Intracranial ictal onset zone in nonlesional lateral temporal lobe epilepsy on scalp ictal EEG

Beyond Stereo-EEG: Is It Worth Combining Stereo-EEG With Other Diagnostic Methods?

SUMMARY

Stereo-EEG is a widely used method to improve the diagnostic precision of presurgical workup in patients with refractory epilepsy. Its ability to detect epileptic activity and identify epileptic networks largely depends on the chosen implantation strategy. Even in an ideal situation, electrodes record activity generated in <10% of the brain and contacts only record from brain tissue in their immediate proximity. In this article, the authors discuss how recording stereo-EEG simultaneously with other diagnostic methods can improve its diagnostic value in clinical and research settings. It can help overcome the limited spatial coverage of intracranial recording and better understand the sources of epileptic activity. Simultaneous scalp EEG is the most widely available method, often used to understand large epileptic networks, seizure propagation, and EEG activity occurring on the contralateral hemisphere. Simultaneous magnetoencephalography allows for more precise source localization and identification of deep sources outside the stereo-EEG coverage. Finally, simultaneous functional MRI can highlight metabolic changes following epileptic activity and help understand the widespread network changes associated with interictal activity. This overview highlights advantages and methodological challenges for all these methods. Clinical use and research applications are presented for each approach.

Who are the Better Candidates for Epilepsy Surgery?

The resective epilepsy surgery can be the effective procedure to get seizure-free outcome in these drug resistant epilepsy (DRE) patients. Class I evidence firmly establishes the superiority of epilepsy surgery over medical treatments in both seizure control and quality of life for DRE patients. For the effective identification of optimal surgical candidates, it's essential to understand the prognostic factors of epilepsy surgery based on the surgical methods employed. Established positive prognostic indicators for temporal resection include the presence of hippocampal sclerosis on magnetic resonance imaging (MRI), focal lesions on MRI, unilateral temporal spikes, concordant ictal electroencephalography (EEG), and a history of prolonged febrile convulsion. Potential negative predictors encompass preoperative secondary generalized tonic-clonic seizures, a normal MRI, postoperative EEG spikes, and age at the time of surgery. For neocortical epilepsy, the prognostic factors identified through multivariate analysis were the presence of a discrete lesion, localized hypometabolism on Fluorodeoxyglucose positron emission tomography (FDG-PET), and localized ictal EEG. A significant correlation was found between achieving a seizure-free outcome in no visible lesion on MRI (MR-negative) epilepsy patients and having concordance in two or more presurgical evaluations, specifically in interictal EEG, ictal EEG, FDG-PET, and ictal single-photon emission computed tomography. There was a marked improvement in the seizure-free outcome in MR-negative temporal lobe epilepsy (TLE) by the application of this strategy. The better surgical candidates for epilepsy surgery are the followings: patients displaying a discrete lesion on MRI with concordant video-EEG monitoring (VEM) results, patients diagnosed with unilateral hippocampal sclerosis who have concordant VEM results, patients with unilateral hippocampal sclerosis but discordant VEM results, patients with focal cortical dysplasia and concordant VEM results, and patients diagnosed with MR-negative TLE who exhibit two or more consistent results from presurgical evaluations.

Optimizing the surgical management of MRI-negative epilepsy in the neuromodulation era

OBJECTIVE

To evaluate the role of intracranial electroencephalography monitoring in diagnosing and directing the appropriate therapy for MRI-negative epilepsy and to present the surgical outcomes of patients following treatment.

METHODS

Retrospective chart review between 2015 - 2021 at a single institution identified forty-eight patients with no lesion on MRI who received surgical intervention for their epilepsy. The outcomes assessed were the surgical treatment performed and the International League Against Epilepsy seizure outcomes at one year of follow up.

RESULTS

Eleven patients underwent surgery without invasive monitoring, including vagus nerve stimulation (10%), deep brain stimulation (8%), laser interstitial thermal therapy (2%) and callosotomy (2%). The remaining 37 patients received invasive monitoring followed by resection (35%), responsive neurostimulation (21%) and deep brain stimulation (15%) or no treatment (6%). At one year postoperatively, 39% were Class 1-2, 36% were Class 3-4 and 24% were Class 5. More patients with Class 1-2 or 3-4 outcomes underwent invasive monitoring (100% and 83% respectively) compared to those with poor outcomes (25%, p < 0.001). Patients with Class 1-2 outcomes more commonly underwent resection or responsive neurostimulation: 69% and 31%, respectively (p < 0.001).

SIGNIFICANCE

The optimal management of MRI-negative focal epilepsy may involve invasive monitoring followed by resection or responsive neurostimulation in most cases, as these treatments were associated with the best seizure outcomes in our cohort. Unless multifocal epileptogenesis is clear from the non-invasive evaluation, invasive monitoring is preferred before pursuing deep brain stimulation or vagal nerve stimulation directly.

Surgical treatment of nonlesional temporal lobe epilepsy

PURPOSE

One-third of the patients with drug-resistant temporal lobe epilepsy (TLE) have a normal MRI, but there are only a few studies regarding the surgical outcomes and the efficacy of anterior temporal lobectomy (ATL) in patients with nonlesional TLE. The objective of this study is to evaluate the surgical outcomes and efficacy of ATL in patients with nonlesional TLE.

METHODS

We included 77 consecutive patients without MRI-identifiable lesions who had undergone surgical resection for drug-resistant TLE. We performed univariate and multivariate logistic regression analyses to identify the predictors of surgical outcomes, and the efficacy of ATL in patients with nonlesional TLE.

RESULTS

More than two-thirds of patients (51/76, 67.3 %) had achieved seizure freedom at the last follow-up. Presence of oroalimentary automatism, localized hypometabolism in FDG-PET, and concordant results in presurgical evaluations were associated with better surgical outcomes. Only 15 out of 77 patients (19.2 %) with nonlesional TLE were treated with ATL, and the surgically resected areas were located within the resection margin of ATL in one-third of the patients (26/77, 33.8 %). Patients with auras suggesting neocortical ictal onset and lateralizing semiological features had a higher chance that their potentially epileptogenic areas were located beyond or outside the resection margin of ATL.

CONCLUSION

Our study showed that the potentially epileptogenic areas were located beyond or outside the margin of the ATL in nearly two-thirds of the patients. Several clinical factors may be useful in predicting the location of an epileptogenic area, which can help optimize a surgical strategy in these patients.

The Risk of Stroke and TIA in Nonstenotic Carotid Plaques: A Systematic Review and Meta-Analysis

BACKGROUND

Severe carotid stenosis carries a high risk of stroke. However, the risk of stroke with nonstenotic carotid plaques (<50%) is increasingly recognized.

PURPOSE

We aimed to summarize the risk of TIA or stroke in patients with nonstenotic carotid plaques.

DATA SOURCES

We performed a comprehensive systematic review and meta-analysis in patients with acute ischemic stroke in whom carotid imaging was performed using MEDLINE and the Cochrane Database, including studies published up to December 2019.

STUDY SELECTION

Included studies had >10 patients with <50% carotid plaques on any imaging technique and reported the incidence or recurrence of ischemic stroke/TIA. High-risk plaque features and the risk of progression to stenosis >50% were extracted if reported.

DATA SYNTHESIS

We identified 31 studies reporting on the risk of ipsilateral stroke/TIA in patients with nonstenotic carotid plaques. Twenty-five studies (n = 13,428 participants) reported on first-ever stroke/TIA and 6 studies (n = 122 participants) reported on the recurrence of stroke/TIA.

DATA ANALYSIS

The incidence of first-ever ipsilateral stroke/TIA was 0.5/100 person-years. The risk of recurrent stroke/TIA was 2.6/100 person-years and increased to 4.9/100 person-years if intraplaque hemorrhage was present. The risk of progression to severe stenosis (>50%) was 2.9/100 person-years (8 studies, n = 448 participants).

LIMITATIONS

Included studies showed heterogeneity in reporting stroke etiology, the extent of stroke work-up, imaging modalities, and classification systems used for characterizing carotid stenosis.

CONCLUSIONS

The risk of recurrent stroke/TIA in nonstenotic carotid plaques is not negligible, especially in the presence of high-risk plaque features. Further research is needed to better define the significance of nonstenotic carotid plaques for stroke etiology.

The Hippocampus and Cortex Together Generate the Scalp EEG Ictal Discharge in Temporal Lobe Epilepsy

PURPOSE

The scalp EEG ictal discharge in temporal lobe epilepsy is reportedly visible only after the intracranial discharge becomes well synchronized and present over 10 to 30 cm of cortex. We investigated the role of the hippocampal formation in the generation of the scalp EEG ictal discharge.

METHODS

Intracranial EEG video monitors were recorded using simultaneous scalp, stereotaxic depth, and subdural strip electrodes in 19 subjects with temporal lobe epilepsy. The location, frequency, morphology, and timing of the initial ictal discharge, and subsequent ictal patterns, were examined in hippocampal formation, medial paleocortex, and lateral temporal neocortex electrocorticographic and scalp temporal EEG recordings.

RESULTS

In every subject, a scalp ictal discharge was visible only after the intracranial ictal discharge had spread to involve the whole temporal lobe (hippocampal formation, medial paleocortex, and lateral temporal neocortex). Beta/gamma frequency and decremental electrocorticographic ictal discharges were never visualized in the EEG. The scalp EEG ictal discharge frequency was 2.4 to 10 Hz and appeared a median of 18 seconds after a faster frequency electrocorticographic initial ictal discharge, once the intracranial discharge slowed to an alpha, theta, or delta frequency.

CONCLUSIONS

In temporal lobe epilepsy, an ictal pattern is not readily visible in the scalp EEG until the intracranial ictal discharge is ≤10 Hz and has propagated from its site of onset to involve the hippocampus, medial paleocortex, and lateral temporal neocortex.

Features of Simultaneous Scalp and Intracranial EEG That Predict Localization of Ictal Onset Zone

OBJECTIVE

To assess the utility of simultaneous scalp EEG in patients with focal epilepsy undergoing intracranial EEG evaluation after a detailed presurgical testing, including an inpatient scalp video EEG evaluation.

METHODS

Patients who underwent simultaneous scalp and intracranial EEG (SSIEEG) monitoring were classified into group 1 or 2 depending on whether the seizure onset zone was delineated or not. Seizures were analyzed using the following 3 EEG features at the onset of seizures latency, location, and pattern.

RESULTS

The criteria showed at least one of the following features when comparing SSIEEG: prolonged latency, absence of anatomical congruence, lack of concordance of EEG pattern in 11.11% (1/9) of the patients in group 1 and 75 % (3/4) of the patients in group 2. These 3 features were not present in any of the 5 patients who had Engel class I outcome compared with 1 of the 2 patients (50%) who had seizure recurrence after resective surgery. The mean latency of seizure onset in scalp EEG compared with intracranial EEG of patients in group 1 was 17.48 seconds (SD = 16.07) compared with 4.33 seconds (SD = 11.24) in group 2 ( P = .03). None of the seizures recorded in patients in group 1 had a discordant EEG pattern in SSIEEG.

CONCLUSION

Concordance in EEG features like latency, location, and EEG pattern, at the onset of seizures in SSIEEG is associated with a favorable outcome after epilepsy surgery in patients with intractable focal epilepsy.

SIGNIFICANCE

Simultaneous scalp EEG complements intracranial EEG evaluation even after a detailed inpatient scalp video EEG evaluation and could be part of standard intracranial EEG studies in patients with intractable focal epilepsy.

Linking MRI postprocessing with magnetic source imaging in MRI-negative epilepsy

OBJECTIVE

MRI-negative (MRI-) pharmacoresistant focal epilepsy (PFE) patients are most challenging for epilepsy surgical management. This study utilizes a voxel-based MRI postprocessing technique, implemented using a morphometric analysis program (MAP), aiming to facilitate detection of subtle focal cortical dysplasia (FCD) in MRI- patients. Furthermore, the study examines the concordance between MAP-identified regions and localization from magnetic source imaging (MSI).

METHODS

Included in this retrospective study were 25 MRI- surgical patients. MAP was performed on T1-weighted MRI, with comparison to a normal database. The pertinence of MAP+ areas was confirmed by MSI, surgical outcome and pathology. Analyses of MAP and MSI were performed blindly from patients' clinical information and independently from each other.

RESULTS

The detection rate of subtle changes by MAP was 48% (12/25). Once MAP+ areas were resected, patients were more likely to be seizure-free (p=0.02). There were no false positives in the 25 age-matched normal controls. Seven patients had a concordant MSI correlate. Patients in whom a concordant area was identified by both MAP and MSI had a significantly higher chance of achieving a seizure-free outcome following complete resection of this area (p=0.008). In the 9 resected MAP+ areas, pathology revealed FCD type IA in 7 and type IIB in 2.

INTERPRETATION

MAP shows promise in identifying subtle FCD abnormalities and increasing the diagnostic yield of conventional MRI visual analysis in presurgical evaluation of PFE. Concordant MRI postprocessing and MSI analyses may lead to the noninvasive identification of a structurally and electrically abnormal subtle lesion that can be surgically targeted.

EEG-fMRI evaluation of patients with mesial temporal lobe sclerosis

This preliminary study sought more information on blood oxygen level dependent (BOLD) activation, especially contralateral temporal/extratemporal spread, during continuous EEG-fMRI recordings in four patients with mesial temporal sclerosis (MTS). In two patients, EEG showed unilateral focal activity during the EEG-fMRI session concordant with the interictal focus previously identified with standard and video-poly EEG. In the other two patients EEG demonstrated a contralateral diffusion of the irritative focus. In the third patient (with the most drug-resistant form and also extratemporal clinical signs), there was an extratemporal diffusion over frontal regions, ipsilateral to the irritative focus. fMRI analysis confirmed a single activation in the mesial temporal region in two patients whose EEG showed unilateral focal activity, while it demonstrated a bilateral activation in the mesial temporal regions in the other two patients. In the third patient, fMRI demonstrated an activation in the supplementary motxor area. This study confirms the most significant activation with a high firing rate of the irritative focus, but also suggests the importance of using new techniques (such as EEG-fMRI to examine cerebral blood flow) to identify the controlateral limbic activation, and any other extratemporal activations, possible causes of drug resistance in MTS that may require a more precise pre-surgical evaluation with invasive techniques.

Nonlesional atypical mesial temporal epilepsy: electroclinical and intracranial EEG findings

OBJECTIVE

Misleading manifestations of common epilepsy syndromes might account for some epilepsy surgery failures, thus we sought to characterize patients with difficult to diagnose (atypical) mesial temporal lobe epilepsy (mTLE).

METHODS

We retrospectively reviewed our surgical database over 12 years to identify patients who underwent a standard anterior temporal lobectomy after undergoing intracranial EEG (ICEEG) evaluation with a combination of depth and subdural electrodes. We carefully studied electroclinical manifestations, neuroimaging data, neuropsychological findings, and indications for ICEEG.

RESULTS

Of 835 patients who underwent anterior temporal lobectomy, 55 were investigated with ICEEG. Ten of these had atypical mTLE features and were not considered to have mTLE preoperatively. All of them had Engel class I outcome for 3 to 7 years (median 3.85). Five reported uncommon auras, and 3 had no auras. Scalp-EEG and nuclear imaging studies failed to provide adequate localization. None had MRI evidence of hippocampal sclerosis. However, ICEEG demonstrated exclusive mesial temporal seizure onset in all patients. Clues suggesting the possibility of mTLE were typical auras when present, anterior temporal epileptiform discharges or ictal patterns, small hippocampi, asymmetrical or ipsilateral temporal hypometabolism on PET, anterior temporal hyperperfusion on ictal SPECT, and asymmetry of memory scores. Histopathology revealed hippocampal sclerosis in 6 patients and gliosis in 2.

CONCLUSIONS

Atypical electroclinical presentation may be deceptive in some patients with mTLE. We emphasize the importance of searching for typical mTLE features to guide ICEEG study of mesial temporal structures in such patients, who may otherwise mistakenly undergo extramesial temporal resections or be denied surgery.

Seizure localization using three-dimensional surface projections of intracranial EEG power

Intracranial EEG (icEEG) provides a critical road map for epilepsy surgery but it has become increasingly difficult to interpret as technology has allowed the number of icEEG channels to grow. Borrowing methods from neuroimaging, we aimed to simplify data analysis and increase consistency between reviewers by using 3D surface projections of intracranial EEG poweR (3D-SPIER). We analyzed 139 seizures from 48 intractable epilepsy patients (28 temporal and 20 extratemporal) who had icEEG recordings, epilepsy surgery, and at least one year of post-surgical follow-up. We coregistered and plotted icEEG β frequency band signal power over time onto MRI-based surface renderings for each patient, to create color 3D-SPIER movies. Two independent reviewers interpreted the icEEG data using visual analysis vs. 3D-SPIER, blinded to any clinical information. Overall agreement rates between 3D-SPIER and icEEG visual analysis or surgery were about 90% for side of seizure onset, 80% for lobe, and just under 80% for sublobar localization. These agreement rates were improved when flexible thresholds or frequency ranges were allowed for 3D-SPIER, especially for sublobar localization. Interestingly, agreement was better for patients with good surgical outcome than for patients with poor outcome. Localization using 3D-SPIER was measurably faster and considered qualitatively easier to interpret than visual analysis. These findings suggest that 3D-SPIER could be an improved diagnostic method for presurgical seizure localization in patients with intractable epilepsy and may also be useful for mapping normal brain function.

Quantitative analysis of simultaneous EEG features during PET studies for childhood partial epilepsy

PURPOSE

To demonstrate the significance of simultaneous electroencephalography (EEG) recording during 2-deoxy-2-[(18)F] fluoro-D-glucose (FDG)-positron emission tomography (PET) in childhood partial epilepsy.

MATERIALS AND METHODS

We included 46 children with partial epilepsy who underwent simultaneous EEG during PET. We compared the epileptogenic area of several EEG features including epileptiform discharges, focal polymorphic slow waves, and electrographic seizures, with the abnormal metabolic region on PET. We also compared the epileptogenic area of simultaneous EEG and PET with findings on magnetic resonance imaging (MRI) and video/EEG, as well as the histopathological diagnosis of the resected cortical area, in eight patients who underwent surgical resection of the epileptogenic area.

RESULTS

Hypometabolic regions on interictal PET were concordant with epileptogenic areas of epileptiform discharges and focal polymorphic slow waves, according to their frequency and/or severity, with odds ratios of 1.35 and 1.81, respectively (p<0.05). Hypermetabolic PET was also concordant with epileptogenic areas of ictal events longer than 20 seconds during the period of FDG uptake. Among the eight patients who underwent surgical resection, six patients, including two with non-lesional MRI, had concordant EEG and PET findings, were confirmed pathologically, and became seizure-free after surgery.

CONCLUSION

Simultaneous EEG is useful in identifying epileptogenic areas due to a high concordance with abnormal PET metabolic areas. Moreover, simultaneous EEG may also prevent false lateralization of PET from postictal and mixed metabolism during ictal events, as well as abnormal hypermetabolism, during frequent interictal epileptiform discharges.

Surgical approaches in nonlesional neocortical epilepsy

There is inherent difficulty in identifying the epileptogenic zone in nonlesional neocortical epilepsy, which leads to the incomplete resection. However, with careful interpretation of other studies including functional neuroimaging and the presence of concordant results, surgical treatment can benefit selected patients with nonlesional neocortical epilepsy. Two recent large studies including ours demonstrated that seizure free outcomes were 47 and 55% for nonlesional TLE, and 41 and 43% for nonlesional extratemporal lobe epilepsy patients. Concordance with two or more presurgical evaluations among interictal EEG, ictal EEG, FDG-PET, and ictal SPECT was significantly related to a seizure-free outcome. However, we should be cautious to the possibility of false localization of ictal EEG or functional neuroimaging in nonlesional neocortical epilepsy. Careful placement of intracranial electrodes on the presumed epileptogenic zone and the adjacent areas should be needed for these patients. The repositioning of intracranial electrodes after the failure in identifying ictal onset zone at the initial intracranial study might identify a new ictal onset zone. Consideration of one-week interval repositioning of intracranial electrodes could be helpful in selected patients. Intracranial EEG is one of the most important procedures in planning surgery and achieving a good surgical outcome in resective epilepsy surgery. Slow propagation and focal or regional ictal onset rather than widespread onset were associated with a seizure-free outcome. Complete resection including the area with initial three second ictal rhythm and interictal abnormalities predicts a good surgical outcome.

Foramen ovale electrodes in the evaluation of epilepsy surgery: conventional and unconventional uses

Foramen ovale (FO) electrodes have been used in the evaluation of epilepsy surgery for more than 25 years. Their traditional application was in patients with mesial temporal lobe epilepsy. Due in part to advances in neuroimaging, their use has declined. We describe our cumulative experience with FO electrodes and use examples to illustrate a range of indications for FO recordings that extend beyond their conventional utility for mesial temporal lobe cases. We also summarize the pros and cons of FO electrodes implantation and attempt to reestablish their utility in presurgical evaluation.

Objective detection of epileptic foci by 18F-FDG PET in children undergoing epilepsy surgery

PET has been used for the presurgical localization of epileptic foci for more than 20 y; still, its clinical role in children with intractable epilepsy remains unclear, largely because of variable analytic approaches and different outcome measures. The purpose of the present study was to evaluate and optimize the performance (lateralization and lobar localization value of epileptic foci) of objective voxel-based analysis of (18)F-FDG PET scans in a pediatric epilepsy population.

METHODS

Twenty children with intractable focal epilepsy (mean age ± SD, 11 ± 4 y; age range, 6-18 y) who underwent interictal (18)F-FDG PET, followed by 2-stage epilepsy surgery with chronic subdural electrocorticographic monitoring, and were seizure-free after surgery were included in this study. PET images were analyzed using both a visual-analysis and a statistical parametric mapping (SPM) method. Lateralization value and performance of lobar localization (in lateral and medial surfaces of all lobes, total of 8 regions in each epileptic hemisphere), calculated for 3 different statistical thresholds, were determined against intracranial electrocorticography-determined seizure-onset region and surgical resection site.

RESULTS

SPM using a statistical threshold of P less than 0.001 provided 100% correct lateralization, which was better than visual assessment (90%). Although visual and SPM analyses (with both P < 0.001 and P < 0.0001 thresholds) performed similarly well (with a sensitivity and specificity of 74% or above) in the localization of seizure-onset regions, SPM detected 7 of 9 seizure-onset regions, mostly in medial cortices, that were missed by visual assessment. Also, SPM performed equally well in both hemispheres, compared with visual analysis, which performed better in the left hemisphere. No statistical difference in performance was observed between visual and SPM analyses of children with abnormal versus normal MRI findings or of children with gliosis versus developmental pathology. Clinical variables, such as age, duration of epilepsy, age of seizure onset, and time between PET and last seizure, showed no correlation with sensitivity or specificity of either visual analysis or SPM analysis.

CONCLUSION

SPM analysis, using a young adult control group, can be used as a complementary objective analytic method in identifying epileptogenic lobar regions by (18)F-FDG PET in children older than 6 y.

Electrical source imaging for presurgical focus localization in epilepsy patients with normal MRI

PURPOSE

Patients with magnetic resonance (MR)-negative focal epilepsy (MRN-E) have less favorable surgical outcomes (between 40% and 70%) compared to those in whom an MRI lesion guides the site of surgical intervention (60-90%). Patients with extratemporal MRN-E have the worst outcome (around 50% chance of seizure freedom). We studied whether electroencephalography (EEG) source imaging (ESI) of interictal epileptic activity can contribute to the identification of the epileptic focus in patients with normal MRI.

METHODS

We carried out ESI in 10 operated patients with nonlesional MRI and a postsurgical follow-up of at least 1 year. Five of the 10 patients had extratemporal lobe epilepsy. Evaluation comprised surface and intracranial EEG monitoring of ictal and interictal events, structural MRI, [(18)F]fluorodeoxyglucose positron emission tomography (FDG-PET), ictal and interictal perfusion single photon emission computed tomography (SPECT) scans. Eight of the 10 patients also underwent intracranial monitoring.

RESULTS

ESI correctly localized the epileptic focus within the resection margins in 8 of 10 patients, 9 of whom experienced favorable postsurgical outcomes.

DISCUSSION

The results highlight the diagnostic value of ESI and encourage broadening its application to patients with MRN-E. If the surface EEG contains fairly localized spikes, ESI contributes to the presurgical decision process.

The role of EEG in epilepsy: a critical review

Electroencephalography (EEG) is the most specific method to define epileptogenic cortex. Its sensitivity and specificity depend on several factors such as age and recording procedures, for example, sleep recordings and activation procedures (hyperventilation, photic stimulation). EEG reveals characteristic findings in several epilepsy syndromes. Rarely, epileptiform discharges are recorded in healthy, particularly young individuals. Ictal video/EEG recording is considered to be critical in localizing the epileptogenic zone. A careful analysis of the first clinical signs and symptoms of a seizure and of the evolution of the seizure symptomatology can provide important localizing clues. Although surface EEG recordings are less sensitive than invasive studies, they provide the best overview and, therefore, the most efficient way to define the approximate localization of the epileptogenic zone. Invasive recordings are used in patients in whom the epileptogenic zone either cannot be located with noninvasive diagnostic methods or is adjacent to eloquent cortex. The most commonly used invasive electrodes are stereotactically implanted depth electrodes and subdural strip or grid electrodes. Foramen ovale and epidural electrodes are of intermediate invasiveness, but less sensitive. Invasive electrodes are subject to sampling errors if misplaced and should be used only after exhaustive noninvasive evaluations have (1) failed to localize the epileptogenic zone and (2) led to a testable hypothesis regarding this localization. Invasive EEG studies are associated with additional risks that are justifiable only if there is a good chance of obtaining essential localizing information and on a potentially resectable area.

Differences in ictal hyperperfusion of limbic-related structures between mesial temporal and neocortical epilepsy

BACKGROUND

Ictal propagation to the cortical and subcortical structures is well known in mesial temporal epilepsy (MTE) and neocortical epilepsy (NE). We analyzed the ictal changes of regional blood flow in limbic-related cortical and subcortical structures to evaluate the differences in ictal propagation patterns between MTE and NE.

METHODS

In 65 patients (mean age, 29.1+/-10.83 years) with intractable MTE (n=46) or NE (n=19), ictal 99m-Tc ethyl cystinate dimer SPECT was performed. A subtracted ictal SPECT coregistered to MRI images was obtained in each patient. The patterns of subtracted ictal hyperperfusion in the limbic, paralimbic and subcortical structures in patients with MTE and NE were compared.

RESULTS

The ictal hyperperfusion of the amygdala, hippocampus, temporal pole and insula were significantly higher in MTE (P<0.01), but cingulate and orbitofrontal hyperperfusion were not. A significant difference in striatal ictal hyperperfusion between the MTE and NE group was found (P=0.035). Ictal hyperperfusion of NE was greater in the superior part (P=0.011), whereas that of MTE was greater in the inferior and middle parts.

CONCLUSIONS

These findings suggest that the ictal propagation of MTE may differ from that of NE via different pathways not only in the cortical regions but also in the subcortical structures.

Epilepsy surgery: eligibility criteria and presurgical evaluation

Epilepsy surgery has benefited from major advances during the last 20 years, thanks to the development of neuroimaging and long-term video-electroencephalographic (EEG) monitoring. However, it remains the case that only a small minority of potential epilepsy surgery candidates will have access to a comprehensive presurgical evaluation. Furthermore, this subset of patients are operated on after an average of 20 to 25 years of epilepsy duration. Among the various reasons that prevent many patients from benefiting from a timely presurgical evaluation, we need to emphasize the role of inaccurate information regarding eligibility criteria and lack of standardized practice. This review aims at providing an indepth discussion of the current views regarding the definition of surgical candidates, and the role of the numerous investigations used in the presurgical evaluation of patients with drug-resistant epilepsy.

The eligibility criteria required to enter a presurgical evaluation in 2008 should be relatively liberal, provided that the patient suffers from disabling seizures unrelated to an idiopathic generalized epileptic syndrome, despite appropriate antiepileptic drug treatment However, the decision as to whether or not to perform a presurgical evaluation must be individualized, and take into account the likelihood of meeting the patient's expectations in terms of outcome. These expectations need to be balanced with the apparent severity of the epileptic condition, the chance of achieving a successful surgical treatment, and the risk of a postoperative neurological, cognitive, or psychiatric deterioration. The roles and specific features of the main types of presurgical investigations are reviewed.

Frontal lobe epilepsy: clinical characteristics, surgical outcomes and diagnostic modalities

OBJECTIVE

To identify surgical prognostic factors and to characterize clinical features according to the location of the intracranial ictal onset zone of frontal lobe epilepsy (FLE) in order to assess the role of various diagnostic modalities, including concordances with presurgical evaluations.

METHODS

We studied 71 FLE patients who underwent epilepsy surgery and whose outcomes were followed for more than 2 years. Diagnoses were established by standard presurgical evaluation.

RESULTS

Clinical manifestations could be categorized into six types: initial focal motor (9 patients), initial versive seizure (15), frontal lobe complex partial seizure (14), complex partial seizure mimicking temporal lobe epilepsy (18), initial tonic elevation of arms (11), and sudden secondary generalized tonic-clonic seizure (4). Thirty-seven patients became seizure-free after surgery. Five patients were deleted in the analysis because of incomplete resection of ictal onset zones. The positive predictive value of interictal EEG, ictal EEG, MRI, PET, and ictal SPECT, respectively were 62.5%, 56.4%, 73.9%, 63.2%, and 63.6%, and the negative predictive value were 46.0%, 44.4%, 53.5%, 44.7%, and 51.7%. No significant relationship was found between the diagnostic accuracy of these modalities and surgical outcome, with the exception of MRI (p=0.029). Significant concordance of two or more modalities was observed in patients who became seizure-free (p=0.011). We could not find any clinical characteristic related to surgical outcome besides seizure frequency. No definite relationship was found between the location of intracranial ictal onset zone and clinical semiology.

CONCLUSION

Although various diagnostic methods can be useful in the diagnosis of FLE, only MRI can predict surgical outcome. Concordance between presurgical evaluations indicates a better surgical outcome.

The Impact of Cerebral Source Area and Synchrony on Recording Scalp Electroencephalography Ictal Patterns

PURPOSE

To determine the cerebral electroencephalography (EEG) substrates of scalp EEG seizure patterns, such as source area and synchrony, and in so doing assess the limitations of scalp seizure recording in the localization of seizure onset zones in patients with temporal lobe epilepsy.

METHODS

We recorded simultaneously 26 channels of scalp EEG with subtemporal supplementary electrodes and 46-98 channels of intracranial EEG in presurgical candidates with temporal lobe epilepsy. We correlated intracranial EEG source area and synchrony at seizure onset with the corresponding scalp EEG. Eighty-six simultaneous intracranial- and scalp-recorded seizures from 23 patients were evaluated.

RESULTS

Thirty-four intracranial ictal discharges (40%) from 9 patients (39%) had sufficient cortical source area (namely > 10 cm(2)) and synchrony at seizure onset to produce a simultaneous or nearly simultaneous focal scalp EEG ictal pattern. Forty-one intracranial ictal discharges (48%) from 10 patients (43%) gradually achieved the necessary source area and synchrony over several seconds to generate a scalp EEG ictal pattern. These scalp rhythms were lateralized, but not localizable as to seizure origin. Eleven intracranial ictal discharges (13%) from 4 patients (17%) recruited the necessary source area, but lacked sufficient synchrony to result in a clearly localized or lateralized scalp ictal pattern.

CONCLUSIONS

Sufficient source area and synchrony are mandatory cerebral EEG requirements for generating scalp-recordable ictal EEG patterns. The dynamic interaction of cortical source area and synchrony at the onset and during a seizure is a primary reason for heterogeneous scalp ictal EEG patterns.

Nuclear Medicine in Pediatric Neurology and Neurosurgery: Epilepsy and Brain Tumors

In pediatric drug-resistant epilepsy, nuclear medicine can provide important additional information in the presurgical localization of the epileptogenic focus. The main modalities used are interictal (18)F-fluorodeoxyglucose positron emission tomography (FDG-PET) and ictal regional cerebral perfusion study with single-photon emission computed tomography (SPECT). Nuclear medicine techniques have a sensitivity of approximately 85% to 90% in the localization of an epileptogenic focus in temporal lobe epilepsy; however, in this clinical setting, they are not always clinically indicated because other techniques (eg, icterictal and ictal electroencephalogram, video telemetry, magnetic resonance imaging [MRI]) may be successful in the identification of the epileptogenic focus. Nuclear medicine is very useful when MRI is negative and/or when electroencephalogram and MRI are discordant. A good technique to identify the epileptogenic focus is especially needed in the setting of extra-temporal lobe epilepsy; however, in this context, identification of the epileptogenic focus is more difficult for all techniques and the sensitivity of the isotope techniques is only 50% to 60%. This review article discusses the clinical value of the different techniques in the clinical context; it also gives practical suggestions on how to acquire good ictal SPECT and interictal FDG-PET scans. Nuclear medicine in pediatric brain tumors can help in differentiating tumor recurrence from post-treatment sequelae, in assessing the response to treatment, in directing biopsy, and in planning therapy. Both PET and SPECT tracers can be used. In this review, we discuss the use of the different tracers available in this still very new, but promising, application of radioisotope techniques.

Temporal lobe surgery in patients with normal MRI

PURPOSE OF REVIEW

The surgical approach to nonlesional temporal lobe epilepsy presents a significant challenge due to uncertainties regarding the extent of resection necessary to result in a seizure-free state. To outline an optimum surgical strategy, an understanding of the clinical and diagnostic presentation of mesial and lateral temporal epilepsy is required in order to properly characterize the location of the ictal onset zone. This review focuses on several methods used to identify this ictal onset zone, with emphasis on the impact each modality has on surgical outcome.

RECENT FINDINGS

Factors predicting an excellent surgical outcome include the presence of a discrete zone of low voltage fast activity and prolonged propagation time on the electroencephalogram, and the absence of metabolic dysfunction in the contralateral temporal lobe. Identifying epileptogenic regions in the temporal lobe using magnetic source imaging is a recent technique that has also yielded promising surgical outcomes. Recent prospective studies have shown that a temporal neocortical resection is very effective in providing a seizure free outcome given strict localization of the ictal onset zone to the lateral temporal region, highlighting the need for accurate characterization of mesial versus lateral nonlesional epilepsy.

SUMMARY

With accurate identification of the ictal onset zone with intracranial electroencephalography, a tailored temporal resection can yield excellent surgical results.

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.

Temporal lobe seizure semiology during wakefulness and sleep

We used regression analysis to compare the semiologic features of temporal lobe seizures that occur during sleep (TLS-S) and wake (TLS-W) in the same patient. Most semiologic features correctly lateralized seizure activity during either sleep or wake. No significant differences were found between TLS-S and TLS-W in the 18 semiologic features analyzed. The diagnostic value of TLS-S semiology is similar to that of TLS-W semiology.

Cortical Substrates of Scalp EEG Epileptiform Discharges

SUMMARY

Scalp EEG is an essential component of epilepsy presurgical evaluation during the lateralization and localization of epileptogenic focus. Scalp EEG epileptiform discharges may either guide direct surgical intervention or provide necessary information to further localize the epileptic focus with intracranial EEG recording. Despite the importance and widespread use of scalp EEG epileptiform discharges, the cortical EEG substrates underlying these spikes and seizure discharges are mostly speculative. Misconceptions are therefore prevalent regarding the necessary cortical area, synchrony, and amplitude required to generate those that are recordable at the scalp. Using contemporary EEG recording techniques such as simultaneous scalp and intracranial EEG recording, the authors' recent studies have shown that the cortical area of epileptiform discharges required for the scalp recording is considerably larger than commonly thought. A cortical area of 10 to 20 cm is often required to generate a scalp recognizable interictal spike or ictal rhythm. Sufficient cortical source area and synchrony are mandatory factors for the corresponding scalp EEG epileptiform recording. The amplitude is primarily dependent on source area and synchrony; therefore it is a less important factor. The authors review the previous literatures in conjunction with their recent investigations on this topic.

The clinical and electrophysiological characteristics of temporal lobe epilepsy with normal MRI

Background and Purpose

To identify the clinical and electrophysiological characteristics of temporal lobe epilepsy (TLE) with normal MRI.

Methods

Twenty-six patients were diagnosed with TLE with normal MRI by stereotaxically implanted depth electrode EEG (SEEG) and quantitative MRI. We divided the patients into anterior or diffuse temporal groups by interictal EEG, into localized, hemispheric or non-lateralized onset groups by ictal scalp EEG, and into focal or regional onset groups by SEEG. The clinical and electrophysiological characteristics were compared with those of 25 TLE patients with unilateral hippocampal atrophy (HA) on MRI. Four patients of TLE with unilateral HA also underwent SEEG.

Results

Patients in the normal MRI group showed a significantly higher frequency of secondarily generalization (225±235, median 160 vs 68±48, median 50, p<0.05), shorter duration of epilepsy (16±10 yrs vs 25.9±7.8 yrs, p<0.001), and less favorable surgical outcome (50% vs 88%, p <0.05) than patients in the unilateral HA group. Also, patients with normal MRI frequently showed diffuse temporal (50% vs 20%, p<0.05) discharges on interictal EEG. The ictal seizure patterns of patients with normal MRI showed less localization to one temporal lobe on scalp EEG (28% vs 65%, p<0.001) and a higher frequency of regional onset on SEEG (68% vs 8%, p<0.001) than patients with unilateral HA.

Conclusions

The characteristics of TLE with normal MRI compared with TLE with unilateral HA are shorter duration of epilepsy, higher frequency of secondarily generalization, and less favorable surgical outcome, suggesting wider areas of temporal lobe involved compared with patients with unilateral HA.

Prognostic Factors in Neocortical Epilepsy Surgery: Multivariate Analysis

PURPOSE

Defining prognostic factors for neocortical epilepsy surgery is important for the identification of ideal candidates and for predicting the prognosis of individual patients. We use multivariate analysis to identify favorable prognostic factors for neocortical epilepsy surgery.

METHODS

One hundred ninety-three neocortical epilepsy patients, including 91 without focal lesions on MRI, were included. Sixty-one had frontal lobe epilepsy (FLE), 80 had neocortical temporal lobe epilepsy (nTLE), 21 had parietal lobe epilepsy (PLE), and 22 had occipital lobe epilepsy (OLE). The primary outcome variable was patient status >or=2 years after surgery (i.e., seizure free or not). Clinical characteristics and the recent presurgical diagnostic modalities were considered as probable prognostic factors. Univariate and standard multiple logistic regression analyses were used to identify favorable prognostic factors.

RESULTS

The seizure-free rate was 57.5%. By univariate analysis, a focal lesion on MRI, localized ictal onset on surface EEG, epilepsies other than FLE, localized hypometabolism on fluorodeoxyglucose-positron emission tomography (FDG-PET), and pathologies other than cortical dysplasia were significantly associated with a seizure-free outcome (p<0.05). Multivariate analysis revealed that a focal lesion on MRI (p=0.003), correct localization by FDG-PET (p=0.007), and localized ictal onset on EEG (p=0.01) were independent predictors of a good outcome.

CONCLUSIONS

The presence of a focal lesion on MRI, correct localized hypometabolism on FDG-PET, or localized ictal rhythms on EEG were identified as predictors of a seizure-free outcome. Our results suggest that these findings allow the selection of better candidates for neocortical epilepsy surgery.

Surgical outcome and prognostic factors of cryptogenic neocortical epilepsy

Surgical treatment of cryptogenic neocortical epilepsy is challenging. The aim of this study was to evaluate surgical outcomes and to identify possible prognostic factors including the results of various diagnostic tools. Eighty-nine patients with neocortical epilepsy with normal magnetic resonance imaging (35 patients with frontal lobe epilepsy, 31 with neocortical temporal lobe epilepsy, 11 with occipital lobe epilepsy, 11 with parietal lobe epilepsy, and 1 with multifocal epilepsy) underwent invasive study and focal surgical resection. Patients were observed for at least 2 years after surgery. The localizing values of interictal electroencephalogram (EEG), ictal scalp EEG, interictal 18F-fluorodeoxyglucose positron emission tomography (FDG-PET), and subtraction ictal single-photon emission computed tomography were evaluated. Seventy-one patients (80.0%) had a good surgical outcome (Engel class 1-3); 42 patients were seizure free. Diagnostic sensitivities of interictal EEG, ictal scalp EEG, FDG-PET, and subtraction ictal single-photon emission computed tomography were 37.1%, 70.8%, 44.3%, and 41.1%, respectively. Localization by FDG-PET and interictal EEG was correlated with a seizure-free outcome. The localizing value of FDG-PET was greatest in neocortical temporal lobe epilepsy. The focalization of ictal onset and also ictal onset frequency in invasive studies were not related to surgical outcome. Concordance with two or more presurgical evaluations was significantly related to a seizure-free outcome.

Ictal SPECT

The localizing value of ictal single-photon emission computed tomography (SPECT) performed with cerebral blood flow agents in patients with epilepsy is based on cerebral metabolic and perfusion coupling. Ictal hyperperfusion is used to localize the epileptogenic zone noninvasively, and is particularly useful in magnetic resonance (MR)-negative partial epilepsy and focal cortical dysplasias. Subtraction ictal SPECT coregistered with MRI (SISCOM) improves the localization of the area of hyperperfusion. Ictal SPECT should always be interpreted in the context of a full presurgical evaluation. Early ictal SPECT injections minimize the problem of seizure propagation and of nonlocalization due to an early switch from ictal hyperperfusion to postictal hypoperfusion during brief extratemporal seizures. The degree of thresholding of SISCOM images affects the sensitivity and specificity of ictal SPECT. Ictal hypoperfusion may reflect ictal inhibition or deactivation. Postictal and interictal SPECT studies are less useful to localize the ictal-onset zone. Statistical parametric mapping analysis of groups of selected ictal-interictal difference images has the potential to demonstrate the evolution of cortical, subcortical, and cerebellar perfusion changes during a particular seizure type, to study seizure-gating mechanisms, and to provide new insights into the pathophysiology of seizures.