Foramen Ovale Electrodes Research Articles

Subclinical mesial temporal lobe activity in Alzheimer’s Disease

AbstractBackgroundAnimal and human studies of Alzheimer’s disease (AD) demonstrated that aberrant network activity with neuronal hyperexcitability plays a role in disease pathogenesis and cognitive functioning. Suppressing network hyperexcitability with levetiracetam improved cognitive functioning in familial AD mice and executive functioning in patients with AD and epileptiform activity. Manifestations of network hyperexcitability are unsensitive biomarkers since they are mostly subclinical and not visible on scalp EEG. (Lam et al. 2019, Vossel et al. 2021, Sanchez et al. 2012). Our purpose was to identify subclinical epileptiform activity in two patients with mild cognitive impairment (MCI) and one patient with moderate AD, with a suspicion of network hyperexcitability based on either spikes on 24‐h scalp EEG or a history suspect for seizures without epileptiform discharges on scalp EEG.MethodThree patients were implanted with bilateral foramen ovale (FO) electrodes to obtain more direct measures of mesial temporal lobe epileptiform activity. Scalp EEG according to the 10‐20 system and polysomnography were simultaneously recorded in all subjects for three to five consecutive days. Polysomnography was scored according to the AASM scoring rules. Persyst 14 software was used for automatic spike detection.ResultInvasive recordings showed interictal epileptic activity in all three patients, which was occasional during wakefulness and abundant during sleep. In one patient two nocturnal unilateral mesiotemporal seizures, characterized by arousal, were recorded during N1 sleep, while at the same time no epileptiform activity was visible on scalp EEG.ConclusionIn patients with AD subclinical mesial temporal epileptiform activity can be present with a normal scalp EEG. Epileptic activity is predominantly found during sleep. Our findings suggest an important role for network hyperexcitability in AD disease pathogenesis.

Prediction of epilepsy surgery outcome using foramen ovale EEG – A machine learning approach

IntroductionPatients with drug-resistant focal epilepsy may benefit from ablative or resective surgery. In presurgical work-up, intracranial EEG markers have been shown to be useful in identification of the seizure onset zone and prediction of post-surgical seizure freedom. However, in most cases, implantation of depth or subdural electrodes is performed, exposing patients to increased risks of complications. MethodsWe analysed EEG data recorded from a minimally invasive approach utilizing foramen ovale (FO) and epidural peg electrodes using a supervised machine learning approach to predict post-surgical seizure freedom. Power-spectral EEG features were incorporated in a logistic regression model predicting one-year post-surgical seizure freedom. The prediction model was validated using repeated 5-fold cross-validation and compared to outcome prediction based on clinical and scalp EEG variables. ResultsForty-seven patients (26 patients with post-surgical 1-year seizure freedom) were included in the study, with 31 having FO and 27 patients having peg onset seizures. The area under the receiver-operating curve for post-surgical seizure freedom (Engel 1A) prediction in patients with FO onset seizures was 0.74 ± 0.23 using electrophysiology features, compared to 0.66 ± 0.22 for predictions based on clinical and scalp EEG variables (p < 0.001). The most important features for prediction were spectral power in the gamma and high gamma ranges. EEG data from peg electrodes was not informative in predicting post-surgical outcomes. ConclusionIn this hypothesis-generating study, a data-driven approach based on EEG features derived from FO electrodes recordings outperformed the predictive ability based solely on clinical and scalp EEG variables. Pending validation in future studies, this method may provide valuable post-surgical prognostic information while minimizing risks of more invasive diagnostic approaches.

Diagnostic and prognostic value of EEG patterns recorded on foramen ovale and epidural peg electrodes

ObjectiveTo describe and assess the significance of EEG characteristics recorded during presurgical video-EEG monitoring (VEM) utilizing foramen ovale (FO) and epidural peg electrodes. MethodsSeizure onset (SOP) and termination pattern morphology and regions, ipsilateral and contralateral latencies, seizure duration, and interictal spike counts were examined in 106 patients (412 seizures). An EEG feature-based logistic regression model predicting one-year post-surgical seizure freedom was assessed using a 5-fold nested cross-validation approach. ResultsFour SOPs and five termination patterns were identified. Seventy-one percent of patients had a single unique SOP, the most common being sharp activity ≤ 13 Hz (28.9% of seizures). Seizures recorded by FO electrodes were associated with SOPs ≤ 13 Hz (OR 1.9, p = .008). Focal-to-bilateral tonic-clonic seizures were associated with SOPs > 13 Hz (p = .04) and bilateral spike and wave termination (p < .001). In patients with temporal lobe epilepsy, logistic regression based prediction of post-surgical outcome had a mean area under the curve of 0.69, with the most important features being SOP, right sided interictal epileptic activity, and contralateral latency. ConclusionsFO and peg recordings yield characteristic EEG patterns. SignificanceEEG features of FO and peg recordings may have diagnostic and prognostic utility in presurgical VEM.

Prolongation of cortical sleep spindles during hippocampal interictal epileptiform discharges in epilepsy patients.

Memory deficits are frequent among patients with epilepsies affecting the temporal lobe. Hippocampal interictal epileptic discharges (hIEDs), the presumed epileptic exaggeration of sharp wave-ripples (SWRs), are known to contribute to memory dysfunction, but the potential underlying mechanism is unknown. The precise temporal coordination between hippocampal SWRs and corticothalamic spindles during sleep is critical for memory consolidation. Moreover, previous investigation indicated that hIEDs induce neocortical spindlelike oscillation. In the present study, we aimed to assess the influence of hIEDs on neocortical spindles. We analyzed the spindle characteristics (duration, amplitude, frequency) of 21 epilepsy patients implanted with foramen ovale (FO) electrodes during a whole night sleep. Scalp sleep spindles were categorized based on their temporal relationship to hIEDs detected on the FO electrodes. Three groups were created: (1) spindles coinciding with hIEDs, (2) spindles "induced" by hIEDs, and (3) spindles without hIED co-occurrence. We found that spindles co-occurring with hIEDs had altered characteristics in all measured properties, lasted longer by 126 ± 48 ms (mean ± SD), and had higher amplitude by 3.4± 3.2μV, and their frequency range shifted toward the higher frequencies within the 13-15-Hz range. Also, hIED-induced spindles had identical oscillatory properties to spindles without any temporal relationships with hIEDs. In more than half of our subjects, clear temporal coherence was revealed between hIEDs and spindles, but the direction of the coupling was patient-specific. We investigated the effect of hippocampal IEDs on neocortical spindle activity and found spindle alterations in cases of spindle-hIED co-occurrence, but not in cases of hIED-initiated spindles. We propose that this is a marker of a pathologic process, where IEDs may have direct effect on spindle generation. It could mark a potential mechanism whereby IEDs disrupt memory processes, and also provide a potential therapeutic target to treat memory disturbances in epilepsy.

Presurgical video-EEG monitoring with foramen ovale and epidural peg electrodes: a 25-year perspective

BackgroundEpilepsy surgery cases are becoming more complex and increasingly require invasive video-EEG monitoring (VEM) with intracranial subdural or intracerebral electrodes, exposing patients to substantial risks. We assessed the utility and safety of using foramen ovale (FO) and epidural peg electrodes (FOP) as a next step diagnostic approach following scalp VEM.MethodsWe analyzed clinical, electrophysiological, and imaging characteristics of 180 consecutive patients that underwent FOP VEM between 1996 and 2021. Multivariate logistic regression was used to assess predictors of clinical and electrophysiological outcomes.ResultsFOP VEM allowed for immediate resection recommendation in 36 patients (20.0%) and excluded this option in 85 (47.2%). Fifty-nine (32.8%) patients required additional invasive EEG investigations; however, only eight with bilateral recordings. FOP VEM identified the ictal onset in 137 patients, compared to 96 during prior scalp VEM, p = .004. Predictors for determination of ictal onset were temporal lobe epilepsy (OR 2.9, p = .03) and lesional imaging (OR 3.1, p = .01). Predictors for surgery recommendation were temporal lobe epilepsy (OR 6.8, p < .001), FO seizure onset (OR 6.1, p = .002), and unilateral interictal epileptic activity (OR 3.8, p = .02). One-year postsurgical seizure freedom (53.3% of patients) was predicted by FO ictal onset (OR 5.8, p = .01). Two patients experienced intracerebral bleeding without persisting neurologic sequelae.ConclusionFOP VEM adds clinically significant electrophysiological information leading to treatment decisions in two-thirds of cases with a good benefit–risk profile. Predictors identified for electrophysiological and clinical outcome can assist in optimally selecting patients for this safe diagnostic approach.

The comparative study of invasive electroencephalography modalities in temporal lobe epilepsy

Introduction. Invasive video-EEG monitoring (invasive EEG) is indicated in patients with refractory focal epilepsy while localization of the epileptogenic zone is unclear. Methods of invasive EEG in different groups of patients demonstrate variable results.Objective: to analyse the results of invasive EEG via subdural and depth electrodes in patients with refractory temporal lobe epilepsy with mesial temporal lobe seizures.Materials and methods. The series of 37 patients who underwent invasive EEG from 2013 to 2020 was retrospectively analysed. The study includes primary adult patients with structural refractory focal epilepsy with mesial temporal lobe seizures without tumor and vascular pathology. Patients were divided onto 3 groups: 1) with foramen ovale electrodes 2) subdural strip electrodes and 3) combination of subdural strips and depths electrodes. The results of anteromedial temporal lobectomy after 6 months were classified according to Engel scale.Results. A group with foramen ovale electrodes included 7 patients, subdural strips – 23, combination – 7. The seizure onset zone was detected in 36 (97 %) cases. Serious complications were observed in 2 (29 %) cases in the group with foramen ovale electrodes. The mean follow-up in 23 (76 %) patients after resective surgery was 28.3 months. Favourable results (Engel I, II) were observed in 4 (80 %) patients with foramen ovale electrodes, in 8 (67 %) patients with subdural electrodes, in 6 (100 %) with combination. Unfavourable results (Engel III, IV) were noted in 1 (20 %) patient with foramen ovale electrode, in 4 (33 %) patients with subdural strips.Conclusion. All the presented modalities of invasive EEG are effective for localizing of seizure onset zone in this category of patients. Foramen ovale electrode using may be limited due to increased risk of complications.

Efficacy of Frameless Radiosurgical Entorhino-Amygdalohippocampectomy for Pharmacoresistant Medial Temporal Lobe Epilepsy

<h3>Purpose/Objective(s)</h3> Evaluate the efficacy and safety of Linac based surgery in the treatment of pharmaco-resistant epilepsies of mesial temporal lobe origin due to medial temporal sclerosis. <h3>Materials/Methods</h3> Retrospective single institution study comprised of 12 MTLE pts. (10 pts. with MT Sclerosis on Dominant side) who underwent SRS between August 2015 to Jan 2019. The epileptogenic substrate was localized using invasive video-EEG monitoring with foramen ovale electrodes, magnetic resonance imaging, metabolic (PET) imaging. All patients underwent detailed neuropsychological testing and the ESI-55 quality-of-life questionnaire. The radiosurgical target included Anterior part of parahippocampal region; entorhinal area adjacent to the collateral sulcus & rhinal sulcus; hippocampus - head & Anterior part and Amygdalofugal part of the amygdaloid complex. Non-coplanar Volumetrically modulated plans or non-isocentric CK plans using variable collimator were generated for prescribed mean dose of 22gy (range: 20-24 Gy), without any dose heterogeneity. Normal structures doses were within acceptable limits. Radiosurgical treatment was performed by using the C-Arm Linac with hexapod robotics (9 pts.) or frameless robotic radiosurgery system with skull tracking (3pts.). A minimum 2-year follow-up period included clinical, neuropsychological, ophthalmic (Visual field charting) & radiologic evaluations. <h3>Results</h3> Median age at diagnosis is 28 yrs (Range: 17-42yrs). At each 6-month follow-up evaluation, the frequency of seizures was significantly smaller than that at the previous visit. The median seizure frequency of 10.25 the month before treatment was reduced to 0.33 at 2 years after treatment. At 2 years, 67% of the patients (8 of 12) were seizure free - 4 Engel class 1A, 3 Engel class 1B, 1 Engel class 2A responses. The median latent interval from SRS to seizure cessation was 9.5 months (range 6–20 months). One patient was immediately seizure free. The median latency in aura cessation was 14 months (range 9–22 months). Morphological changes, edema, on magnetic resonance imaging were visible at 10 months (median) after SRS (range 7–22 months) in nearly all patients. During the onset period of these radiological changes, three patients experienced headache associated, in two cases, with nausea and vomiting. In these three patients the signs resolved immediately after short course of low doses of steroids. No cases of permanent neurological deficit (except three cases of monosymptomatic visual field deficit), or morbidity, or mortality were observed <h3>Conclusion</h3> Initial experience of Frameless SRS, as non-invasive technique, for MTLE provides benefits especially in young patients requiring high social skills with dominant lobe involvement in the traditional realms of safety with short- to middle-term efficacy & superior neuropsychological outcomes, similar to published series in GKS studies but with appreciated realms of patient acceptance, comfort and costs. Further long-term follow up is required.

Clinical semiology of temporal lobe seizures in preschool children: contribution of invasive recording to anatomical classification.

Focal seizure semiology is often inadequately studied, specifically in preschool children. Among drug-resistant epilepsies amenable to surgery, temporal lobe seizure semiology has been widely described in this age group. Nevertheless, a systematic anatomo-electroclinical study has never been performed. We retrospectively reviewed the charts of patients younger than six years old at the time of video-EEG recording who were operated on for temporal lobe epilepsy in our centre between 2010 and 2016. In order to describe the electroclinical semiology and establish anatomo-clinical correlations, we reviewed all the recorded seizures on scalp and invasive video-EEG and analysed pre- and postsurgical clinical data, MRI scans, and surgical and pathological data. We classified patients into the following four anatomical groups: mesio-temporal, temporal lateral, polar, and mesio-lateral, and for each group we selected video-EEG samples for educational purposes. Twenty-eight patients fulfilled the selection criteria. Twenty-three patients (82%) were explored with invasive electrodes that consisted of foramen ovale electrodes in 11 (39%) and stereoelectroencephalography in 12 (43%). The majority of the 53% of patients with mesio-temporal epilepsies had specific ictal semiology, as described in adults. The others had subtle seizures or seizures limited to apnoea. The other groups also had some features comparable to adults, although no child reported the classic auras of lateral epilepsies. In total, 11% had infantile spasms (IS); post-ictal examination provided lateralization signs in 28%. With a mean post-surgical follow-up duration of 5.5 years, 89% of the patients were classified as Engel Class I. Preschool children were shown to have non-specific seizures, notably subtle events or IS. However, careful video-EEG analysis can provide arguments for localizing the epileptogenic zone within the temporal lobe in most cases. Seizures with apnoea are characteristic of mesial temporal onset in patients with long-term epilepsy-associated tumours.

Highly consistent temporal lobe interictal spike networks revealed from foramen ovale electrodes

ObjectiveA major challenge that limits understanding and treatment of epileptic events from mesial temporal structures comes from our inability to detect and map interictal networks reproducibly using scalp electrodes. Here, we developed a novel approach to map interictal spike networks and demonstrate their relationships to seizure onset and lesions in patients with foramen ovale electrode implantations. MethodsWe applied the direct Directed Transfer Function to reveal interictal spike propagation from bilateral foramen ovale electrodes on 10 consecutive patients and co-registered spatially with both seizure onset zones and temporal lobe lesions. ResultsHighly reproducible, yet unique interictal spike networks were seen for each patient (correlation: 0.93 ± 0.13). Interictal spikes spread in both anterior and posterior directions within each temporal lobe, often reverberating between sites. Spikes propagated to the opposite temporal lobe predominantly through posterior pathways. Patients with structural lesions (N = 4), including tumors and sclerosis, developed reproducible spike networks adjacent to their lesions that were highly lateralized compared to patients without lesions. Only 5% of mesial temporal lobe spikes were time-locked with scalp electrode spikes. Our preliminary observation on two lesional patients suggested that along with lesion location, Interictal spike networks also partially co-registered with seizure onset zones suggesting interrelationship between seizure onset and a subset of spike networks. ConclusionsThis is the first demonstration of patient-specific, reproducible interictal spike networks in mesial temporal structures that are closely linked to both temporal lobe lesions and seizure onset zones. SignificanceInterictal spike connectivity is a novel approach to map epileptic networks that could help advance invasive and non-invasive epilepsy treatments.

Surgery in intractable epilepsy-physicians' recommendations and patients' decisions.

To identify demographic and clinical variables independently associated with patients' decisions against their physicians' recommendations for resective epilepsy surgery or further scalp video-EEG monitoring (sca-VEM), semi-invasive (sem-)VEM with foramen ovale and/or peg electrodes, and invasive (in-)VEM. Consecutive patients, who underwent presurgical assessment with at least one sca-VEM between 2010 and 2014, were included into this retrospective analysis. Multivariate analysis was used to identify independent variables associated with patients' decisions. Within the study period, 352 patients underwent 544 VEM sessions comprising 451 sca-, 36 sem-, and 57 in-VEMs. Eventually, 96 patients were recommended resective surgery, and 106 were ineligible candidates; 149 patients denied further necessary VEMs; thus, no decision could be made. After sca- or additional sem-VEM, nine out of 51 eligible patients (17.6%) rejected resection. One hundred and ten patients were recommended in-VEM, 52 of those (47.2%) declined. Variables independently associated with rejection of in-VEM comprised intellectual disability (OR 4.721, 95% CI 1.047-21.284), extratemporal focal aware non-motor seizures ("aura") vs. no "aura" (OR 0.338, 95% CI 0.124-0.923), and unilateral or bilateral vs. no MRI lesion (OR 0.248, 95% CI 0.100-0.614 and 0.149, 95% CI 0.027-0.829, respectively). During and after presurgical evaluation, patients with intractable focal epilepsy declined resections and intracranial EEGs, as recommended by their epileptologists, in almost 20% and 50% of cases. This calls for early and thorough counseling of patients on risks and benefits of epilepsy surgery. Future prospective studies should ask patients in depth for specific reasons why they decline their physicians' recommendations.

Intracranial EEG monitoring methods

Resective surgery is considered to be the best option towards achieving seizure-free state in drug-resistant epilepsy. Intracranial EEG (iEEG) is necessary if the seizure-onset zone is localized near to an eloquent cortical area, or if the results of presurgical examinations are discordant, or if an extratemporal epilepsy patient is MRI-negative. Nowadays, 3 kinds of electrodes are used: (1) foramen ovale (FO) electrodes; (2) subdural strip or grid electrodes (SDG); (3) deep electrodes (stereo-electroencephalographia, SEEG). The usage of FO electrode is limited to bitemporal cases. SDG and SEEG have a distinct philosophical approach, different advantages and disadvantages. SDG is appropriate for localizing seizure-onset zones on hemispherial or interhemispherial surfaces; it is preferable if the seizure-onset zone is near to an eloquent cortical area. SEEG is excellent in exploration of deeper cortical structures (depths of cortical sulci, amygdala, hippocampus), although a very precise planning is required because of the low spatial sampling. The chance for seizure-freedom is relatively high performing both methods (SDG: 55%, SEEG: 64%), beside a tolerable rate of complications.

New Approaches to Studying Silent Mesial Temporal Lobe Seizures in Alzheimer's Disease.

Silent seizures were discovered in mouse models of Alzheimer's disease over 10 years ago, yet it remains unclear whether these seizures are a salient feature of Alzheimer's disease in humans. Seizures that arise early in the course of Alzheimer's disease most likely originate from the mesial temporal lobe, one of the first structures affected by Alzheimer's disease pathology and one of the most epileptogenic regions of the brain. Several factors greatly limit our ability to identify mesial temporal lobe seizures in patients with Alzheimer's disease, however. First, mesial temporal lobe seizures can be difficult to recognize clinically, as their accompanying symptoms are often subtle or even non-existent. Second, electrical activity arising from the mesial temporal lobe is largely invisible on the scalp electroencephalogram (EEG), the mainstay of diagnosis for epilepsy in this population. In this review, we will describe two new approaches being used to study silent mesial temporal lobe seizures in Alzheimer's disease. We will first describe the methodology and application of foramen ovale electrodes, which captured the first recordings of silent mesial temporal lobe seizures in humans with Alzheimer's disease. We will then describe machine learning approaches being developed to non-invasively identify silent mesial temporal lobe seizures on scalp EEG. Both of these tools have the potential to elucidate the role of silent seizures in humans with Alzheimer's disease, which could have important implications for early diagnosis, prognostication, and development of targeted therapies for this population.

Preoperative MRI findings and prediction of diagnostic utility of foramen ovale electrodes.

Foramen ovale electrodes (FOEs) are a minimally invasive method to localize mesial temporal seizures in cases in which noninvasive methods are inconclusive. The objective of this study was to identify factors predicting the ability of FOEs to yield a diagnosis in order to determine optimal candidates for this procedure. All cases of diagnostic investigations performed with FOEs at the authors' institution between 2005 and 2017 were reviewed. FOE investigation was defined as diagnostic if it led to a treatment decision. Demographic and clinical variables for diagnostic and nondiagnostic investigations were compared using a Wilcoxon rank-sum test for continuous variables and Fisher's exact test for categorical variables. Ninety-three patients underwent investigations performed with FOEs during the study period and were included in the study. FOE investigation was diagnostic in 75.3% of cases. Of patients who underwent anterior temporal lobectomy following diagnostic FOE evaluation, 75.9% were Engel class I at last follow-up (average 40.1 months). When the diagnostic and nondiagnostic FOE groups were compared, patients who had diagnostic investigations were more likely to be male (57.1% male vs 26.1% in the nondiagnostic group, p = 0.015). They were also more likely to have temporal lesions on preoperative MRI (p = 0.018). FOEs are a useful, minimally invasive diagnostic modality resulting in a treatment decision in 75% of cases. Male patients and patients with temporal lesions on MRI may be most likely to benefit from FOE investigation.

S83. Sensitivity of quantitative EEG for seizure identification in intracranial recordings

Introduction Previous work has shown that review of quantitative EEG (QEEG) alone has reasonable sensitivity for identifying seizure onset. The objectives of this study are: (1) To evaluate whether QEEG can be also used to identify seizures in intracranial EEG recordings. (2) To explore whether QEEG can be used to triage the review of raw EEG records and assist in prioritizing the review of records with the highest probability of seizure Methods 66 one-hour EEG epochs were chosen from 6 temporal lobe epilepsy patients who underwent intracranial monitoring using bilateral foramen ovale (FO) electrodes. 33 clips were seizure free, and 33 clips contained at least one seizure, for a total of 36 seizures. Seizure onset was determined by review of the raw EEG by an epilepsy attending. We generated standard QEEG panels using Persyst (Prescott, AZ). Three blinded reviewers were asked to: (1) mark each area of concern for seizure on these QEEG panels and (2) rank the likelihood of that area containing a seizure as “probable,” “possible,” or “unlikely.” Epochs with no marked areas of concern were considered to be “low risk” for containing a seizure. Seizure identification was considered positive for an epoch if there was an area of concern marked and there was also a seizure anywhere within the 1 h-clip. We examined sensitivity and specificity as a function of the highest likelihood assigned to any area of concern. However, analysis of the “possible” and “unlikely” groups were combined because their sensitivity was not statistically different. Results Marking an epoch with at least one “probable” seizure had 52% sensitivity and 94% specificity for that one-hour epoch actually containing a seizure. Marking an epoch with at least one “possible” or “unlikely” seizure had 40% sensitivity and 73% specificity. Marking an epoch with any area of concern (“probable,” “possible”, or “unlikely”) had 92% sensitivity and 67% specificity, which implies that one-hour epochs containing a seizure had only a 8% false negative rate for being classified “low risk.” Conclusion QEEG can be used to identify seizures in standardized intracranial recordings, such as FO electrodes, and can also classify a sub-set of epochs as either very likely or very unlikely to contain a seizure. This suggests that QEEG can be used to triage the review of the intracranial raw EEG recordings, so the review of higher yield studies can be prioritized over lower yield records.

Surgical treatment of epilepsy in patients with mediobasal temporal cavernous malformations

Epilepsy is a frequent clinical manifestation of cavernous malformations (CMs) of the mediobasal temporal region (MBTR). Surgical removal of CMs is an excellent technique for treating associated epilepsy and may range from pure lesionectomy to tailored resection of the temporal lobe. The study purpose was to determine the optimal surgical management for epilepsy in patients with CMs of the MBTR. We retrospectively analyzed the clinical data, neuroimaging findings, surgical techniques, and surgical outcomes in 11 patients with epilepsy and CMs of the MBTR. All patients underwent video-electroencephalography, magnetic resonance imaging, and computed tomography in the pre- and postoperative periods. Nine patients underwent preoperative implantation of foramen ovale electrodes. In all cases, surgery was accompanied by electrocorticography (ECoG). CMs were located in the anterior MBTR in 7 cases, anterior and middle thirds of the MBTR in 1 case, middle third in 2 cases, and middle and posterior thirds in 1 case. In 8 patients, preoperative monitoring revealed a seizure onset area in the MBTR. These patients underwent cavernomectomy with ECoG-guided resection of the hemosiderin ring and adjacent tissue using the pterional (4 cases) or supracerebellar transtentorial approach (4). In 3 cases, anterior temporal lobectomy with cavernomectomy was additionally used due to spreading of pathological activity to the lateral temporal neocortex. Seizure control after surgery was excellent in 7 patients (class 1 ILAE) and good in 4 (class 2 ILAE). Surgery in patients with epilepsy caused by CMs of the MBTR should be performed based on non-invasive and invasive presurgical evaluation. If the seizure onset area is located in the MBTR, lesionectomy with ECoG-guided resection of the adjacent temporal cortical areas can be performed using the pterional or supracerebellar transtentorial approach. Lateral spread of epileptic activity requires cavernomectomy and anterior temporal lobectomy.

Interictal Epileptiform Activity in the Foramen Ovale Electrodes of a Frontotemporal Dementia Patient.

Frontotemporal dementia (FTD) is a frequent cause of cognitive decline. While epilepsy is an important comorbidity of Alzheimer’s disease, we lack studies on its presence in FTD. We report on an FTD patient with transient, short-term changes of behavior and cognitive performance suggesting non-convulsive epilepsy. Video-EEG recording with foramen ovale (FO) electrodes revealed mesio-temporal epileptiform potentials, undetectable by scalp leads. We also found beta spindles in the FO electrodes, not described in the literature. We conclude that video-EEG monitoring with FO electrodes might usefully complement the assessment of dementia-associated epilepsy opening new perspectives in dementia-research.

S197 Foramen ovale electrodes in epilepsy

Temporal lobe epilepsy is the most prevalent type of epileptic syndrome, and also the most frequent type of surgically treatable epilepsy. But in many cases the laterality is difficult to prove based on non-invasive investigations and scalp EEG. There is broad evidence that suggest that the seizure onset zone may be on the opposite side compared to where the seizure is seen on the scalp EEG due to rapid spread to the contralateral side. Based on these data a technique was developed first by Wieser et al., which allowed direct corticographic recordings from the medial temporal structures without performing invasive surgery. The implantation of foramen ovale (FO) electrode proved to be safe, efficient and very useful in many cases where the seizure onset zone could not be determined on non-invasive data alone. In the last 20 years we have implanted FO electrodes in more tha 100 patients with success. We find using FO electrodes useful in answering the following questions: • Is there an unilateral onset? (laterality vs. Bilateral indenpendent onset?). • Evidence of seizure start in the mediobasal region? (localisation?). • Where is the seizure onset zone over the mediobasal. surface along the anterior – posterior axis? • Seizure propagation routes?

P372 Inference of intra and inter hippocampal directed causal relationships based on foramen ovale recordings

Objectives The traditional method for seizure onset zone determination is based on the temporal order in the appearance of pathological waveforms. However, this method assumes, that the well observable pathological waveforms are really the first signs of those processes, which play important role in the seizure generation. Our objective is to develop and apply new mathematical methods to identify the causal relations between brain areas during epileptic activity. Methods The Convergent Cross Mapping method of Sugihara was modified to reveal temporal dynamics of causal connections, and applied to current source density distribution data calculated from multichannel foramen ovale electrode recordings. Results The causality analysis revealed directed connections between the two hippocampi and specific changes in intra and inter-hippocampal connections comparing interictal and ictal periods: intrahippocampal connections get stronger and spread away spatially, while inter-hippocampal connections show more complex temporal development patterns during the seizures: the functional connectivity decreases during initial phase of the seizures in some cases, while increases during the full blown seizures. Onset of the tonic phases accompanied by temporal increase of the causal connections between the two hippocampi as well. Conclusions New mathematical analysis method, such as Convergent Cross Mapping raise the possibility of objective determination the causal relationships at the initiation of the seizure, without specific assumptions about the waveforms. Significance Causality analysis can support more precise identification of seizure onset zones, thus results in better surgical outcome.

SCOPE-mTL: A non-invasive tool for identifying and lateralizing mesial temporal lobe seizures prior to scalp EEG ictal onset.

In mesial temporal lobe (mTL) epilepsy, seizure onset can precede the appearance of a scalp EEG ictal pattern by many seconds. The ability to identify this early, occult mTL seizure activity could improve lateralization and localization of mTL seizures on scalp EEG. Using scalp EEG spectral features and machine learning approaches on a dataset of combined scalp EEG and foramen ovale electrode recordings in patients with mTL epilepsy, we developed an algorithm, SCOPE-mTL, to detect and lateralize early, occult mTL seizure activity, prior to the appearance of a scalp EEG ictal pattern. Using SCOPE-mTL, 73% of seizures with occult mTL onset were identified as such, and no seizures that lacked an occult mTL onset were identified as having one. Predicted mTL seizure onset times were highly correlated with actual mTL seizure onset times (r=0.69). 50% of seizures with early mTL onset were lateralizable prior to scalp ictal onset, with 94% accuracy. SCOPE-mTL can identify and lateralize mTL seizures prior to scalp EEG ictal onset, with high sensitivity, specificity, and accuracy. Quantitative analysis of scalp EEG can provide important information about mTL seizures, even in the absence of a visible scalp EEG ictal correlate.

Silent hippocampal seizures and spikes identified by foramen ovale electrodes in Alzheimer's disease

We directly assessed mesial temporal activity in two Alzheimer’s disease (AD) patients without a history or EEG evidence of seizures, using intracranial foramen ovale electrodes. We detected clinically silent hippocampal seizures and epileptiform spikes during sleep, a period when both were most likely to interfere with memory consolidation. These index cases support a model in which early development of occult hippocampal hyperexcitability may contribute to the pathogenesis of AD.