Beyond the lesion: the epileptogenic networks around cavernous angiomas

An imaging review of the hippocampus and its common pathologies

The hippocampus is a complex structure located in the mesial temporal lobe that plays a critical role in cognitive and memory-related processes. The hippocampal formation consists of the dentate gyrus, hippocampus proper, and subiculum, and its importance in the neural circuitry makes it a key anatomic structure to evaluate in neuroimaging studies. Advancements in imaging techniques now allow detailed assessment of hippocampus internal architecture and signal features that has improved identification and characterization of hippocampal abnormalities. This review aims to summarize the neuroimaging features of the hippocampus and its common pathologies. It provides an overview of the hippocampal anatomy on magnetic resonance imaging and discusses how various imaging techniques can be used to assess the hippocampus. The review explores neuroimaging findings related to hippocampal variants (incomplete hippocampal inversion, sulcal remnant and choroidal fissure cysts), and pathologies of neoplastic (astrocytoma and glioma, ganglioglioma, dysembryoplastic neuroepithelial tumor, multinodular and vacuolating neuronal tumor, and metastasis), epileptic (mesial temporal sclerosis and focal cortical dysplasia), neurodegenerative (Alzheimer's disease, progressive primary aphasia, and frontotemporal dementia), infectious (Herpes simplex virus and limbic encephalitis), vascular (ischemic stroke, arteriovenous malformation, and cerebral cavernous malformations), and toxic-metabolic (transient global amnesia and opioid-associated amnestic syndrome) etiologies.

Kindling in humans: Does secondary epileptogenesis occur?

The relevance of secondary epileptogenesis for human epilepsy remains a controversial subject decades after it was first described in animal models. Whether or not a previously normal brain region can become independently epileptogenic through a kindling-like process has not, and cannot, be definitely proven in humans. Rather than reliance on direct experimental evidence, attempts to answering this question must depend on observational data. In this review, observations based largely upon contemporary surgical series will advance the case for secondary epileptogenesis in humans. As will be argued, hypothalamic hamartoma-related epilepsy provides the strongest case for this process; all the stages of secondary epileptogenesis can be observed. Hippocampal sclerosis (HS) is another pathology where the question of secondary epileptogenesis frequently arises, and observations from bitemporal and dual pathology series are explored. The verdict here is far more difficult to reach, in large part because of the scarcity of longitudinal cohorts; moreover, recent experimental data have challenged the claim that HS is acquired consequent to recurrent seizures. Synaptic plasticity more than seizure-induced neuronal injury is the likely mechanism of secondary epileptogenesis. Postoperative running-down phenomenon provides the best evidence that a kindling-like process occurs in some patients, evidenced by its reversal. Finally, a network perspective of secondary epileptogenesis is considered, as well as the possible role for subcortical surgical interventions.

Stereo-Encephalographic Presurgical Evaluation of Temporal Lobe Epilepsy: An Evolving Science

Drug-resistant epilepsy is present in nearly 30% of patients. Resection of the epileptogenic zone has been found to be the most effective in achieving seizure freedom. The study of temporal lobe epilepsy for surgical treatment is extensive and complex. It involves a multidisciplinary team in decision-making with initial non-invasive studies (Phase I), providing 70% of the required information to elaborate a hypothesis and treatment plans. Select cases present more complexity involving bilateral clinical or electrographic manifestations, have contradicting information, or may involve deeper structures as a part of the epileptogenic zone. These cases are discussed by a multidisciplinary team of experts with a hypothesis for invasive methods of study. Subdural electrodes were once the mainstay of invasive presurgical evaluation and in later years most Comprehensive Epilepsy Centers have shifted to intracranial recordings. The intracranial recording follows original concepts since its development by Bancaud and Talairach, but great advances have been made in the field. Stereo-electroencephalography is a growing field of study, treatment, and establishment of seizure pattern complexities. In this comprehensive review, we explore the indications, usefulness, discoveries in interictal and ictal findings, pitfalls, and advances in the science of presurgical stereo-encephalography for temporal lobe epilepsy.

Long-Term Management of Seizures after Surgical Treatment of Supratentorial Cavernous Malformations : A Retrospective Single Centre Study

Objective

Seizure recurrence after the first-ever seizure in patients with a supratentorial cerebral cavernous malformation (CCM) is almost certain, so the diagnosis and treatment of epilepsy is justified. The optimal method of management of these patients is still a matter of debate. The aim of our study was to identify factors associated with postoperative seizure control and assess the surgical morbidity rate.

Methods

We retrospectively analysed 45 consecutive patients with a supratentorial CCM and symptomatic epilepsy in a single centre. Pre- and postoperative epidemiological data, seizure-related patient histories, neuroimaging results, surgery details and outcomes were obtained from hospital medical records. Seizure outcomes were assessed at least 12 months after surgery.

Results

Thirty-five patients (77.8%) were seizure free at the long-term follow-up (Engel class I); six (13,3%) had rare, nocturnal seizures (Engel class II); and four (8.9%) showed meaningful improvement (Engel class III). In 15 patients (33%) in the Engel I group; it was possible to discontinue antiepileptic medication. Although there was not statistical significance, our results suggest that patients can benefit from early surgery. No deaths occurred in our study, and mild postoperative neurologic deficits were observed in two patients (4%) at the long-term follow-up.

Conclusion

Surgical resection of CCMs should be considered in all patients with a supratentorial malformation and epilepsy due to the favourable surgical results in terms of the epileptic seizure control rate and low postoperative morbidity risk, despite the use of different predictors for the seizure outcome.

Earlier Age at Surgery for Brain Cavernous Angioma-Related Epilepsy May Achieve Complete Seizure Freedom without Aid of Anti-Seizure Medication

Background: The present study hypothesized that some factors may distinguish between patients with a brain cavernous angioma (BCA), who were free from anti-seizure medication (ASM), and patients who still required ASMs postoperatively. The purpose of the study was thus to identify factors associated with ceasing ASMs for patients with drug-resistant epilepsy secondary to BCA, who underwent BCA removal surgery. Methods: We divided patients into those with drug-resistant epilepsy secondary to BCA who achieved complete seizure freedom without ASMs a year after surgery (No-ASM group) (International League Against Epilepsy (ILAE) classification class I with no epileptiform discharges), and others (ASM group) (ILAE classification ≤ II and/or epileptiform discharges). We statistically compared groups in terms of: (1) age at operation; (2) history of epilepsy; (3) size of BCA; and (4) location of BCA. Results: Overall, a year after the surgery, the No-ASM group comprised 12 patients (48%), and the ASM group comprised 13 patients (52%). In both multi- and univariate logistic regression analyses, age at BCA removal surgery correlated significantly with the No-ASM group (p = 0.043, p = 0.019), but history of epilepsy did not (p = 0.581, p = 0.585). Conclusions: Earlier age at surgery for patients with drug-resistant epilepsy is encouraged to achieve complete seizure freedom without the need for ASMs when the cause of epilepsy is BCA.

Role of i-CT, i-US, and Neuromonitoring in Surgical Management of Brain Cavernous Malformations and Arteriovenous Malformations: A Case Series

OBJECTIVE

We retrospectively reviewed the institutional experience in patients who underwent microsurgical resection of cavernous malformations (CMs) or arteriovenous malformations (AVMs) using a multimodal intraoperative protocol including neuronavigation, intraoperative ultrasound (i-US), computed tomography (i-CT), and neuromonitoring.

METHODS

Twenty-four patients (14 male), with a mean age of 47.5 years (range 27-73), have been included: 20 of them suffered from CMs and 4 suffered from AVMs. Neuromonitoring was used in 18 cases, when lesions were located in eloquent areas; 2 patients underwent awake craniotomy. First, an i-CT scan with and without contrast was acquired after patient positioning. Navigated B-mode ultrasound acquisition was carried out after dural opening to identify the lesion (CMs or AVMs nidus). Following identification and resection of vascular lesions, postcontrast i-CT (or CT-angio) was performed to detect and localize any small or calcified remnant (in cases of CMs) or residual vessels feeding the nidus (in cases of AVMs).

RESULTS

In 5 cases of CMs and in 1 case of AVM, i-CT identified small residual lesions. In these cases, new i-CT images were uploaded into the navigation system and used for further resection. i-US was useful before starting transsulcal or transcortical approach to identify the lesions and guide the trajectory of the approach. However, several artifacts were observed during subsequent steps of dissection, making image interpretation difficult.

CONCLUSIONS

The combination of different intraoperative real-time imaging modalities (i-CT and i-US), coupled with neuromonitoring, in the surgical management of vascular lesions, particularly if located in eloquent areas, has a positive impact on clinical outcome.

The accuracy of quantitative EEG biomarker algorithms depends upon seizure onset dynamics

OBJECTIVE

To compare the performance of different ictal quantitative biomarkers of the seizure onset zone (SOZ) across many seizures in a cohort of consecutive patients with a variety of seizure onset patterns.

METHODS

The Epileptogenicity Index (EI, a measure of fast activity) and Slow Polarizing Shift index (SPS, a measure of infraslow activity) were calculated for 212 seizures (22 patients). After stratification by onset pattern, median index values inside and outside the SOZ were compared in aggregate and for each of the onset patterns. Receiver Operating Characteristic (ROC) curves were constructed to compare the performance of each index.

RESULTS

Median values of EI (0.056 vs 0.0087), SPS (0.27 vs 0.19), and CI (0.21 vs 0.12) were significantly higher for contacts inside the SOZ, all p < 0.0001. Analysis of AUC showed variable performance of these indices across seizure types, although AUC for EI and SPS was generally greatest for seizures with fast activity at onset.

CONCLUSIONS

All indices were significantly higher for contacts inside the SOZ; however, the performance of these indices varied depending on the pattern of seizure onset.

SIGNIFICANCE

These findings suggest that future studies of quantitative biomarkers of the SOZ should account for seizure onset pattern.

Seizure outcome in patients with cavernous malformation after early surgery

OBJECTIVE

To describe seizure outcome and complications in patients with cavernous malformations (CM) undergoing early versus late surgery.

METHODS

A database was created for all CM patients who presented with seizure referred to the neurosurgical clinic at an academic center. A telephone survey and chart review were conducted to evaluate for preoperative and postoperative seizure frequency. Postoperative seizure-free outcome of patients who had ≤2 preoperative seizures versus those that had >2 preoperative seizures was compared.

RESULTS

A total of 35 CM patients were included for analysis. Nineteen patients had ≤2 preoperative seizures and 16 patients had >2 preoperative seizures, six of them drug resistant for over two years. Among the ≤2 seizure group, 15 had only a single seizure before surgical resection. 94.7% of patients with ≤2 preoperative seizures and 62.5% of patients with >2 preoperative seizures were seizure free one year following surgical resection (p = 0.019). 78.9% of patients with ≤2 preoperative seizures and 25% of patients with >2 preoperative seizures were able to wean off AEDs (p < 0.001). Among those patients who had a single preoperative seizure, 100% of patients were seizure free at one year.

CONCLUSIONS

Early surgical resection for CM patients who present after a CM-related seizure is an effective, well tolerated treatment and has good chance to offer seizure freedom without the need for long-term antiepileptic medications. Outcome for patients operated with only one or two preoperative seizures may lead to better results than patients who delay the procedure.

Data-driven method to infer the seizure propagation patterns in an epileptic brain from intracranial electroencephalography

Surgical interventions in epileptic patients aimed at the removal of the epileptogenic zone have success rates at only 60-70%. This failure can be partly attributed to the insufficient spatial sampling by the implanted intracranial electrodes during the clinical evaluation, leading to an incomplete picture of spatio-temporal seizure organization in the regions that are not directly observed. Utilizing the partial observations of the seizure spreading through the brain network, complemented by the assumption that the epileptic seizures spread along the structural connections, we infer if and when are the unobserved regions recruited in the seizure. To this end we introduce a data-driven model of seizure recruitment and propagation across a weighted network, which we invert using the Bayesian inference framework. Using a leave-one-out cross-validation scheme on a cohort of 45 patients we demonstrate that the method can improve the predictions of the states of the unobserved regions compared to an empirical estimate that does not use the structural information, yet it is on the same level as the estimate that takes the structure into account. Furthermore, a comparison with the performed surgical resection and the surgery outcome indicates a link between the inferred excitable regions and the actual epileptogenic zone. The results emphasize the importance of the structural connectome in the large-scale spatio-temporal organization of epileptic seizures and introduce a novel way to integrate the patient-specific connectome and intracranial seizure recordings in a whole-brain computational model of seizure spread.

All that glitters: Contribution of stereo-EEG in patients with lesional epilepsy

OBJECTIVE

To determine the contribution of stereo-EEG for localization purpose in patients with a visible lesion on MRI.

BACKGROUND

Intracranial EEG is often used to localize the epileptogenic focus in patients with non-lesional focal epilepsy. Its role in cases where a lesion is visible on MRI can be even more complex and the relationship between the lesion and the seizure onset has rarely been addressed.

METHODS

All consecutive patients between February 2013 and May 2018 who underwent stereo-EEG and had a lesion visible on MRI were included. We assessed the localization of the seizure onset and its relationship with the lesion. Clinical, radiological, and electrographic analyses were performed.

RESULTS

Stereo-EEG revealed a seizure onset with either partial or no overlap with the lesion seen on MRI in 42 (56 %) of the 75 lesions included. Mesial temporal sclerosis was the only lesion type associated with an exclusively lesional seizure onset (p = 0.003).

CONCLUSION

Epilepsy surgery in MRI-positive cases should rely not only the results of lesions seen on MRI, which might be potentially misleading; SEEG is a gold standard method in these cases to define resective borders.

Long-term outcomes of reoperations in epilepsy surgery

OBJECTIVE

To analyze longitudinal seizure outcomes following epilepsy surgery, including reoperations, in patients with intractable focal epilepsy.

METHODS

Clinicoradiological characteristics of patients who underwent epilepsy surgery from 1995 to 2016 with follow-up of ≥1 year were reviewed. In patients undergoing reoperations, the latest resection was considered the index surgery. The primary outcome was complete seizure freedom (Engel I) at last follow-up. Potentially significant outcome variables were first identified using univariate analyses and then fit in multivariate Cox proportional hazards models.

RESULTS

Of 898 patients fulfilling study criteria, 110 had reoperations; 92 had one resection prior to the index surgery and 18 patients had two or more prior resective surgeries. Two years after the index surgery, 69% of patients with no prior surgeries had an Engel score of I, as opposed to only 42% of those with one prior surgery, and 33% of those with two or more prior resections (P < .001). Among surgical outcome predictors, the number of prior epilepsy surgeries, female sex, lesional initial magnetic resonance imaging, no prior history of generalization, and pathology correlated with better seizure outcomes on univariate analysis. However, only sex (P = .011), history of generalization (P = .016), and number of prior surgeries (P = .002) remained statistically significant in the multivariate model.

SIGNIFICANCE

Although long-term seizure control is possible in patients with failed prior epilepsy surgery, the chances of success diminish with every subsequent resection. Outcome is additionally determined by inherent biological markers (sex and secondary generalization tendency), rather than traditional outcome predictors, supporting a hypothesis of "surgical refractoriness."

Focal epilepsy caused by single cerebral cavernous malformation (CCM) is associated with regional and global resting state functional connectivity (FC) disruption

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To our knowledge, this is the first study to report resting state functional connectivity (FC) abnormalities associated with focal epilepsy caused by a single cerebral cavernous malformation (CCM).

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We show, by comparing to the data acquired from the age and gender matched control group, that this type of focal epilepsy is associated with the disruption of the normal regional and global FC. The disruption includes a decrease in the coactivation between the region surrounding the CCM lesion, i.e., the lesional region, and its homotopic counterpart, a reduction in FC between the lesional region and the rest of the brain, and decreased FC among the default mode network (DMN).

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These changes may be alleviated or reversed after the surgical resection of the CCM and the epileptogenic zone has successfully stopped recurrent seizures.

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Finally, the severity of the FC disruption in the brain tissue adjacent to the CCM may be used to delineate the epileptogenic zone and to aid the surgical resection.

Epilepsy, including the type with focal onset, is increasingly viewed as a disorder of the brain network. Here we employed the functional connectivity (FC) metrics estimated from the resting state functional MRI (rsfMRI) to investigate the changes of brain network associated with focal epilepsy caused by single cerebral cavernous malformation (CCM). Eight CCM subjects and 21 age and gender matched controls were enrolled in the study. Seven of 8 CCM subjects underwent surgical resection of the CCM and became seizure free and 4 of the surgical subjects underwent a repeat rsfMRI study. We showed that there was both regional and global disruption of the FC values among the CCM subjects including decreased in homotopic FC (HFC) and global FC (GFC) in the regions of interest (ROIs) where the CCMs were located. There was also the disruption of the default mode network (DMN) especially the FC between the middle prefrontal cortex (MPFC) and the right lateral parietal cortex (LPR) among these individuals. We observed the trend of alleviation of these disruptions after the individual has become seizure free from the surgical resection of the CCM. Using a voxel-based approach, we found the disruption of the HFC and GFC in the brain tissue immediately adjacent to the CCM and the severity of the disruption appeared inversely proportional to the distance of the brain tissue to the lesion. Our findings confirm the disruption of normal brain networks from focal epilepsy, a process that may be reversible with successful surgical treatments rendering patients seizure free. Some voxel-based metrics may help identify the epileptogenic zone and guide the surgical resection.

Surgical outcomes in patients with epilepsy after viral encephalitis: contribution of SEEG study

Background

Nowadays, few surgery analysis has been reported in cases of epilepsy after viral encephalitis(VE). Herein, this study was to evaluate the efficacy of surgery and capability of stereoelectroencephalography (SEEG) in the definition of the epileptogenic zone (EZ) after VE, and also to explore the relationship between the SEEG features and the surgical outcomes.

Methods

We retrospectively analyzed 10 surgically treated patients that identified to suffer from epilepsy secondary to VE using SEEG, and investigated the SEEG features associated with surgical outcomes in these patients. Besides visual analysis, we used the epileptogenicity index (EI), a semi-quantitative and supplementary tool to evaluate the validity of SEEG in the context of VE.

Results

Among the 10 operated patients, 3 of them became completely seizure-free. The patients who got totally seizure free or significant improvement, the seizure onset was located either in the antero-mesial temporal structures or focal gyrus; patients who got worthwhile improvement or no improvement, the seizure started from multiple brain lobes. The number of electrodes classified as epileptogenic visually involved were closely correlated with EI positive onses.Anatomic areas defined and shown as EZ on MRI by visual assessment were also defined as epileptogenic by the EI in these cases.

Conclusion

Apart from exploring the surgical outcome related to epilepsy after VE, we also bring insight into the relationship between the SEEG features and surgical outcome with the application of the supplementary methods.

Temporal Lobe Cavernous Malformation Caused Epileptic Amnesic Episodes and Mild Cognitive Impairment

Neuropathological features in Alzheimer's disease (AD) are amyloid β (Aβ) deposits and neurofibrillary changes. AD is characterized by memory impairment and cognitive dysfunction, with some reports associating these impairments with hyperexcitability caused by Aβ in the medial temporal lobe. Epileptic seizures are known to be common in AD. We encountered a 65-year-old patient with cavernous malformation (CM) in the right temporal lobe who exhibited epileptic amnesia (EA) and AD-like symptoms. Scalp electroencephalography (EEG), including long-term video-EEG, showed no interictal discharges, but intraoperative subdural electrode (SE) recording from the right parahippocampal area showed frequent epileptiform discharges. Neuropathologically, senile plaques were found in the surrounding normal cortex of the CM. Postoperatively, the patient has remained free of EA and AD-like symptoms since total removal of the CM. This is the first surgical case report to confirm temporal lobe hyperexcitability associated with EA and AD-like symptoms.

The Surgical and Cognitive Outcomes of Focal Cortical Dysplasia

Focal cortical dysplasia (FCD) is the major cause of intractable focal epilepsy in childhood leading to epilepsy surgery. The overall seizure freedom after surgery ranges between 50–75% at 2 years after surgery and the long-term seizure freedom remain relatively stable. Seizure outcome after surgery depends on a various factors such as pathologic etiologies, extent of lesion, and types of surgery. Therefore, seizure outcome after surgery for FCD should be analyzed carefully considering cohorts’ characteristics. Studies of pediatric epilepsy surgery emphasize the early surgical intervention for a better cognition. Early surgical intervention and cessation of seizure activity are important for children with intractable epilepsy. However, there are limited data on the cognitive outcome after surgery in pediatric FCD, requiring further investigation. This paper reviews the seizure and cognitive outcomes of epilepsy surgery for FCD in children. Several prognostic factors influencing seizure outcome after surgery will be discussed in detail.

Surgical resection of cavernous angioma located within eloquent brain areas: International survey of the practical management among 19 specialized centers

PURPOSE

The practical management of cavernous angioma located within eloquent brain area before, during and after surgical resection is poorly documented. We assessed the practical pre-operative, intra-operative, and post-operative management of cavernous angioma located within eloquent brain area.

METHOD

An online survey composed of 61 items was sent to 26 centers to establish a multicenter international retrospective cohort of adult patients who underwent a surgical resection as the first-line treatment of a supratentorial cavernous angioma located within or close to eloquent brain area.

RESULTS

272 patients from 19 centers (mean 13.6 ± 16.7 per center) from eight countries were included. The pre-operative management varied significantly between centers and countries regarding the pre-operative functional assessment, the pre-operative epileptological assessment, the first given antiepileptic drug, and the time to surgery. The intra-operative environment varied significantly between centers and countries regarding the use of imaging systems, the use of functional mapping with direct electrostimulations, the extent of resection of the hemosiderin rim, the realization of a post-operative functional assessment, and the time to post-operative functional assessment. The present survey found a post-operative improvement, as compared to pre-operative evaluations, of the functional status, the ability to work, and the seizure control.

CONCLUSIONS

We observed a variety of practice between centers and countries regarding the management of cavernous angioma located within eloquent regions. Multicentric prospective studies are required to solve relevant questions regarding the management of cavernous angioma-related seizures, the timing of surgery, and the optimal extent of hemosiderin rim resection.

Is additional mesial temporal resection necessary for intractable epilepsy with cavernous malformations in the temporal neocortex?

PURPOSE

Cavernous malformation (CM) in the temporal neocortex causes intractable epilepsy. Whether to resect additional mesial temporal structures in addition to the lesionectomy is a still controversial issue. To clarify the need for the procedure, we retrospectively analyzed pre- and postoperative clinical data of patients with surgically removed CM.

SUBJECTS AND METHODS

We included data from 18 patients with CM in the temporal neocortex who presented with intractable epilepsy. Eleven patients of our early series were treated with extended resection, i.e., lesionectomy and the resection of additional mesial temporal structures. Seven patients underwent lesionectomy, i.e., removal of the CM and of hemosiderin-stained surrounding brain tissue. Pathological assessments of the resected hippocampus were performed. Chronic intracranial electroencephalography (EEG) recordings were obtained in 6 patients. We performed perioperative neuropsychological assessments in all patients.

RESULTS

The seizure outcome was recorded as Engel class I in 17 patients (94.4%); Ia = 12 (66.7%) Ib = 2 (11.1%), Ic = 1 (5.6%), Id = 2 (11.1%), and class IIb in one patient (5.6%). Adding resection of the mesial temporal structures to lesionectomy did not alter the seizure outcome. Pathology of hippocampus revealed limited neuronal loss in CA4. Ictal onsets in the ipsilateral lateral cortex were detected in all 6 patients who underwent intracranial EEG. In 4 patients each, we also detected ictal onsets from the ipsilateral mesial temporal structures and from the contralateral temporal lobe. Postoperatively, in the patients where their CM was located in the language-dominant hemisphere (n = 10), the full-scale intelligence quotient (IQ) and the performance IQ increased (p < 0.05), whereas the verbal memory (WMS-R) deteriorated in two of 5 patients.

CONCLUSION

Excellent seizure outcomes were obtained even the lesionectomy alone. To confirm appropriate surgical strategy for lateral temporal CM with intractable epilepsy, further studies in large sample size are needed.

Presurgical intracranial investigations in epilepsy surgery

Identification and localization of the "epileptogenic process" in the brain of patients with drug-resistant epilepsy for surgical cure is the goal of presurgical investigations. Intracranial recordings are required when conflicting data between seizure clinical semiology and EEG prevent precise localization within one hemisphere or lateralization, when a visible lesion on MRI seems unrelated to the electroclinical data, or in MRI-negative cases. Two methods are currently used. The objective of the subdural grid electrocorticography with or without depth electrodes (SDG/DE) is the best possible identification of the area of onset of spontaneous seizures and localization of the eloquent cortex. The objective of stereoelectroencephalography (SEEG) is to define the epileptogenic zone (configured as a network) and its relation to an unmasked lesion. Two-dimensional (SDG) and three-dimensional (SEEG) brain sampling dictate different strategies for noninvasive presurgical phase I goals as well as for data analysis. SEEG must resolve several potential localization hypotheses in a manner that cannot be achieved with SDG. SDG operates through brain surface coverage, unlike SEEG, which samples networks. SDG estimates the extent of cortical resection through a lobar or sublobar localization of ictal onset and constraints from functional mapping. SEEG defines a tailored resection according to the results of anatomo-electro-clinical correlations in stereotaxic space that will guide the ablation of the epileptogenic zone. SEEG is currently expanding faster than SDG. The prerequisites (especially in the preimplantation hypothetical strategy) and technical tools (especially stimulation and functional mapping) in the two methods are very different. This chapter presents a comparative review of the rationale, indications, electrode implantation strategies, interpretation, and surgical decision making of these two approaches of presurgical evaluation for epilepsy surgery.

Stereoelectroencephalography Versus Subdural Electrodes for Localization of the Epileptogenic Zone: What Is the Evidence?

Accurate and safe localization of epileptic foci is the crux of surgical therapy for focal epilepsy. As an initial evaluation, patients with drug-resistant epilepsy often undergo evaluation by noninvasive methods to identify the epileptic focus (i.e., the epileptogenic zone (EZ)). When there is incongruence of noninvasive neuroimaging, electroencephalographic, and clinical data, direct intracranial recordings of the brain are often necessary to delineate the EZ and determine the best course of treatment. Stereoelectroencephalography (SEEG) and subdural electrodes (SDEs) are the 2 most common methods for recording directly from the cortex to delineate the EZ. For the past several decades, SEEG and SDEs have been used almost exclusively in specific geographic regions (i.e., France and Italy for stereo-EEG and elsewhere for SDEs) for virtually the same indications. In the last decade, however, stereo-EEG has started to spread from select centers in Europe to many locations worldwide. Nevertheless, it is still not the preferred method for invasive localization of the EZ at many centers that continue to employ SDEs exclusively. Despite the increased dissemination of the SEEG method throughout the globe, important questions remain unanswered. Which method (SEEG or SDEs) is superior for identification of the EZ and does it depend on the etiology of epilepsy? Which technique is safer and does this hold for all patient populations? Should these 2 methods have equivalent indications or be used selectively for different focal epilepsies? In this review, we seek to address these questions using current invasive monitoring literature. Available meta-analyses of observational data suggest that SEEG is safer than SDEs, but it is less clear from available data which method is more accurate at delineating the EZ.

Seizure activity per se does not induce tissue damage markers in human neocortical focal epilepsy

OBJECTIVE

The contribution of recurring seizures to the progression of epileptogenesis is debated. Seizure-induced brain damage is not conclusively demonstrated either in humans or in animal models of epilepsy. We evaluated the expression of brain injury biomarkers on postsurgical brain tissue obtained from 20 patients with frequent seizures and a long history of drug-resistant focal epilepsy.

METHODS

The expression patterns of specific glial, neuronal, and inflammatory molecules were evaluated by immunohistochemistry in the core of type II focal cortical dysplasias (FCD-II), at the FCD boundary (perilesion), and in the adjacent normal-appearing area included in the epileptogenic region. We also analyzed surgical specimens from cryptogenic patients not presenting structural alterations at imaging.

RESULTS

Astroglial and microglial activation, reduced neuronal density, perivascular CD3-positive T-lymphocyte clustering, and fibrinogen extravasation were demonstrated in the core of FCD-II lesions. No pathological immunoreactivity was observed outside the FCD-II or in cryptogenetic specimens, where the occurrence of interictal and ictal epileptiform activity was confirmed by either stereo-electroencephalography or intraoperative electrocorticography.

INTERPRETATION

Recurrent seizures do not induce the expression of brain damage markers in nonlesional epileptogenic cortex studied in postsurgical tissue from cryptogenic and FCD patients. This evidence argues against the hypothesis that epileptiform activity per se contributes to focal brain injury, at least in the neocortical epilepsies considered here. Ann Neurol 2017;82:331-341.

Hypothalamic hamartoma: Epileptogenesis beyond the lesion?

The discovery of intrinsic epileptogenicity of the hypothalamic hamartoma (HH) marked a new area in understanding the associated clinical syndrome, often manifesting as progressive epileptic encephalopathy. However, therapeutic procedures targeting the HH proved to be inefficient to cure seizures in up to 50% of cases, whereas in cases with partial improvement, the electroclinical patterns of persisting seizures suggest an involvement of distant cortical regions. The concept of kindling-like secondary epileptogenesis has been suggested as a possible underlying mechanism. Yet the role of the hypothalamic lesion in the pathophysiology of the syndrome remains debatable. In the Strasbourg-Kork series, the best outcomes were obtained when the duration of epilepsy before endoscopic HH surgery did not exceed 10 years. In two patients with HH ablation followed at a later time by a temporal lobectomy, only this second surgical step allowed complete seizure freedom. These findings suggest the existence of an independent, third stage of secondary epileptogenesis in human. In the Grenoble series, stereotactic intracerebral recordings (stereo electroencephalography [SEEG]) of five HH cases demonstrated that gelastic/dacrystic seizures were correlated with discharges within the HH, whereas other seizure types were related to discharges affecting cortical regions, which sometimes seemed to be triggered by HH. In the Marseille series, two cases explored by SEEG provided evidence of extended epileptogenicity outside the limits of the HH, forming complex epileptogenic networks, with HH still triggering clusters of neocortical seizures in the first, but not obligatory involved in spontaneous seizures in the second case. Taken together, our data argue for the existence of dynamic ictal network organization, with possible "kindling-like" relationships between the HH and the neocortex or widespread epileptogenesis. Despite the existence of secondary epileptogenesis, the epileptogenic zone could still be limited to the hamartoma, for which early surgical treatment should be pragmatically considered as a first surgical step.

High-frequency oscillations detected in ECoG recordings correlate with cavernous malformation and seizure-free outcome in a child with focal epilepsy: A case report

Epilepsy associated with cavernous malformation (CM) often requires surgical resection of seizure focus to achieve seizure‐free outcome. High‐frequency oscillations (HFOs) in intracranial electroencephalogram (EEG) are reported as potential biomarkers of epileptogenic regions, but to our knowledge there are no data on the existence of HFOs in CM‐caused epilepsy. Here we report our experience of the identification of the seizure focus in a 3‐year‐old pediatric patient with intractable epilepsy associated with CM. The electrocorticographic recordings were obtained from a 64‐contact grid over 2 days in the epilepsy monitoring unit (EMU). The spatial distribution of HFOs and epileptic spikes were estimated from recording segments right after the electrode placement, during sleep and awake states separately. The HFO distribution showed consistency with the perilesional region; the location of spikes varied over days and did not correlate with the lesion. The HFO spatial distribution was more compact in sleep state and pinpointed the contacts sitting on the CM border. Following the resection of the CM and the hemosiderin ring, the patient became seizure‐free. This is the first report describing HFOs in a pediatric patient with intractable epilepsy associated with CM and shows their potential in identifying the seizure focus.

Pathology of cavernous malformations

Cavernous malformations (CMs) are low-pressure angiographically occult lesions, composed of blood-filled sinusoidal locules known as "caverns." Although these lesions were once believed to be congenital in nature, there is compelling evidence to support de novo formation of CMs as well. They can occur as sporadic lesions or be inherited in an autosomal-dominant phenotype in familial forms of the disease. The pathophysiology of CMs is commonly believed to be due to abnormal vascular pathology. Three genes, CCM1, CCM2, and CCM3, have been extensively studied for their role in vascular pathology, resulting in abnormal angiogenesis and compromising the structural integrity of vessel endothelial cell. The expression of growth factors has been researched to gain insight into the dynamic behavior of CM lesions. Gross and microscopic images are utilized in this chapter to illustrate the pathologic findings of these lesions. Ultrastructural analysis demonstrates the aberrations in CM endothelial cells and structural integrity that may provide better understanding into how and why these lesions have a propensity to hemorrhage.

Non-invasive Evaluation for Epilepsy Surgery

Epilepsy surgery is aimed to remove the brain tissues that are indispensable for generating patient's epileptic seizures. There are two purposes in the pre-operative evaluation: localization of the epileptogenic zone and localization of function. Surgery is planned to remove possible epileptogenic zone while preserving functional area. Since no single diagnostic modality is superior to others in identifying and localizing the epileptogenic zone, multiple non-invasive evaluations are performed to estimate the location of the epileptogenic zone after concordance between evaluations. Essential components of non-invasive pre-surgical evaluation of epilepsy include detailed clinical history, long-term video-electroencephalography monitoring, epilepsy-protocol magnetic resonance imaging (MRI), and neuropsychological testing. However, a significant portion of drug-resistant epilepsy is associated with no or subtle MRI lesions or with ambiguous electro-clinical signs. Additional evaluations including fluoro-deoxy glucose positron emission tomography (FDG-PET), magnetoencephalography and ictal single photon emission computed tomography can play critical roles in planning surgery. FDG-PET should be registered on three-dimensional MRI for better detection of focal cortical dysplasia. All diagnostic tools are complementary to each other in defining the epileptogenic zone, so that it is always important to reassess the data based on other results to pick up or confirm subtle abnormalities.