Lateralization of temporal lobe foci: depth versus subdural electrodes

Accuracy and Utility of Frameless Stereotactic Placement of Stereoelectroencephalography Electrodes

BACKGROUND

Successful surgery for epilepsy hinges on identification of the epileptogenic focus. Stereoelectroencephalography (sEEG) is the most effective way to identify most seizure foci. There are multiple methods of inserting depth electrodes, including frame-based, frameless, and robot-assisted techniques. Studies have shown the accuracy of frame-based and robotic-assisted techniques to be statistically similar, while only one study has detailed the frameless sEEG insertion technique.

METHODS

Patients underwent placement of sEEG depth electrodes using frameless stereotaxy from September 2019 to September 2021 at Geisinger Medical Center by a single surgeon. Seizure history, electrode placement accuracy relative to the planned trajectories, surgical times, success rate of identifying the epileptogenic focus, and subsequent seizure control rates after surgical treatment were documented.

RESULTS

Data were available for 21 patients and 181 electrodes inserted using the VarioGuide frameless stereotactic system. Each insertion took an average of 14.5 minutes per lead. Average entry variance was 2.7 mm with an average target variance of 4.6 mm. The epileptogenic focus was identified in 19 of 21 patients, and further surgical treatment was performed in 18 of 21 patients (85.7%).

CONCLUSIONS

VarioGuide frameless stereotaxy for sEEG placement is comparable to frame-based and robotic-assisted techniques with statistically similar rates of epileptic focus identification. Lead placement accuracy is slightly lower and time per lead is slightly higher relative to robot-assisted surgeries. When a robot system is unavailable, surgeons can consider using a frameless stereotactic technique for sEEG insertion, allowing patients to benefit from a similarly high rate of epileptic zone identification.

Combined Depth and Subdural Electrodes for Lateralization of the Ictal Onset Zone in Mesial Temporal Lobe Epilepsy with Hippocampal Sclerosis

(1) Objective: This study aimed to explore the efficacy of conventional invasive techniques in confirming unilateral seizure onset localization in mesial temporal lobe epilepsy with hippocampal sclerosis (MTLE-HS) and to investigate the association between electrode type and intracranial electroencephalography (EEG) pattern. (2) Methods: This retrospective study encompasses patients diagnosed with MTLE-HS who underwent an invasive study prior to an anterior temporal lobectomy (ATL). Intracranial EEG features were assessed for 99 seizure events from 25 selected patients who achieved seizure remission with ATL after an invasive study using bilateral combined depth and subdural electrodes. Their findings were compared to those of 21 seizure events in eight patients who exhibited suboptimal seizure outcomes. (3) Results: For the distribution of electrodes that recorded the ictal onset, hippocampal depth electrodes recorded 96% of all seizure events, while subdural electrodes recorded 52%. Among the seizures recorded in subdural electrodes, 49% were localized in medial electrodes, with only 8% occurring in lateral electrodes. The initiation of seizures exclusively detected in hippocampal depth electrodes was associated with successful seizure remission, whereas those solely recorded in the lateral strip electrodes were often linked to refractory seizures after ATL. (4) Conclusions: These findings emphasize the importance of employing a combination of depth and subdural electrodes in invasive studies for patients with MTLE-HS to enhance the accuracy of lateralization. This also cautions against sole reliance on subdural electrodes without depth electrodes, which could lead to inaccurate localization.

Changing concepts in presurgical assessment for epilepsy surgery

Candidates for epilepsy surgery must undergo presurgical evaluation to establish whether and how surgical treatment can stop seizures without causing neurological deficits. Various techniques, including MRI, PET, single-photon emission CT, video-EEG, magnetoencephalography and invasive EEG, aim to identify the diseased brain tissue and the involved network. Recent technical and methodological developments, encompassing both advances in existing techniques and new combinations of technologies, are enhancing the ability to define the optimal resection strategy. Multimodal interpretation and predictive computer models are expected to aid surgical planning and patient counselling, and multimodal intraoperative guidance is likely to increase surgical precision. In this Review, we discuss how the knowledge derived from these new approaches is challenging our way of thinking about surgery to stop focal seizures. In particular, we highlight the importance of looking beyond the EEG seizure onset zone and considering focal epilepsy as a brain network disease in which long-range connections need to be taken into account. We also explore how new diagnostic techniques are revealing essential information in the brain that was previously hidden from view.

Patient phenotypes and clinical outcomes in invasive monitoring for epilepsy: An individual patient data meta-analysis

OBJECTIVE

Invasive monitoring provides valuable clinical information in patients with drug-resistant epilepsy (DRE). However, there is no clear evidence indicating either stereoelectroencephalography (SEEG) or subdural electrodes (SDE) as the optimal method. Our goal was to examine differences in postresection seizure freedom rates between SEEG- and SDE-informed resective epilepsy surgeries. Additionally, we aimed to determine potential clinical indicators for SEEG or SDE monitoring in patients with drug-resistant epilepsy.

METHODS

A systematic literature review was performed in which we searched for primary articles using keywords such as "electroencephalography", "intracranial grid", and "epilepsy." Only studies containing individual patient data (IPD) were included for analysis. A one-stage IPD meta-analysis was performed to determine differences in rates of seizure freedom (International League Against Epilepsy (ILAE) guidelines and Engel classification) and resection status between SEEG and SDE patients. A Cox proportional-hazards regression was performed to determine the effect of time on seizure freedom status. Additionally, a principal component analysis was performed to investigate primary drivers of variance between these two groups.

RESULTS

This IPD meta-analysis compared differences between SEEG and SDE invasive monitoring techniques in 595 patients from 33 studies. Our results demonstrate that while there was no difference in seizure freedom rates regardless of resection (p = 0.0565), SEEG was associated with a lower rate of resection compared with SDE (82.00% SEEG, 92.74% SDE, p = 0.0002). Additionally, while SDE was associated with a higher rate of postresection seizure freedom (54.04% SEEG, 64.32% SDE, p = 0.0247), the difference between seizure freedom rates following SEEG- or SDE-informed resection decreased with long-term follow-up. A principal component analysis showed that cases resulting in SEEG were associated with lower risk of morbidity than SDE cases, which were strongly collinear with multiple subpial transections, anterior temporal lobectomy, amygdalectomy, and hippocampectomy.

SIGNIFICANCE

In this IPD meta-analysis of SEEG and SDE invasive monitoring techniques, SEEG and SDE were associated with similar rates of seizure freedom at latest follow-up. The former was associated with lower rates of resection. Furthermore, the clinical phenotypes of patients undergoing SEEG monitoring was associated with lower rates of complications. Future long-term prospective registries of IPD are promising options for clarifying the differences in these intracranial monitoring techniques as well as the unique patient phenotypes that may be associated with their indication.

Up and Down States of Cortical Neurons in Focal Limbic Seizures

Recent work suggests an important role for cortical-subcortical networks in seizure-related loss of consciousness. Temporal lobe seizures disrupt subcortical arousal systems, which may lead to depressed cortical function and loss of consciousness. Extracellular recordings show ictal neocortical slow waves at about 1 Hz, but it is not known whether these simply represent seizure propagation or alternatively deep sleep-like activity, which should include cortical neuronal Up and Down states. In this study, using in vivo whole-cell recordings in a rat model of focal limbic seizures, we directly examine the electrophysiological properties of cortical neurons during seizures and deep anesthesia. We found that during seizures, the membrane potential of frontal cortical secondary motor cortex layer 5 neurons fluctuates between Up and Down states, with decreased input resistance and increased firing rate in Up states when compared to Down states. Importantly, Up and Down states in seizures are not significantly different from those in deep anesthesia, in terms of membrane potential, oscillation frequency, firing rate, and input resistance. By demonstrating these fundamental similarities in cortical electrophysiology between deep anesthesia and seizures, our results support the idea that a state of decreased cortical arousal may contribute to mechanisms of loss of consciousness during seizures.

Method of invasive monitoring in epilepsy surgery and seizure freedom and morbidity: A systematic review

OBJECTIVE

Invasive monitoring is sometimes necessary to guide resective surgery in epilepsy patients, but the ideal method is unknown. In this systematic review, we assess the association of postresection seizure freedom and adverse events in stereoelectroencephalography (SEEG) and subdural electrodes (SDE).

METHODS

We searched three electronic databases (MEDLINE, Embase, and CENTRAL [Cochrane Central Register of Controlled Trials]) from their inception to January 2018 with the keywords "electroencephalography," "intracranial grid," and "epilepsy." Studies that presented primary quantitative patient data for postresection seizure freedom with at least 1 year of follow-up or complication rates of SEEG- or SDE-monitored patients were included. Two trained investigators independently collected data from eligible studies. Weighted mean differences (WMDs) with 95% confidence interval (CIs) were used as a measure of the association of SEEG or SDE with seizure freedom and with adverse event outcomes.

RESULTS

Of 11 462 screened records, 48 studies met inclusion criteria. These studies reported on 1973 SEEG patients and 2036 SDE patients. Our systematic review revealed SEEG was associated with 61.0% and SDE was associated with 56.4% seizure freedom after resection (WMD = +5.8%, 95% CI = 4.7-6.9%, P = .001). Furthermore, SEEG was associated with 4.8% and SDE was associated with 15.5% morbidity (WMD = -10.6%, 95% CI = -11.6 to -9.6%, P = .001). SEEG was associated with 0.2% mortality and SDE was associated with 0.4% mortality (WMD = -0.2%, 95% CI = -0.3 to -0.1%, P = .001).

SIGNIFICANCE

In this systematic review of SEEG and SDE invasive monitoring techniques, SEEG was associated with fewer surgical resections yet better seizure freedom outcomes in those undergoing resections. SEEG was also associated with lower mortality and morbidity than SDE. Clinical studies directly comparing these modalities are necessary to understand the relative rates of seizure freedom, morbidity, and mortality associated with these techniques.

Accuracy of frameless image-guided implantation of depth electrodes for intracranial epilepsy monitoring

OBJECTIVE

Various techniques are available for stereotactic implantation of depth electrodes for intracranial epilepsy monitoring. The goal of this study was to evaluate the accuracy and effectiveness of frameless MRI-guided depth electrode implantation.

METHODS

Using a frameless MRI-guided stereotactic approach (Stealth), depth electrodes were implanted in patients via burr holes or craniotomy, mostly into the medial temporal lobe. In all cases in which it was possible, postoperative MR images were coregistered to planning MR images containing the marked targets for quantitative analysis of intended versus actual location of each electrode tip. In the subset of MR images done with sufficient resolution, qualitative assessment of anatomical accuracy was performed. Finally, the effectiveness of implanted electrodes for identifying seizure onset was retrospectively examined.

RESULTS

Sixty-eight patients underwent frameless implantation of 413 depth electrodes (96% to mesial temporal structures) via burr holes by one surgeon at 2 institutions. In 36 patients (203 electrodes) planning and postoperative MR images were available for quantitative analysis; an additional 8 procedures with 19 electrodes implanted via craniotomy for grid were also available for quantitative analysis. The median distance between intended target and actual tip location was 5.19 mm (mean 6.19 ± 4.13 mm, range < 2 mm-29.4 mm). Inaccuracy for transtemporal depths was greater along the electrode (i.e., deep), and posterior, whereas electrodes inserted via an occipital entry deviated radially. Failure to localize seizure onset did not result from implantation inaccuracy, although 2 of 62 patients (3.2%)-both with electrodes inserted occipitally-required reoperation. Complications were mostly transient, but resulted in long-term deficit in 2 of 68 patients (3%).

CONCLUSIONS

Despite modest accuracy, frameless depth electrode implantation was sufficient for seizure localization in the medial temporal lobe when using the orthogonal approach, but may not be adequate for occipital trajectories.

Use of an intraventricular strip electrode for mesial temporal monitoring in children with medically intractable epilepsy

OBJECTIVE The objective of this study was to evaluate mesial temporal electroencephalographic (EEG) monitoring, using an intraventricular strip electrode (IVSE) along the ventricular surface of the hippocampus, in children with medically intractable epilepsy. METHODS The authors reviewed 10 consecutive cases in which subdural electrode placements and mesial temporal monitoring were recommended. The median age of the patients was 12.7 years (range 4.5-19.3 years). Both grids and IVSE were placed in all patients. The 4-contact IVSE was used in 5 cases, and the 6-contact IVSE in the other 5 cases. The median number of contacts, including IVSE contacts, was 122 (range 66-181). A total of 182 seizures were analyzed. RESULTS The IVSE localized seizure-onset zones in 8 patients. The seizure-onset zone was identified exclusively by IVSE in 3 patients and was simultaneous in IVSE and subdural electrodes in 5 patients. Among the 5 patients with simultaneous onset on both IVSE and subdural electrodes, 4 had basal temporal onset and one had orbitofrontal and lateral midtemporal onset. In the remaining 2 patients, the absence of IVSE seizure onset permitted sparing of mesial temporal structures. An Engel Class Ia outcome was achieved in 9 of 10 cases. No complication was associated with IVSE placement. CONCLUSIONS Intracranial monitoring using IVSE offers an alternative in terms of quality of EEG recording. IVSE was useful in children who already required open craniotomy for intracranial monitoring over an extensive network of hyper-excitability.

Depth versus subdural temporal electrodes revisited: Impact on surgical outcome after resective surgery for epilepsy

OBJECTIVE

To study retrospectively the impact of electrode modality (subdural or depth electrodes) during presurgical assessment on surgical outcome after temporal lobectomy.

METHODS

The study included 17 patients assessed with depth electrodes and 57 with bitemporal subdural strips.

RESULTS

MRI showed a larger proportion of bilateral pathology in patients undergoing depth recordings (29.41% versus 3.5%, p=0.00069). Among the operated patients, those undergoing depth electrode recordings showed better outcome at one year after surgery (11/12 versus 22/33; p=0.046). This difference disappears at longest follow up (10/12 versus 22/33; p=0.138). Moreover, the probability of undergoing surgery and having good outcome after assessment with intracranial recordings is higher for the depth electrode group at one-year follow up (11/17 versus 22/57; p=0.029) but statistical differences decrease to a trend for the longest follow up (10/17 versus 22/57; p=0.069). No other statistical differences were noted between subdural and depth electrodes. Depth electrodes showed lower complication rates than subdural electrodes.

CONCLUSION

Both depth and subdural electrodes are effective for presurgical assessment of temporal lobe epilepsy.

SIGNIFICANCE

Assessment with depth electrodes is associated with slightly increased likelihood of surgery and marginally better surgical outcome at one year follow up which disappears for longer follow up periods. Initial assessment with depth electrodes would have avoided a second implantation in 15% of patients.

High frequency oscillations for lateralizing suspected bitemporal epilepsy

OBJECTIVE

In some cases of single focus epilepsy, conventional video electroencephalography (EEG) cannot reveal the epileptogenic focus even when intracranial electrodes are used. Here, we tested whether analyzing high frequency oscillations (HFOs) can be used to determine the ictal onset zone in suspected bitemporal epilepsy and improve seizure outcome.

METHODS

We prospectively studied 13 patients with refractory temporal seizures who were treated over a 4-year period and underwent bilateral placement of intracranial electrodes. Subdural strips were used in all cases, and depth electrodes were implanted into mesial temporal lobes in 10 patients. The mean patient age was 30.92 years, and 30.7% of patients were male. Patients were monitored by conventional and wide-band frequency amplifiers.

RESULTS

Conventional invasive EEG monitoring of interictal periods showed bilateral epileptiform abnormalities in 12 patients (92.3%) and unilateral epileptiform abnormalities in one (7.7%), and monitoring of ictal periods revealed unilateral seizure origins in nine patients (69.2%) and bilateral origins in four (30.8%). In contrast, high frequency invasive EEG monitoring of interictal periods showed bilateral HFOs in seven patients (53.8%) and unilateral HFOs in six (46.2%), and monitoring of ictal periods revealed unilateral HFOs in all 10 patients who were tested. Three patients were not monitored during ictal periods because of time limitations. All 13 patients subsequently underwent a standard unilateral temporal lobectomy and have been followed-up for a minimum of 12 months. Eleven (84%) had a Class I outcome, one (8%) a Class II outcome, and one a Class III outcome.

SIGNIFICANCE

Bilateral placement of subdural strip and depth electrodes for seizure monitoring in patients with suspected bitemporal epilepsy is both safe and effective. Monitoring high frequency oscillations can help determine the laterality of the onset zone when localization using conventional EEG or brain MRI fails.

Subdural electrodes in focal epilepsy surgery at a typical academic epilepsy center

PURPOSE

To assess the use of subdural intracranial EEG (iEEG) on postoperative outcomes at an epilepsy center.

METHODS

Ninety-one epilepsy patients underwent iEEG. Array design was compared with noninvasive EEG with over 1 year of outcome. Patient demographics, implanted brain sites, electrodes, contacts/site, and surgical location were correlated with outcome. Fisher exact test and logistics regression were used to evaluate significance (P ≤ 0.05).

RESULTS

Of ninety-one women, 55 (mean age, 32.3 years; range, 11-60) underwent tailored iEEG. Seventy of ninety-one (76.9%) resections (70% temporal) yielded 24/91 (26.4%) seizure free (SF). Strips (57.1%), grids (5.5%), or both (37.4%) for iEEG use was commonly bilateral (58.2%; 65.3% bitemporal) but did not predict outcome (P = NS). A lesion (28/91) did predict a SF outcome (42.9%). The iEEG localized 45.7% of seizures beyond scalp EEG and changed the localization or lateralization in 75.7% of resected patients. Electrode design, localization, lateralization, and site of resection did not correlate with outcome (P = NS). Overall, iEEG use portended a non-SF outcome (P ≤ 0.0001).

CONCLUSIONS

The use of iEEG selected 46% additional patients for surgery, yet only 26% became SF. A magnetic resonance imaging lesion predicted a SF or seizure-improved outcome. Although iEEG changed the localization and lateralization of scalp ictal EEG in three quarters of patients, its use was a negative predictor for a favorable outcome. Preoperative counseling should emphasize expectations for seizure reduction in patients requiring iEEG.

Multimodal effective connectivity analysis reveals seizure focus and propagation in musicogenic epilepsy

Dynamic causal modeling (DCM) is a method to non-invasively assess effective connectivity between brain regions. 'Musicogenic epilepsy' is a rare reflex epilepsy syndrome in which seizures can be elicited by musical stimuli and thus represents a unique possibility to investigate complex human brain networks and test connectivity analysis tools. We investigated effective connectivity in a case of musicogenic epilepsy using DCM for fMRI, high-density (hd-) EEG and MEG and validated results with intracranial EEG recordings. A patient with musicogenic seizures was examined using hd-EEG/fMRI and simultaneous '256-channel hd-EEG'/'whole head MEG' to characterize the epileptogenic focus and propagation effects using source analysis techniques and DCM. Results were validated with invasive EEG recordings. We recorded one seizure with hd-EEG/fMRI and four auras with hd-EEG/MEG. During the seizures, increases of activity could be observed in the right mesial temporal region as well as bilateral mesial frontal regions. Effective connectivity analysis of fMRI and hd-EEG/MEG indicated that right mesial temporal neuronal activity drives changes in the frontal areas consistently in all three modalities, which was confirmed by the results of invasive EEG recordings. Seizures thus seem to originate in the right mesial temporal lobe and propagate to mesial frontal regions. Using DCM for fMRI, hd-EEG and MEG we were able to correctly localize focus and propagation of epileptic activity and thereby characterize the underlying epileptic network in a patient with musicogenic epilepsy. The concordance between all three functional modalities validated by invasive monitoring is noteworthy, both for epileptic activity spread as well as for effective connectivity analysis in general.

Invasive EEG explorations

The Wada test was adapted from the procedure described by Wada in 1964. It still has a role in the prognostic evaluation of memory disorders after mesial temporal lobectomy. The test consists of injecting a short-acting anesthetic into one hemisphere, under continuous EEG monitoring and during carotid catheterization, to verify the function of contralateral structures. Intracranial EEG recordings deliver signals with few artifacts, and which are quite specific of the zone explored. Three types of electrodes are in common use: (a) foramen ovale (FO) electrodes: electrodes can be inserted directly, without any stereotactic procedure, to provide easy and comparative EEG recordings of the lower and middle portions of the temporal lobe close to the hippocampus. These allow validation of the temporal lobe origin of seizures using FO electrodes recording coupled with scalp EEG; (b): subdural strip or grip electrodes. This relatively aggressive technique carries infectious and hemorrhagic risks and does not allow the exploration of deep cortical structures. However, it permits precise functional cortical mapping via electrical stimulation because of dense and regular positioning of electrodes over the cortical convexity; (c) stereotactically implanted depth electrodes (stereo-electroencephalography [SEEG]). Electrodes are individually planned and inserted within the brain parenchyma through small burr holes. This technique is less aggressive than subdural grid exploration. However it offers relatively limited spatial sampling that may be less well adapted to precise functional evaluation. It allows recording from deep cortical structures and can be argued to be the gold standard of presurgical EEG exploration.

[French guidelines on electroencephalogram]

Electroencephalography allows the functional analysis of electrical brain cortical activity and is the gold standard for analyzing electrophysiological processes involved in epilepsy but also in several other dysfunctions of the central nervous system. Morphological imaging yields complementary data, yet it cannot replace the essential functional analysis tool that is EEG. Furthermore, EEG has the great advantage of being non-invasive, easy to perform and allows control tests when follow-up is necessary, even at the patient's bedside. Faced with the advances in knowledge, techniques and indications, the Société de Neurophysiologie Clinique de Langue Française (SNCLF) and the Ligue Française Contre l'Épilepsie (LFCE) found it necessary to provide an update on EEG recommendations. This article will review the methodology applied to this work, refine the various topics detailed in the following chapters. It will go over the summary of recommendations for each of these chapters and underline proposals for writing an EEG report. Some questions could not be answered by the review of the literature; in those cases, an expert advice was given by the working and reading groups in addition to the guidelines.

Stereoelectroencephalography: surgical methodology, safety, and stereotactic application accuracy in 500 procedures

BACKGROUND

Stereoelectroencephalography (SEEG) methodology, originally developed by Talairach and Bancaud, is progressively gaining popularity for the presurgical invasive evaluation of drug-resistant epilepsies.

OBJECTIVE

To describe recent SEEG methodological implementations carried out in our center, to evaluate safety, and to analyze in vivo application accuracy in a consecutive series of 500 procedures with a total of 6496 implanted electrodes.

METHODS

Four hundred nineteen procedures were performed with the traditional 2-step surgical workflow, which was modified for the subsequent 81 procedures. The new workflow entailed acquisition of brain 3-dimensional angiography and magnetic resonance imaging in frameless and markerless conditions, advanced multimodal planning, and robot-assisted implantation. Quantitative analysis for in vivo entry point and target point localization error was performed on a sub--data set of 118 procedures (1567 electrodes).

RESULTS

The methodology allowed successful implantation in all cases. Major complication rate was 12 of 500 (2.4%), including 1 death for indirect morbidity. Median entry point localization error was 1.43 mm (interquartile range, 0.91-2.21 mm) with the traditional workflow and 0.78 mm (interquartile range, 0.49-1.08 mm) with the new one (P < 2.2 × 10). Median target point localization errors were 2.69 mm (interquartile range, 1.89-3.67 mm) and 1.77 mm (interquartile range, 1.25-2.51 mm; P < 2.2 × 10), respectively.

CONCLUSION

SEEG is a safe and accurate procedure for the invasive assessment of the epileptogenic zone. Traditional Talairach methodology, implemented by multimodal planning and robot-assisted surgery, allows direct electrical recording from superficial and deep-seated brain structures, providing essential information in the most complex cases of drug-resistant epilepsy.

Occipitotemporal hippocampal depth electrodes in intracranial epilepsy monitoring: safety and utility

Object

Intracranial monitoring for epilepsy has been proven to enhance diagnostic accuracy and provide localizing information for surgical treatment of intractable seizures. The authors investigated the usefulness of hippocampal depth electrodes in the era of more advanced imaging techniques.

Methods

Between 1988 and 2010, 100 patients underwent occipitotemporal hippocampal depth electrode (OHDE) implantation as part of invasive seizure monitoring, and their charts were retrospectively reviewed. The authors' technique involved the stereotactically guided (using the Leksell model G frame) implantation of a 12-contact depth electrode directed along the long axis of the hippocampus, through an occipital twist drill hole.

Results

Of the 100 patients (mean age 35.0 years [range 13–58 years], 51% male) who underwent intracranial investigation, 84 underwent resection of the seizure focus. Magnetic resonance imaging revealed mesial temporal sclerosis (MTS) in 27% of patients, showed abnormal findings without MTS in 55% of patients, and showed normal findings in 18% of patients. One patient developed a small asymptomatic occipital hemorrhage around the electrode tract. The use of OHDEs enabled epilepsy resection in 45.7% of patients who eventually underwent standard or selective temporal lobe resection. The hippocampal formation was spared during surgery because data obtained from the depth electrodes showed no or only secondary involvement in 14% of patients with preoperative temporal localization. The use of OHDEs prevented resections in 12% of patients with radiographic evidence of MTS. Eighty-three percent of patients who underwent resection had Engel Class I (68%) or II (15%) outcome at 2 years of follow-up.

Conclusions

The use of OHDEs for intracranial epilepsy monitoring has a favorable risk profile, and in the authors' experience it proved to be a valuable component of intracranial investigation. The use of OHDEs can provide the sole evidence for resection of some epileptogenic foci and can also result in hippocampal sparing or prevent likely unsuccessful resection in other patients.

Role of electroencephalography in presurgical evaluation of temporal lobe epilepsy

Surgery remains a therapeutic option for patients with medically refractory epilepsy. Comprehensive presurgical evaluation includes electroencephalography (EEG) and video EEG in identifying patients who are likely to benefit from surgery. Here, we discuss in detail the utility of EEG in presurgical evaluation of patients with temporal lobe epilepsy along with illustrative cases.

Evidence-based review on epilepsy and driving

OBJECTIVE

The aim of this study was to synopsize the evidence on predictors of crashes and driving status in people with epilepsy (PWE).

METHODS

Evidence-based review of the published English literature was the method used. We searched various databases and extracted data from 16 (of 77) primary studies. On the basis of American Academy of Neurology criteria, we assigned each study a class of evidence (I-IV, where I indicates the highest level of evidence) and made recommendations (Level A: predictive or not; Level B: probably predictive or not; Level C: possibly predictive or not; Level U: no recommendations).

RESULTS

For PWE, the following characteristics are considered useful: For identifying crash risk, epilepsy (level B) and short seizure-free intervals (≥3 months) (Level C) are not predictive of motor vehicle crash (MVC). For self/proxy-reported crash risk, epilepsy surgery (Level B), seizure-free intervals (6-12 months) (Level B), few prior non-seizure-related crashes (Level B), and regular antiepileptic drug adjustments (Level B) are protective against crashes; seizures contribute to MVCs (Level C); mandatory reporting does not contribute to reduced crashes (Level C). No recommendations for reliable auras, age, and gender (Level U), as data are inadequate to make determinations. For self-reported driving or licensure status, employment and epilepsy surgery are predictive of driving (Level C); there are no recommendations for antiepileptic drug use, self-reported driving, gender, age, receiving employment benefits, or having reduced seizure frequency (Level U).

CONCLUSION

Limitations, that is, heterogeneity among studies, examining the English literature from 1994 to 2010, must be considered. Yet, this is the first evidence-based review to synopsize the current PWE and driving literature and to provide recommendation(s) to clinicians and policy makers. Class I studies, matched for age and gender, yielding Level A recommendations are urgently needed to define the risks, benefits, and causal factors underlying driving performance issues in PWE.

Frameless stereotactic endoscope-assisted transoccipital hippocampal depth electrode placement: cadaveric demonstration of a new approach

PURPOSE

Hippocampal recording using depth electrodes is indicated in a small subgroup of patients with medically intractable seizures. There are several conventional techniques for implantation of hippocampal depth electrodes. We describe a new method for hippocampal depth electrode placement using an image-guided endoscopic transoccipital route. This technique is simple and effective, eliminating several drawbacks of conventional techniques.

METHODS

One silicone-injected cadaver head was used. A rigid endoscope sheath was inserted through a transoccipital corridor into the atrium of the lateral ventricle and then advanced to the temporal horn. Each of the hemispheres was cannulated. The hippocampus was identified visually, and a depth electrode was inserted into the substance of the hippocampus along its long axis under direct vision.

RESULTS

In both hemispheres we were able to successfully implant the depth electrode within the hippocampus. The advantages of our technique over conventional approaches are (1) there is no need for frame-based stereotaxy, thus reducing operating time and patient discomfort, (2) the electrodes are inserted into the hippocampus under direct endoscopic visualization, reducing the chance of injury to vascular structures, (3) there is no need to insert a larger cannula into the hippocampus before placement of the electrodes, reducing trauma to the hippocampus, and (4) the number of electrodes within the hippocampus can be assessed at the end of the procedure, reducing malposition.

CONCLUSION

We believe that image-guided endoscopic transoccipital hippocampal depth electrode placement can be performed with precision equal or superior to conventional techniques but without their major disadvantages.

Bilateral intracranial electrodes for lateralizing intractable epilepsy: efficacy, risk, and outcome

OBJECTIVE

Medically refractory epilepsy is amenable to neurosurgical intervention if the epileptogenic focus is accurately localized. If the scalp video-electroencephalography (EEG) and magnetic resonance imaging are nonlateralizing, yet a single focus is suspected, video-EEG monitoring with bilateral intracranial electrode placement is helpful to lateralize the ictal onset zone. We describe the indications, risks, and utility of such bilateral surveys at our institution.

METHODS

We retrospectively reviewed 26 patients with medically refractory seizures who were treated over a 5-year period and underwent bilateral placement of intracranial electrodes. Subdural strips were used in all cases, and additional stereotactic implantation of depth electrodes into mesial temporal lobes occurred in 50%. The mean patient age was 37.7 years, and 65.4% of patients were male.

RESULTS

The most common indication for bilateral invasive monitoring was bilateral ictal onsets on surface video-EEG (76.9%), followed by frequent interictal spikes contralateral to a single ictal focus (7.7%). Intracranial monitoring lasted an average of 8.2 days, with ictal events recorded in all cases. Ten patients (38.5%) subsequently underwent more extensive unilateral monitoring via implantation of subdural and depth electrodes through a craniotomy. A therapeutic procedure was performed in 17 patients (65.4%), whereas 1 patient underwent a palliative corpus callosotomy (3.8%). Nine patients underwent a resection without unilateral invasive mapping. Reasons for no therapeutic surgery (n = 8) included multifocal onsets, failing the Wada test, refusal of further treatment, and negative intraoperative electrocorticogram. There was 1 surgical complication, involving a retained electrode fragment that was removed in a separate minor procedure. Of the 26 patients, 15 (57.7%) are now seizure-free or have seizure disorders that have substantially improved (modified Engel classes I and II). Of the 17 patients who underwent a potentially curative surgery, 13 (76.5%) were Engel classes I and II.

CONCLUSION

Bilateral placement of subdural strip and depth electrodes for epilepsy monitoring in patients with nonlateralizing scalp EEG and/or discordant imaging studies but clinical suspicion for focal seizure origin is both safe and effective. Given the safety and efficacy of this procedure, epileptologists should have a low threshold to consider bilateral implants for suitable patients.

Consciousness and epilepsy: why are complex-partial seizures complex?

Why do complex-partial seizures in temporal lobe epilepsy (TLE) cause a loss of consciousness? Abnormal function of the medial temporal lobe is expected to cause memory loss, but it is unclear why profoundly impaired consciousness is so common in temporal lobe seizures. Recent exciting advances in behavioral, electrophysiological, and neuroimaging techniques spanning both human patients and animal models may allow new insights into this old question. While behavioral automatisms are often associated with diminished consciousness during temporal lobe seizures, impaired consciousness without ictal motor activity has also been described. Some have argued that electrographic lateralization of seizure activity to the left temporal lobe is most likely to cause impaired consciousness, but the evidence remains equivocal. Other data correlates ictal consciousness in TLE with bilateral temporal lobe involvement of seizure spiking. Nevertheless, it remains unclear why bilateral temporal seizures should impair responsiveness. Recent evidence has shown that impaired consciousness during temporal lobe seizures is correlated with large-amplitude slow EEG activity and neuroimaging signal decreases in the frontal and parietal association cortices. This abnormal decreased function in the neocortex contrasts with fast polyspike activity and elevated cerebral blood flow in limbic and other subcortical structures ictally. Our laboratory has thus proposed the "network inhibition hypothesis," in which seizure activity propagates to subcortical regions necessary for cortical activation, allowing the cortex to descend into an inhibited state of unconsciousness during complex-partial temporal lobe seizures. Supporting this hypothesis, recent rat studies during partial limbic seizures have shown that behavioral arrest is associated with frontal cortical slow waves, decreased neuronal firing, and hypometabolism. Animal studies further demonstrate that cortical deactivation and behavioral changes depend on seizure spread to subcortical structures including the lateral septum. Understanding the contributions of network inhibition to impaired consciousness in TLE is an important goal, as recurrent limbic seizures often result in cortical dysfunction during and between epileptic events that adversely affects patients' quality of life.

Remote effects of focal hippocampal seizures on the rat neocortex

Seizures have both local and remote effects on nervous system function. Whereas propagated seizures are known to disrupt cerebral activity, little work has been done on remote network effects of seizures that do not propagate. Human focal temporal lobe seizures demonstrate remote changes including slow waves on electroencephalography (EEG) and decreased cerebral blood flow (CBF) in the neocortex. Ictal neocortical slow waves have been interpreted as seizure propagation; however, we hypothesize that they reflect a depressed cortical state resembling sleep or coma. To investigate this hypothesis, we performed multimodal studies of partial and secondarily generalized limbic seizures in rats. Video/EEG monitoring of spontaneous seizures revealed slow waves in the frontal cortex during behaviorally mild partial seizures, contrasted with fast polyspike activity during convulsive generalized seizures. Seizures induced by hippocampal stimulation produced a similar pattern, and were used to perform functional magnetic resonance imaging weighted for blood oxygenation and blood volume, demonstrating increased signals in hippocampus, thalamus and septum, but decreases in orbitofrontal, cingulate, and retrosplenial cortex during partial seizures, and increases in all of these regions during propagated seizures. Combining these results with neuronal recordings and CBF measurements, we related neocortical slow waves to reduced neuronal activity and cerebral metabolism during partial seizures, but found increased neuronal activity and metabolism during propagated seizures. These findings suggest that ictal neocortical slow waves represent an altered cortical state of depressed function, not propagated seizure activity. This remote effect of partial seizures may cause impaired cerebral functions, including loss of consciousness.

Novel use of a custom stereotactic frame for placement of depth electrodes for epilepsy monitoring

The authors describe the first reported application of a miniature, customized, one-time use, skull-mounted stereotactic frame for the implantation of depth electrodes for epilepsy monitoring.

Using a platform template, 4 skull fiducial markers were placed 1 week prior to surgery. A brain MR image and a CT scan were subsequently obtained. All planning (longitudinal trajectories into the hippocampi) was done preoperatively using personal computers in the office. No further workstation planning was necessary on the day of the operation. The StarFix microTargeting Platform system was secured to the previously implanted skull fiducial screws. Pin fixation was not required. The platform was used to identify the area of entry for the depth electrodes on the right and left sides. On each side, a 12-contact depth electrode was advanced to the depth of the targets without difficulty. A temporal craniotomy was then performed to place subdural electrodes.

The desired location of the electrodes was confirmed on postoperative imaging studies. There were no complications associated with the electrode implantation. The depth electrodes demonstrated symmetrical, robust coverage of each hippocampus, with epileptiform discharges observed bilaterally.

This first application of the StarFix platform for placing depth electrodes for epilepsy monitoring was both safe and feasible. With this technique, the patient does not need to be pinned or placed in a head holder, no imaging or computer planning is required on the day of implantation (which means there is no time pressure when the meticulous target/trajectory planning is done), and with bilateral posterior implants both bur holes can be made simultaneously. For these reasons this system may be preferable to existing methods of depth electrode implantation.

Insertion of subdural strip electrodes for the investigation of temporal lobe epilepsy

✓ Temporal lobe epilepsy (TLE) is the most common type of surgically treatable epilepsy, with a considerable number of patients needing invasive electroencephalography monitoring. The authors describe a surgical technique used in the placement of subdural strip electrodes for coverage of the temporal lobe. The electrodes are inserted through an enlarged temporooccipital bur hole using fluoroscopic guidance. With this technique, subdural electrode strips can be safely placed to cover the mesial, inferior, and lateral temporal surfaces, and the seizure focus can be lateralized and localized within the temporal lobe. The technique does not require the use of a craniotomy, stereotactic frame, or neuronavigation systems. The authors compare this technique with previous descriptions of subdural electrode placement for the evaluation of TLE.

Usefulness of intracranial EEG in the decision process for epilepsy surgery

BACKGROUND AND PURPOSE

In patients with discordant results, non-localizing EEG, or bitemporal seizure onset, intracranial monitoring is done to confirm the seizure onset. Our aim was to assess the yield of intracranial recordings in patients with different clinical scenarios.

METHODS

The records of all patients who underwent prolonged intracranial EEG monitoring (IEM) at the London Health Sciences Centre, University of Western Ontario, Canada, between 1993 and 1999, identified using our EEG patient database in continuous use since December 1972, were reviewed. Patients were analyzed in the following groups according to perceived increasing degrees of uncertainty of epileptic zone localization-group 1: lesion on MRI congruent with focal ictal and interictal scalp EEG, but findings are subtle and of low level of certainty (n=13), group 2: focal MRI, focal ictal and multifocal interictal scalp EEG (n=11), group 3: focal MRI, non-localizing or incongruent scalp EEG (n=73), group 4: normal of multifocal MRI, focal ictal scalp EEG (n=11), group 5: multifocal MRI, non-localizing scalp EEG (n=18), and group 6: normal MRI, multifocal scalp EEG (n=36).

RESULTS

One hundred and seventy one patients underwent IEM at the London Health Sciences Centre between 1993 and 1999. All patients had localization-related epilepsy, plus or minus secondary generalization. IEM was helpful overall in 86% of patients, in 69% of group 1, 36% of group 2, 90% of group 3, 81% of group 4, 100 of group 5 and 92% of group 6.

CONCLUSIONS

Our study shows that the yield of the IEM was higher in the groups of patients with lack of congruence between the MRI and the scalp EEG. The yield was lower in patients with congruent but subtle or uncertain scalp EEG and MRI findings.

The use of multiplanar trajectory planning in the stereotactic placement of depth electrodes

OBJECTIVE

To assess the value of multiplanar reconstruction software in trajectory planning for depth electrode insertion in medically refractory epilepsy.

METHODS

A series of 29 patients undergoing frame-based hippocampal depth electrode insertion were identified. In 19 patients, preoperative trajectory planning was conducted in axial, coronal, and sagittal planes using standard-axis software. In 10 patients, preoperative trajectory planning was conducted with multiplanar reconstruction software. Postoperative magnetic resonance imaging scans were evaluated to study the quality of insertion. Target accuracy was assessed by measuring the mean shortest distance to strictly defined hippocampal borders in the coronal plane ("coronal deviation"). Additionally, the number of electrode contacts placed within the amygdalohippocampal structure was assessed.

RESULTS

With the use of multiplanar reconstruction software, there was a statistically insignificant increase in coronal deviation (standard-axis software group, 0.09 +/- 0.50 mm; multiplanar reconstruction group, 0.37 +/- 1.16 mm). However, the use of multiplanar planning strategies resulted in approximately one additional electrode contact inserted in the amygdalohippocampal structure (standard-axis software group, 3.42 +/- 0.89; multiplanar reconstruction group, 4.36 +/- 0.93; P < 0.01).

CONCLUSION

The use of reconstructed planes in preoperative trajectory planning allows for the insertion of additional electrode contacts within the target structure.

Placement of subdural electrode grids for seizure focus localization in patients with a large arachnoid cyst. Technical note

Subdural electrode arrays are placed to localize seizure foci for possible resection. The procedure is usually straightforward when an electrode grid array is placed on the brain convexity but can become complicated if the surface on which the grids are applied is not convex. Arachnoid cysts can be associated with seizures, but their topography presents a challenge to standard techniques for the placement of subdural grids. The authors report on a technique for electrode grid placement that successfully localized seizure foci in the depths of arachnoid cysts in two patients. Subdural grids were placed to conform to the concave cyst cavity. They were held in place with rolled gelatin foam padding, which filled the arachnoid cyst. The padding was removed before removing the electrode grids and resecting the seizure focus. Although arachnoid cysts present a technical challenge when seizure foci are located within the cyst cavity, the technique of packing the cyst cavity with gelatin foam provides good electrode contact on the concave cyst wall, allowing adequate seizure focus localization.

Ictal neocortical slowing in temporal lobe epilepsy

BACKGROUND

Temporal lobe epilepsy (TLE) may affect brain regions outside the temporal lobe, causing impaired neocortical function during seizures.

METHODS

The authors selected 11 consecutive patients with mesial TLE and hippocampal sclerosis who underwent intracranial EEG monitoring and had no seizures during a follow-up period of at least 1 year after temporal lobe resection. Secondarily generalized seizures were excluded, and up to three seizures were analyzed per patient (31 seizures total). Electrode contacts were assigned to one of nine cortical regions based on MRI surface reconstructions. EEG during seizures was analyzed for specific patterns including low-voltage fast (LVF), rhythmic polyspike, spike-wave, irregular slowing, and postictal suppression.

RESULTS

Mesial and lateral temporal contacts on the side of seizure onset showed significant increases in ictal patterns such as LVF and polyspike activity, followed by postictal suppression. Bilateral frontal and ipsilateral parietal cortex exhibited large amplitude irregular slow waves during seizures. This frontoparietal slowing persisted into the postictal period. Perirolandic and occipital cortex were relatively spared. These EEG patterns were accompanied by bland staring, minor automatisms, and unresponsiveness or amnesia in the majority of patients studied.

CONCLUSIONS

Prominent irregular slowing occurs in bilateral frontal and ipsilateral parietal association cortex during and after temporal lobe seizures. EEG slowing in the frontoparietal association cortex may signify physiologic impairment that contributes to widespread altered cerebral function during partial seizures.

Detection of abnormal cerebral excitability by coincident stimulation and recording

OBJECTIVE

A method for mapping brain excitability and detecting abnormalities, by concurrently stimulating and recording 'focal' compound responses through one microelectrode, was evaluated in three rat epilepsy models in comparison with distal stimulation of perforant path afferents.

METHODS

A fixed trajectory from neocortex to dentate gyrus was mapped under halothane anesthesia. Several weeks earlier, tetanus toxin or vehicle was microinjected into the dentate polymorphic layer, or else rats were genetically epilepsy-prone (GEPR-9) or epilepsy-resistant (GERR-0). Other (unmapped) rats received acute penicillin microinjections within the dentate granular layer.

RESULTS

Focal responses, although widespread, proved largest in the dentate (>+/-0.5 mV). Tetanus toxin diminished focal responses near the microinjection site versus vehicle-microinjected (66%) or contralateral controls (55%), but enhanced them elsewhere in the dentate. It enhanced distal responses at all hippocampal locations. Focal but not distal responses were higher in GEPR-9 than in GERR-0 rats at widespread forebrain locations (mean 233%). Penicillin facilitated both focal and distal dentate responses, but the focal facilitation peaked sooner (about 75 versus 180 min).

CONCLUSIONS

Focal responses better uncover pervasive or discrete excitability differences.

SIGNIFICANCE

Focal mapping may aid in diagnostic imaging and intraoperative targeting, offering high resolution, rapid performance, low stimulus currents and minimal invasion.

Use of an anteromedial subdural strip electrode in the evaluation of medial temporal lobe epilepsy. Technical note

The temporal lobe is the most common site of partial epilepsy that is amenable to surgical therapy, and therefore ictal localization in this region is important. The authors describe the application of an anteromedial subdural strip electrode for the evaluation of epilepsy originating from the medial temporal lobe. This strip is advanced around the temporal pole and underneath the lesser wing of the sphenoid bone as it follows the medial temporal lobe contour. The advantages of this method of placement are the consistent path and reliable final position of the strip along the medial basal temporal lobe surface. This method allows adequate coverage of the parahippocampal gyrus along its long axis extending posterior to the level of the collicular plate. This technique has been used with no complications during intracranial monitoring of more than 100 patients with presumed temporal lobe epilepsy.

Complications of invasive subdural grid monitoring in children with epilepsy

OBJECT

This study was performed to evaluate the complications of invasive subdural grid monitoring during epilepsy surgery in children.

METHODS

The authors retrospectively reviewed the records of 35 consecutive children with intractable localization-related epilepsy who underwent invasive video electroencephalography (EEG) with subdural grid electrodes at The Hospital for Sick Children between 1996 and 2001. After subdural grid monitoring and identification of the epileptic regions, cortical excisions and/or multiple subpial transections (MSTs) were performed. Complications after these procedures were then categorized as either surgical or neurological. There were 17 male and 18 female patients whose mean age was 11.7 years. The duration of epilepsy before surgery ranged from 2 to 17 years (mean 8.3 years). Fifteen children (43%) had previously undergone surgical procedures for epilepsy. The number of electrodes on the grids ranged from 40 to 117 (mean 95). During invasive video EEG, cerebrospinal fluid leaks occurred in seven patients. Also, cerebral edema (five patients), subdural hematoma (five patients), and intracerebral hematoma (three patients) were observed on postprocedural imaging studies but did not require surgical intervention. Hypertrophic scars on the scalp were observed in nine patients. There were three infections, including one case of osteomyelitis and two superficial wound infections. Blood loss and the amounts of subsequent transfusions correlated directly with the size and number of electrodes on the grids (p < 0.001). Twenty-eight children derived significant benefit from cortical resections and MSTs, with a more than 50% reduction of seizures and a mean follow-up period of 30 months.

CONCLUSIONS

The results of this study indicate that carefully selected pediatric patients with intractable epilepsy can benefit from subdural invasive monitoring procedures that entail definite but acceptable risks.