniques. Functional neuronavigation (f-NN) is the integration of fMRI and stereotactic technologies and provides interactive data regarding the localization of the motor cortex. This report presented a case with a dysembryoplastic neuroepithelial tumor (DNET), which was removed using f-NN and electrocorticography (ECoG) techniques. A 19-year-old patient with intractable complex partial and secondary generalized seizures was presented; MRI revealed a low-grade tumor located in the right parietal region just behind the motor cortex, and a contralateral temporal arachnoid cyst. Ictal EEG demonstrated the right parietal origin of the seizures. The patient underwent a right parietal craniotomy and tumor excision using f-NN and ECoG techniques intraoperatively. ECoG findings correlated with the epileptogenicity of the parietal lesion. The postoperative course was uneventful; no postoperative deficit was observed. The patient was seizure-free at the 8-month follow-up. Pathological examination reported the lesion as DNET. The authors concluded that ictal EEG had a very important role in the identification of the epileptogenic focus in cases with multiple brain lesions. Preservation of the functional cortex was the most prominent aim during lesional surgery for epilepsy. Intraoperative mapping using f-NN and ECoG supported the orientation of the neurosurgeon to the functional and epileptogenic cortical areas, thus increasing the safety and efficacy of surgical procedures.

Maesawa et al. (2016) stated that few studies have examined the clinical characteristics of patients with lesions in the deep parietal operculum facing the sylvian fissure, the region recognized as the secondary somatosensory area (SII). Moreover, surgical approaches in this region are challenging. These investigators reported on a patient presenting with SII epilepsy with a tumor in the left deep parietal operculum. The patient was a 24-year-old man who suffered daily partial seizures with extremely uncomfortable dysesthesia and/or occasional pain on his right side. MRI revealed a tumor in the medial aspect of the anterior transverse parietal gyrus, surrounding the posterior insular point. Long-term video-EEG monitoring with scalp electrodes (for determination of epileptogenesis) failed to show relevant changes to seizures. Resection with cortical and subcortical mapping under awake conditions was performed. A negative response to stimulation was observed at the subcentral gyrus during language and somatosensory tasks; thus, the transcortical approach (specifically, a trans-subcentral gyral approach) was used through this region. Subcortical stimulation at the medial aspect of the anterior parietal gyrus and the posterior insula around the posterior insular point elicited strong dysesthesia and pain on his right side, similar to the manifestation of his seizure. The tumor was completely removed and pathologically diagnosed as pleomorphic xanthoastrocytoma. His epilepsy disappeared without neurological deterioration postoperatively. In this case study, three points were clinically significant. First, the clinical manifestation of this case was quite rare, although still representative of SII epilepsy. Second, the location of the lesion made surgical removal challenging, and the trans-subcentral gyral approach was useful when intraoperative mapping was performed during awake surgery. Third, intraoperative mapping demonstrated that the patient experienced pain with electrical stimulation around the posterior insular point. Thus, this report demonstrated the safe and effective use of the trans-subcentral gyral approach during awake surgery to resect deep parietal opercular lesions, clarified electrophysiological characteristics in the SII area, and achieved successful tumor resection with good control of epilepsy.

Yao et al. (2018) noted that using intraoperative ECoG to identify epileptogenic areas and improve postoperative seizure control in patients with low-grade gliomas (LGGs) remains inconclusive. These researchers retrospectively reported on a surgical strategy that was based on intraoperative ECoG monitoring. A total of 108 patients with LGGs presenting at the onset of refractory seizures were included. Patients were divided into two groups. In Group I, all patients underwent gross-total resection (GTR) combined with resection of epilepsy areas guided by intraoperative ECoG, while patients in Group II underwent only GTR. Tumor location, tumor side, tumor size, seizure-onset features, seizure frequency, seizure duration, preoperative anti-epileptic drug therapy, intraoperative electrophysiological monitoring, postoperative Engel class, and histological tumor type were compared between the two groups. Univariate analysis demonstrated that tumor location and intraoperative ECoG monitoring correlated with seizure control. There were 30 temporal lobe tumors, 22 frontal lobe tumors, and 2 parietal lobe tumors in Group I, with 18, 24, and 12 tumors in those same lobes, respectively, in Group II (p < 0.05). In Group I, 74.07% of patients were completely seizure-free (Engel Class I), while 38.89% in Group II (p < 0.05). In Group I, 96.30% of the patients achieved satisfactory postoperative seizure control (Engel Class I or II), compared with 77.78% in Group II (p < 0.05). Intraoperative ECoG monitoring indicated that in patients with temporal lobe tumors, most of the epileptic discharges (86.7%) were detected at the anterior part of the temporal lobe. In these patients with epilepsy discharges located at the anterior part of the temporal lobe, satisfactory postoperative seizure control (93.3%) was achieved after resection of the tumor and the anterior part of the temporal lobe. The authors concluded that intraoperative ECoG monitoring provided the exact location of epileptogenic areas and significantly improved postoperative seizure control of LGGs. In patients with temporal lobe LGGs, resection of the anterior temporal lobe with epileptic discharges was sufficient to control seizures.

Maesawa et al. (2018) stated that epilepsy surgery aims to control epilepsy by resecting the epileptogenic region while preserving function. In some patients with epileptogenic foci in and around functionally eloquent areas, awake surgery is implemented. These investigators analyzed the surgical outcomes of such patients and discussed the clinical application of awake surgery for epilepsy. They examined a total of 5 consecutive patients, in whom these researchers performed lesionectomy for epilepsy with awake craniotomy, with postoperative follow-up of greater than 2 years. All patients showed clear lesions on MRI in the right frontal (n = 1), left temporal (n = 1), and left parietal lobe (n = 3). Intraoperatively, under awake conditions, sensorimotor mapping was performed; primary motor and/or sensory areas were successfully identified in 4 cases, but not in 1 case of temporal craniotomy. Language mapping was performed in 4 cases, and language areas were identified in 3 cases. In 1 case with a left parietal arteriovenous malformation (AVM) scar, language centers were not identified, probably because of a functional shift. Electrocorticograms (ECoGs) were recorded in all cases, before and after resection; ECoG information changed surgical strategy during surgery in 2 of 5 cases. Postoperatively, no patient demonstrated neurological deterioration. Seizures disappeared in 4 of 5 cases (Engel Class I), but recurred after 2 years in the remaining patient due to tumor recurrence. Therefore, for patients with epileptogenic foci in and around functionally eloquent areas, awake surgery allowed maximal resection of the foci; intraoperative ECoG evaluation and functional mapping allowed functional preservation. This led to improved seizure control and functional outcomes.

Electroencephalography (EEG) Monitoring During WADA Testing

Tu et al. (2015) noted that the intra-carotid amobarbital or Wada procedure is a component of the pre-surgical evaluation for refractory epilepsy, during which monitoring the onset and offset of transient anesthetic effects is critical. These researchers characterized changes in 8 quantitative measures during 26 Wada tests, which included alpha, beta, theta, and delta powers, alpha/delta power ratio, beta/delta power ratio, median amplitude-integrated EEG, and 90% spectral edge frequency (SEF90), and correlated them with contralateral hemiplegia. They found that on the side of injection, delta and theta powers, alpha/delta power ratio, beta/delta power ratio, and SEF90 peaked within 1 minute following injection of 70 to 150 mg amobarbital or 4 to 7 mg methohexital. When contralateral arm strength returned to 3/5, delta power and amplitude-integrated EEG decayed on average 24% and 19%, respectively, for amobarbital, similar to that of methohexital (27% and 18%). The authors concluded that because delta power resolution most closely mirrored that of the hemiplegia and amplitude-integrated EEG had the highest signal-to-noise ratio, these quantitative values appeared to be the best measures for decay of anesthetic effects. Moreover, these researchers stated that the increase in alpha power persisted longest; thus, it may be the best measure of late residual anesthetic effects.

Passarelli et al. (2015) examined the effect of contralateral electrographic involvement on memory performance (measured by neuropsychological and Wada memory testing) in patients with epilepsy associated with unilateral mesial temporal sclerosis (MTS). These investigators studied 51 patients with medically refractory epilepsy associated with unilateral MTS (27 women, 30/51, left MTS) who were submitted to prolonged non-invasive video-EEG monitoring and bilateral Wada testing. According to ictal electrographic involvement, patients were classified as having contralateral ictal involvement when one or more seizures evolved with rhythmic activity in the temporal region contralateral to the MTS, or exclusive ipsilateral ictal involvement if all seizures showed ictal EEG activity exclusively on the MTS side. Wada testing involved a 12-item memory paradigm. The Wada memory asymmetry score was calculated for each patient by subtracting the number of recalled items after injection on the lesion side from the number of recalled items after contralateral injection. Expected asymmetry (EA) was considered if the Wada memory asymmetry was > 0, and symmetrical or reversed memory asymmetry (S-RA) when ≤ 0. Neuropsychological testing was applied in the 51 patients and in 40 healthy controls (HCs). Verbal memory was examined with the Rey Auditory Verbal Learning Test (RAVLT), considering the number of recalled items on immediate recall after the initial 5 consecutive encoding trials (RAVLT 6), a post-interference delayed (30 minutes) recall (RAVLT 7), and recall after 7 days. Non-verbal memory was tested with Wechsler Memory Scale-III (WMS-III) Faces subtests 1 and 2. Groups did not differ in demographic, clinical, and video-EEG monitoring variables. S-RA was observed more frequently in the group with contralateral ictal involvement (57.2% versus 27.0%; p = 0.03). Logistic regression analysis considering demographic, clinical, hippocampal volume, and video-EEG monitoring variables showed contralateral ictal involvement as the only independent variable associated with S-RA (coefficient = 1.32, p = 0.029, OR of 3.77; 95% CI: 1.1 to 12.47). Furthermore, the patient group with contralateral ictal EEG involvement displayed worse verbal and nonverbal memory scores compared to HCs. The authors concluded that in this cohort of unilateral MTS patients, contralateral ictal involvement was associated with decreased memory performance on Wada and on neuropsychological testing.

Bogaarts et al. (2016) stated that the Wada test is commonly used to evaluate language and memory lateralization in candidates for epilepsy surgery. The spatial Brain Symmetry Index (BSI) quantifies inter-hemispheric differences in the EEG, and its use has been shown to be feasible during Wada testing. These researchers developed a method for the quantification of EEG asymmetry that matches visual assessments of the EEG better than BSI. A total of 53 patients' EEG data, with a total of 85 injections, were analyzed. In a step-wise, data-driven manner, multiple electrode and frequency band combinations were examined. Eventually, BSI, calculated using only the frontal electrodes F3 and F4, was combined with a temporal measure of delta power in the central electrodes, C3 and C4, into a new measure: cBSI. Using the area under the ROC curve (AUC), these investigators showed that cBSI performed significantly better relative to BSI (median AUC of 0.98 versus 0.96, p = 0.0015, Wilcoxon signed-rank test). The authors concluded that these findings showed that asymmetry detection was significantly improved by combining temporal with spatial quantitative EEG measures. These researchers noted that in the future, their combined quantitative EEG (qEEG) measure could allow for a more objective way of monitoring EEG asymmetry, thus increasing the feasibility of using EEG as a monitoring tool during the Wada test. These investigators stated that future studies are needed to validate their cBSI method in real time in the operating room or radiology suite. The authors listed the following highlights of this study:

• A qEEG measure combining temporal and spatial EEG changes during Wada testing better agrees with visual assessment of EEG slowing.
• Quantification of EEG changes per EEG channel and frequency band during the Wada test can reduce subjectivity of EEG interpretation.
• Not all EEG electrodes and frequency bands are equally important for the detection of EEG slowing during Wada testing

Danoun et al. (2021) stated that the Wada test is used to evaluate language lateralization and memory performance following inactivation of an isolated cerebral hemisphere. Methohexital, a short-acting barbiturate, has been employed for the induction of interictal discharges during intraoperative corticography. In a retrospective study, these investigators reported a new finding of activation of lateralized periodic discharges (LPDs) following methohexital injection. They reviewed 174 consecutive adult patients who underwent Wada testing in preparation for epilepsy surgery (n = 129, 74%) or brain tumor resection (n = 45, 26%) at the University of Michigan to determine the frequency of induced periodic discharges by methohexital. A total of 4 epilepsy patients (2.29%) had methohexital-induced LPDs within a median of 2 seconds (1 to 99 seconds) of the injection and lasting a median of 4 minutes (3 to 10 minutes) after a total of 7 injections. All LPDs occurred ipsilateral to the injection hemisphere in the known region of interictal epileptiform discharges. LPDs were not induced in brain tumor patients. In 1 patient, LPDs occurred during memory testing, and this patient's memory performance was below expectation based on pre-test neuropsychological testing. The authors concluded that methohexital could induce LPDs in the ipsilateral hemisphere and that this could potentially affect memory performance. These investigators stated that this observation demonstrated that concurrent EEG monitoring during the Wada test is important and that induced discharges should be considered when interpreting Wada test results.

Castro-Lima et al. (2023) compared memory outcomes after surgery for unilateral hippocampal sclerosis (HS)-associated epilepsy in patients with unilateral and bilateral ictal electrographic involvement. These researchers prospectively evaluated HS patients aged 18 to 55 years with an IQ of 70 or higher. Left (L) and right (R) surgical groups underwent non-invasive video-EEG monitoring and Wada testing. They classified patients as ipsilateral if ictal EEG was restricted to the HS side, or bilateral if at least 1 seizure onset occurred contralaterally to the HS, or if ictal discharge evolved to the opposite temporal region. Patients who declined surgery served as controls. Memory was assessed on 2 occasions with the Rey Auditory-Verbal Learning Test and Rey Visual-Design Learning Test. Baseline neuropsychological test scores were compared between groups. Pre- and postoperative scores were compared within each group. Reliable change index Z-scores (RCI) were obtained using controls as references and compared between surgical groups. These investigators evaluated 64 patients. Subjects were classified as follows: L-ipsilateral (n = 9), L-bilateral (n = 15), L-control (n = 9), R-ipsilateral (n = 10), R-bilateral (n = 9), and R-control (n = 12). On preoperative evaluation, memory performance did not differ among surgical groups. Right HS patients did not present postoperative memory decline. The L-ipsilateral group presented postoperative decline on immediate (p = 0.036) and delayed verbal recall (p = 0.011), while the L-bilateral group did not decline. The L-ipsilateral group had lower RCI Z-scores, indicating delayed verbal memory decline compared to the L-bilateral group (p = 0.012). The authors concluded that dominant HS patients with bilateral ictal involvement presented less pronounced postoperative verbal memory decline compared to patients with exclusive ipsilateral ictal activity. Surgery was indicated in these patients regardless of memory impairment on neuropsychological testing, since resection of the left sclerotic hippocampus could result in cessation of contralateral epileptiform activity and, thus, improved memory function.

Prediction of Emergence Agitation After Sevoflurane Anesthesia

Jang and colleagues (2018) noted that emergence agitation (EA) is common after sevoflurane anesthesia, but there are no definite predictors. In a prospective, predictive study, these researchers examined if intra-operative EEG can indicate the occurrence of EA in children. EEG-derived parameters (spectral edge frequency 95, beta, alpha, theta, and delta power) were measured at 1.0 minimum alveolar concentration (MAC) and 0.3 MAC of end-tidal sevoflurane (EtSEVO) in 29 patients. EA was evaluated using an EA score (EAS) in the post-anesthetic care unit on arrival (EAS 0) and at 15 and 30 minutes after arrival (EAS 15 and EAS 30). The correlation between EEG-derived parameters and EAS was analyzed using Spearman correlation, and receiver-operating characteristic curve analysis was used to measure the predictability.  EA occurred in 11 patients. The alpha power at 1.0 MAC of EtSEVO was correlated with EAS 15 and EAS 30. The theta/alpha ratio at 0.3 MAC of EtSEVO was correlated with EAS 30. The area under the receiver-operating characteristic curve of percentage of alpha bands at 0.3 MAC of EtSEVO and the occurrence of EA was 0.672. The authors concluded that children showing high-alpha powers and low theta powers (= low theta/alpha ratio) during emergence from sevoflurane anesthesia were at high risk of EA in the post-anesthetic care unit. These preliminary findings need to be validated by well-designed studies.

Prediction of Postoperative Delirium

Fritz and colleagues (2016) stated that post-operative delirium is a common complication associated with increased morbidity and mortality, longer hospital stays, and greater health care expenditures. Intraoperative EEG slowing has been associated previously with post-operative delirium, but the relationship between intra-operative EEG suppression and post-operative delirium has not been investigated. In this observational cohort study, a total of 727 adult patients who received general anesthesia with planned intensive care unit (ICU) admission were included. Duration of intraoperative EEG suppression was recorded from a frontal EEG channel (FP1 to F7). Delirium was assessed twice-daily on post-operative days 1 through 5 with the Confusion Assessment Method for the ICU. Thirty days after surgery, quality of life (QOL), functional independence, and cognitive ability were measured using the Veterans RAND 12-item survey, the Barthel index, and the PROMIS Applied Cognition-Abilities-Short Form 4a survey. Post-operative delirium was observed in 162 (26%) of 619 patients assessed. When these researchers compared patients with no EEG suppression with those divided into quartiles based on duration of EEG suppression, patients with more suppression were more likely to experience delirium (χ(4) = 25, p < 0.0001). This effect remained significant after these investigators adjusted for potential confounders (odds ratio [OR] for log(EEG suppression) 1.22 (99% confidence interval [CI]:, 1.06 to 1.40, p = 0.0002] per 1-minute increase in suppression); EEG suppression may have been associated with reduced functional independence (Spearman partial correlation coefficient -0.15, p = 0.02); but not with changes in QOL or cognitive ability. Predictors of EEG suppression included greater end-tidal volatile anesthetic concentration and lower intra-operative opioid dose. The authors concluded that EEG suppression is an independent risk factor for post-operative delirium. Moreover, they stated that future studies should examine if anesthesia titration to minimize EEG suppression decreases the incidence of post-operative delirium.

1. because this was an observational study, the findings cannot indicate whether the relationship between EEG suppression and delirium is causal. Delirium was assessed as part of routine clinical care, and such assessments have limited sensitivity despite high specificity,
2. some patients either left the ICU prior to the first delirium assessment or were sedated at all assessment time points,
3. the post-discharge outcomes may be limited due to incomplete survey responses, particularly because patients who experienced post-operative delirium were less likely to return the survey,
4. the Barthel Index was not performed pre-operatively, and thus it was not possible to distinguish whether patients who experienced EEG suppression had reduced functional independence before surgery as well, and
5. this study also restricted its focus to patients with planned ICU admission after surgery, so care should be taken when applying these results to a broader surgical patient population.

This  research group is currently conducting the ENGAGES clinical trial (NCT02241655), which may shed further light on the association between intra-operative burst suppression and post-operative delirium.

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References