Source localization of mesial temporal interictal epileptiform discharges: Correlation with intracranial foramen ovale electrode recordings

OBJECTIVE: We have investigated the localization accuracy of low-resolution electromagnetic tomography (LORETA) for mesial temporal interictal epileptiform discharges (IED) on a statistical basis by using clinical electroencephalographic (EEG) data of simultaneous scalp and intracranial foramen ovale (FO) electrode recordings. METHODS: We retrospectively analyzed the IED of 15 patients who underwent presurgical assessment for intractable temporal lobe epilepsy. All patients have subsequently undergone amygdalohippocampectomy. The scalp signals were averaged time-locked to the peak activity in bilateral 10-contact FO electrode recordings. Source modeling was carried out by using statistical non-parametric mapping (SNPM) of LORETA values and by calculating raw LORETA values of averaged IED. The results were compared to intracranial data obtained from FO electrode recordings. RESULTS: Two thousand six hundred and fifteen discharges could be attributed to 19 different patterns of intracranial mesial temporal IED. SNPM of LORETA revealed confined ipsilateral mesial temporal solutions for 14 (73.7%) and no significant solutions for five (26.3%) of these patterns. Raw LORETA current density distributions of the 19 averaged IED patterns revealed ipsilateral basal to lateral temporal solutions for the 14 IED patterns with a sufficient signal to noise ratio (SNR), but spurious results for those five IED with a low SNR. CONCLUSIONS: SNPM of LORETA but not LORETA analysis of averaged IED patterns accurately localizes the source generators of mesial temporal IEDs. SIGNIFICANCE: SNPM of raw LORETA values might be appropriate for localizing restricted mesial temporal lobe sources.     

Propagation of interictal discharges in temporal lobe epilepsy: Correlation of spatiotemporal mapping with intracranial foramen ovale electrode recordings

OBJECTIVE: We have investigated intracerebral propagation of interictal epileptiform discharges (IED) in patients with mesial temporal lobe epilepsy (MTLE) by using spatiotemporal source maps based on statistical nonparametric mapping (SNPM) of low resolution electromagnetic tomography (LORETA) values. METHODS: We analyzed 30 patterns of IED recorded simultaneously with scalp and intracranial foramen ovale (FO) electrodes in 15 consecutive patients with intractable MTLE. The scalp EEG signals were averaged time-locked to the peak activity in bilateral 10-contact FO electrode recordings. SNPM was applied to LORETA values and spatiotemporal source maps were created by allocating the t-values over time to their corresponding Brodmann areas. Propagation was defined as secondary statistically significant involvement of distinct cortical areas separated by >15 ms. The results were correlated with intracranial data obtained from FO electrode recordings and with scalp EEG recordings. All patients underwent subsequent amygdalo-hippocampectomy and outcome was assessed one year after surgery. RESULTS: We found mesial to lateral propagation in 6/30 IED patterns (20%, four patients), lateral to mesial propagation in 4/30 IED patterns (13.3%, four patients) and simultaneous (within 15 ms) activation of mesial and lateral temporal areas in 6/30 IED patterns (20%, five patients). Propagation generally occurred within 30 ms and was always limited to ipsilateral cortical regions. Nine/30 IED patterns (30%) showed restricted activation of mesial temporal structures and no significant solutions were found in 5/30 IED patterns (16.7%). There was no clear association between the number or characteristics of IED patterns and the postsurgical outcome. CONCLUSIONS: Spatiotemporal mapping of SNPM LORETA accurately describes mesial to lateral temporal propagation of IED, and vice versa, which commonly occur in patients with MTLE. SIGNIFICANCE: Intracerebral propagation must be considered when using non-invasive source algorithms in patients with MTLE. Spatiotemporal mapping might be useful for visualizing this propagation.     

Source localization of interictal epileptiform discharges: comparison of three different techniques to improve signal to noise ratio.

OBJECTIVE: To investigate the localization accuracy of low-resolution electromagnetic tomography (LORETA) for mesial temporal interictal epileptiform discharges (IED) using a new relative averaging (RELAVG) technique for noise reduction. METHODS: We analyzed 19 patterns of mesial temporal IED recorded simultaneously with scalp and foramen ovale (FO) electrodes in 15 consecutive patients who underwent presurgical assessment for intractable temporal lobe epilepsy. The scalp signals were time-locked to the peak activity in the FO electrode recordings and source modeling was performed using the RELAVG technique. Random noise of various amounts was then applied. The results were compared to intracranial data obtained from the FO electrode recordings and to LORETA source solutions obtained using two other approaches to improve signal to noise ratio (SNR): statistical non-parametric mapping (SNPM) and the commonly applied averaging (AVG) technique. RESULTS: The RELAVG technique allowed for reasonable mesial temporal localization in 52.6% (10/19) of IED patterns, compared with 73.7% (14/19) using SNPM. The AVG technique provided no strictly mesial temporal solutions. Nine of the IED patterns revealed relative current density quotient changes >10; all of these were accurately localized by RELAVG into mesial temporal structures. Increasing amounts of white and physiological noise had no influence on the accuracy of RELAVG and SNPM solutions, whereas AVG source reconstructions became progressively spurious. CONCLUSION: The RELAVG technique and SNPM, but not the commonly used AVG technique, allow for reasonable source localization of mesial temporal IED. SNPM is the most accurate but also the most time-consuming noise reduction technique. SIGNIFICANCE: The RELAVG LORETA technique might provide a simple and fast semi-quantitative alternative for localizing IED with low single to noise ratio.     

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.     

Parietal lobe source localization and sensitivity to hyperventilation in a patient with subclinical rhythmic electrographic discharges of adults (SREDA).

OBJECTIVE: Subclinical rhythmic electrographic discharges of adults (SREDA) is currently considered a benign EEG pattern of uncertain significance. The underlying cortical sources and generating mechanisms are unknown. We performed a source localization analysis of SREDA with the aim of better understanding this unusual EEG pattern. METHODS: Multiple spontaneous episodes of typical SREDA were recorded in a patient during continuous EEG monitoring. Additional SREDA episodes were induced by hyperventilation. Source localization was carried out using statistical non-parametric mapping (SNPM) of low resolution electromagnetic tomography (LORETA). RESULTS: SNPM of both time- and frequency-domain LORETA revealed a widespread biparietal cortical origin of SREDA, the anatomical distribution of which included the parietal operculum and the known vascular watershed areas between anterior, middle and posterior cerebral arteries. Vigorous deep hyperventilation induced SREDA on three of four attempts. Mean duration of the hyperventilation-induced SREDA was approximately three times longer than spontaneous events. CONCLUSIONS: Investigations in this patient with typical SREDA revealed hyperventilation sensitivity and a posterior hemispheric source localization maximal in the parietal cortex bilaterally, in large part overlying the anatomical distribution of the vascular watershed areas. SIGNIFICANCE: The source localization results and sensitivity to hyperventilation suggest some sort of association between cerebral vascular supply and SREDA, as originally proposed by Naquet et al. [Naquet R, Louard C, Rhodes J, Vigouroux M. A propos de certaines décharges paroxystiques du carrefour temporo-pariéto-occipital. Leur activation par l'hypoxie. Rev Neurol 1961;105:203-207.].     

Cortical current density reconstruction of interictal epileptiform activity in temporal lobe epilepsy.

OBJECTIVE: To investigate the value of cortical current density (CCD) reconstruction in localizing intracranial generators of interictal epileptiform activity in mesial and lateral temporal lobe epilepsy (TLE). METHODS: Non-linear minimum L(1)-norm CCD reconstruction (with current sources restricted to the individual cortical surface and a realistic boundary element method (BEM) head model) was used to localize and to study the propagation of interictal epileptiform EEG activity in 13 pre-surgical patients with TLE. RESULTS: In all but one patient with mesial temporal lesions, an initial activation maximum corresponding to the ascending part of averaged sharp waves was found in the ipsilateral anterior basolateral temporal lobe, mostly extending up to the affected mesial structures whose resection rendered the patients seizure-free. In all 3 patients with lateral temporal lesions, the activation was initially confined to temporal neocortex immediately adjacent to the epileptogenic lesion. Towards the peak of sharp waves, two patients showed a propagation of interictal activity to anterior and posterior and partly contralateral temporal regions. A conventional EEG analysis based on amplitude maxima or phase reversal would have missed the initial onset zone. CONCLUSIONS: The findings demonstrate that CCD reconstruction can be a valuable additional non-invasive component in the multimodal pre-surgical evaluation of epilepsy patients.     

Temporal Epileptogenesis: Localizing Value of Scalp and Subdural Interictal and Ictal EEG Data

PURPOSE: To determine the value of scalp epileptiform EEG data and subdural interictal spikes in localizing temporal epileptogenesis among patients requiring invasive recordings. For this delineation, we related such factors to site of subdural seizure origin in 27 consecutive patients. METHODS: Patients with temporal lobe epilepsy whose non-invasive lateralizing data were inconclusive and therefore required subdural electroencephalography were studied. All patients had (a) 24-h scalp telemetered EEGs, (b) adequate bitemporal subdural placements with an inferomesial line extending from a posterior burr hole anteriorly to <2.5 cm from anterior uncus and a lateral line reaching within 2.5 cm of the temporal tip, and (c) > or =2 subdurally recorded seizures. RESULTS: Three hundred one (96%) of 314 subdurally recorded clinical seizures involving all 27 patients arose from a discrete focus; 266 (85%) arose from mesial temporal regions, which was the origin of the majority of seizures in 24 (89%) patients. The majority of subdural seizures arose ipsilateral to the majority of scalp EEG spikes in 22 (81%) of 27, and most subdural seizures of 15 (75%) of 20 arose ipsilateral to scalp seizures. Lateralization of interictal subdural spikes correlated with that of subdural seizures in 74-92% of patients, depending on the method of spike compilation: for example, most subdural seizures arose from the same lobe of most consistent principal temporal spikes in 92% of patients. These indices of epileptogenesis also appeared more commonly on the side of effective (> or =90% improvement) temporal lobectomy than contralaterally in the following proportions: most consistent principal subdural spikes, 86% of patients ipsilateral vs. 9% contralateral; scalp-recorded clinical seizures, 55% vs. 18%; scalp EEG spikes, 45% vs. 9%. CONCLUSIONS: Even among patients whose scalp data are sufficiently complex to require invasive recording for clarification, lateralization of temporal scalp interictal and ictal epileptiform activity and subdural interictal spikes should be included when assessing the side of temporal epileptogenesis.     

Intracranial volume conduction of cortical spikes and sleep potentials recorded with deep brain stimulating electrodes.

OBJECTIVE: To examine interictal epileptiform and sleep potentials recorded intracranially from deep brain stimulation (DBS) electrodes in patients treated with DBS for epilepsy. Specifically, this study sought to determine whether the DBS-recorded potentials represent: (a) volume conduction from surface neocortical discharges or (b) transsynaptic propagation along cortical-subcortical pathways with local generation of the subcortical potentials near the DBS targets. METHODS: Six patients with intractable epilepsy treated with thalamic DBS of the central median nucleus (CM; one patient) or anterior thalamus (5 patients) who had focal interictal spikes were studied. Sleep potentials were also studied in a 7th patient with Parkinson disease treated with DBS of the subthalamic nucleus (STN). RESULTS: Focal interictal cortical spikes recorded by scalp electroencephalography (EEG) were recorded synchronously, but with opposite polarity, from the DBS electrodes in CM as well as the more superficial anterior thalamic contacts situated in the anterior nucleus (AN) and dorsal medial nucleus (DM). In referential montages, the subcortical potentials were of highest amplitude ipsilateral to the focal cortical spikes, with a small but reproducible amplitude decrement present at each electrode contact more distant from the cortical source, irrespective of the specific DBS target. Subcortical sleep potentials (K-complexes and sleep spindles) were also recorded synchronously and with inverse polarity compared to the corresponding scalp potentials, and appeared in a similar fashion at all subcortical sites sampled by the DBS electrodes. Amplitude attenuation in the thalamus of intracranial volume conducted potentials with increasing distance from their cortical spike sources was measured at approximately 5-10 microV/mm. DISCUSSION: Recent reports on scalp-CM or scalp-STN EEG recordings in patients treated with DBS for epilepsy have interpreted the intracranial waveforms as evidence of transsynaptic cortical-subcortical transmission across neuroanatomical pathways presumed to be involved in the generation of sleep potentials (Clin. Neurophysiol. 113 (2002) 25) and epileptiform activity (Clin. Neurophysiol. 113 (2002) 1391). However, our results show that the intracranial spikes recorded from DBS electrodes in various regions of the thalamus (CM, AN and DM) represent subcortical volume conduction of the synchronous cortical spikes recorded with scalp EEG. The same is true for the intracranial reflections of scalp EEG sleep potentials recorded from DBS electrodes in CM, AN, DM and STN. These interictal DBS waveforms thus cannot be used to support hypotheses of specific cortical-subcortical pathways of neural propagation or subcortical generation of the DBS-recorded potentials associated with scalp EEG interictal spikes and sleep potentials. SIGNIFICANCE: Detailed analysis of the intracranial potentials recorded from DBS electrodes in association with scalp EEG spikes and sleep discharges shows that the intracranial waveforms represent volume conduction from discharges generated in the neocortex and not, as has been suggested, locally generated activity resulting from cortical-subcortical neural propagation.     

Interictal, unifocal spikes in refractory extratemporal epilepsy predict ictal origin and postsurgical outcome.

OBJECTIVES: To evaluate the significance of exclusively unifocal, unilateral, interictal epileptiform patterns on scalp electroencephalography (EEG) in surgical candidates with medically intractable extratemporal epilepsy. METHODS: We reviewed 126 patients with refractory extratemporal partial seizures who underwent epilepsy surgery at our center. All were followed for at least 2 years after resections. Surgery was based on ictal EEG recordings. We examined ictal onsets and surgical outcome in subjects whose preoperative, interictal scalp EEGs during long-term monitoring (LTM) demonstrated only unilateral, well-defined focal discharges, and outcome in patients whose interictal EEGs during LTM showed bilateral, non-localized, or multifocal epileptiform patterns. RESULTS: We found that 26 subjects exhibited only unilateral, unifocal, interictal epileptiform patterns. In all 26 cases (100%) clinical seizures arose from the regions expected by the interictal findings (P<0.0001, Sign test). At last follow-up 77% (20/26) of these patients were seizure-free, while 23% (6/26) had >75% reduction in seizures. This compares to the remaining patients, of whom 34% (34/100) were seizure-free, 41% (41/100) had >75% reduction in seizures, and 25% (25/100) had <75% reduction in seizures (P=0.0001, Fisher's Exact test). CONCLUSIONS: Strictly unifocal, interictal epileptiform patterns on scalp EEG, though seen in a minority of subjects, may be an important, independent factor in evaluating subjects with intractable extratemporal, localization-related epilepsy for surgical therapy. This finding is highly predictive of both ictal onsets and successful postsurgical outcome.     

Intracranial EEG substrates of scalp EEG interictal spikes

PURPOSE: To determine the area of cortical generators of scalp EEG interictal spikes, such as those in the temporal lobe epilepsy. METHODS: We recorded simultaneously 26 channels of scalp EEG with subtemporal supplementary electrodes and 46 to 98 channels of intracranial EEG in 16 surgery candidates with temporal lobe epilepsy. Cerebral discharges with and without scalp EEG correlates were identified, and the area of cortical sources was estimated from the number of electrode contacts demonstrating concurrent depolarization. RESULTS: We reviewed approximately 600 interictal spikes recorded with intracranial EEG. Only a very few of these cortical spikes were associated with scalp recognizable potentials; 90% of cortical spikes with a source area of >10 cm(2) produced scalp EEG spikes, whereas only 10% of cortical spikes having <10 cm(2) of source area produced scalp potentials. Intracranial spikes with <6 cm(2) of area were never associated with scalp EEG spikes. CONCLUSIONS: Cerebral sources of scalp EEG spikes are larger than commonly thought. Synchronous or at least temporally overlapping activation of 10-20 cm(2) of gyral cortex is common. The attenuating property of the skull may actually serve a useful role in filtering out all but the most significant interictal discharges that can recruit substantial surrounding cortex.     

Usefulness of Unilateral Interictal Sharp Waves of Temporal Lobe Origin in Prolonged Video-EEG Monitoring Studies

The value of EEG interictal epileptiform activity in predicting location of the seizure focus remains controversial. In 64 patients, scalp video-EEG monitoring studies showed one or two ipsilateral interictal foci in the temporal lobe. The site of these interictal foci correlated with location of the seizure focus recorded during prolonged video-electrocorticography (ECoG) with use of subdural grids placed under the mesiobasal temporal region and over the lateral temporal convexity. Our findings suggest that unilateral anterotemporal interictal foci can accurately predict location of seizure onset. This is also true in patients with two ipsilateral temporal interictal foci, provided that the dominant focus is localized in anterotemporal regions. We believe that in such patients invasive recordings are not warranted, but we caution against sole use of interictal epileptiform criteria for localization of the seizure focus. Correlation with clinical information, ictal EEG, neuropsychometric, and neu-roimaging studies is required before performance of epilepsy surgery.     

Is ictal recording mandatory in temporal lobe epilepsy? Not when the interictal electroencephalogram and hippocampal atrophy coincide

OBJECTIVE: To investigate the concordance between scalp electroencephalogram (EEG) lateralization and side of hippocampal atrophy in patients with temporal lobe epilepsy (TLE). METHODS: We studied 184 consecutive patients with TLE without lesions other than those compatible with mesial temporal sclerosis. In this study, we studied specifically hippocampal atrophy and the results of scalp EEG investigation. Patients were classified according to the localization of interictal epileptiform discharges as unilateral, bilateral asymmetric, and bilateral symmetric. The EEG seizure onsets were also classified separately as unilateral, bilateral asymmetric, and bilateral symmetric. The hippocampal atrophy was determined by volumetric measurements using high-resolution magnetic resonance imaging (MRIVol). RESULTS: Only 3% of patients had discordance between the ictal and interictal EEG lateralizations; however, none of these had unilateral interictal EEG abnormalities. Interictal EEGs were considered unilateral in 62.0% of patients, bilateral asymmetric in 31.5%, and bilateral symmetric in 6.5%. Ictal EEGs were considered unilateral in 63.5% of patients, bilateral asymmetric in 30.0%, and bilateral symmetric in 6.5%. The MRIVol showed unilateral hippocampal atrophy in 60.9% of patients, bilateral asymmetric hippocampal atrophy in 19.0%, symmetric hippocampal atrophy in 3.8%, and normal volumes in 16.3%. There was a significant concordance between MRIVol lateralization and both interictal and ictal EEG lateralization (P<.001). All patients with unilateral hippocampal atrophy had concordant interictal and ictal EEG lateralization. Six (18.2%) of the 33 patients with bilateral asymmetric hippocampal atrophy had MRI lateralization discordant with EEG lateralization. CONCLUSIONS: We found a strong concordance between EEG and MRIVol lateralization in patients with TLE. Unilateral hippocampal atrophy predicted ipsilateral interictal epileptiform abnormalities and ipsilateral seizure onsets with no false lateralization. Previous studies in addition to the present series support that a concordant outpatient EEG evaluation in patients with TLE and unilateral hippocampal atrophy would obviate the need for inpatient EEG monitoring.     

Intracerebral propagation of interictal activity in partial epilepsy: implications for source localisation.

The hypothesis that focal scalp EEG and MEG interictal epileptiform activity can be modelled by single dipoles or by a limited number of dipoles was examined. The time course and spatial distribution of interictal activity recorded simultaneously by surface electrodes and by electrodes next to mesial temporal structures in 12 patients being assessed for epilepsy surgery have been studied to estimate the degree of confinement of neural activity present during interictal paroxysms, and the degree to which volume conduction and neural propagation take part in the diffusion of interictal activity. Also, intrapatient topographical correlations of ictal onset zone and deep interictal activity have been studied. Correlations between the amplitudes of deep and surface recordings, together with previous reports on the amplitude of scalp signals produced by artificially implanted dipoles suggest that the ratio of deep to surface activity recorded during interictal epileptiform activity on the scalp is around 1:2000. This implies that most such activity recorded on the scalp does not arise from volume conduction from deep structures but is generated in the underlying neocortex. Also, time delays of up to 220 ms recorded between interictal paroxysms at different recording sites show that interictal epileptiform activity can propagate neuronally within several milliseconds to relatively remote cortex. Large areas of archicortex and neocortex can then be simultaneously or sequentially active via three possible mechanisms: (1) by fast association fibres directly, (2) by fast association fibres that trigger local phenomena which in turn give rise to sharp/slow waves or spikes, and (3) propagation along the neocortex. The low ratio of deep-to-surface signal on the scalp and the simultaneous activation of large neocortical areas can yield spurious equivalent dipoles localised in deeper structures. Frequent interictal spike activities can also take place independently in areas other than the ictal onset zone and their interictal propagation to the surface is independent of their capacity to trigger seizures. It is concluded that: (1) the deep-to-surface ratios of electromagnetic fields from deep sources are extremely low on the scalp; (2) single dipoles or a limited number of dipoles are not adequate for surgical assessment; (3) the correct localisation of the onset of interictal activity does not necessarily imply the onset of seizures in the region or in the same hemisphere. It is suggested that, until volume conduction and neurophysiological propagation can be distinguished, semiempirical correlations between symptomatology, surgical outcome, and detailed presurgical modeling of the neocortical projection patterns by combined MEG, EEG, and MRI could be more fruitful than source localization with unrealistic source models.     

Analysis of the dynamics and origin of epileptic activity in patients with tuberous sclerosis evaluated for surgery of epilepsy

OBJECTIVE: The epilepsies associated with the tuberous sclerosis complex (TSC) are very often refractory to medical therapy. Surgery for epilepsy is an effective alternative when the critical link between the localization of seizure onset in the scalp and a particular cortical tuber can be established. In this study we perform analysis of ictal and interictal EEG to improve such link. METHODS: The ictal and interictal recordings of four patients with TSC undergoing surgery for epilepsy were submitted to independent component analysis (ICA), followed by source analysis, using the sLORETA algorithm. The localizations obtained for the ictal EEG and for the average interictal spikes were compared. RESULTS: The ICA of ictal EEG produced consistent results in different events, and there was good agreement with the tubers that were successfully removed in three of the four patients (one patient refused surgery). In some patients there was a large discrepancy between the localization of ictal and interictal sources. The interictal activity produced more widespread source localizations. CONCLUSIONS: The use of ICA of ictal EEG followed by the use of source analysis methods in four cases of epilepsy and TSC was able to localize the epileptic generators very near the lesions successfully removed in surgery for epilepsy. SIGNIFICANCE: The ICA of ictal EEG events may be a useful add-on to the tools used to establish the connection between epileptic scalp activity and the cortical tubers originating it, in patients with TSC considered for surgery of epilepsy.     

Low resolution electromagnetic tomography analysis of ictal EEG patterns in mesial temporal lobe epilepsy with hippocampal sclerosis

ObjectiveTo investigate the difference in the spatial distribution of scalp initial ictal discharge (IID) patterns in mesial temporal lobe epilepsy with hippocampal sclerosis (HS–MTLE).MethodsScalp ictal EEG data in 22 seizure-free patients after temporal lobectomy with amygdalo-hippocampectomy were classified as follows: a regular 5–9 Hz rhythm with a restricted temporal/subtemporal distribution (type 1, 11 patients), or an irregular 2–5 Hz rhythm with a widespread fronto-temporal distribution (type 2, 11 patients). EEG data were fragmented into segments of 1.28 s, both at ictal onset and at baseline. The LORETA solution of three frequency bands was compared between ictal and baseline using statistical non-parametric mapping (p < 0.01).ResultsThe LORETA solution of 5–9 Hz in type 2 had wider cortical activity in the ipsilateral fronto-temporal area, compared to type 1 with activation of the ipsilateral focal mesial and lateral temporal regions. The LORETA solution of 10–13 Hz in both types showed increased activity in the fronto-temporal area, which was wider in type 2 than type 1. Increased cortical activity of <5 Hz was not observed in type 1, whereas increased cortical activity was observed in the bilateral anterior frontal area in type 2.ConclusionsThe cortical source distribution in HS–MTLE may depend on scalp IID frequency. The neural generators of 5–13 Hz may be important for the formation of the ictal onset zone in both ictal patterns.SignificanceSpatial distributions in HS–MTLE patients differ with scalp IID frequency.     

Automated electrical source imaging with scalp EEG to define the insular irritative zone: Comparison with simultaneous intracranial EEG

Objective To evaluate the accuracy of automated interictal low-density electrical source imaging (LD-ESI) to define the insular irritative zone (IZ) by comparing the simultaneous interictal ESI localization with the SEEG interictal activity.Methods Long-term simultaneous scalp electroencephalography (EEG) and stereo-EEG (SEEG) with at least one depth electrode exploring the operculo-insular region(s) were analyzed. Automated interictal ESI was performed on the scalp EEG using standardized low-resolution brain electromagnetic tomography (sLORETA) and individual head models. A two-step analysis was performed: i) sublobar concordance between cluster-based ESI localization and SEEG-based IZ; ii) time-locked ESI-/SEEG analysis. Diagnostic accuracy values were calculated using SEEG as reference standard. Subgroup analysis was carried out, based on the involvement of insular contacts in the seizure onset and patterns of insular interictal activity.Results Thirty patients were included in the study. ESI showed an overall accuracy of 53% (C.I. 29–76%). Sensitivity and specificity were calculated as 53% (C.I. 29–76%), 55% (C.I. 23–83%) respectively. Higher accuracy was found in patients with frequent and dominant interictal insular spikes.Conclusions LD-ESI defines with good accuracy the insular implication in the IZ, which is not possible with classical interictal scalp EEG interpretation.SignificanceAutomated LD-ESI may be a valuable additional tool to characterize the epileptogenic zone in epilepsies with suspected insular involvement.     

Bilateral mesial temporal lobe epilepsy: comparison of scalp EEG and hippocampal MRI-T2 relaxometry

OBJECTIVE: Bilateral hippocampal abnormality is frequent in mesial temporal lobe sclerosis and might affect outcome in epilepsy surgery. The objective of this study was to compare the lateralization of interictal and ictal scalp EEG with MRI T2 relaxometry. MATERIAL AND METHODS: Forty-nine consecutive patients with intractable mesial temporal lobe epilepsy (MTLE) were studied with scalp EEG/video monitoring and MRI T2 relaxometry. RESULTS: Bilateral prolongation of hippocampal T2 time was significantly associated with following bitemporal scalp EEG changes: (i) in ictal EEG left and right temporal EEG seizure onsets in different seizures, or, after regionalized EEG onset, evolution of an independent ictal EEG over the contralateral temporal lobe (left and right temporal asynchronous frequencies or lateralization switch; P = 0.002); (ii) in interictal EEG both left and right temporal interictal slowing (P = 0.007). Bitemporal T2 changes were not, however, associated with bitemporal interictal epileptiform discharges (IED). Lateralization of bilateral asymmetric or unilateral abnormal T2 findings were associated with initial regionalization of the ictal EEG in all but one patient (P < 0.005), with lateralization of IED in all patients (P < 0.005), and with scalp EEG slowing in 28 (82,4%) of 34 patients (P = 0.007). CONCLUSION: Our data suggest that EEG seizure propagation is more closely related to hippocampal T2 abnormalities than IED. Interictal and ictal scalp EEG, including the recognition of ictal propagation patterns, and MRI T2 relaxometry can help to identify patients with bitemporal damage in MTLE. Further studies are needed to estimate the impact of bilateral EEG and MRI abnormal findings on the surgical outcome.     

Focal fast rhythmic epileptiform discharges on scalp EEG in a patient with cortical dysplasia.

A distinctive scalp electroencephalographic (EEG) pattern of focal fast rhythmic epileptiform discharges (FREDs) in a 23-year-old man with symptomatic localization related epilepsy is presented. Magnetic resonance imaging (MRI) of the brain revealed cortical dysplasia over the right temporal region where the peculiar EEG abnormality was detected. We suggest that this characteristic EEG pattern may be predictive of focal cortical dysplasia (FCD). A brief review of the rhythmic EEG abnormalities in FCDs is also presented.     

Posterior quadrant epilepsy surgery: Predictors of outcome

PurposeTo identify predictors of seizure recurrence following posterior quadrant epilepsy surgery.MethodsBetween 1983 and 2008, 43 medically refractory epilepsy patients underwent posterior quadrant epilepsy surgery. Epilepsy surgery involved the occipital lobe in all cases; some cases also included resection of the adjacent parietal or temporal cortex. Using a logistic regression model, we evaluated the relationship between outcome (Engel class I–IV) and 5 outcome predictors: absence of a visual aura, a temporal lobe type aura, versive head movement unaccompanied by a visual aura, non-focal interictal scalp EEG, and surgical pathology other than low grade tumor or cortical dysplasia. We also determined the relative risk for significant post-operative cognitive decline of Wechsler intelligence test score among those receiving complete lobectomies compared to those receiving partial lobectomies.ResultsOverall, outcome was favorable at 1 year following surgery: 22 (51.2%) patients Engel class I, 10 (24%) patients Engel class II, 5 (12%) patients Engel class III, and 6 (14%) patients Engel class IV. The 3 best univariate predictors of seizure recurrence were versive head movement unaccompanied by visual aura, non-focal interictal scalp EEG, and pathology other than low grade tumor or cortical dysplasia. A multivariate predictor combining temporal lobe type aura, versive head movement unaccompanied by visual aura, non-focal interictal scalp EEG, and pathology other than low grade tumor or cortical dysplasia was optimum. Complete lobectomy significantly increased the risk of post-operative decline of Wechsler intelligence score.ConclusionsThese findings indicate that posterior quadrant epilepsy surgery may provide sustained seizure control. A multivariate model combining temporal lobe type aura, versive head movement unaccompanied by a visual aura, non-focal interictal scalp EEG, and pathology other than low grade tumor or cortical dysplasia may contribute to predicting seizure recurrence following posterior quadrant epilepsy surgery. The extent of cortical resection may predict significant cognitive decline in post-operative Wechsler intelligence score.     

Analysis of EEG patterns and genotypes in patients with Angelman syndrome

We prospectively analyzed EEGs from participants in the ongoing NIH Rare Diseases Clinical Research Network Angelman Syndrome Natural History Study. Of the one-hundred-sixty enrolled patients (2006-2010), 115 had complete data (58 boys, median age 3.6 years). Distinct EEG findings were intermittent rhythmic delta waves (83.5%), interictal epileptiform discharges (74.2%), intermittent rhythmic theta waves (43.5%), and posterior rhythm slowing (43.5%). Centro-occipital and centro-temporal delta waves decreased with age (p=0.01, p=0.03). There were no specific correlations between EEG patterns and genotypes. A classification tree allowed the prediction of deletions class-1 (5.9 Mb) in patients with intermittent theta waves in <50% of EEG and interictal epileptiform abnormalities; UPD, UBE3A mutation or imprinting defects in patients with intermittent theta in <50% of EEG without interictal epileptiform abnormalities; deletions class-2 (5.0 Mb) in patients with >50% theta and normal posterior rhythm; atypical deletions in patients with >50% theta but abnormal posterior rhythm. EEG patterns are important biomarkers in Angelman syndrome and may suggest the underlying genetic etiology.