Methohexital activation of epileptogenic foci during acute electrocorticography

Effect of anesthesia on electrocorticography for localization of epileptic focus: Literature review and future directions

Intraoperative electrocorticography (ECoG) is a useful technique to guide resections in epilepsy surgery and is mostly performed under general anesthesia. In this systematic literature review, we seek to investigate the effect of anesthetic agents on the quality and reliability of ECoG for localization of the epileptic focus. We conducted a systematic search using PubMed and EMBASE until January 2019, aiming to review the effects of anesthesia on ECoG yield. Fifty-eight studies were included from 1016 reviewed. There are favorable reports for dexmedetomidine and remifentanil during ECoG recording. There is inadequate, or sometimes conflicting, evidence to support using enflurane, isoflurane, sevoflurane, and propofol. There is evidence to avoid halothane, nitrous oxide, etomidate, ketamine, thiopental, methohexital, midazolam, fentanyl, and alfentanil due to undesired effects. Depth of anesthesia, intraoperative awareness, and surgical outcomes were not consistently evaluated. Available studies provide helpful information about the effect of anesthesia on ECoG to localize the epileptic focus. The proper use of anesthetic agents and careful dose titration, and effective communication between the neurophysiologist and anesthesiologist based on ECoG activity are essential in optimizing recordings. Anesthesia is a crucial variate to consider in the design of studies investigating ECoG and related biomarkers.

Methohexital-Induced Seizure in a Patient Undergoing Conscious Sedation

BACKGROUND

Methohexital is a short-acting barbiturate used for procedural sedation in the emergency department (ED). As with other sedatives, adverse effects with methohexital include excess sedation and hypotension, but this agent can also lower the seizure threshold. We report a patient who developed a generalized seizure after administration of methohexital.

CASE REPORT

A 60-year-old man presented to the ED by ambulance with chest pain and shortness of breath. Paramedics had administered adenosine for supraventricular tachycardia without conversion before arrival to the ED. He had no history of seizures. His initial vital signs in the ED included heart rate of 189 beats/min with a supraventricular rhythm, blood pressure 137/108 mm Hg, respiration 22 breaths/min, and oxygen saturation of 98% on room air. It was decided to attempt synchronized electrical cardioversion, and methohexital 1 mg/kg (120 mg) was administered over 2 min for moderate sedation. Within 15 s of methohexital administration, the patient developed a generalized seizure that lasted for 90 s. After seizure termination, he was successfully cardioverted, returned to his previous baseline level of consciousness within 20 min, and discharged without further problems with a follow-up referral to neurology. WHY SHOULD AN EMERGENCY PHYSICIAN BE AWARE OF THIS?: Methohexital is a short-acting barbiturate used for moderate sedation. Its adverse effects are unique in that it can lower the seizure threshold in some patients. Alternative agents for sedation should be considered in individuals with possible seizure disorders.

Etomidate activates epileptic high frequency oscillations

OBJECTIVE

The short acting anesthetic etomidate has been shown to provoke epileptic spikes and rarely seizures. Influence of etomidate on the occurrence of epileptic HFO (high frequency oscillations) however is unknown. An HFO inducing effect of etomidate would allow further validation of the substance as a provocation measure in presurgical evaluation as well as provide insights into the common mechanisms of HFO, spike and seizure generation.

METHODS

We retrospectively analyzed EEG data from four patients who underwent etomidate activation during invasive video-EEG monitoring with subdural strip electrodes. Spikes were manually selected in raw data, HFO in band pass filtered data (80-250Hz). Rate and spatial distribution of HFO and spikes in three segments were compared: immediately after etomidate administration, as well as during slow wave sleep and while awake.

RESULTS

Rates of HFO and spikes increased significantly after etomidate administration: Overall average rates of spikes were 9.7/min during sleep, 10/min while awake and 61.4/min after etomidate. Average HFO rates were 9.5/min during sleep, 8.3/min while awake and 24.4/min after etomidate (p<0.001, non-parametric ANOVA). Spatial distributions of HFO and spikes after administration of etomidate were consistent with the seizure onset zone (SOZ) and area of resection when available (SOZ: two patients; resection: one patient; no information: one patient). Except for spurious events, no additional HFO and spike foci were seen with activation.

CONCLUSIONS

Etomidate administration activates spikes and HFO. Spatial distributions do not extend beyond electrodes showing spikes and HFO without Etomidate and seem consistent with the epileptic network.

SIGNIFICANCE

Etomidate activation is a safe procedure to provoke not only epileptic spikes but also HFO, which were shown to have a high specificity for the SOZ.

The anesthetic considerations of intraoperative electrocorticography during epilepsy surgery

Epilepsy surgery is a well-established therapeutic intervention for patients with medically refractory seizures. Success of epilepsy surgery depends on the accurate localization and complete removal of the epileptogenic zone. Despite the advances in presurgical localization modalities, electrocorticography is still used in approximately 60% to 70% of the epilepsy centers in North America to guide surgical resection of the epileptogenic lesion and to assess for completeness of surgery. In this review, we discuss the principles and intraoperative use of electrocorticography, the effect of anesthetic drugs on electrocorticography, and the use of pharmacoactivation for intraoperative localization of epileptogenic zone.

Anesthetic management of the patient with epilepsy or prior seizures

PURPOSE OF REVIEW

Epilepsy is a clinical disorder of paroxysmal recurring seizures, the diagnosis excluding alcohol or drug withdrawal seizures or such recurring exogenous events as repeated insulin-induced hypoglycemia. Epilepsy has a profound impact on each individual diagnosed with this disease.

RECENT FINDINGS

New antiepileptic drugs (AEDs) have been a major change in the approach to management of patients with epilepsy. These drugs tend to have fewer significant drug interactions and less severe side effects. Nonetheless, first-generation AEDs are still widely used. Propofol and desflurane have reliable anticonvulsant effects, whereas remifentanil in larger doses and sevoflurane appear to support epileptiform activity, although the clinical significance of these observations is unclear.

SUMMARY

The primary concerns for providing anesthesia to the patient with epilepsy are the capacity of anesthetics to modulate or potentiate seizure activity and the interaction of anesthetic drugs with AEDs. Proconvulsant and anticonvulsant properties have been reported for virtually every anesthetic such that these properties become elements of the anesthetic plan in the patient with epilepsy. Moreover, AEDs have many physiologic and pharmacologic effects that can have an impact on an anesthetic.

Language assessment in Wada test: comparison of methohexital and amobarbital

INTRODUCTION

Methohexital has replaced amobarbital during Wada testing at many centers. The objective of our study was to compare the use of methohexital and amobarbital during Wada testing regarding language and memory lateralization quotients as well as speech arrest times.

METHODS

A chart review of 582 consecutive patients undergoing 1041 Wada-procedures was performed (left=60, right=63, bilateral=459). Language lateralization was calculated based on duration of speech arrest using a laterality index, defined as (L-R)/(L+R). Memory lateralization was expressed as percentage of retained objects and laterality quotient.

RESULTS

Language and memory lateralization revealed a similar distribution with amobarbital and methohexital. Speech arrest after left and right-sided injection was significantly longer in the amobarbital group as compared to the methohexital group. Language lateralization did not differ in the two groups. Percentage of retained memory items was higher in the methohexital group and there were fewer presented test items in the methohexital group.

DISCUSSION

Language and memory testing during the Wada test can successfully be performed with methohexital instead of amobarbital. The shorter half-life of methohexital allows repeated injections and shorter interhemispheric testing intervals, but also shortens the testing window.

Presurgical evaluation of epilepsy by brain diffusion: MR-detected effects of flumazenil on the epileptogenic focus

PURPOSE

After focal status epilepticus, focal alterations of the apparent diffusion coefficient (ADC) have been demonstrated in the epileptogenic zone by using diffusion-weighted magnetic resonance (MR)imaging (DWI). Effects of flumazenil on an epileptogenic focus have been demonstrated by EEG recordings, but not by functional MRI. We hypothesized that dynamic spatiotemporal alterations of brain diffusion of the epileptogenic focus after application of flumazenil will be detectable by DWI and correlate with the epileptogenic zone.

METHODS

Twelve adult patients considered for epilepsy surgery with medically intractable temporal lobe epilepsy (TLE; n = 7), extratemporal lobe epilepsy (ETE; n = 2), and TLE+ETE (n = 3) were prospectively examined with DWI interictally (serving as baseline) and 10 min after application of 1 mg flumazenil i.v.

RESULTS

The baseline interictal ADC was significantly elevated in the hippocampus on the ictogenic side in the patients with TLE (p = 0.002) as compared with healthy volunteers. The following changes of the mean ADC were seen in different regions of interest (ROIs) after injection of flumazenil: decreases in the hippocampus on the seizure-onset side by 14.8% (p = 0.005); decreases in the parahippocampal gyrus on both sides by 6.8% (epileptogenic side; p = 0.044) or 7.9% (nonepileptogenic side; NS), respectively; decreases in the cortex on the nonictogenic side by 7.9% (p = 0.047); and no significant changes of the ADC in the other ROIs.

CONCLUSIONS

ADC decreases measured after application of flumazenil were seen in the seizure-onset zone as revealed by EEG and structural MRI and are an indicator of focus localization in patients with TLE.

Sodium methohexital (brevital) as an anesthetic in the Wada test

PURPOSES

We report our experience with sodium methohexital (Brevital) as an anesthetic used in the Wada test for language and memory in 86 epilepsy surgery patients (173 procedures).

METHODS

The methods are compared with those of the more commonly used anesthetic sodium amobarbital (Amytal).

RESULTS

Despite differences between the methohexital and amobarbital test protocols, the behavioral and neurologic effects of the two anesthetics are similar. Because of the brief duration of methohexital, two successive injections are made on each side rather than one, to lengthen the time available for testing both language and memory. Behavioral and EEG indices return to baseline more quickly and more completely with methohexital than with amobarbital, allowing several repetitions of the procedure without incremental drowsiness, and the total time taken for the procedure is less with methohexital than with amobarbital.

CONCLUSIONS

The results of language and memory testing in the Wada test are equivalent for amobarbital and methohexital, except that methohexital has a briefer duration of action and is associated with less sedation.

Activation of interictal spiking in mesiotemporal lobe epilepsy by propofol-induced sleep

The objective of this study was to test whether low-dose propofol increases the number of interictal spikes in patients with mesiotemporal lobe epilepsy, and to determine whether this is the result of intrinsic properties and is restricted to the primary epileptogenic focus. Controlled infusion of propofol in step-up/-down target concentrations of 0, 0.3, 0.6, and 0.8 mg/L was administered to 10 patients during a 3.5-hour daytime EEG registration. The number of spikes were counted and related to propofol concentration and sleep level. Results were compared with a spontaneous, nocturnal first sleep cycle in 9 of 10 patients. All patients entered nonrapid eye movement 1 sleep during propofol administration, and 8 reached nonrapid eye movement 2 sleep. In 7 patients who showed spikes, spikes were related to sleep (P < 0.05) and not to increasing (P = 0.1) or decreasing (P = 0.5) propofol concentration. Six of nine patients showed more spikes during spontaneous (nocturnal) sleep than during propofol-induced sleep. Contralateral spiking was not suppressed selectively. Low-dose propofol is a safe means of increasing spiking in these patients because it induces sleep. There were no signs of an intrinsic epileptogenicity of propofol or a selective effect on ipsilateral spikes. Controlled sleep induction will increase the yield of interictal spikes during short interictal recordings such as in magnetoencephalography.

Alfentanil-induced activation: a promising tool in the presurgical evaluation of temporal lobe epilepsy patients

Pharmacologic activation of epileptic foci has been used experimentally with the hope that it may accelerate the presurgical evaluation of patients with medically intractable epilepsy. In this article, we will review the existing literature on these activating tests giving emphasis on the opioid analogs, and particularly alfentanil. Alfentanil is an opioid analog with rapid anesthetic effect, which has been known to trigger epileptiform discharges in epilepsy patients. 58 temporal lobe epilepsy (TLE) patients were studied with alfentanil activation during electrocorticography, at the Epilepsy Surgery Unit (ING, Brazil). An increase of the interictal epileptiform discharges was observed originating from hippocampal and parahippocampal regions (96.5%). To a lesser extent, alfentanil activated the basal and lateral temporal regions. Electrographic seizures were observed in 38%. In addition, we performed continuous video-EEG (VT/EEG) monitoring, with scalp and bilateral foramen ovale electrodes, in 12 TLE patients. The results of spontaneously observed seizures were compared with the electrographic changes following alfentanil activation (50-75 microg/kg, i.v.). In seven cases, alfentanil triggered focal electrographic seizures, ipsilaterally to the side generating the spontaneous seizures and in two patients it produced bilateral sequential activation of the temporal lobes. Ictal SPECTs during the alfentanil test showed hyperperfusion at the lateral temporal regions, ipsilaterally to the activated area or bilaterally. In summary, our study confirms the activating effect of alfentanil, and provides a strong evidence for its selective activating effect on the temporal lobes of TLE patients. The ictal SPECT during alfentanil activation did not offer any additional advantage for the localization of the ictal onset.

Determinants of seizure threshold in ECT: benzodiazepine use, anesthetic dosage, and other factors

The electrical dosage of the ECT stimulus impacts on efficacy and cognitive side effects, yet seizure threshold (ST) may vary as much as 50-fold across patients. It would be desirable to predict ST on the basis of patient and treatment characteristics. In particular, concerns have been raised that benzodiazepine use and higher dosage of barbiturate anesthetics elevate ST. In a three-site study, ST was quantified at the first ECT session using an identical empirical titration procedure in 294 patients who met RDC and DSM-IIIR criteria for a major depressive episode. ST varied over a 35-fold range across patients treated with right unilateral (RUL) (n = 267) and bilateral (BL) (n = 27) ECT. Higher ST was associated with BL electrode placement (p = 0.001). Among patients treated with RUL ECT, univariate analyses indicated that higher ST was associated with advanced age (p < 0.001), male gender (p < 0.001), greater burden of medical illness (p < 0.001), weight (p < 0.01), duration of mood disorder (p < 0.01), and history of previous ECT (p < 0.05). Average lorazepam dose in the 48 hours prior to ECT was not associated with ST, but was associated with decreased seizure duration (p < 0.01). Absolute, but not weight-adjusted, methohexital dose was associated with ST (p < 0.01). Multivariate analyses in patients treated with unilateral ECT showed that only 27.6% of the variance in ST (p < 0.0001) could be predicted. In the multivariate analyses, only age (p = 0.0001), gender (p = 0.01), and methohexital dose (p = 0.0001) were independently related to ST. Low dosage of lorazepam and methohexital dosage below 1 mg/kg are unlikely to impact on ST. Given the limited capacity to predict ST, empirical titration remains the only accurate method to determine electrical dosage in RUL ECT.

Long-term inpatient audiovisual scalp EEG monitoring

Long-term audiovisual scalp EEG monitoring is an essential diagnostic tool for the evaluation of paroxysmal disorders. The definitive classification of both nonepileptic and epileptic events is often possible only with the use of this technique. Assessment of response to treatment and the noninvasive presurgical localization of seizure foci are other important uses. The optimization of both clinical semiology and electrophysiologic data obtained from such studies is the subject of significant research efforts. Outcomes studies and advanced EEG analysis research should ultimately serve to minimize the cost of this valuable technique as well as maximizing its utility.

Clonidine- and methohexital-induced epileptiform discharges detected by magnetoencephalography (MEG) in patients with localization-related epilepsies

PURPOSE

During presurgical evaluation, 14 patients with medically intractable focal epilepsies underwent magnetoencephalographic (MEG) recordings to localize the epileptogenic focus. To increase the number of epileptiform discharges required for MEG analysis, methohexital a short-acting barbiturate that is known to activate epileptiform activity, was used. Additionally, we investigated the spike-provoking properties of clonidine in comparison to methohexital.

METHODS

After oral premedication with clonidine, short-lasting anesthesia was provided by intravenously administered methohexital. The number and location of epileptiform MEG discharges were assessed after clonidine premedication and during methohexital anesthesia. Results were compared with baseline MEG recordings.

RESULTS

Methohexital increased the frequency of focal epileptiform discharges in eight of 13 patients (one of the 14 patients did not receive methohexital after premedication with clonidine). Additionally, premedication with clonidine was found to increase focal epileptiform discharges in nine of 14 patients. When compared with baseline MEG recordings, recordings after treatment with both clonidine premedication and methohexital anesthesia showed a significant increase in the total number of epileptiform signals and the number of spikes contributing to MEG source localizations.

CONCLUSIONS

This study confirms the selective proconvulsant effects of methohexital on the epileptogenic focus as suggested previously by EEG and electrocorticogram (ECoG) investigations. Additionally, our data establish for the first time that clonidine increases epileptiform activity in patients with seizure disorders. These results indicate that clonidine is suited as an activating agent for the localization of epileptogenic foci by means of MEG. This effect of clonidine on specific epileptic activity also indicates that clonidine should be used with caution as an antihypertensive drug in patients with seizure disorders.

Clonidine and methohexital-induced epileptic magnetoencephalographic discharges in patients with focal epilepsies

PURPOSE

During presurgical evaluation, 14 patients with medically intractable focal epilepsies underwent magnetoencephalographic (MEG) recordings for focus localization. To increase the number of epileptic discharges required for MEG analysis, we administered methohexital (MHT), a short-acting barbiturate known to provoke epileptic activity. We also investigated the spike-provoking properties of clonidine in comparison with MHT.

METHODS

Patients were briefly anesthetized with intravenously administered MHT after being premedicated orally with clonidine. Numbers and locations of epileptic MEG discharges were assessed after clonidine premedication as well as during MHT anesthesia. Results were compared with baseline MEG recordings.

RESULTS

MHT increased the frequency of focal epileptic discharges in 8 of 13 patients ( of the 14 patients did not receive MHT after premedication with clonidine). Premedication with clonidine also increased focal epileptic discharges in 9 of 14 patients. The numbers of epileptic signals and numbers of spikes contributing to MEG source localizations were significantly increased in MEG recordings under both treatment conditions (clonidine premedication and MHT anesthesia) as compared with baseline MEG recordings.

CONCLUSIONS

Our results confirmed the selective proconvulsive effects of MHT on the epileptic focus, as previously suggested by EEG and electrocorticographic (ECoG) investigations. However, our present data establish for the first time that clonidine increases epileptic activity in patients with seizure disorders and indicate that clonidine is suitable as an activating agent for localization of epileptogenic foci by MEG. This effect of clonidine on specific epileptic activity also indicates that specific care must be taken when clonidine is used as an antihypertensive drug in patients with seizure disorders.

Localisation of epileptic foci with electric, magnetic and combined electromagnetic models

We compare the localisation of epileptic foci by means of (1) EEG, (2) magnetoencephalography (MEG) and (3) combined EEG/MEG data in a group of patients suffering from pharmaco-resistant focal epilepsy. Individual epileptic events were localised by means of a moving dipole model in a 4-shell spherical head approximation. A patient's epileptic activity was summarised by calculating the spatial density distribution (DD) of all localised events, and the centre of gravity of DD was considered the most likely locus of seizure generation. To verify these loci a subgroup of 6 patients was selected, in which seizures could be related to a clearly identifiable lesion in MRI. On average, the combined EEG/MEG approach resulted in the smallest error (1.8 cm distance between calculated locus and the nearest lesion border); using only MEG yielded the largest error (2.4 cm), while EEG resulted in an intermediate value (2.2 cm). In the individual patients, EEG/MEG would also rank intermediate, but never worst. In summary, combining EEG/MEG appears to be a more robust approach to localisation than using only EEG or only MEG. Finally, we also report on the use of the barbiturate methohexital as a safe method of increasing the number of spike events during an EEG/MEG recording session.

Induction of burst-suppression and activation of epileptiform activity after methohexital and selective amygdalo-hippocampectomy

Electrocorticography (ECOG) compared the effects of methohexital (MTH) and selective amygdalo-hippocampectomy (selAH) upon lateral temporal neocortical epileptiform activity (EA) in 31 patients with mesial temporal epilepsy. Pre-excision ECOG showed independent neocortical EA before MTH in 12/31 and after MTH in 18/31. MTH (20-50 mg) activated neocortical EA in 12 cases and induced burst-suppression (BS) over temporal neocortex in 14/31. Post-excision ECOG showed neocortical EA in 21/31 and BS in 27/31: compared with pre-excision ECOG before MTH, selAH activated neocortical EA in 15 cases. Significant correlations were found between presence of pre-excisional neocortical EA and presence of post-excisional neocortical EA (P < 0.001) and between activation of pre-excisional neocortical EA by MTH and activation of (post-excisional) neocortical EA by selAH (P < 0.006). Presence or severity of BS in the post-excision ECOG was not correlated with presence, absence or activation of post-excisional EA. Presence of neocortical EA was significantly correlated with a higher pre-operative seizure frequency (P < 0.001) but not with duration of epilepsy nor surgical outcome. Both MTH and selAH can induce neocortical BS, likely through chemical and surgical disconnection of cortex, respectively. Unrelated to induction of BS, MTH and selAH appear to decrease threshold for expression of neocortical EA in a similar fashion.

Possibilities and limitations of magnetic source imaging of methohexital-induced epileptiform patterns in temporal lobe epilepsy patients

The usefulness of MEG-based techniques in lateralizing and localizing the epileptogenic area was investigated in the present study. Spontaneous and methohexital-induced spikes were studied in a group of 15 patients with temporomesial epilepsy using a 37-channel neuromagnetometer. The accuracy of the magnetic source imaging was compared to the results of electrocorticographic (ECoG) recordings. Differences of drug-induced spike densities in the MEG recordings between both sides confirmed a similar lateralizing power of the MEG and ECoG recordings. Source location analyses based on a moving dipole model resp. a rotating dipole model were performed using a spherical head model. After subdivision of the volume of each patient's head, 8 cm3 cubicles containing at least 3 source locations were projected onto the individual MRI scan and resulted in source locations within or close to the presurgically defined primary epileptogenic area only in 3 of the 15 patients. Spike induction by methohexital has the advantage of shortening the recording period as compared to recordings of interictal epileptiform discharges. However, the correlation analyses of spike densities from MEG and ECoG recordings and the source location analyses from MEG recordings indicate that spike generated in deep temporomesial structures may escape the MEG registration.

Electrocorticography and stimulation

Although acute electrocorticography (ECoG) is routinely used during epilepsy surgery there is little agreement as to its value nor criteria for its interpretation. Specific issues are reviewed on the basis of the literature and personal studies: does failure to resect the entire irritative zone prejudice seizure control, and are residual discharges predictive of failure; does activation of the ECoG by intravenous barbiturates provide information of clinical value; does intraoperative electrical stimulation help to improve localisation or avoid postoperative deficits; is the ECoG of value for monitoring functional procedures; can the value of ECoG be increased by new interpretive approaches? It is suggested that resection of the entire area of interictal discharge is not essential for satisfactory surgical outcome, but a distinction may need to be made between those discharging regions that function as pacemakers and those in which ECoG spikes appear secondarily. There is also evidence that, apart from any consideration of determining the area resected, the topography of epileptiform discharge may be predictive of pathology and surgical outcome. It is concluded that more detailed topographic and quantitative analysis of the ECoG is required before its value in planning surgery can be determined or objective interpretive criteria established.

Effect of barbiturate on epileptiform activity: comparison between intravenous and oral administration. Sphenoidal, zygomatic and temporal recordings

The present study was performed in order to compare: 1) the differences between oral and intravenous barbiturate on interictal epileptiform activity (sharp-waves and spikes) in the EEG, and 2) interictal epileptiform activity in the sphenoidal electrode compared to the temporal and zygomatic electrodes (an electrode placed at the cutaneous entry of the sphenoidal electrode) during intravenous barbiturate administration in patients with epilepsy. Two procedures were performed: 1) an oral pentobarbital sleep induction with 10-20 electrode placement including a zygomatic electrode, and 2) an intravenous thiopental sleep induction with the same electrode placement including a sphenoidal electrode. Thirty eight patients with complex partial seizures were included. During the oral pentobarbital procedure 34 of 38 (90%) patients showed interictal epileptiform activity compared with 22 of 38 (55%) patients during the intravenous thiopental procedure (p < 0.005). A interictal epileptiform focus was observed in 33 (87%) patients in the oral procedure and in 19 (50%) patients in the intravenous procedure (p < 0.01). Interictal epileptiform activity recorded in the sphenoidal electrode was also recorded in the zygomatic electrode. Except from two patients a good correlation was observed between the zygomatic electrodes and the F7/F8 electrodes. We conclude that administration of intravenous thiopental offers no advantage compared to the administration of oral pentobarbital as an activating procedure, and for standard interictal EEG recordings with sleep activation procedures, suitable places scalp electrodes including a zygomatic electrode with the use of oral pentobarbital may be sufficient.

Effect of isoflurane (Forane) on intraoperative electrocorticogram

Isoflurane, an inhalation agent often used for general anesthesia during craniotomy, has been reported to suppress spike activity in the intraoperative electrocorticogram (ECoG) during epilepsy surgery. We studied the effect of isoflurane concentrations of 0.25, 0.5, 0.75, 1, and 1.25% on the number of spike bursts per 5-min epochs in 15 patients undergoing ECoG during epilepsy surgery. N2O in O2 was maintained at 50% in 10 patients, at 60% in 2, and at 70% in 3. End tidal CO2 concentration was maintained in the hypocarbic range, and analgesia was maintained with the narcotic alfentanil in the range of 0.5-2 micrograms/min. The median number of spikes for each isoflurane concentration was 29 (range 3-107) at 0.25%, 27 (range 2-73) at 0.5%; 29 (range 5-90) at 0.75%, 33 (range 2-100) at 1%, and 40 (range 32-140) in 5 patients who tolerated 1.25% without occurrence of burst suppression pattern. No significant difference (Student's paired t test) was noted in the number of spikes for each isoflurane concentration. Therefore, if isoflurane concentrations are maintained between 0.25 and 1.25% or before burst suppression pattern occurs and N2O/O2 is maintained in the 50-70% range, isoflurane has no significant effect on spike activity.

EEG manifestations during ECT: effects of electrode placement and stimulus intensity

This study examined the ictal electroencephalographic (EEG) characteristics of four forms of electroconvulsive therapy (ECT) known to differ in efficacy. Previously, we demonstrated that titrated, low-dose right unilateral ECT reliably produces generalized seizures of adequate duration, but is remarkably weak in antidepressant effects. Using a new rating scale, we found that specific features of the ictal and immediate postictal EEG varied significantly with ECT stimulus intensity and electrode placement. The low-dose right unilateral condition differed from more effective forms of ECT in having the longest polyspike phase duration, averaging twice that of the other conditions; it was also the condition least likely to manifest bioelectric suppression immediately following seizure termination. In contrast, high-dose bilateral ECT--a treatment with particularly rapid antidepressant effects--resulted in the greatest peak slow-wave amplitude in both hemispheres. Total seizure duration did not differ among the four treatment conditions. These findings indicate that seizure duration is not a useful marker of therapeutic efficacy, and instead provide preliminary evidence that other features of the EEG may be more useful markers of treatment adequacy.

Electrocorticography in patients with medically intractable temporal lobe seizures. II. Quantification of epileptiform discharges following successive stages of resective surgery

We quantified retrospectively the interictal epileptiform discharges (EDs) detected visually in the electrocorticograms (ECoGs) of 42 patients undergoing successive stages of anterior temporal lobectomy for medically intractable temporal lobe seizures (TLS). Following first resection sparing the hippocampus (H) and the parahippocampal gyrus (PHG), EDs were recorded on both structures in all patients and by far exceeded in amount those on residual lateral infratemporal and lateral temporal cortices. Frequently, EDs occurred apparently simultaneously but with opposite polarities on the H and the PHG, but more complex relationships were also evident in most individuals. These features likely reflected abnormal post-synaptic activity generated at different locations and cortical depths within the H, PHG, or both. Quantification of epileptiform activity and the effects of selective anterior hippocampectomy or parahippocampectomy suggested that both the H and PHG had remarkable epileptogenic potential. Levels of epileptiform activity were not significantly different in the H and PHG and in the H of subjects with and without H sclerosis. After final resection, including the amygdaloid nucleus (AN), anterior H and PHG, interictal EDs were present, although markedly diminished, in 35 patients. Postresection foci were significantly less numerous and extensive, and attained smaller maximal voltages, than did foci before and after first resection.

Use of increased anesthetic dose prior to electroconvulsive therapy to prevent postictal excitement

The authors report on three patients who developed severe postictal excitement at several consecutive electroconvulsive therapy (ECT) treatments. In all three cases, an increase in the anesthetic (or equivalent medication) dosage prior to ECT prevented the emergence of postictal excitement at subsequent treatments. This strategy, among others, should be considered in the management of patients who repeatedly manifest this phenomenon during a course of ECT.

Localization of the epileptic focus during methohexital-induced anesthesia

A short anesthesia was provided by methohexital for painless percutaneous removal of subdural electrodes in 27 patients with medically intractable, complex partial seizures who had undergone invasive preoperative evaluation. Electrocorticographic(ECoG) recordings performed before and during the narcosis were submitted to visual (n = 27) and computerized (n = 3) analysis to obtain additional information about the location of the epileptic focus (or foci). The following observations were made: focal epileptiform potentials were induced in 24 of 27 patients (89%); (b) in 20 of these 24, the induced spikes appeared amid or were followed by isoelectric or subdelta activity. These spikes were characterized by high amplitude as well as rhythmic and synchronized appearance over a circumscribed focal area (or areas) known to be spontaneously epileptogenic. This phenomenon, termed spike-burst-suppression (SBS) pattern, allowed identification of the primary epileptic focus in many patients with temporal lobe epilepsy. Furthermore, it indicated a good outcome of epilepsy surgery. Computerized analysis of the induced synchronized spikes (n = 3) allowed further delineation of the primary site of epileptogenicity and quantitative comparison of multiple epileptic generators. ECoG recording during deep methohexital-induced narcosis is a valuable tool for lateralization and delineation of the primary epileptogenic focus.

Interictal spike detection comparing subdural and depth electrodes during electrocorticography

We compared the ability of subdural and depth electrodes to detect and localize interictal epileptiform discharges (IEDs) in the temporal lobe. Sixteen patients had simultaneous intraoperative recordings with depth and subdural electrodes while undergoing anterior temporal lobe resections under local anesthesia for medically intractable seizures. IEDs that were focal (detected at just 1 or 2 electrode contacts) typically registered at the nearest contact, regardless of type. IEDs that were regional (engaging more than 2 electrode contacts) typically appeared simultaneously at both electrode types. Neither method was better able to indicate whether an IED was mesial or lateral, posterior or anterior. Subdural and depth electrodes seem to provide complementary information on the location of IEDs within the temporal lobe.

Methohexital (Brevital) effect on electrocorticogram may be misleading

Intravenous (i.v.) methohexital (MTH, Brevital) was found to have an effect on the intraoperative electrocorticogram (ECOG) of 63 patients who had temporal lobectomies performed under general anesthesia for intractable complex partial seizures. In the preresection ECOG, MTH increased the frequency of spikes in 78%, the area of cortical spiking in 30% and induced seemingly "new" spike foci in 43%. Similar although less dramatic changes occurred in the final (i.e., postresection) ECOG. Whether these changes induced by MTH, specifically the new spike foci, are significant was assessed by correlating surgical results with the presence of "residual spikes" (i.e., after all resections, not spontaneously occurring but activated by MTH). Surprisingly, nine patients with residual "MTH-spikes" did not have any postoperative seizures whereas two had some. This raises the question of whether MTH effects are significant overall. Caution is advised in the use of MTH in intraoperative assessment of interictal spike fields, especially when new spike foci are activated. Further study of the possibility of false activation, with a larger series, is advised.

Activation of the epileptic focus during intracarotid amobarbital test. Electrocorticographic registration via subdural electrodes

During presurgical evaluation, bilateral intracarotid amobarbital tests were performed in 21 patients (42 tests) to establish the lateralization of cerebral speech dominance as well as memory function. All patients suffered from long-standing, medically intractable, complex partial seizures. Electrocorticographic recording during the investigation was carried out via bilaterally implanted subdural electrodes. The aim of the study was to assess amobarbital-induced effects on the primary epileptic focus, determined throughout presurgical evaluation. Following the application of amobarbital the primary epileptic focus was selectively activated in 10 and preferentially activated in 3 patients suffering from temporal lobe epilepsy. Specific responses of the primary epileptic focus consisted of: (i) induction of a spike-burst-suppression pattern, mainly in the mesio-basal aspects of the focal temporal lobe (n = 13); (ii) induction of spikes or sharp waves following contralateral intracarotid injection (n = 10); (iii) late induction of spikes or sharp waves following ipsilateral injection (n = 4); (iv) loss or marked reduction of drug-induced beta activity over the primary epileptic focus (n = 14) following ipsilateral or contralateral injection. In conclusion, electrocorticographic recording during the intracarotid amobarbital test contributes valuable information about the primary epileptic focus in a high proportion of patients.

Clinical features, pathogenesis and management of drug-induced seizures

Many classes of pharmacological agents have been implicated in cases of drug-induced seizures. The list includes antidepressant drugs, lithium salts, neuroleptics, antihistamines (H1-receptor antagonists), anticonvulsants, central nervous system stimulants, general and local anaesthetics, antiarrhythmic drugs, narcotic and non-narcotic analgesics, non-steroidal anti-inflammatory drugs, antimicrobial agents, antifungal agents, antimalarial drugs, antineoplastic drugs, immunosuppressive drugs, radiological contrast agents and vaccines. For each of these classes of drugs, this article offers a revision of the literature and emphasises in particular the frequency of the adverse reaction, its clinical presentation, its presumed epileptogenic mechanism and the therapeutic strategy for the management of drug-induced seizures. An attempt is also made to distinguish seizures induced by standard dosages from those provoked by accidental or self-induced intoxication. For some classes of drugs such as antidepressants, neuroleptics, central nervous system stimulants (e.g. theophylline, cocaine, amphetamines) and beta-lactam antibiotics, seizures are a well recognised adverse reaction, and a large body of literature has been published discussing exhaustively the major aspects of the issue; sufficient data are available also for the other classes of pharmacological agents mentioned above. In contrast, several other drugs [e.g. allopurinol, digoxin, cimetidine, protirelin (thyrotrophin releasing hormone), bromocriptine, domperidone, insulin, fenformin, penicillamine, probenecid, verapamil, methyldopa] have not been studied thoroughly under this aspect, and the only source of information is the occasional case report. This review does not address the issue of seizures induced by drug withdrawal.