Indications and outcome of ictal recording with intracerebral and subdural electrodes in refractory complex partial seizures

Using high-frequency oscillations from brief intraoperative neural recordings to predict the seizure onset zone

BACKGROUND

While high-frequency oscillations (HFOs) and their stereotyped clusters (sHFOs) have emerged as potential neuro-biomarkers for the rapid localization of the seizure onset zone (SOZ) in epilepsy, their clinical application is hindered by the challenge of automated elimination of pseudo-HFOs originating from artifacts in heavily corrupted intraoperative neural recordings. This limitation has led to a reliance on semi-automated detectors, coupled with manual visual artifact rejection, impeding the translation of findings into clinical practice.

METHODS

In response, we have developed a computational framework that integrates sparse signal processing and ensemble learning to automatically detect genuine HFOs of intracranial EEG data. This framework is utilized during intraoperative monitoring (IOM) while implanting electrodes and postoperatively in the epilepsy monitoring unit (EMU) before the respective surgery.

RESULTS

Our framework demonstrates a remarkable ability to eliminate pseudo-HFOs in heavily corrupted neural data, achieving accuracy levels comparable to those obtained through expert visual inspection. It not only enhances SOZ localization accuracy of IOM to a level comparable to EMU but also successfully captures sHFO clusters within IOM recordings, exhibiting high specificity to the primary SOZ.

CONCLUSIONS

These findings suggest that intraoperative HFOs, when processed with computational intelligence, can be used as early feedback for SOZ tailoring surgery to guide electrode repositioning, enhancing the efficacy of the overall invasive therapy.

Reliability of visual review of intracranial electroencephalogram in identifying the seizure onset zone: A systematic review and implications for the accuracy of automated methods

Visual review of intracranial electroencephalography (iEEG) is often an essential component for defining the zone of resection for epilepsy surgery. Unsupervised approaches using machine and deep learning are being employed to identify seizure onset zones (SOZs). This prompts a more comprehensive understanding of the reliability of visual review as a reference standard. We sought to summarize existing evidence on the reliability of visual review of iEEG in defining the SOZ for patients undergoing surgical workup and understand its implications for algorithm accuracy for SOZ prediction. We performed a systematic literature review on the reliability of determining the SOZ by visual inspection of iEEG in accordance with best practices. Searches included MEDLINE, Embase, Cochrane Library, and Web of Science on May 8, 2022. We included studies with a quantitative reliability assessment within or between observers. Risk of bias assessment was performed with QUADAS-2. A model was developed to estimate the effect of Cohen kappa on the maximum possible accuracy for any algorithm detecting the SOZ. Two thousand three hundred thirty-eight articles were identified and evaluated, of which one met inclusion criteria. This study assessed reliability between two reviewers for 10 patients with temporal lobe epilepsy and found a kappa of .80. These limited data were used to model the maximum accuracy of automated methods. For a hypothetical algorithm that is 100% accurate to the ground truth, the maximum accuracy modeled with a Cohen kappa of .8 ranged from .60 to .85 (F-2). The reliability of reviewing iEEG to localize the SOZ has been evaluated only in a small sample of patients with methodologic limitations. The ability of any algorithm to estimate the SOZ is notably limited by the reliability of iEEG interpretation. We acknowledge practical limitations of rigorous reliability analysis, and we propose design characteristics and study questions to further investigate reliability.

Stereotyped high-frequency oscillations discriminate seizure onset zones and critical functional cortex in focal epilepsy

High-frequency oscillations in local field potentials recorded with intracranial EEG are putative biomarkers of seizure onset zones in epileptic brain. However, localized 80-500 Hz oscillations can also be recorded from normal and non-epileptic cerebral structures. When defined only by rate or frequency, physiological high-frequency oscillations are indistinguishable from pathological ones, which limit their application in epilepsy presurgical planning. We hypothesized that pathological high-frequency oscillations occur in a repetitive fashion with a similar waveform morphology that specifically indicates seizure onset zones. We investigated the waveform patterns of automatically detected high-frequency oscillations in 13 epilepsy patients and five control subjects, with an average of 73 subdural and intracerebral electrodes recorded per patient. The repetitive oscillatory waveforms were identified by using a pipeline of unsupervised machine learning techniques and were then correlated with independently clinician-defined seizure onset zones. Consistently in all patients, the stereotypical high-frequency oscillations with the highest degree of waveform similarity were localized within the seizure onset zones only, whereas the channels generating high-frequency oscillations embedded in random waveforms were found in the functional regions independent from the epileptogenic locations. The repetitive waveform pattern was more evident in fast ripples compared to ripples, suggesting a potential association between waveform repetition and the underlying pathological network. Our findings provided a new tool for the interpretation of pathological high-frequency oscillations that can be efficiently applied to distinguish seizure onset zones from functionally important sites, which is a critical step towards the translation of these signature events into valid clinical biomarkers.awx374media15721572971001.

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.

The role of SISCOM in preoperative evaluation for patients with epilepsy surgery: A meta-analysis

PURPOSE

To assess the specific value of subtraction ictal and inter-ictal SPECT co-registered to MRI (SISCOM) in identifying the epileptogenic zone (EZ) and predicting postoperative outcomes in epileptic surgical patients.

METHOD

A meta-analysis of studies published from January 1995 to June 2015 was conducted through a comprehensive literature search, and 11 studies were included. R software was first used to calculate a pooled positive rate, concordant rate and positive predictive value (PPV) for good outcomes. Stata software was then used to explore the relationship between SISCOM localization and surgical outcomes, including a subgroup analysis for extra-temporal lobe epilepsy.

RESULTS

The unweighted positive and concordant rates of SISCOM were 85.9% and 65.3%, respectively. In 142 MRI-negative patients, the SISCOM positive rate was 83.8%. The pooled PPV of 178 surgical patients with concordant SISCOM was 56%. In the meta-analysis of 275 surgical patients, the seizure-free odds ratio was 3.28-times higher in concordant than in non-concordant SISCOM patients [95%CI (1.90, 5.67)]. For extra-temporal lobe epilepsy, the seizure-free odds ratio was 2.44-times higher in concordant than in non-concordant SISCOM patients [95%CI (1.34, 4.43)].

CONCLUSION

Our data indicate that SISCOM has moderate sensitivity in localizing the epileptogenic zone and can provide complementary information when MRI is negative. Furthermore, SISCOM localization concordant with the gold standard demonstrates slightly higher predictive value for good surgical outcomes. Further research is required to explore the influence of SISCOM localization results in temporal lobe versus extra-temporal lobe epilepsy.

Mesial temporal lobe epilepsy: a distinct electroclinical subtype of temporal lobe epilepsy

Mesial temporal lobe epilepsy is a common subtype of temporal lobe epilepsy. Its most common cause is hippocampal sclerosis, which contributes to its distinct electroclinical phenotype that is seen commonly in the epilepsy monitoring unit setting. The common electrophysiological data show anterior temporal interictal sharp waves as well as rhythmic theta activity in the same localization. While the electrophysiological data can at times be misleading, its stereotyped and characteristic semiology can often allow for accurate diagnosis on its own. As patients with mesial temporal lobe epilepsy often fail medical therapy, surgical therapy can be considered. Early accurate diagnosis in these patients is essential for optimal care.

Computer-Aided Diagnosis and Localization of Lateralized Temporal Lobe Epilepsy Using Interictal FDG-PET

Interictal FDG-PET (iPET) is a core tool for localizing the epileptogenic focus, potentially before structural MRI, that does not require rare and transient epileptiform discharges or seizures on EEG. The visual interpretation of iPET is challenging and requires years of epilepsy-specific expertise. We have developed an automated computer-aided diagnostic (CAD) tool that has the potential to work both independent of and synergistically with expert analysis. Our tool operates on distributed metabolic changes across the whole brain measured by iPET to both diagnose and lateralize temporal lobe epilepsy (TLE). When diagnosing left TLE (LTLE) or right TLE (RTLE) vs. non-epileptic seizures (NES), our accuracy in reproducing the results of the gold standard long term video-EEG monitoring was 82% [95% confidence interval (CI) 69-90%] or 88% (95% CI 76-94%), respectively. The classifier that both diagnosed and lateralized the disease had overall accuracy of 76% (95% CI 66-84%), where 89% (95% CI 77-96%) of patients correctly identified with epilepsy were correctly lateralized. When identifying LTLE, our CAD tool utilized metabolic changes across the entire brain. By contrast, only temporal regions and the right frontal lobe cortex, were needed to identify RTLE accurately, a finding consistent with clinical observations and indicative of a potential pathophysiological difference between RTLE and LTLE. The goal of CADs is to complement - not replace - expert analysis. In our dataset, the accuracy of manual analysis (MA) of iPET (∼80%) was similar to CAD. The square correlation between our CAD tool and MA, however, was only 30%, indicating that our CAD tool does not recreate MA. The addition of clinical information to our CAD, however, did not substantively change performance. These results suggest that automated analysis might provide clinically valuable information to focus treatment more effectively.

Mesial temporal lobe epilepsy

Temporal lobe epilepsy (TLE) is the most common form of adult localization-related epilepsy. Hippocampal onset accounts for at least 80% of all temporal lobe seizures. The electroencephalogram (EEG) of mesial TLE contains interictal features often associated with anterior temporal epileptiform discharges with a maximal voltage over the basal temporal electrodes. Localized ictal patterns on scalp EEGs characteristically reveal unilateral 5- to 9-Hz rhythmic ictal theta or alpha epileptiform activity maximal in the anterior temporal scalp electrodes. Invasive-scalp EEG comparisons have yielded direct information about mesial temporal sources and their corresponding electrical fields. Refinement of macroscopic spatial and the temporal resolution suggest that a more precise seizure localization may exist beyond 1- to 35-Hz frequencies observed in routine scalp recording. Defining the focal areas of ictogenesis within the medial temporal lobe demonstrates a rich connection to a broad network that goes beyond the medial structures and even the temporal lobe itself. Advanced electrophysiologic application in TLE may further our understanding of ictogenesis to perfect surgical treatment and to elucidate the neurophysiologic corollaries of epileptogensis itself.

Noninvasive predictors of subdural grid seizure localization in children with nonlesional focal epilepsy

PURPOSE

Subdural grid evaluation (SDE) in refractory focal epilepsy aims to precisely define the ictal onset zone and map eloquent cortex. In a small but significant proportion of children, SDE shows multifocal or diffuse, rather than focal, seizure onset. Resective epilepsy surgery is denied, or is unsuccessful, in the majority of such patients. The authors investigated whether the noninvasive data could be abstracted to predict subsequent SDE electrographic outcome (focal vs. multifocal/diffuse ictal onset).

METHODS

The authors retrospectively reviewed charts of 66 children with refractory focal epilepsy undergoing SDE at Cleveland Clinic over a 7-year period, studied previously by Pestana Knight et al. A semiquantitative "score" summarizing the localizing value and concordance between selected noninvasive investigations (interictal and ictal EEG; positron emission tomography [PET], and/or single-photon emission computed tomography [SPECT]), as well as Bayesian predictors of individual investigations and their combinations, were adapted from the study of Kalamangalam et al to the subset of patients with nonlesional cranial MRI.

RESULTS

Forty (60.6%) patients had a single MRI brain lesion, 7 (10.6%) had bilateral or diffuse MRI changes, and 19 (28.8%) were nonlesional. Subdural grid evaluation ictal onset was nonfocal in four patients in the first group (10%) and in two patients (28.5%) in the second group. One patient in the third (nonlesional) group was excluded because of incomplete data. In the remainder (n = 18), SDE ictal onset was multifocal or diffuse in 5 (27.8%) and focal in 13 (72.2%). Focality on SDE was positively correlated with higher noninvasive scores in the nonlesional patient group (χ test, P < 0.025). Bayesian predictors in this group were highest for concordance between the interictal and ictal scalp EEG (likelihood ratio = 3.85). Considered separately, interictal and ictal EEG were of equivalent predictive value (likelihood ratio = 2.3 and 2.1, respectively). Metabolic imaging was the least useful modality.

CONCLUSIONS

(1) Diffuse or multifocal ictal onsets on SDE are almost three times as likely in nonlesional patients as in those with a single definite MRI brain lesion. (ii) The noninvasive data of children with nonlesional brain MRI may be summarized by a score that rewards localizing information and intermodality concordance: low-scoring patients are more likely to exhibit diffuse or multifocal ictal onset on subsequent SDE. (iii) Bayesian likelihood ratios predictive of ictal focality on SDE are highly favorable for concordant scalp interictal-ictal EEG combinations. (iv) Decision-theoretic methods of this type may find use in the selection of nonlesional pediatric presurgical candidates offered SDE.

Ictal intracranial recording from a 'burned-out hippocampus'

OBJECTIVE

To present a case involving a discrepancy in the presurgical data of a patient suffering from pharmacoresistant mesial temporal lobe epilepsy.

CLINICAL PRESENTATION AND INTERVENTION

A 47-year-old, female patient with complex partial seizures since her twenties came to be evaluated in the Epilepsy Surgery Unit. The ictal electroencephalogram suggested a left temporal epileptogenic zone and the magnetic resonance image showed an abnormality in the right mesial temporal lobe. Intracranial monitoring revealed a pacemaker zone in the right hippocampus that discharged fast spreading to the left mesial temporal lobe, a phenomenon known as 'burned-out hippocampus'.

CONCLUSION

The intracranial recording, even though it is an invasive procedure, was necessary for the presurgical evaluation of our patient. This case demonstrates the risks of using surface electroencephalography to determine localization of epileptogenic zones.

Outcome of no resection after long-term subdural electroencephalography evaluation in children with epilepsy

OBJECT

The aim of this study was to identify the reasons for and predictors of no resection of the epileptogenic zone in children with epilepsy who had undergone long-term invasive subdural grid electroencephalography (SDG-EEG) evaluation.

METHODS

The authors retrospectively reviewed the consecutive medical records of children (< 19 years of age) who had undergone SDG-EEG evaluation over a 7-year period (1997-2004). To determine the predictors of no resection, the authors obtained the clinical characteristics and imaging and EEG findings of children who had no resection after long-term invasive SDG-EEG evaluation and compared these data with those in a group of children who did undergo resection. They describe the indications for SDG-EEG evaluation and the reasons for no resection in these patients.

RESULTS

Of 66 children who underwent SDG-EEG evaluation, 9 (13.6%) did not undergo subsequent resection (no-resection group; 6 males). Of these 9 patients, 6 (66.7%) had normal neurological examinations and 5 (55.6%) had normal findings on brain MR imaging. Scalp video EEG localized epilepsy to the left hemisphere in 6 of the 9 patients and to the right hemisphere in 2; it was nonlocalizable in 1 of the 9 patients. Indications for SDG-EEG in the no-resection group were ictal onset zone (IOZ) localization (9 of 9 patients), motor cortex localization (5 of 9 patients), and language area localization (4 of 9 patients). Reasons for no resection after SDG-EEG evaluation were the lack of a well-defined IOZ in 5 of 9 patients (4 multifocal IOZs and 1 nonlocalizable IOZ) and anticipated new permanent postoperative neurological deficits in 7 of 9 patients (3 motor, 2 language, and 2 motor and language deficits). Comparison with the resection group (57 patients) demonstrated that postictal Todd paralysis in the dominant hand was the only variable seen more commonly (χ(2) = 4.781, p = 0.029) in the no-resection group (2 [22.2%] of 9 vs 2 [3.5%] of 57 patients). The no-resection group had a larger number of SDG electrode contacts (mean 126. 5 ± 26.98) as compared with the resection group (100.56 ± 25.52; p = 0.010). There were no significant differences in the demographic data, seizure characteristics, scalp and invasive EEG findings, and imaging variables between the resection and no-resection groups.

CONCLUSIONS

Children who did not undergo resection of the epileptogenic zone after SDG-EEG evaluation were likely to have normal neurological examinations without preexisting neurological deficits, a high probability of a new unacceptable permanent neurological deficit following resection, or multifocal or nonlocalizable IOZs. In comparison with the group that underwent resection after SDG-EEG, a history of Todd paralysis in the dominant hand and arm was the only predictor of no resection following SDG-EEG evaluation. Data in this study will help to better select pediatric patients for SDG-EEG and to counsel families prior to epilepsy surgery.

Hippocampal sclerosis in temporal lobe epilepsy: findings at 7 T¹

PURPOSE

To determine if ultrahigh-field-strength magnetic resonance (MR) imaging can be used to detect subregional hippocampal alterations.

MATERIALS AND METHODS

Subjects provided written consent to participate in this prospective institutional review board-approved HIPAA-compliant study. T1- and T2-weighted 7-T brain MR images were acquired in 11 healthy subjects and eight patients with temporal lobe epilepsy (TLE). In all subjects, images were qualitatively examined for evidence of hippocampal atrophy, signal change, and malrotation with the Bernasconi definition, and digitations of the hippocampal heads were counted (agreement was measured with the κ statistic). Data were analyzed quantitatively with manual subregional hippocampal body segmentation. Subregional data in individual subjects with TLE were compared with data in control subjects to detect deviation from the control range for volume measures on each side and with asymmetry indexes.

RESULTS

All eight patients with TLE had hippocampal abnormalities on the epileptogenic side. Subregional analysis revealed selective lateral Ammon horn atrophy in six patients and diffuse Ammon horn and dentate gyrus atrophy in one patient. Paucity of hippocampal digitations occurred on the epileptogenic side in all patients with TLE and also on the contralateral side in three patients (interrater κ value, 0.80). Hippocampal malrotation was observed in three patients with TLE and four control subjects.

CONCLUSION

Ultrahigh-field-strength MR imaging permitted detection of selectively greater Ammon horn atrophy in patients with TLE and hippocampal sclerosis. Paucity of digitations is a deformity of the hippocampal head that was detected independent of hippocampal atrophy in patients with mesial TLE.

Safety and utility of supplemental depth electrodes for localizing the ictal onset zone in pediatric neocortical epilepsy

OBJECT

Depth electrodes provide a better sampling of sulci and regions of cortex that lie tangential to the plane of subdural electrodes. The aim of this study was to evaluate the utility of supplemental depth electrodes in the surgical treatment of pediatric patients with neocortical epilepsy.

METHODS

Cases involving 12 consecutive pediatric patients (mean age [SD] 10.9 ± 4.4 years) were reviewed. Focal resective surgery (in 9 cases) or functional hemispherectomy (in 3 cases) was performed after intracranial monitoring. The mean total number of electrodes was 118 ± 29; the mean numbers of grid, strip, and depth electrodes were 95 ± 27, 10 ± 6, and 13 ± 5, respectively The most common pathological condition was focal cortical dysplasia.

RESULTS

In 4 cases, depth electrodes demonstrated the ictal onset zone in an area not easily accessible by grids or strips (in the basal temporal region in 3 cases and the upper opercular region in 1 case). In 3 of these 4 cases, the ictal onset zone was defined exclusively by the depth electrodes. In each of these 3 cases, the surface electrodes (on grids or strips) demonstrated early propagation but not ictal onset. In 9 cases, the depth electrodes also demonstrated the early propagation zone. The information about the ictal onset zone and the early propagation zone helped to provide additional information that affected the extent (in 7 cases) or depth (in 3 cases) of the resection. The proportion of the electrodes involved in resection relative to the total number of implanted electrodes was low (mean ± SD, 0.26 ± 0.09). Nine patients (75.0%) became seizure free (Engel class IA outcome) after surgery (mean duration of follow-up 25.7 ± 4.29 months). There were no surgical complications related to intracranial electroencephalography monitoring.

CONCLUSIONS

Most patients (75.0%) became seizure free after extensive monitoring and more limited resection of seizure-onset regions. Supplemental depth electrodes contribute to improved outcome by providing information about the ictal onset zone that is not accessible by grid or strip electrodes in some cases. The supplemental depth electrodes conferred an extra dimension of depth to the analysis, which allowed for successful outcome with more limited resection.

Presurgical epilepsy localization with interictal cerebral dysfunction

Localization of interictal cerebral dysfunction with 2-[(18)F]fluoro-2-D-deoxyglucose (FDG) positron emission tomography (PET) and neuropsychological examination usefully supplements electroencephalography (EEG) and brain magnetic resonance imaging (MRI) in planning epilepsy surgery. In MRI-negative mesial temporal lobe epilepsy, correlation of temporal lobe hypometabolism with extracranial ictal EEG can support resection without prior intracranial EEG monitoring. In refractory localization-related epilepsies, hypometabolic sites may supplement other data in hypothesizing likely ictal onset zones in order to intracranial electrodes for ictal recording. Prognostication of postoperative seizure freedom with FDG PET appears to have greater positive than negative predictive value. Neuropsychological evaluation is critical to evaluating the potential benefit of epilepsy surgery. Cortical deficits measured with neuropsychometry are limited in lateralizing and localizing value for determination of ictal onset sites, however. Left temporal resection risks iatrogenic verbal memory deficits and dysnomia, and neuropsychological findings are useful in predicting those at greatest risk. Prognostication of cognitive risks with resection at other sites is less satisfactory.

Predictive factors for outcome of invasive video-EEG monitoring and subsequent resective surgery in patients with refractory epilepsy

OBJECTIVE

This is a descriptive study of patients who underwent invasive video-EEG monitoring (IVEM) at Ghent University Hospital. The aim of the study is to identify predictive factors for outcome of IVEM and resective surgery (RS). These factors may optimize the patient flow following the non-invasive presurgical evaluation towards IVEM and RS or other treatments.

PATIENTS AND METHODS

Over the past 16 years, 68/710 refractory epilepsy patients included in the presurgical evaluation protocol (M/F 41/27, mean age 33 years) underwent IVEM at Ghent University Hospital. Patient features and follow-up data were collected from the patients' medical files and the electronic patient database at the neurology and neurosurgery department. Predictive factors for IVEM outcome were identified by comparing features of patients with a positive IVEM outcome (i.e. ictal onset zone identification) and patients with a negative IVEM outcome. Predictive factors for RS outcome were identified by comparing features of patients with Engel class I and patients with Engel class II-IV outcome.

RESULTS

In 56/68 patients (82%) IVEM outcome was positive. The occurrence of a seizure-free interval in the patient's history and a non-localizing ictal scalp EEG in patients with a structural abnormality on MRI (p<0.05) were predictive factors for a negative IVEM outcome. 32/68 patients underwent RS. In 22/32 (70%) patients RS resulted in an Engel class I outcome. A structural abnormality on MRI was a predictive factor for a positive RS outcome in patients in whom a focal or regional focus was resected (p<0.05).

CONCLUSION

This study shows that IVEM identifies one or more ictal onset zone(s) in up to 80% of patients. The potential of IVEM to identify the ictal onset zone is unlikely in patients with a seizure-free interval in their medical history and a non-localizing ictal scalp EEG during the non-invasive presurgical evaluation. Half of these patients underwent RS with long-term seizure freedom in 70%. Patients with structural MRI lesions have the highest chance of seizure freedom. These findings may contribute to the optimization of patient management during both the invasive and non-invasive presurgical work-up.

Advances in intracranial monitoring

Intracranial monitoring using electroencephalography (IC-EEG) continues to play a critical role in the assessment of patients with medically intractable localization-related epilepsy. There has been minimal change in grid or electrode design in the last 15–20 years, and the surgical approaches for implantation are unchanged. Intracranial monitoring using EEG allows detailed definition of the region of ictal onset and defines the epileptogenic zone, particularly with regard to adjacent potentially eloquent tissue. Recent developments of IC-EEG include the coregistration of functional imaging data such as magnetoencephalography to the frameless navigation systems. Despite significant inherent limitations that are often overlooked, IC-EEG remains the gold standard for localization of the epileptogenic cortex. Intracranial electrodes take a variety of different forms and may be placed either in the subdural (subdural strips and grids, depth electrodes) or extradural spaces (sphenoidal, peg, and epidural electrodes). Each form has its own advantages and shortcomings but extensive subdural implantation of electrodes is most common and is most comprehensively discussed. The indications for intracranial electrodes are reviewed.

Chronic subdural electrodes in the management of epilepsy

Subdural electrodes play a very important role in the evaluation of a percentage of patients being considered for epilepsy surgery. Electrical activity at very low and very high frequencies, beyond the practical range of scalp EEG, can be recorded subdurally and may contain considerable information not available non-invasively. The recording and stimulating procedures for using chronically implanted subdural electrodes to localize the epileptogenic zone and map eloquent functions of the human cortex are well established, and complication rates are low. Complications include infections, CSF leak, and focal neurologic deficits, all of which tend to be increased with a higher number of electrodes and longer duration of recordings. Careful consideration of the risks and benefits should be coupled with a firm hypothesis about the epileptogenic zone derived from the non-invasive components of the epilepsy workup to guide the decision about whether and where to implant subdural electrodes. When they are employed to answer a specific question in an individual patient, subdural electrodes can optimize the clinical outcome of a candidate for epilepsy surgery.

The contribution of 18F-FDG PET in preoperative epilepsy surgery evaluation for patients with temporal lobe epilepsy

OBJECTIVE

To assess the predictive diagnostic added value of positron emission tomography (PET) in preoperative epilepsy surgery evaluation for patients with temporal lobe epilepsy (TLE).

METHODS

A meta-analysis of publications from 1992 to 2006 was performed. Forty-six studies were identified that met inclusion criteria presenting detailed diagnostic test results and a classified postoperative outcome. Studies exclusively reporting on patients with brain tumors or on children were excluded.

RESULTS

The analyses were complicated by significant differences in study design and often by lack of precise patient data. Ipsilateral PET hypometabolism showed a predictive value of 86% for good outcome. The predictive value was 80% in patients with normal MRI and 72% in patients with non-localized ictal scalp EEG. In a selected population of 153 TLE patients with a follow-up of >12 months PET correlated well with other non-invasive diagnostic tests, but none of the odds ratios of any test combination was significant.

CONCLUSION

Our data confirm that ipsilateral PET hypometabolism may be an indicator for good postoperative outcome in presurgical evaluation of drug-resistant TLE, although the actual diagnostic added value remained questionable and unclear. PET does not appear to add value in patients localized by ictal scalp EEG and MRI. Prospective studies limited to non-localized ictal scalp EEG or MRI-negative patients are required for validation.

The role of 1H magnetic resonance spectroscopy in pre-operative evaluation for epilepsy surgery. A meta-analysis

PURPOSE

We aimed to assess the additional pre-operative value of (1)H MRS in identifying the epileptogenic zone (EZ) for epilepsy surgery by performing a meta-analysis considering publications from 1992 to 2003.

METHODS

From an extensive computer and hand search 22 studies were included. For inclusion, studies had to report post-operative outcome and detailed diagnostic test results for each individual patient. Studies exclusively reporting on patients with brain tumors or on children were excluded.

RESULTS

Great heterogeneity among studies regarding methodological and technical aspects and concerning evaluation and interpretation of data was observed. Only patients with intractable temporal lobe epilepsy were presented. Sixty-four percent of all patients and 72% of patients with good outcome had an ipsilateral MRS abnormality concordant with the EZ. The positive predictive value of all patients with ipsilateral MRS abnormality for good outcome was 82%. An odds ratio weighted by inverse variance showed a 4.891 better chance of seizure free outcome [CI=1.965-12.172; Q=2.748; d.f.=5; critical chi2-value=11.07] in patients with an ipsilateral MRS abnormality when compared to patients with bilateral MRS abnormalities. Data for MRI-negative patients were conflicting. One study stressed a role for MRS in patients with bilateral hippocampal atrophy at MRI.

CONCLUSIONS

MRS still remains a research tool with clinical potential. Our findings indicate the connection of ipsilateral MRS abnormality to good outcome. The ability for prediction of post-operative outcome may depend on the assessed population. Prospective studies limited to non-localized ictal scalp EEG or MRI-negative patients are required for validation of these data.

Stereoelectroencephalography in the presurgical evaluation of focal epilepsy: a retrospective analysis of 215 procedures

OBJECTIVE

To report on indications, surgical technique, results, and morbidity of stereoelectroencephalography (SEEG) in the presurgical evaluation of patients with drug-resistant focal epilepsy.

METHODS

Two-hundred fifteen stereotactic implantations of multilead intracerebral electrodes were performed in 211 patients (4 patients were explored twice), who showed variable patterns of localizing incoherence among electrical (interictal/ictal scalp electroencephalography), clinical (ictal semeiology), and anatomic (magnetic resonance imaging [MRI]) investigations. MRI scanning showed a lesion in 134 patients (63%; associated with mesial temporal sclerosis in 7) and no lesion in 77 patients (37%; with mesial temporal sclerosis in 14 patients). A total of 2666 electrodes (mean, 12.4 per patient) were implanted (unilaterally in 175 procedures and bilaterally in 40). For electrode targeting, stereotactic stereoscopic cerebral angiograms were used in all patients, coupled with a coregistered three-dimensional MRI scan in 108 patients.

RESULTS

One hundred eighty-three patients (87%) were scheduled for resective surgery after SEEG recording, and 174 have undergone surgery thus far. Resections sites were temporal in 47 patients (27%), frontal in 55 patients (31.6%), parietal in 14 patients (8%), occipital in one patient (0.6%), rolandic in one patient (0.6%), and multilobar in 56 patients (32.2%). Outcome on seizures (Engel's classification) in 165 patients with a follow-up period of more than 12 months was: Class I, 56.4%; Class II, 15.1%; Class III, 10.9%; and Class IV, 17.6%. Outcome was significantly associated with the results of MRI scanning (P = 0.0001) and with completeness of lesion removal (P = 0.038). Morbidity related to electrode implantation occurred in 12 procedures (5.6%), with severe permanent deficits from intracerebral hemorrhage in 2 (1%) patients.

CONCLUSION

SEEG is a useful and relatively safe tool in the evaluation of surgical candidates when noninvasive investigations fail to localize the epileptogenic zone. SEEG-based resective surgery may provide excellent results in particularly complex drug-resistant epilepsies.

Seizure outcome after epilepsy surgery in patients with normal preoperative MRI

OBJECTIVE

To determine outcome after epilepsy surgery in patients with normal preoperative magnetic resonance imaging (MRI).

METHODS

24 adult and paediatric patients with normal preoperative MRIs were studied. They underwent epilepsy surgery between 1994 and 2001 and had at least one year of follow up.

RESULTS

At the most recent follow up, nine patients (37%) were seizure-free and 18 (75%) had at least a 90% reduction in seizure frequency with weekly or monthly seizures. Seizure freedom was not significantly different after resections in frontal (5/9) or temporal regions (4/13) (p = 0.24, Fisher's exact test), or among patients with or without localising features on EEG, PET, or ictal SPECT. Subdural grids, used in 15 of 24 patients, helped tailor resections but were not associated with differences in outcome. Histopathology showed cortical dysplasia in 10 patients (42%), non-specific findings in 13 (54%), and hippocampal sclerosis in one (4%). Cortical dysplasia was seen in seven patients with frontal resection (78%) and non-specific findings in nine (69%) with temporal resection. Seizure outcome did not differ on the basis of location of resection or histopathology.

CONCLUSIONS

While these results were less favourable than expected for patients with focal epileptogenic lesions seen on MRI, they represented worthwhile improvement for this patient population with high preoperative seizure burden. In this highly selected group, no single test or combination of tests further predicted postoperative seizure outcome.

The role of positron emission tomography with [18F]fluorodeoxyglucose in the evaluation of the epilepsies

Cerebral glucose metabolic mapping using positron emission tomography (PET) and 2-[18F]fluoro-2-deoxyglucose (FDG) has been extensively studied in the epilepsies. Regions of interictal glucose hypometabolism are highly associated with cerebral sites of seizure generation-propagation in focal epilepsies. The volume of reduced glucose metabolism is often widespread and even bilateral in focal epilepsies, although ictal onset zones typically are located at the sites of most severe hypometabolism within a larger volume of hypometabolism.

Positron emission tomography and single photon emission computed tomography in epilepsy care

Radiopharmaceutical brain imaging is clinically applied in planning resective epilepsy surgery. Cerebral sites of seizure generation-propagation are highly associated with regions of hyperperfusion during seizures, and with glucose hypometabolism interictally. For surgical planning in epilepsy, the functional imaging modalities currently established are ictal single photon emission computed tomography (SPECT) with [(99m)Tc]technetium-hexamethylpropyleneamine oxime (HMPAO) or with [(99m)Tc]technetium-ethylene cysteine dimer (ECD), and interictal positron emission tomography (PET) with 2-[(18)F]fluoro-2-deoxyglucose (FDG). Ictal SPECT and interictal FDG PET can be used in presurgical epilepsy evaluations to reliably: (1) determine the side of anterior temporal lobectomy, and in children the area of multilobar resection, without intracranial electroencephalographic recording of seizures; (2) select high-probability sites of intracranial electrode placement for recording ictal onsets; and, (3) determine the prognosis for complete seizure control following anterior temporal lobe resection. Coregistration of a patient's structural (magnetic resonance) and functional images, and statistical comparison of a patient's data with a normal data set, can increase the sensitivity and specificity of these SPECT and PET applications to the presurgical evaluation.

Interrater reliability among epilepsy centers: multicenter study of epilepsy surgery

PURPOSE

To measure the interrater reliability of presurgical testing and surgical decisions among epilepsy centers.

METHODS

Seven centers participating in an ongoing, prospective multicenter study of resective epilepsy surgery agreed to conform to a detailed protocol regarding presurgical evaluation and surgery. To assess quality assurance, each center independently reviewed 21 randomly selected surgical cases for preoperative study lateralization and localization, and surgical decisions. Interrater reliability was assessed by using intraclass correlation coefficients (ICCs), validated for use with multiple raters, and calculated in a two-way random model based on absolute agreement.

RESULTS

Agreement for ICC values: > or = 0.75, excellent; 0.60-0.74, good; 0.40-0.59, fair; < or = 0.39, poor. One center was excluded for missing data. Agreement was excellent for extracranial EEG lateralization (0.8039), magnetic resonance imaging (MRI) lateralization (0.9521) and localization (0.9130), Wada lateralization (0.9453), and intracranial EEG localization (0.7905). Agreement was good for extracranial EEG localization (0.7384) and neuropsychological testing lateralization (0.7178) and localization (0.6891). Consensus about the decision to perform intracranial monitoring was fair (0.5397), in part reflecting one center's tendency toward intracranial monitoring. Overall agreements on whether to perform surgery (0.8311) and specific surgery recommended (0.8164) were excellent.

CONCLUSIONS

High interrater reliability among six epilepsy centers was present for interpretation of most components of presurgical testing. Although consensus for the decision to perform intracranial monitoring was only fair, agreements for the ultimate decision about resective surgery and specific choice of resection were excellent. We believe that this study demonstrates the feasibility of implementing multicenter protocols for neurologic management, especially those involving localization, as well as protocols combining study results with clinical decision making.

Long-term EEG monitoring: a clinical approach to electrophysiology

Long-term electroencephalographic monitoring (LTM) is the capability of recording the EEG over long periods of time and not a specific duration. Prolonged EEG recording is used primarily for epilepsy monitoring, but LTM is also used in the intensive care unit, the operating room, and in the emergency department. The purpose of LTM is to expand the limited time sampling associated with shorter "routine" EEG recording. Audiovisual monitoring may also be used in conjunction with LTM to evaluate simultaneously a specific clinical behavior that may or may not be associated with EEG alteration. This is typically performed in a hospital setting for safety and ancillary testing purposes. LTM is used most frequently in the diagnosis and management of seizures and "spells," but has also gained wider application in the evaluation of sleep disorders, cerebrovascular disease, psychiatric conditions, and movement disorders. Computer-assisted LTM systems that process, analyze, compress, and store data digitally have become widely available in clinical practice both in the hospital as well as outside the hospital when the patient is ambulatory.

[Neurofunctional tests in presurgical strategies for partial epilepsies]

The presurgical evaluation of drug-resistant partial epilepsies primarily relies on two major investigations, including a long term video-EEG monitoring which aimed at recording the patient's typical seizures, and a specifically designed high quality magnetic resonance imaging (MRI). The latter demonstrates an abnormality within the epileptogenic lobe in the majority of cases, which might not, however, necessarily match the epileptogenic zone. Numerous functional neuro-imaging techniques have been progressively added to the pre-surgical evaluation of refractory partial epilepsies, such as the study of cerebral glucose metabolism, benzodiazepine receptor availability, and methionine incorporation using positron emission tomography (PET), the evaluation of ictal cerebral blood flow changes using single photon emission computerized tomography (SPECT), the measurement of N-acetyl-aspartate concentration with magnetic resonance spectroscopy, and the mapping of eloquent areas using functional MRI. These investigations can help to confirm the origin of seizure onset previously suggested by MRI and electro-clinical data, and provide independent prognostic information regarding the chance of a successful surgical treatment. Moreover, functional neuro-imaging data can have a critical diagnostic value when MRI is strictly normal or shows multifocal abnormalities. However, the variety and rapid evolution of functional neuro-imaging techniques makes it difficult to propose a standard protocol. Finally, it remains mandatory to proceed to an intracranial EEG investigation in a substantial number of patients, including the majority of those suffering from an extra-temporal epilepsy.