Intracranial EEG in Temporal Lobe Epilepsy: Location of Seizure Onset Relates to Degree of Hippocampal Pathology

Multi-modal characterization and simulation of human epileptic circuitry

Temporal lobe epilepsy is the fourth most common neurological disorder, with about 40% of patients not responding to pharmacological treatment. Increased cellular loss is linked to disease severity and pathological phenotypes such as heightened seizure propensity. While the hippocampus is the target of therapeutic interventions, the impact of the disease at the cellular level remains unclear. Here, we show that hippocampal granule cells change with disease progression as measured in living, resected hippocampal tissue excised from patients with epilepsy. We show that granule cells increase excitability and shorten response latency while also enlarging in cellular volume and spine density. Single-nucleus RNA sequencing combined with simulations ascribes the changes to three conductances: BK, Cav2.2, and Kir2.1. In a network model, we show that these changes related to disease progression bring the circuit into a more excitable state, while reversing them produces a less excitable, "early-disease-like" state.

Surgical outcome and prognostic factors in epilepsy patients with MR-negative focal cortical dysplasia

Objective

Focal cortical dysplasia (FCD) represents a heterogeneous group of disorders of the cortical formation and is one of the most common causes of epilepsy. Magnetic resonance imaging (MRI) is the modality of choice for detecting structural lesions, and the surgical prognosis in patients with MR lesions is favorable. However, the surgical prognosis of patients with MR-negative FCD is unknown. We aimed to evaluate the long-term surgical outcomes and prognostic factors in MR-negative FCD patients through comprehensive presurgical data.

Methods

We retrospectively reviewed data from 719 drug-resistant epilepsy patients who underwent resective surgery and selected cases in which surgical specimens were pathologically confirmed as FCD Type I or II. If the epileptogenic focus and surgical specimens were obtained from brain areas with a normal MRI appearance, they were classified as MR-negative FCD. Surgical outcomes were evaluated at 2 and 5 years, and clinical, neurophysiological, and neuroimaging data of MR-negative FCD were compared to those of MR-positive FCD.

Results

Finally, 47 MR-negative and 34 MR-positive FCD patients were enrolled in the study. The seizure-free rate after surgery (Engel classification I) at postoperative 2 year was 59.5% and 64.7% in the MR-negative and positive FCD groups, respectively (p = 0.81). This rate decreased to 57.5% and 44.4% in the MR-negative and positive FCD groups (p = 0.43) at postoperative 5 years. MR-negative FCD showed a higher proportion of FCD type I (87.2% vs. 50.0%, p = 0.001) than MR-positive FCD. Unilobar cerebral perfusion distribution (odds ratio, OR 5.41) and concordance of interictal epileptiform discharges (OR 5.10) were significantly associated with good surgical outcomes in MR-negative FCD.

Conclusion

In this study, MR-negative and positive FCD patients had a comparable surgical prognosis, suggesting that comprehensive presurgical evaluations, including multimodal neuroimaging studies, are crucial for obtaining excellent surgical outcomes even in epilepsy patients with MR-negative FCD.

Multiple hippocampal transections for refractory pediatric mesial temporal lobe epilepsy: seizure and neuropsychological outcomes

OBJECTIVE

Temporal lobe epilepsy (TLE) is the most common focal epilepsy across adult and pediatric age groups. It is also the most amenable to surgery, with excellent long-term seizure outcome. Most TLE cases have an epileptogenic zone in the mesial temporal structures, namely the hippocampus. Resecting the dominant hippocampus has been shown to be associated with significant verbal memory deficits, especially in patients with intact verbal memory scores presurgically. Multiple hippocampal transection (MHT) is a relatively new surgical technique designed to interrupt the longitudinal hippocampal circuitry involved in seizure propagation yet preserve the circular fibers involved in memory function. This technique has been used to treat mesial TLE in both dominant- and nondominant-hemisphere cases, almost exclusively in adults. It has been applied to normal and sclerotic hippocampi.

METHODS

In this study, information on 3 pediatric patients who underwent MHT for mesial TLE at Children's Wisconsin between 2017 and 2018 is included. Clinical, electroencephalographic, and neuropsychological features and outcomes are described in detail.

RESULTS

MRI revealed a tumor in the amygdala with a normal hippocampus in 1 patient and hippocampal sclerosis in 2 patients. All patients underwent stereoelectroencephalography confirming the involvement of the hippocampus in seizure onset. MHTs were completed under intraoperative monitoring, with amygdala and temporal tip resection in all patients due to early spread to these regions. All patients had excellent seizure outcomes at 1 year, and 2 of the 3 patients remain seizure free at last follow-up (range 20-36 months), all with stable or improved neuropsychological profiles, including verbal memory.

CONCLUSIONS

MHT is a relatively new surgical procedure designed to preserve essential memory circuitry while disrupting seizure propagation pathways in the hippocampus. A growing body of literature shows good seizure and neuropsychological results, but mainly in adults. This is the first series of MHTs used exclusively in children at one medical center, showcasing excellent seizure control and preservation of neuropsychological functioning. One of the patients is also the first described to have MHT in the setting of an amygdalar tumor abutting the hippocampus, further expanding the pathological setting in which MHT can be used effectively.

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.

Prognostic Role of Functional Neuroimaging after Multilobar Resection in Patients with Localization-Related Epilepsy

To investigate the usage of functional neuroimaging as a prognostic tool for seizure recurrence and long-term outcomes in patients with multilobar resection, we recruited 90 patients who received multilobar resections between 1995 and 2013 with at least 1-year follow-up (mean 8.0 years). All patients were monitored using intracranial electroencephalography (EEG) after pre-surgical evaluation. Clinical data (demographics, electrophysiology, and neuroimaging) were reviewed retrospectively. Surgical outcomes were evaluated at 1, 2, 5 years after surgery, and at the end of the study. After 1 year, 56 patients (62.2%) became Engel class I and at the last follow-up, 47 patients (52.2%) remained seizure-free. Furthermore, non-localized 18F-fluorodeoxyglucose positron emission tomography (PET), identifying hypometabolic areas not concordant with ictal onset zones, significantly correlated with seizure recurrence after 1 year. Non-lesional magnetic resonance imaging (MRI) and left-sided resection correlated with poor outcomes. In the last follow-up, non-localized PET and left-sided resection significantly correlated with seizure recurrence. Both localized PET and ictal-interictal SPECT subtraction co-registered to MR (SISCOM) predicted good surgical outcomes in the last follow-up (69.2%, Engel I). This study suggests that PET and SISCOM may predict postoperative outcomes for patients after multilobar epilepsy and shows comparable long-term surgical outcomes after multilobar resection.

Functional neuroimaging findings in patients with lateral and mesio-lateral temporal lobe epilepsy; FDG-PET and ictal SPECT studies

The differentiation of combined mesial and lateral temporal onset of seizures (mesio-lateral TLE, MLTLE) from lateral TLE (LTLE) is critical to achieve good surgical outcomes. However, the functional neuroimaging features in LTLE patients based on the ictal onset zone utilizing intracranial EEG (iEEG) in a large series have not been investigated. We enrolled patients diagnosed with MLTLE (n = 35) and LTLE (n = 53) based on the site of ictal onset zone from iEEG monitoring. MLTLE is defined when ictal discharges originate from the mesial and lateral temporal cortices independently, whereas seizures of LTLE arise exclusively from the lateral temporal cortex. Compared to patients with LTLE, patients with MLTLE were more likely to have 18F- fluorodeoxyglucose positron emission tomography (FDG-PET) hypometabolism and hyperperfusion on ictal single-photon emission computed tomography (SPECT) restricted to the temporal areas. MLTLE patients had more frequent aura or secondarily generalized seizures than LTLE patients. No significant differences were found in scalp EEG, MRI, and Wada asymmetry between groups. The overall seizure-free rate was good (73.8%, mean follow-up = 9.7 years), which was not different (Engel class I, 74.3% in MLTLE vs. 73.6% in LTLE). Postsurgical memory function was spared in LTLE patients, while visual memory was impaired in MLTLE patients when their mesial temporal structures were sufficiently resected. It suggests that functional neuroimaging (interictal PET and ictal and interictal SPECT) may play a crucial role to differentiate between MLTLE and LTLE.

Grey and white matter abnormalities in temporal lobe epilepsy with and without mesial temporal sclerosis

Temporal lobe epilepsy with (TLE-mts) and without (TLE-no) mesial temporal sclerosis display different patterns of cortical neuronal loss, suggesting that the distribution of white matter damage may also differ between the sub-groups. The purpose of this study was to examine patterns of white matter damage in TLE-mts and TLE-no and to determine if identified changes are related to neuronal loss at the presumed seizure focus. The 4 T diffusion tensor imaging (DTI) and T1-weighted data were acquired for 22 TLE-mts, 21 TLE-no and 31 healthy controls. Tract-based spatial statistics (TBSS) was used to compare fractional anisotropy (FA) maps and voxel-based morphometry (VBM) was used to identify grey matter (GM) volume atrophy. Correlation analysis was conducted between the FA maps and neuronal loss at the presumed seizure focus. In TLE-mts, reduced FA was identified in the genu, body and splenium of the corpus callosum, bilateral corona radiata, cingulum, external capsule, ipsilateral internal capsule and uncinate fasciculus. In TLE-no, FA decreases were identified in the genu, the body of the corpus callosum and ipsilateral anterior corona radiata. The FA positively correlated with ipsilateral hippocampal volume. Widespread extra-focal GM atrophy was associated with both sub-groups. Despite widespread and extensive GM atrophy displaying different anatomical patterns in both sub-groups, TLE-mts demonstrated more extensive FA abnormalities than TLE-no. The microstructural organization in the corpus callosum was related to hippocampal volume in both patients and healthy subjects demonstrating the association of these distal regions.

Heritability of subcortical volumetric traits in mesial temporal lobe epilepsy

Objectives

We aimed to 1) determine if subcortical volume deficits are common to mesial temporal lobe epilepsy (MTLE) patients and their unaffected siblings 2) assess the suitability of subcortical volumetric traits as endophenotypes for MTLE.

Methods

MRI-based volume measurements of the hippocampus, amygdala, thalamus, caudate, putamen and pallidium were generated using an automated brain reconstruction method (FreeSurfer) for 101 unrelated ‘sporadic’ MTLE patients [70 with hippocampal sclerosis (MTLE+HS), 31 with MRI-negative TLE], 83 unaffected full siblings of patients and 86 healthy control subjects. Changes in the volume of subcortical structures in patients and their unaffected siblings were determined by comparison with healthy controls. Narrow sense heritability was estimated ipsilateral and contralateral to the side of seizure activity.

Results

MTLE+HS patients displayed significant volume deficits across the hippocampus, amygdala and thalamus ipsilaterally. In addition, volume loss was detected in the putamen bilaterally. These volume deficits were not present in the unaffected siblings of MTLE+HS patients. Ipsilaterally, the heritability estimates were dramatically reduced for the volume of the hippocampus, thalamus and putamen but remained in the expected range for the amygdala. MRI-negative TLE patients and their unaffected siblings showed no significant volume changes across the same structures and heritability estimates were comparable with calculations from a healthy population.

Conclusions

The findings indicate that volume deficits for many subcortical structures in ‘sporadic’ MTLE+HS are not heritable and likely related to acquired factors. Therefore, they do not represent suitable endophenotypes for MTLE+HS. The findings also support the view that, at a neuroanatomical level, MTLE+HS and MRI-negative TLE represent two distinct forms of MTLE.

Electroencephalography in mesial temporal lobe epilepsy: a review

Electroencephalography (EEG) has an important role in the diagnosis and classification of epilepsy. It can provide information for predicting the response to antiseizure drugs and to identify the surgically remediable epilepsies. In temporal lobe epilepsy (TLE) seizures could originate in the medial or lateral neocortical temporal region, and many of these patients are refractory to medical treatment. However, majority of patients have had excellent results after surgery and this often relies on the EEG and magnetic resonance imaging (MRI) data in presurgical evaluation. If the scalp EEG data is insufficient or discordant, invasive EEG recording with placement of intracranial electrodes could identify the seizure focus prior to surgery. This paper highlights the general information regarding the use of EEG in epilepsy, EEG patterns resembling epileptiform discharges, and the interictal, ictal and postictal findings in mesial temporal lobe epilepsy using scalp and intracranial recordings prior to surgery. The utility of the automated seizure detection and computerized mathematical models for increasing yield of non-invasive localization is discussed. This paper also describes the sensitivity, specificity, and predictive value of EEG for seizure recurrence after withdrawal of medications following seizure freedom with medical and surgical therapy.

Interictal regional delta slowing is an EEG marker of epileptic network in temporal lobe epilepsy

PURPOSE

Several studies have suggested that interictal regional delta slowing (IRDS) carries a lateralizing and localizing value similar to interictal spikes and is associated with favorable surgical outcomes in patients with temporal lobe epilepsy (TLE). However, whether IRDS reflects structural dysfunction or underlying epileptic activity remains controversial. The objective of this study is to determine the cortical electroencephalography (EEG) correlates of scalp-recorded IRDS, in so doing, to further understand its clinical and biologic significances.

METHODS

We examined the cortical EEG substrates of IRDS with electrocorticography (ECoG-IRDS) and delineated the spatiotemporal relationship between ECoG-IRDS and both interictal and ictal discharges by recording simultaneously scalp and intracranial EEG in 18 presurgical candidates with TLE.

KEY FINDINGS

Our results demonstrated that ECoG-IRDS is typically a mixture of delta/theta slowing and spike-wave potentials. ECoG-IRDS was predominantly recorded from basal and anterolateral temporal cortex, occasionally in mesial, posterior temporal, and extratemporal regions. Abundant IRDS was most commonly observed in patients with neocortical temporal lobe epilepsy (NTLE), whereas infrequent to moderate IRDS was usually observed in patients with mesial temporal lobe epilepsy (MTLE). The anatomic distribution of ECoG-IRDS was highly correlated with the irritative and seizure-onset zones in 10 patients with NTLE. However, it was poorly correlated with the irritative and seizure-onset zones in the 8 patients with MTLE.

SIGNIFICANCE

These findings demonstrate that IRDS is an EEG marker of epileptic network in patients with TLE. Although IRDS and interictal/ictal discharges likely arise from the same neocortical generator in patients with NTLE, IRDS in patients with MTLE may reflect a network disease that involves temporal neocortex.

Widespread extrahippocampal NAA/(Cr+Cho) abnormalities in TLE with and without mesial temporal sclerosis

MR spectroscopy has demonstrated extrahippocampal NAA/(Cr+Cho) reductions in medial temporal lobe epilepsy with (TLE-MTS) and without (TLE-no) mesial temporal sclerosis. Because of the limited brain coverage of those previous studies, it was, however, not possible to assess differences in the distribution and extent of these abnormalities between TLE-MTS and TLE-no. This study used a 3D whole brain echoplanar spectroscopic imaging (EPSI) sequence to address the following questions: (1) Do TLE-MTS and TLE-no differ regarding severity and distribution of extrahippocampal NAA/(Cr+Cho) reductions? (2) Do extrahippocampal NAA/(Cr+Cho) reductions provide additional information for focus lateralization? Forty-three subjects (12 TLE-MTS, 13 TLE-no, 18 controls) were studied with 3D EPSI. Statistical parametric mapping (SPM2) was used to identify regions of significantly decreased NAA/(Cr+Cho) in TLE groups and in individual patients. TLE-MTS and TLE-no had widespread extrahippocampal NAA/(Cr+Cho) reductions. NAA/(Cr+Cho) reductions had a bilateral fronto-temporal distribution in TLE-MTS and a more diffuse, less well defined distribution in TLE-no. Extrahippocampal NAA/(Cr+Cho) decreases in the single subject analysis showed a large inter-individual variability and did not provide additional focus lateralizing information. Extrahippocampal NAA/(Cr+Cho) reductions in TLE-MTS and TLE-no are neither focal nor homogeneous. This reduces their value for focus lateralization and suggests a heterogeneous etiology of extrahippocampal spectroscopic metabolic abnormalities in TLE.

Electroencephalographic features of temporal lobe epilepsy

Electroencephalography (EEG) is an important tool for diagnosing, lateralizing and localizing temporal lobe seizures. In this paper, we review the EEG characteristics of temporal lobe epilepsy (TLE). Several "non-standard" electrodes may be needed to further evaluate the EEG localization, Ictal EEG recording is a major component of preoperative protocols for surgical consideration. Various ictal rhythms have been described including background attenuation, start-stop-start phenomenon, irregular 2-5 Hz lateralized activity, and 5-10 Hz sinusoidal waves or repetitive epileptiform discharges. The postictal EEG can also provide valuable lateralizing information. Postictal delta can be lateralized in 60% of patients with TLE and is concordant with the side of seizure onset in most patients. When patients are being considered for resective surgery, invasive EEG recordings may be needed. Accurate localization of the seizure onset in these patients is required for successful surgical management.

Network-level analysis of cortical thickness of the epileptic brain

Temporal lobe epilepsy (TLE) characterized by an epileptogenic focus in the medial temporal lobe is the most common form of focal epilepsy. However, the seizures are not confined to the temporal lobe but can spread to other, anatomically connected brain regions where they can cause similar structural abnormalities as observed in the focus. The aim of this study was to derive whole-brain networks from volumetric data and obtain network-centric measures, which can capture cortical thinning characteristic of TLE and can be used for classifying a given MRI into TLE or normal, and to obtain additional summary statistics that relate to the extent and spread of the disease. T1-weighted whole-brain images were acquired on a 4-T magnet in 13 patients with TLE with mesial temporal lobe sclerosis (TLE-MTS), 14 patients with TLE with normal MRI (TLE-no), and 30 controls. Mean cortical thickness and curvature measurements were obtained using the FreeSurfer software. These values were used to derive a graph, or network, for each subject. The nodes of the graph are brain regions, and edges represent disease progression paths. We show how to obtain summary statistics like mean, median, and variance defined for these networks and to perform exploratory analyses like correlation and classification. Our results indicate that the proposed network approach can improve accuracy of classifying subjects into two groups (control and TLE) from 78% for non-network classifiers to 93% using the proposed approach. We also obtain network "peakiness" values using statistical measures like entropy and complexity-this appears to be a good characterizer of the disease and may have utility in surgical planning.

Outcome after cortico-amygdalo-hippocampectomy in patients with temporal lobe epilepsy and normal MRI

RATIONALE

We describe seizure and neuropsychological outcome obtained after CAH in patients with TLE and normal MRI evaluated in the modern imaging era.

METHODS

Forty-five adult consecutive patients with TLE and normal MRI were studied. All patients had neuropsychological testing, interictal and ictal EEG recordings and MRI. They were divided into two groups: Group 1 (n=18), included patients in whom non-invasive neurophysiological evaluation was lateralizing and Group 2 (n=27) included patients with non-lateralizing neurophysiological data who were submitted to invasive recordings.

RESULTS

Seventy-seven percent of the Group 1 patients were rated as Engel I; 11% were rated as Engel II and 11% as Engel III. In Group 2, there were 57% of patients seizure-free, 26% in Engel II and 14% in Engel III. Pre-operatively, mean general IQ was 82 and 78 in Groups 1 and 2, respectively; post-operatively, mean general IQ was respectively 86 and 71. Some degree of verbal memory decline was noted in all patients submitted to dominant temporal lobe resection in both Groups 1 and 2. At last follow-up visit, 22% of Group 1 and 11% of Group 2 patients were receiving no antiepileptic drugs (AED).

CONCLUSIONS

Our data showed that patients with TLE and normal MRI could get good surgical results after CAH although 60% of them would need invasive recordings and their results regarding seizure control and cognition were worse than those obtained in patients with MRI defined temporal lobe lesions. Caution should be taken in offering dominant temporal lobe resection to this subset of patients.

Involvement of the thalamocortical network in TLE with and without mesiotemporal sclerosis

PURPOSE

The thalamus plays an important role in seizure propagation in temporal lobe epilepsy (TLE). This study investigated how structural abnormalities in the focus, ipsilateral thalamus and extrafocal cortical structures relate to each other in TLE with mesiotemporal sclerosis (TLE-MTS) and without hippocampal sclerosis (TLE-no).

METHODS

T₁ and high-resolution T₂ images were acquired on a 4T magnet in 29 controls, 15 TLE-MTS cases, and 14 TLE-no. Thalamus volumes were obtained by warping a labeled atlas onto each subject's brain. Deformation-based morphometry was used to identify regions of thalamic volume loss and FreeSurfer for cortical thickness measurements. CA1 volumes were obtained from high-resolution T₂ images. Multiple regression analysis and correlation analyses for voxel- and vertex-based analyses were performed in SPM2 and FreeSurfer.

RESULTS

TLE-MTS had bilateral volume loss in the anterior thalamus, which was correlated with CA1 volume and cortical thinning in the mesiotemporal lobe. TLE-no had less severe volume loss in the dorsal lateral nucleus, which was correlated with thinning in the mesiotemporal region but not with extratemporal thinning.

DISCUSSION

The findings suggest that seizure propagation from the presumed epileptogenic focus or regions close to it into the thalamus occurs in TLE-MTS and TLE-no and results in circumscribed neuronal loss in the thalamus. However, seizure spread beyond the thalamus seems not to be responsible for the extensive extratemporal cortical abnormalities in TLE.

Widespread neocortical abnormalities in temporal lobe epilepsy with and without mesial sclerosis

PURPOSE

Extrafocal structural abnormalities have been consistently described in temporal lobe epilepsy (TLE) with mesial temporal lobe sclerosis (TLE-MTS). In TLE without MTS (TLE-no) extrafocal abnormalities are more subtle and often require region of interest analyses for their detection. Cortical thickness measurements might be better suited to detect such subtle abnormalities than conventional whole brain volumetric techniques which are often negative in TLE-no. The aim of this study was to seek and characterize patterns of cortical thinning in TLE-MTS and TLE-no.

METHODS

T1 weighted whole brain images were acquired on a 4 T magnet in 66 subjects (35 controls, 15 TLE-MTS, 16 TLE-no). Cortical thickness measurements were obtained using the FreeSurfer software routine. Group comparisons and correlation analyses were done using the statistical routine of FreeSurfer (FDR, p=0.05).

RESULTS

TLE-MTS and TLE-no showed both widespread temporal and extratemporal cortical thinning. In TLE-MTS, the inferior medial and posterior temporal regions were most prominently affected while lateral temporal and opercular regions were more affected in TLE-no. The correlation analysis showed a significant correlation between the ipsilateral hippocampal volume and regions of thinning in TLE-MTS and between inferior temporal cortical thickness and thinning in extratemporal cortical regions in TLE-no.

CONCLUSION

The pattern of thinning in TLE-no was different from the pattern in TLE-MTS. This finding suggests that different epileptogenic networks could be involved in TLE-MTS and TLE and further supports the hypothesis that TLE-MTS and TLE-no might represent two distinct TLE syndromes.

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.

Metabolic changes in temporal lobe structures measured by HR-MAS NMR at early stage of electrogenic rat epilepsy

The purpose of this study was to determine cerebral metabolic profile changes in response to electric stimulation to the right dorsal hippocampus (HPC) for the establishment of an epileptic rat model. Electroencephalogram measurements and behavioral results indicated that the experimental rats were in an early stage of epilepsy. Metabolites were determined by high-resolution magic-angle-spinning nuclear magnetic resonance (HR-MAS NMR) spectroscopy of the following intact brain tissue: bilateral hippocampi, entorhinal cortices (ECs), and temporal lobes (TLs). The NMR data was statistically analyzed using principal component analysis (PCA). Results demonstrated that metabolic profiles were significantly different between the experimental and sham rats in the bilateral hippocampi and the ipsilateral EC. Significant increases in total creatine in the ipsilateral HPC and alanine in the ipsilateral TL were measured (p<0.05). Some metabolite levels were disturbed in the bilateral HPC-EC loops. In the sham group, glutamate and choline concentrations were significantly higher or lower in the ipsilateral EC than bilateral hippocampi, respectively (p<0.01). However, such differences were not observed in the experimental group. In addition, N-acetylaspartate levels in the experimental group were significantly less in the ipsilateral HPC than in bilateral ECs (p<0.05). The level of myo-inositol in the ipsilateral EC significantly increased in the experimental group, compared to the contralateral EC (p<0.05). These results may provide metabolic information about temporal lobe structures to provide more knowledge about epileptic abnormalities at the early stage.

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.

Voxel-based T2 relaxation rate measurements in temporal lobe epilepsy (TLE) with and without mesial temporal sclerosis

INTRODUCTION

Quantitative measurements of T(2) relaxation in the hippocampus for focus lateralization in mesial temporal lobe epilepsy (mTLE) are well established. Less is known to what degree such relaxation abnormalities also affect regions beyond the ipsilateral hippocampus. Therefore, the aim of this study was to characterize extent and distribution pattern of extrahippocampal relaxation abnormalities in TLE with (TLE-MTS) and without MRI evidence of mesial-temporal sclerosis (TLE-no).

METHODS

Double spin echo images (TE1/2: 20/80 ms) acquired in 24 TLE-MTS and 18 TLE-no were used to calculate relaxation rate maps. These maps were analyzed by SPM2 and by selecting regions of interest (ROI) in the hippocampus and several extrahippocampal brain regions.

RESULTS

In TLE-MTS, the results of the SPM and ROI analysis were in good agreement and showed the most severe relaxation rate decreases in the ipsilateral hippocampus but also in other ipsilateral temporal regions, orbitofrontal, and parietal regions and to a lesser degree in contralateral frontal regions. The relaxation rate decreases in TLE-no were confined to small regions in the ipsilateral anterior inferior and medial temporal lobe in the SPM analysis while ROI analysis showed additional regions in the ipsilateral hippocampus, amygdala, and anterior cingulate.

CONCLUSION

TLE-MTS showed extensive, widespread but predominantly ipsilateral temporal and also extratemporal T(2) relaxation rate decreases. In contrast, the findings of the SPM and ROI analyses in TLE-no suggested that if relaxation rate decreases are present, they are less uniform and generally milder than in TLE-MTS. This further supports the hypothesis that TLE-no is a distinct clinicopathological entity from TLE-MTS and probably heterogeneous in itself.

Corpora amylacea in mesial temporal lobe epilepsy: Clinico-pathological correlations

PURPOSE

To investigate the electro-clinical significance of premature accumulation of corpora amylacea (CoA) in the resected hippocampus of patients with medically refractory mesial temporal lobe epilepsy with hippocampal sclerosis (MTLE-HS).

METHODS

We compared the clinical and EEG characteristics, and post-operative seizure outcome of 373 (mean age 29.4 years, range 7-55 years) surgically treated MTLE-HS patients with (MTLE-HS-CoA(+), n=129 [34.5%]) and without (MTLE-HS-CoA(-), n=244 [65.5%]) CoA.

RESULTS

Age at surgery was significantly higher and duration of epilepsy before surgery was significantly longer for MTLE-HS-CoA(+) patients compared to MTLE-HS-CoA(-) patients. Although the distribution of interictal epileptiform EEG abnormalities did not differ, type 1 ictal EEG pattern was more frequent in MTLE-HS-CoA(+) patients. Among the 21 patients with major interictal psychosis detected prior to epilepsy surgery, 19 (90.5%) belonged to MTLE-HS-CoA(+) group. Schizophrenia-like psychosis was most prevalent. The post-operative seizure-free outcome was comparable, but significantly more MTLE-HS-CoA(-) patients were free of antiepileptic drugs.

CONCLUSIONS

Overall, our observations support the hypothesis that the pathological process in MTLE-HS is progressive. MTLE-HS-CoA(+) patients are predisposed to increased psychiatric morbidity. In vivo detection of hippocampal CoA accumulation in the future will help us to understand the neurobiological significance of this phenomenon.

Correlation between temporal pole MRI abnormalities and surface ictal EEG patterns in patients with unilateral mesial temporal lobe epilepsy

OBJECTIVE

The objective of this retrospective study is to analyze ictal patterns observed during continuous Video-EEG monitoring in patients with temporal lobe epilepsy (TLE) due to unilateral hippocampal sclerosis (HS), and to correlate these EEG patterns to temporal pole abnormalities observed on magnetic resonance imaging exams.

METHODS

We analyzed 147 seizures from 35 patients with TLE and unilateral HS. Ictal patterns were classified and correlated to signal abnormalities and volumetric measures of the temporal poles. Volume differences over 10% were considered abnormal.

RESULTS

The most frequent type of ictal pattern was rhythmic theta activity (RTA), encountered in 65.5% of the seizures. Rhythmic beta activity (RBA) was observed in 11% of the seizures, localized attenuation in 8%, interruption of epileptiform discharges in 6%, repetitive discharges in 5.5%, and rhythmic delta activity (RDA) in 4%. Sixty-six percent of the patients presented signal abnormalities in the temporal pole that were always ipsilateral to the HS. Sixty percent presented significant asymmetry of the temporal poles consisting of reduced volume that was also always ipsilateral to HS. Although patients with RTA as the predominant ictal pattern tended to present asymmetry of temporal poles (p=0.305), the ictal EEG pattern did not correlate with temporal pole asymmetry or signal abnormalities.

CONCLUSIONS

RTA is the most frequent initial ictal pattern in patients with TLE due to unilateral HS. Temporal pole signal changes and volumetric reduction were commonly found in this group of patients, both abnormalities appearing always ipsilateral to the HS. However, neither temporal pole volume reduction nor signal abnormalities correlated with the predominant ictal pattern, suggesting that the temporal poles are not crucially involved in the process of epileptogenesis.

Epileptogenicity of cortical dysplasia in temporal lobe dual pathology: an electrophysiological study with invasive recordings

Hippocampal sclerosis is often associated with macroscopic or microscopic dysplasia in the temporal neocortex (TN). The relevance of such a dual pathology with regard to epileptogenesis is unclear. This study investigates the role of both pathologies in the generation of ictal and interictal activity. Ictal (113 seizures) and interictal data from invasive EEG recordings with simultaneous depth electrodes in the hippocampus and subdural electrodes over the TN were analysed retrospectively in 12 patients with variable degrees of hippocampal sclerosis and different types of histologically confirmed temporal cortical dysplasia [all male, age at epilepsy onset <1-29 years (mean 9.6 years), age when invasive recordings were performed 6-50 years (mean 28.2 years)]. Of the seizures 41.3% arose from the amygdala/hippocampus complex (AHC), 34.7% from the TN, 22% were simultaneously recorded from AHC and TN (indeterminate seizure onset), and 2% from other regions. In three patients, seizure onset was recorded only from the AHC. In patients with severe hippocampal sclerosis only 12% of the seizures arose from the TN, whereas in patients with mild hippocampal sclerosis 58% arose from the TN. The type of cortical dysplasia, however, did not predict seizure onset in the AHC or TN. Propagation time from the TN to the AHC tended to be shorter (mean 7.4 s) than vice versa (mean 13.7 s). The most common initial ictal patterns in the AHC were rhythmic beta activity (<25 Hz) and repetitive sharp waves, and in the TN were fast activity (>25 Hz) and repetitive sharp waves. The interictal patterns over the TN were similar to those seen over extratemporal focal cortical dysplasias. Simultaneous recordings from the hippocampus and the TN strongly suggest that dysplastic tissue in the TN is often epileptogenic. The quantitative contribution of the hippocampus to seizure generation corresponded with the degree of hippocampal pathology, whereas different subtypes of cortical dysplasia did not affect its relative contribution to seizure generation and even mild forms of dysplasia were epileptogenic.