Localization Of Seizure Onset Research Articles

Identification of Focal Epileptogenic Networks in Generalized Epilepsy Using Brain Functional Connectivity Analysis of Bilateral Intracranial EEG Signals.

Simultaneous bilateral onset and bi-synchrony epileptiform discharges in electroencephalogram (EEG) remain hallmarks for generalized seizures. However, the possibility of an epileptogenic focus triggering rapidly generalized epileptiform discharges has been documented in several studies. Previously, a new multi-stage surgical procedure using bilateral intracranial EEG (iEEG) prior to and post complete corpus callosotomy (CC) was developed to uncover seizure focus in non-lateralizing focal epilepsy. Five patients with drug-resistant generalized epilepsy who underwent this procedure were included in the study. Their bilateral iEEG findings prior to complete CC showed generalized epileptiform discharges with no clear lateralization. Nonetheless, the bilateral ictal iEEG findings post complete CC indicated lateralized or localized seizure onset. This study hypothesized that brain functional connectivity analysis, applied to the pre CC bilateral iEEG recordings, could help identify focal epileptogenic networks in generalized epilepsy. The results indicated that despite diffuse epileptiform discharges, focal features can still be observed in apparent generalized seizures through brain connectivity analysis. The seizure onset localization/lateralization from connectivity analysis demonstrated a good agreement with the bilateral iEEG findings post complete CC and final surgical outcomes. Our study supports the role of focal epileptic networks in generalized seizures.

Sleep-wake distribution and circadian patterns of epileptic seizures in children.

Epilepsy is one of the most common chronic neurologic disorders. Daily periodicity of epileptic seizures has been known for over a century. The diurnal patterns of epileptic seizures have also been observed in studies. To investigate the sleep/wake cycle, day/night, and 24-h periodicity of various seizure subtypes and seizure onset localizations in children. We analyzed the clinical seizures of 170 consecutive epilepsy patients who underwent video-electroencephalography (EEG) monitoring over the last 5 years. Semiology of the seizures was classified according to the semiological seizure classification. Origin of the seizures was defined by the onset of ictal activity on EEG. Seizures were evaluated in terms of occurrence during the day (06:00-18:00h) or night (18:00-06:00h), in wakefulness or in sleep, and within a 3-h time interval throughout 24h. A total of 909 seizures were analyzed. Auras, dialeptic, myoclonic, hypomotor, atonic seizures, and epileptic spasms occurred more frequently in wakefulness; tonic, clonic, and hypermotor seizures occurred more frequently in sleep. Auras, dialeptic, and atonic seizures and epileptic spasms occurred more often during daytime; hypermotor seizures occurred more often at night. Generalized seizures were seen most frequently in wakefulness (between 12:00 and 18:00h); frontal lobe seizures were seen at night and in sleep (between 24:00 and 03:00h); temporal lobe seizures were seen in wakefulness (between 06:00 and 09:00h and between 12:00 and 15:00h); occipital seizures were seen during daytime and in wakefulness (between 09:00 and 12:00h and between 15:00 and 18:00h, respectively); parietal seizures were seen mostly during daytime. Seizures in children occur in specific circadian patterns and in specific sleep/wake distributions depending on seizure onset location and semiology.

Visual and semiautomated evaluation of epileptogenicity in focal cortical dysplasias - An intracranial EEG study.

The aim of the study was the evaluation of the added value of depth to subdural electrodes in delineating epileptogenicity of focal cortical dysplasias (FCDs) and to test the Epileptogenicity Index (EI) in this setting. Fifteen patients with FCD underwent iEEG with subdural and depth electrodes. Visual/EI analysis was performed in up to three habitual seizures per patient. Visual analysis: Grid onset seizures (n=10) started in electrodes overlying the lesion in 7 and remote from it in 3 cases. Depth onset seizures (n=7) affected only intralesional contacts in 4, intra- and extralesional in 2, and exclusively extralesional in 1 patient. Seizures started in depth and grid contacts simultaneously in 2 cases. EI analysis: The EI completely confirmed visual localization of seizure onset in 8 cases and depicted ictal onset-time accurately in 13. Beta/gamma ictal patterns were most reliably captured. Impact on surgical decision: Resection outline differed from MRI lesion in 7 patients based on grid and in three based on depth electrode information. In FCD, seizures can be generated within gyral/deep tissue appearing normal on imaging. Investigating FCD with subdural and depth electrodes is efficient to outline the seizure onset zone. The EI is a helpful additional tool to quantify epileptogenicity. Specific ictal patterns are prerequisite for reliable results.

Circadian Patterns of Epileptiform Discharges Differ by Seizure Onset Location During Chronic, Long-Term Intracranial Monitoring (P5.249)

Circadian Patterns of Epileptiform Discharges Differ by Seizure Onset Location During Chronic, Long-Term Intracranial Monitoring (P5.249)

Resecting critical nodes from an epileptogenic circuit in refractory focal-onset epilepsy patients using subtraction ictal SPECT coregistered to MRI.

OBJECTIVE The purpose of this study was to assess the positive predictive value of postresection outcomes obtained by presurgical subtracted ictal SPECT in patients with lesional (MRI positive) and nonlesional (MRI negative) refractory extratemporal lobe epilepsy (ETLE) and temporal lobe epilepsy (TLE). Specifically, outcomes were compared between partial versus complete resection of the regions of transient hyperperfusion identified using subtraction ictal SPECT coregistered to MRI (SISCOM) in relation to the ictal onset zone (IOZ) that was confirmed by electrocorticography (ECoG). That is, SISCOM was used to understand the long-term postsurgical outcomes following resection of the IOZ that overlapped with 1 or more regions of ictal onset-associated transient hyperperfusion. METHODS The study cohort included 44 consecutive patients with refractory ETLE or TLE who were treated between 2002 and 2013 and underwent presurgical evaluation using SISCOM. Concordance was determined between SISCOM localization and the IOZ on the basis of ECoG monitoring. In addition, the association between the extent of the resection site overlapping with the SISCOM signal and postresection outcomes were assessed. Postsurgical follow-up was longer than 24 months in 39 of 44 patients. RESULTS The dominant SISCOM signals were concordant with ECoG and overlapped the resection site in 32 of 44 (73%) patients (19 ETLE and 13 TLE patients), and 20 of 32 (63%) patients became seizure free. In all 19 ETLE patients with concordant SISCOM and ECoG results, the indicated location of ictal onset on ECoG was completely resected; 11 of 19 patients (58%) became seizure free (Engel Class I). In all 13 TLE patients with concordant SISCOM and ECoG results, the indicated ECoG focus was completely resected; 9 of 13 patients (69%) became seizure free (Engel Class I). Complete resection of the SISCOM signal was found in 7 of 34 patients (21%). Of these 7 patients, 5 patients (72%) were seizure free (Engel Class I). Partial resection of the SISCOM signal was found in 16 of 34 patients (47%), and 10 of these 16 patients (63%) were seizure free (Engel Class I) after more than 24 months of follow-up. CONCLUSIONS Concordance between 1 or more SISCOM regions of hyperperfusion with ECoG and at least partial resection of the dominant SISCOM signal in this refractory epilepsy cohort provided additional useful information for predicting long-term postresection outcomes. Such regions are likely critical nodes in more extensive, active, epileptogenic circuits. In addition, SPECT scanner technology may limit the sensitivity of meaningful SISCOM signals for identifying the maximal extent of the localizable epileptogenic network.

Spike pattern recognition by supervised classification in low dimensional embedding space.

Epileptiform discharges in interictal electroencephalography (EEG) form the mainstay of epilepsy diagnosis and localization of seizure onset. Visual analysis is rater-dependent and time consuming, especially for long-term recordings, while computerized methods can provide efficiency in reviewing long EEG recordings. This paper presents a machine learning approach for automated detection of epileptiform discharges (spikes). The proposed method first detects spike patterns by calculating similarity to a coarse shape model of a spike waveform and then refines the results by identifying subtle differences between actual spikes and false detections. Pattern classification is performed using support vector machines in a low dimensional space on which the original waveforms are embedded by locality preserving projections. The automatic detection results are compared to experts’ manual annotations (101 spikes) on a whole-night sleep EEG recording. The high sensitivity (97 %) and the low false positive rate (0.1 min−1), calculated by intra-patient cross-validation, highlight the potential of the method for automated interictal EEG assessment.

Exploring the time–frequency content of high frequency oscillations for automated identification of seizure onset zone in epilepsy

Objective. High frequency oscillations (HFOs) in intracranial electroencephalography (iEEG) recordings are considered as promising clinical biomarkers of epileptogenic regions in the brain. The aim of this study is to improve and automatize the detection of HFOs by exploring the time–frequency content of iEEG and to investigate the seizure onset zone (SOZ) detection accuracy during the sleep, awake and pre-ictal states in patients with epilepsy, for the purpose of assisting the localization of SOZ in clinical practice. Approach. Ten-minute iEEG segments were defined during different states in eight patients with refractory epilepsy. A three-stage algorithm was implemented to detect HFOs in these segments. First, an amplitude based initial detection threshold was used to generate a large pool of HFO candidates. Then distinguishing features were extracted from the time and time–frequency domain of the raw iEEG and used with a Gaussian mixture model clustering to isolate HFO events from other activities. The spatial distribution of HFO clusters was correlated with the seizure onset channels identified by neurologists in seven patient with good surgical outcome. Main results. The overlapping rates of localized channels and seizure onset locations were high in all states. The best result was obtained using the iEEG data during sleep, achieving a sensitivity of 81%, and a specificity of 96%. The channels with maximum number of HFOs identified epileptogenic areas where the seizures occurred more frequently. Significance. The current study was conducted using iEEG data collected in realistic clinical conditions without channel pre-exclusion. HFOs were investigated with novel features extracted from the entire frequency band, and were correlated with SOZ in different states. The results indicate that automatic HFO detection with unsupervised clustering methods exploring the time–frequency content of raw iEEG can be efficiently used to identify the epileptogenic zone with an accurate and efficient manner.

Ictal onset on intracranial EEG: Do we know it when we see it? State of the evidence.

A major limitation of intracranial electroencephalography (iEEG) is recording from a confined region. This may falsely localize seizure onset if the distinction between ictal onset zone, proximity, and spread is unclear, or if the ictal rhythm is not clearly identified. Delineation of the ictal onset zone is crucial for surgical success. We appraised the evidence to determine whether specific iEEG ictal patterns are associated with the ictal onset zone. We searched Embase for articles in English until September 30, 2014, with MeSH keywords related to intracranially implanted electrodes and seizures. Two authors independently screened abstracts, reviewed full text articles, and abstracted data. The association between seizure outcome and type of ictal onset pattern (IOP), and its extent, location, and spread were explored visually or by univariate analysis when sufficient data were provided. Methodologic quality of each study was assessed. We reviewed 1,987 abstracts from which 21 articles were analyzed. Fifteen IOPs were reported. Low frequency high amplitude repetitive spiking (LFRS) was the most frequently reported IOP by studies that dealt with mesial temporal lobe epilepsy (mTLE) and investigated with depth electrodes. In neocortical epilepsy, low voltage fast activity (LVFA) was the most commonly described IOP. Delta activity was an infrequently reported IOP and was described mostly as a spread pattern. LFRS is associated with good surgical outcome in mTLE and has a strong relation with mesial temporal pathology and its severity. LVFA is associated with neocortical temporal epilepsy and focal LVFA is associated with better surgical outcome. Electrodecrement may be associated with regional or widespread onsets. Rhythmic delta is a propagation rhythm rather than an IOP. Focal IOPs and slower propagation times are associated with better outcomes. The quality of the studies is suboptimal and there are methodological problems. Interobserver agreement is poorly documented.

The performance of the spatiotemporal Kalman filter and LORETA in seizure onset localization.

The assumption of spatial-smoothness is often used to solve the bioelectric inverse problem during electroencephalographic (EEG) source imaging, e.g., in low resolution electromagnetic tomography (LORETA). Since the EEG data show a temporal structure, the combination of the temporal-smoothness and the spatial-smoothness constraints may improve the solution of the EEG inverse problem. This study investigates the performance of the spatiotemporal Kalman filter (STKF) method, which is based on spatial and temporal smoothness, in the localization of a focal seizure's onset and compares its results to those of LORETA. The main finding of the study was that the STKF with an autoregressive model of order two significantly outperformed LORETA in the accuracy and consistency of the localization, provided that the source space consists of a whole-brain volumetric grid. In the future, these promising results will be confirmed using data from more patients and performing statistical analyses on the results. Furthermore, the effects of the temporal smoothness constraint will be studied using different types of focal seizures.

P52 – 2932: Sleep-wake distribution and circadian patterns of epileptic seizures in children

Objective To investigate the sleep/wake cycle, day/night and 24-hour periodicity of various seizure subtypes and seizure onset localizations in children. Methods We analyzed the clinical seizures of 170 consecutive epilepsy patients who underwent video-electroencephalography (EEG) monitoring over the last 5-year period. Semiology of the seizures was classified according to the semiological seizure classification. Origin of the seizures was defined by the onset of ictal activity in EEG. Seizures were evaluated in terms of occurence during the day (06:00–18:00) or night (18:00–06:00), in wakefulness or sleep and within the 3-hour time interval throughout 24 hours. Results A total of 909 seizures were analyzed. While auras, dialeptic, myoclonic, hypomotor, atonic seizures and epileptic spasms occurred more frequently in wakefulness; tonic, clonic, hypermotor seizures occurred more frequently in sleep. Auras, dialeptic, atonic seizures and epileptic spasms occurred more often during daytime, hypermotor seizures occurred more often at night. Generalized seizures were seen most frequently in wakefulness (between 12:00–18:00 h), frontal lob seizures were seen at night and in sleep (between 24:00–6:00 h), temporal lobe seizures were seen in wakefulness (between 06:00–09:00 h and 12:00 am-15:00 h), occipital seizures were seen during daytime and in wakefulness (between 09:00–12:00 h and 15:00–18:00 h), parietal seizures were seen mostly during daytime. Conclusion Seizures in children occur in specific circadian patterns and in specific sleep/wake distributions depending on seizure onset location and semiology. Recognition of these daily patterns of seizures provides new diagnostic and therapeutic options. To investigate the sleep/wake cycle, day/night and 24-hour periodicity of various seizure subtypes and seizure onset localizations in children. We analyzed the clinical seizures of 170 consecutive epilepsy patients who underwent video-electroencephalography (EEG) monitoring over the last 5-year period. Semiology of the seizures was classified according to the semiological seizure classification. Origin of the seizures was defined by the onset of ictal activity in EEG. Seizures were evaluated in terms of occurence during the day (06:00–18:00) or night (18:00–06:00), in wakefulness or sleep and within the 3-hour time interval throughout 24 hours. A total of 909 seizures were analyzed. While auras, dialeptic, myoclonic, hypomotor, atonic seizures and epileptic spasms occurred more frequently in wakefulness; tonic, clonic, hypermotor seizures occurred more frequently in sleep. Auras, dialeptic, atonic seizures and epileptic spasms occurred more often during daytime, hypermotor seizures occurred more often at night. Generalized seizures were seen most frequently in wakefulness (between 12:00–18:00 h), frontal lob seizures were seen at night and in sleep (between 24:00–6:00 h), temporal lobe seizures were seen in wakefulness (between 06:00–09:00 h and 12:00 am-15:00 h), occipital seizures were seen during daytime and in wakefulness (between 09:00–12:00 h and 15:00–18:00 h), parietal seizures were seen mostly during daytime. Seizures in children occur in specific circadian patterns and in specific sleep/wake distributions depending on seizure onset location and semiology. Recognition of these daily patterns of seizures provides new diagnostic and therapeutic options.

The relationship between seizure onset zone and ictal tachycardia: An intracranial EEG study

ObjectivesSeizures are often accompanied by ictal tachycardia, which, when pronounced, is one of the cardiac arrhythmias associated with sudden unexpected death in epilepsy (SUDEP). We examined the relationship between the lateralization and localization of seizure onset and development of ictal tachycardia. MethodsWe identified patients who underwent bi-hemispheric intracranial EEG recording over a period of 18months. Two to four consecutive seizures were reviewed for each patient. ResultsFifty-seizures from 19 consecutive patients were analyzed. Forty seizures (80%) developed tachycardia (>20% increase from baseline), but laterality at seizure onset did not predict its occurrence (p=0.168).Bi-laterality at ictal onset was associated with early ictal tachycardia (<10s) (p=0.0208).Seizures out of sleep developed tachycardia faster (mean 19.7s vs. 68.2s, p=0.0067), but the state of alertness was not predictive of the development of tachycardia within 10s of seizure onset.Temporal and/or orbito-frontal lobe involvement was associated with tachycardia when compared to any other lobar combinations at ictal onset (p=0.0073). ConclusionLaterality at seizure onset does not predict the occurrence of ictal tachycardia. Involvement of the temporal and orbito-frontal cortex, spread to the contralateral hemisphere and state of alertness, may define the degree and rate of autonomic changes. SignificanceOur results help clarify the autonomic control during seizures and offer potential use for future studies in SUDEP risk and automatic seizure detection.

The Role of Neuropsychological Evaluation in Epilepsy Surgery Candidates

Neuropsychological evaluations of preoperative epilepsy surgical candidates have been a routine portion of the multidisciplinary evaluation at most epilepsy centers for decades. Neuropsychology assessments have been explored as a means to predict and identify postoperative cognitive deficits after resections (chiefly temporal lobe), and in numerically quantifying those changes that do occur. In this manuscript, we describe the typical tools used, including tests of general intelligence, tests of language, tests of visuospatial cognition, and tests of verbal memory. Evidence for or against the use of the neuropsychological evaluation as a means to localize seizure onset, or as an adjunct for ictal focus lateralization is also included.

Diagnostic outcome of surgical revision of intracranial electrode placements for seizure localization.

We aimed to determine the yield of revising intracranially implanted electrodes and the factors contributing to the yield. Patients were identified from the Mayo Clinic Epilepsy Surgery Database between 1997 and 2010. Twenty patients had revision of intracranial electrode placements because initial implantation did not localize seizure onset adequately. Seizures were captured in 18 of 20 patients who underwent intracranial electrode revision, of which 10 (55.6%) showed localized seizure onset that led to a surgical resection. Seizures were improved in 9 of 10 patients who underwent resection; of these, five were seizure free. The only factors found to be statistically significant in localizing ictal onset zone after revised implantation were prior focal scalp interictal discharges and an initial intracranial EEG showing ictal onset at the edge of the electrode grid. No permanent complication was associated with revised implantation, but one patient had transient apraxia of the right foot. Revised implantation could be useful in selected patients with inadequate seizure localization on initial intracranial EEG. Resective surgery was performed in 50% of patients who underwent revision of intracranial electrodes with the majority of these patients experiencing an improvement in seizure control.

Study on concordance of ictal and interictal epileptiform activity in patients with tuberous sclerosis complex

To analyze the relationship between the ictal onset zone and dominant interictal epileptiform foci in tuberous sclerosis complex (TSC) patients. Clinical data of 20 patients with TSC which had epileptic seizures during Video-EEG monitoring was assessed. Consistency and dominance of focal interictal epileptiform activity and the ictal onset zone were identified. Concordance between interictal and ictal findings was analyzed. Of the 20 patients, 7 were female, and 13 were male. The age of epilepsy onset was from 15 d to 6 years. The Video-EEG monitoring age was from 6 months to 11 years. Family history was found in three cases. Abnormality in neuroimaging existed in 17 of 18 patients who were examined. Interictal EEG showed hypsarrhythmia in 3 patients, multifocal epileptiform activity with a dominant focus in 12 patients, both focal and generalized discharges in 2 patients, and only focal discharges in 3 patients. The seizures types during EEG monitoring included epileptic spasms, partial seizure, atypical absence, and generalized or focal myoclonic seizure. The most common seizure type was partial seizure and then epileptic spasms. EEG in 4 patients with epileptic spasms showed ictal generalized discharges and interictal hypsarrhythmia or generalized discharges. Clinical manifestation of epileptic spasms was asymmetric in 3 patients. Lateralization and location of interictal and ictal discharges were consistent in 2 of the 3 patients, while only lateralization consistency in 1 of the 3 patients. Partial seizures as the only seizure type were monitored in 13 patients. Of the 13 patients, lateralization and location of interictal and ictal discharges were inconsistent in 2 patients (15%), consistent in 8 patients (62%), lateralization or location consistent in 2 patients (15%). One case could not be analyzed because of uncertainty of lateralization and location of seizure onset. In the majority of patients with TSC, multifocal interictal epileptiform activity is present, in which a most dominance of focal epileptiform activity could be found. For some epileptic seizures or the majority of partial seizures, the ictal onset zone is concordant with the dominance of focal interictal epileptiform foci. The concordance might have positioning reference significance for preoperative evaluation of epilepsy surgery.

Using ictal high-frequency oscillations (80–500 Hz) to localize seizure onset zones in childhood absence epilepsy: A MEG study

This study aimed to use ictal high-frequency oscillations (HFOs) ranging from 80Hz to 500Hz to locate seizure onset zones in childhood absence epilepsy (CAE) using non-invasive magnetoencephalography (MEG). Ten drug-naïve children with CAE were studied using a 275-channel MEG system. MEG data were digitized at a sampling rate of 6000Hz. HFO spectral power in real-time spectrograms was assessed using Morlet continuous wavelet transform. Magnetic sources were volumetrically localized through dynamic magnetic source imaging with a slide window. HFOs were identified in all patients. The total time of fast ripples (250–500Hz) was greater than that of ripples (80–250Hz) during absence seizures. The rate of fast ripples was associated with seizure frequency. HFO duration was significantly longer when co-occurring with spikes than when occurring independently, and the maximum frequency of HFOs co-occurring with spikes was higher than that of HFOs occurring independently. HFOs were predominantly localized in the medial prefrontal cortex (MPFC), whereas spikes were widespread to a variety of regions during the absence seizures. Compared with spikes, HFOs appeared to be more focal. The findings indicate that HFOs in the MPFC have a primary function in initializing epileptic activity in CAE.

Invasive electroencephalography monitoring: Indications and presurgical planning.

Electroencephalography (EEG) remains a “gold standard” for defining seizures; hence identification of epileptogenic zone for surgical treatment of epilepsy requires precise electrographic localization of the seizures. Routine scalp EEG recording is not sufficient in many instances, such as extratemporal lobe epilepsy or non-lesional temporal lobe epilepsy. In these individuals EEG recording from proximity of the seizure focus is necessary, which can be achieved by placing electrodes on the surface or in the substance of the brain. As this process requires invasive procedures (usually necessitating surgical intervention) EEG obtained via these electrodes is defined as invasive electroencephalography (iEEG). As only limited areas of the brain can be covered by these electrodes in an individual, precise targeting of the presumed seizure onset location is crucial. The presurgical planning includes where to place electrodes, which type of the electrodes to choose and planned duration of the intracranial recording. Though there are general principles that guide such endeavor, each center does it slightly differently depending upon the various technologies available to them and expertise and preferences of the epilepsy surgery team. Here we describe our approach to iEEG recording. We briefly describe the background, types of iEEG recording and rationale for each, various electrode types, and scenarios where iEEG might be useful. We also describe planning of iEEG recording once the need has been established as well as our decision making process of deciding about location of electrode placement, type of electrodes to use, length of recording, choice of arrays, mapping of eloquent cortex and finally surgical planning and decisions.

Influence of age and location of ictal onset on postoperative outcome in patients with localization-related epilepsy

Introduction. Localization-related epilepsy is a significant concern for the middle-aged and older population given the lesser cognitive reserve and the greater likelihood of adverse systemic antiepileptic drug effects. Epilepsy surgery for this age group has the potential for greater postoperative morbidity and mortality, a protracted rehabilitation with its associated economic burden, progressive cognitive and memory decline and a lesser chance of becoming seizure-free after the procedure. Methods. A retrospective institutional archival review of 120 patients with medically refractory partial epilepsy of both temporal and extratemporal epilepsy was performed. Comparative assessments for seizure cessation and neuropsychological effects were made for those younger and older than 50 years and those with epilepsy durations of greater and less than 20 years. Results. A comparison of surgical outcomes by age group identified that Engel Class I outcomes were attained in 26 of 35 (74%) patients in the older group compared with 49 of 85 (58%) patients in the younger group (p = 0.087). Of the 11 patients aged 60 years or older, 91% attained an Engel Class I outcome, significantly higher than those under 60 years (60%; p = 0.041). Stratification by age of the entire group, otherwise, demonstrated no significant differences by Engel class outcome. Considering both transient and permanent disabilities, the overall complication rate was 14.2% and was similar in patients less than 50 years of age (15.3%) and those aged 50 years or greater (11.4%; p = 0.58). Verbal memory decline was more notable in the older population and for those with epilepsy duration of greater than 20 years. Conclusion. This study demonstrates that epilepsy surgery in the older population has similar seizure-free outcomes and complications when compared with younger patients for both mesial temporal and extratemporal origins. Caution regarding postoperative memory decline in the older population must be stressed.

Intravenous recording of intracranial, broadband EEG

The most direct evaluation of human brain activity has been obtained from intracranial electrodes placed either on the surface of the brain or inserted into the brain to record from deep brain structures. Currently, the placement of intracranial electrodes implies transcranial surgery, either through a burr hole or a craniotomy, but the high degree of invasiveness and potential for morbidity of such major surgical procedures limits the applicability of intracranial recording. The vascular system provides a natural avenue to reach many brain regions that currently are reached by transcranial approaches, along with deep brain structures that cannot be reached via a transcranial approach without significant risk. To determine the applicability of intravascular approaches to high-frequency intracranial monitoring, a catheter containing multiple macro- and micro-electrodes was placed into the superior sagittal sinus of anesthetized pigs in parallel with clinical, subdural electrode grids to record epileptiform activity induced by direct, cortical injection of penicillin and to record responses to electrical stimulation. Intravascular electrodes recorded epileptiform spikes with similar magnitudes and waveshapes to those obtained by surface electrodes, both for macroelectrodes and microelectrodes, including the spatiotemporal evolution of epileptiform activity, suggesting that intravascular electrodes might provide localizing information regarding seizure foci. Sinusoidal electrical stimulation showed that intravascular electrodes provide sufficient broadband fidelity to record high-frequency, physiological events that may also prove useful in localizing seizure onset zones. As intravascular techniques have transformed cardiology, so intravascular neurophysiology may transform intracranial monitoring, in general, and the treatment of epilepsy, in particular.

LOCALIZATION OF EPILEPTIC FOCI USING MULTIMODALITY NEUROIMAGING

Approximately 30% of epilepsy patients are medically intractable. Epilepsy surgery may offer cure or palliation, and neuromodulation and direct drug delivery are being developed as alternatives. Successful treatment requires correct localization of seizure onset zones and understanding surrounding functional cortex to avoid iatrogenic disability. Several neurophysiologic and imaging localization techniques have inherent individual weaknesses which can be overcome by multimodal analysis. We review common noninvasive techniques, then illustrate the value of multimodal analysis to localize seizure onset for targeted treatment.

Mapping preictal and ictal haemodynamic networks using video-electroencephalography and functional imaging

Ictal patterns on scalp-electroencephalography are often visible only after propagation, therefore rendering localization of the seizure onset zone challenging. We hypothesized that mapping haemodynamic changes before and during seizures using simultaneous video-electroencephalography and functional imaging will improve the localization of the seizure onset zone. Fifty-five patients with ≥2 refractory focal seizures/day, and who had undergone long-term video-electroencephalography monitoring were included in the study. 'Preictal' (30 s immediately preceding the electrographic seizure onset) and ictal phases, 'ictal-onset'; 'ictalestablished' and 'late ictal', were defined based on the evolution of the electrographic pattern and clinical semiology. The functional imaging data were analysed using statistical parametric mapping to map ictal phase-related haemodynamic changes consistent across seizures. The resulting haemodynamic maps were overlaid on co-registered anatomical scans, and the spatial concordance with the presumed and invasively defined seizure onset zone was determined. Twenty patients had typical seizures during functional imaging. Seizures were identified on video-electroencephalography in 15 of 20, on electroencephalography alone in two and on video alone in three patients. All patients showed significant ictal-related haemodynamic changes. In the six cases that underwent invasive evaluation, the ictal-onset phase-related maps had a degree of concordance with the presumed seizure onset zone for all patients. The most statistically significant haemodynamic cluster within the presumed seizure onset zone was between 1.1 and 3.5 cm from the invasively defined seizure onset zone, which was resected in two of three patients undergoing surgery (Class I post-surgical outcome) and was not resected in one patient (Class III post-surgical outcome). In the remaining 14 cases, the ictal-onset phase-related maps had a degree of concordance with the presumed seizure onset zone in six of eight patients with structural-lesions and five of six non-lesional patients. The most statistically significant haemodynamic cluster was localizable at sub-lobar level within the presumed seizure onset zone in six patients. The degree of concordance of haemodynamic maps was significantly better (P < 0.05) for the ictal-onset phase [entirely concordant/concordant plus (13/20; 65%) + some concordance (4/20; 20%) = 17/20; 85%] than ictal-established [entirely concordant/concordant plus (5/13; 38%) + some concordance (4/13; 31%) = 9/13; 69%] and late ictal [concordant plus (1/9; 11%) + some concordance (4/9; 44%) = 5/9; 55%] phases. Ictal propagation-related haemodynamic changes were also seen in symptomatogenic areas (9/20; 45%) and the default mode network (13/20; 65%). A common pattern of preictal changes was seen in 15 patients, starting between 98 and 14 s before electrographic seizure onset, and the maps had a degree of concordance with the presumed seizure onset zone in 10 patients. In conclusion, preictal and ictal haemodynamic changes in refractory focal seizures can non-invasively localize seizure onset at sub-lobar/gyral level when ictal scalp-electroencephalography is not helpful.