Stereoencephalography Research Articles

The impact of skull thickness on pediatric stereoencephalography electrode implantation and technical considerations.

One consideration in pediatric stereoencephalography (SEEG) is decreased skull thicknesses compared with adults, which may limit traditional bolt-based anchoring of electrodes. The authors aimed to investigate the safety profile, complication rates, and technical adaptations of placing SEEG electrodes in pediatric patients. The authors retrospectively reviewed all patients aged 12 years or younger at the time of SEEG implantation at their institution. Postimplantation CT scans were used to measure skull thickness at the entry point of each SEEG lead. Postimplantation lead accuracy was also assessed. Fifty-three patients were reviewed. The median skull thickness was 4.1 (interquartile range [IQR] 3.15-5.2) mm. There were 5 total complications: 1 retained bolt fragment, 3 asymptomatic subdural hematomas, and 1 asymptomatic intracranial hemorrhage. Median radial error from the lead target was 3.5 (IQR 2.24-5.25) mm. Linear regression analysis revealed that increasing skull thickness decreased the deviation from the intended target, implying an improved accuracy to target at thicker skull entry points; this trended towards improved accuracy, but did not achieve statistical significance (p = 0.54). This study found a 1.9% hardware complication rate and a 9.4% asymptomatic hemorrhage rate. Suturing electrodes to the scalp may represent a reasonable option if there are concerns of young age or a thin skull. These data indicate that invasive SEEG evaluation is safe among patients 12 years old or younger.

Stereoelectroencephalography Implantation Using Frameless Neuronavigation and Varioguide: Prospective Analysis of Accuracy and Safety in a Case Series of 11 Patients

Stereoencephalography (SEEG) is becoming a widespread diagnostic procedure for drug-resistant epilepsy investigation. Techniques include frame-based and robot-assisted implantation, and more recently, frameless neuronavigated systems (FNSs). Despite its recent use, the accuracy and safety of FNS are still under investigation. To assess in a prospective study the accuracy and safety of a specific FNS use for SEEG implantation. Twelve patients who underwent SEEG implantation using FNS (Varioguide [Brainlab]) were included in this study. Data were collected prospectively and included demographic data, postoperative complications, functional results, and implantation characteristics (i.e., duration and number of electrodes). Further analysis included accuracy at entry point and target using measurements of the euclidean distance between planned and actual trajectories. Eleven patients underwent SEEG-FNS implantation from May 2019 to March 2020. One patient did not undergo surgery because of a bleeding disorder. The mean target deviation was 4.06 mm, and mean entry point deviation was 4.2 mm, with insular electrodes significantly more deviated. Results excluding insular electrodes showed a mean target deviation of 3.66 mm and a mean entry point deviation of 3.77 mm. No severe complications occurred; a few mild to moderate adverse events were reported (1 superficial infection, 1 seizure cluster, and 3 transient neurologic impairments). The mean implantation duration by electrodes was 18.5 minutes. Implantation of depth electrodes for SEEG using FNS seems to be safe, but larger prospective studies are needed to validate these results. Accuracy is sufficient for noninsular trajectories but warrant caution for insular trajectories with statistically significantly less accuracy.

Peri-ictal hypoxemia during temporal lobe seizures: A SEEG study.

Focal seizures originating from the temporal lobe are commonly associated with peri‐ictal hypoxemia (PIH). During the course of temporal lobe seizures, epileptic discharges often not only spread within various parts of the temporal lobe but also possibly insula and frontal lobe. The link between spatial propagation of the seizure discharges and PIH is still unclear. The present study investigates the involvement of several brain structures including medial temporal structures, temporal pole, anterior insula, and frontal cortex in the occurrence of PIH. Using quantitative indices obtained during SEEG (stereoencephalography) recordings in 38 patients, we evaluated the epileptogenicity, the spatial propagation, and functional connectivity between those structures during seizures leading to PIH. Multivariate statistical analyses of SEEG quantitative indices showed that temporal lobe seizures leading to PIH are characterized by a strong involvement of amygdala and anterior insula during seizure propagation and a more widespread involvement of medial temporal lobe structures, lateral temporal lobe, temporal pole, and anterior cingulate at the end of the seizures. On the contrary, seizure‐onset zone was not associated with PIH occurrence. During seizure propagation, anterior insula, temporal pole, and temporal lateral neocortex activities were correlated with intensity of PIH. Lastly, PIH occurrence was also related to a widespread increase of synchrony between those structures. Those results suggest that PIH occurrence during temporal lobe seizures may be related to the activation of a widespread network of cortical structures, among which amygdala and anterior insula are key nodes.

Assessing differential representation of hand movements in multiple domains using stereo-electroencephalographic recordings

Invasive brain-computer interfaces (BCI) have made great progress in the reconstruction of fine hand movement parameters for paralyzed patients, where superficial measurement modalities including electrocorticography (ECoG) and micro-array recordings are mostly used. However, these recording techniques typically focus on the signals from the sensorimotor cortex, leaving subcortical regions and other cortical regions related to the movements largely unexplored. As an intracranial recording technique for the presurgical assessments of brain surgery, stereo-encephalography (SEEG) inserts depth electrodes containing multiple contacts into the brain and thus provides the unique opportunity for investigating movement-related neural representation throughout the brain. Although SEEG samples neural signals with high spatial-temporal resolutions, its potential of being used to build BCIs has just been realized recently, and the decoding of SEEG activity related to hand movements has not been comprehensively investigated yet. Here, we systematically evaluated the factors influencing the performance of movement decoding using SEEG signals recorded from 32 human subjects performing a visually-cued hand movement task. Our results suggest that multiple regions in both lateral and depth directions present significant neural selectivity to the task, whereas the sensorimotor area, including both precentral and postcentral cortex, carries the richest discriminative neural information for the decoding. The posterior parietal and prefrontal cortex contribute gradually less, but still rich sources for extracting movement parameters. The insula, temporal and occipital cortex also contains useful task-related information for decoding. Under the cortex layer, white matter presents decodable neural patterns but yields a lower accuracy (42.0 ± 0.8%) than the cortex on average (44.2 ± 0.8%, p<0.01). Notably, collectively using neural signals from multiple task-related areas can significantly enhance the movement decoding performance by 6.9% (p<0.01) on average compared to using a single region. Among the different spectral components of SEEG activity, the high gamma and delta bands offer the most informative features for hand movements reconstruction. Additionally, the phase-amplitude coupling strength between these two frequency ranges correlates positively with the performance of movement decoding. In the temporal domain, maximum decoding accuracy is first reached around 2 s after the onset of movement commands. In sum, this study provides valuable insights for the future motor BCIs design employing both SEEG recordings and other recording modalities.

SYLVIUS: A multimodal and multidisciplinary platform for epilepsy surgery

Background and objectiveWe present SYLVIUS, a software platform intended to facilitate and improve the complex workflow required to diagnose and surgically treat drug-resistant epilepsies. In complex epilepsies, additional invasive information from exploration with stereoencephalography (SEEG) with deep electrodes may be needed, for which the input from different diagnostic methods and clinicians from several specialties is required to ensure diagnostic efficacy and surgical safety. We aim to provide a software platform with optimal data flow among the different stages of epilepsy surgery to provide smooth and integrated decision making. MethodsThe SYLVIUS platform provides a clinical workflow designed to ensure seamless and safe patient data sharing across specialities. It integrates tools for stereo visualization, data registration, transfer of electrode plans referred to distinct datasets, automated postoperative contact segmentation, and novel DWI tractography analysis. Nineteen cases were retrospectively evaluated to track modifications from an initial plan to obtain a final surgical plan, using SYLVIUS. ResultsThe software was used to modify trajectories in all 19 consulted cases, which were then imported into the robotic system for the surgical intervention. When available, SYLVIUS provided extra multimodal information, which resulted in a greater number of trajectory modifications. ConclusionsThe architecture presented in this paper streamlines epilepsy surgery allowing clinicians to have a digital clinical tool that allows recording of the different stages of the procedure, in a common multimodal 2D/3D setting for participation of different clinicians in defining and validating surgical plans for SEEG cases.

Neuro-oscillatory tracking of low- and high-level musico-acoustic features during naturalistic music listening: Insights from an intracranial electroencephalography study.

Studies investigating the neural processing of musico-acoustic features have tended to do so using highly controlled musical stimuli. However, it is increasingly argued that failing to use naturalistic stimuli limits the extent to which findings from lab studies can be extrapolated to rich and varied real-world experiences. Here, we recorded electrical brain activity from 8 epileptic patients, implanted for pre-surgical evaluation with Stereo-encephalography (SEEG), while they listened to pieces from the western tonal music repertoire. We estimated the sound intensity and key and pulse clarity of the stimuli using a toolbox for automatic extraction of musico-acoustic features. We then used partial-correlation analyses to examine the patterns of neuro-oscillatory activity associated with the processing of these features. Our results showed clear tracking of sound intensity in high-gamma and alpha frequency bands in posterior superior temporal gyrus, reflecting neural firing and the transfer of auditory information from the thalamus to auditory cortices, respectively. Patterns of partial correlations, in line with our hypotheses, also suggested limbic and inferior frontal cortical tracking of tonal and rhythmic uncertainty, albeit without the robustness shown for sound intensity tracking in auditory areas. The study provides an important contribution to the existing literature in its adherence to the call for a greater use of ecologically valid stimuli in neuroscientific investigations of music listening. Our results, specifically, have implications for research on the neural processing of musical uncertainty and for future studies seeking to use intracranial EEG to examine naturalistic music processing.

Ictal swearing network confirmed by stereoencephalography: a case report.

Ictal swearing, characterized by episodic utterance of deistic, visceral and other taboo words and phrases, is a rare manifestation of epilepsy. Limited manifestation of that elucidate neural network about manifestation of. Here we report a 43-year-old right-handed man who has suffered from refractory epilepsy manifestation of ictal swearing. The stereoencephalography(SEEG) investigation located the seizure onset zone in left orbito-frontal cortex and the involvement of left orbito-frontal and right hippocampus in the propagation of epileptic activities. Surgical resection has made the patient seizure-free. The present case gives us better understanding of these ictal phenomena and helps to optimize the placement of SEEG electrodes in refractory epilepsy patient with ictal swearing.

Application of robot-assisted stereoencephalography electrode implantation in epilepsy surgery

Objective To explore the precision, efficiency and safety of robot-assisted stereoencephalography (SEEG) electrode implantation applied in epilepsy surgery. Methods A retrospective analysis was conducted on 23 patients of refractory epilepsy who were admitted to Beijing Institute of Functional Neurosurgery, Xuanwu Hospital of Capital Medical University from February 2018 to June 2018 and underwent robot-assisted SEEG electrode implantation ('robot’ group). A total of 25 refractory epilepsy patients admitted from October 2017 to January 2018 underwent CRW stereotactic apparatus-guided SEEG implantation and served as control ('CRW’ group). Continuous thin-slice CT was conducted post operation in each case, based on which the distance between the actual and planned target of each electrode trajectory was measured and set as operational deviation.Operational duration for each electrode implantation and incidences of postoperative complications were documented. The parameters as above were compared between the 2 groups. Results In the 'robot’ group, a total of 179SEEG electrodes with 2 642 contacts were implanted in this series (23 cases), with the median of 8 (5-10) electrodes with 120 (76-136)contacts in each patient.In the 'CRW’ group, 25 patients underwent implantation of 172 electrodes with 2 520 contacts, with the median of 7 (6-9)electrodes with 100 (60-128) contacts in each patient.The operational deviation of each electrode was 1.4 (0.4-9.3) mm ('robot’ group) and 1.4 (0-4.6) mm ('CRW’ group), respectively, which had no statistical difference (P>0.05). Compared with 'CRW’ group, the 'robot’ group had significantly shorter operational duration[8(5.9-12.8) min vs. 14.7 (10.3-18.5) min, P<0.01] for each electrode implantation. In the 'robot’ group, there were 2 cases (8.7%) of asymptomatic intracranial hemorrhage. No cases of infection or other complications were observed. In the 'CRW’ group, there were 1 case (4.0%) of intracerebral hemorrhage causing contralateral paralysis, 1 case (4.0%) of asymptomatic hemorrhage and no cases of other complications. Conclusions Robot-assisted technique seems to be able to remarkedly increase the efficiency of SEEG electrode implantation with relatively good precision and safety, which could thus be recommended for clinical use and promotion. Key words: Epilepsy; Neurosurgical procedures; Robot; Stereotaxic techniques; Intracranial electrode

Achieving a cure for hypothalamic hamartomas: aSisyphean quest?

Achieving a cure for hypothalamic hamartomas: aSisyphean quest?

Accuracy of intracranial electrode placement for stereoencephalography: A systematic review and meta-analysis.

Stereoencephalography (SEEG) is a procedure in which electrodes are inserted into the brain to help define the epileptogenic zone. This is performed prior to definitive epilepsy surgery in patients with drug-resistant focal epilepsy when noninvasive data are inconclusive. The main risk of the procedure is hemorrhage, which occurs in 1-2% of patients. This may result from inaccurate electrode placement or a planned electrode damaging a blood vessel that was not detected on the preoperative vascular imaging. Proposed techniques include the use of a stereotactic frame, frameless image guidance systems, robotic guidance systems, and customized patient-specific fixtures. Using the Preferred Reporting Items for Systematic Reviews and Meta-Analysis (PRISMA) guidelines, a structured search of the PubMed, Embase, and Cochrane databases identified studies that involve the following: (1) SEEG placement as part of the presurgical workup in patients with (2) drug-resistant focal epilepsy for which (3) accuracy data have been provided. Three hundred twenty-six publications were retrieved, of which 293 were screened following removal of duplicate and non-English-language studies. Following application of the inclusion and exclusion criteria, 15 studies were included in the qualitative and quantitative synthesis of the meta-analysis. Accuracies for SEEG electrode implantations have been combined using a random-effects analysis and stratified by technique. The published literature regarding accuracy of SEEG implantation techniques is limited. There are no prospective controlled clinical trials comparing different SEEG implantation techniques. Significant systematic heterogeneity exists between the identified studies, preventing any meaningful comparison between techniques. The recent introduction of robotic trajectory guidance systems has been suggested to provide a more accurate method of implantation, but supporting evidence is limited to class 3 only. It is important that new techniques are compared to the previous "gold-standard" through well-designed and methodologically sound studies before they are introduced into widespread clinical practice.

Speech mapping using depth electrodes: The “electric Wada”

ObjectivesThe Wada test is the gold standard for identification of the side of speech dominance but is associated with risk of angiography and requires patient cooperation. We report a method to verify speech laterality using implanted electrodes in patients undergoing stereo-encephalography (SEEG). Patients and methodsFour pediatric patients with frontal lobe drug-resistant epilepsy underwent implantation of multiple SEEG electrodes using framed stereotaxy to identify the epileptogenic zone, including electrodes placed bilaterally into the pars opercularis of each inferior frontal gyrus from a superior parasagittal entry point. Biphasic stimulation at 50Hz, 0.3ms pulse width, up to 10mA was administered to distal contacts during a reading task. ResultsSpeech arrest upon stimulation was obtained when stimulating distal contacts in the inferior frontal gyrus on one side with no effect on the contralateral side, establishing language laterality in every patient. This information was used to plan subsequent surgical treatment. There were no adverse events of implantation or stimulation. ConclusionsBrain mapping using SEEG electrodes is feasible and can be used to lateralize speech dominance. In cases of frontal lobe epilepsy in which electrodes will be implanted bilaterally for recording of ictal activity, this technique may make the Wada test unnecessary.

Ictal dancing associated with hippocampal sclerosis

Ictal dancing is a very rare seizure semiology. Five cases have been previously reported in the literature [1–4]. Even though one case report investigated with stereo-encephalography (SEEG) showing intracranial ictal EEG change arising from right prefrontal region, particularly right dorsolateral prefrontal area in patients with ictal singing associated with dancing, underlying mechanisms and the neural basis of ictal dancing remain unknown [4]. This report presents one more case of ictal dancing in a temporal lobe epilepsy (TLE) patient with pathologically-confirmed hippocampal sclerosis (HS). A 27-year-old right-handed woman suffered from medically intractable TLE. Her seizure began at the age of 10 and consisted of (1) isolated aura; (2) aura with automotor seizure; and (3) secondarily generalized tonic clonic seizure (GTC). Aura is described as palpitation and fear. Despite treatment with several antiepileptic drugs (AEDs), her seizure persisted on a weekly basis. She also had one episode of simple febrile convulsion at the age of 3. She was admitted to the epilepsy monitoring unit in our center for pre-surgical evaluation. Four habitual seizures were recorded. Three of them were aura with automotor seizure. The other was initially similar but was followed by a GTC. All four seizures consistently started with autonomic (palpitation) and affective (fear) aura then followed by oral and bimanual automotor seizure. During two seizures, different patterns of associated complex motor behavior including acting out a dance move was observed

A novel method for spatial source localization using ECoG and SEEG recordings in human epilepsy patients

In recent decades, there has been increasing interest in surgical treatments of patients with pharmacologically intractable epileptic seizures. In some cases, noninvasive methods do not sufficiently localize epileptogenic foci and invasive methods of presurgical evaluation are necessary. In some patients, craniotomy is required, where-in electrodes are placed directly over the cortex - these recordings are referred to as electrocorticography (ECoG). Another invasive method is based on intra-cortical depth electrodes which are stereotactically steered into deep cerebral structure - these are referred to as stereoencephalography (SEEG). Based on these recordings of electrical potentials at corresponding electrode positions, the spatial location of the sources of epileptic activity in the brain, that is to be lesioned, needs to be estimated. Improving the precision of locating these sources - referred to as source imaging, derived from ECoG and SEEG recordings remains a critical goal in the field [1]. Based on our previous studies [2], we propose a new method - kESI (kernel Electrical Source Imaging), which takes into account realistic brain morphology and spatial variations in brain conductivity. kESI can localize multiple sources, and is flexible to arbitrary electrode positions. Therefore this method can be effectively used for a specific patient's case. The core of kESI is in the construction of kernel functions requiring computation of the potentials generated in the brain by numerous basis functions covering the probed volume. To test the robustness of the method, we generated ground truth data using a simplified spherical brain model - with uniform conductivity, and placed a dipole source inside it (Figure ​(Figure1A).1A). The electrodes were placed on the surface of the sphere, and were also placed inside the spherical volume emulating ECoG and SEEG style recordings respectively. The potentials generated at these electrodes were computed using Finite Element Methods (FEM) in FEniCS software, the mesh was generated in gmsh. In kESI, this FEM model was used to compute the potentials generated by the basis functions, and hence obtain the reconstructed sources. To evaluate the accuracy, the root mean squared difference (RMS) values between the ground-truth and the reconstructed sources from kESI was computed in the orthogonal planes passing through the dipole (Figure ​(Figure1B).1B). This was found to be 0.158 when using 652 electrodes (ECoG & SEEG), and 0.248 when using 100 electrodes (only ECoG) (Figure ​(Figure1C1C). Figure 1 A) Shows the VTK rendering of the simplified 3D brain model, the red-blue spheres indicate the placement of a dipole inside the brain. B) Shows the two orthogonal planes passing through the dipole (red-blue spheres) and origin C) Reconstruction of sources ...

The start and development of epilepsy surgery in Europe: a historical review.

Epilepsy has not always been considered a brain disease, but was believed to be a demonic possession in the past. Therefore, trepanation was done not only for medical but also for religious or spiritual reasons, originating in the Neolithic period (3000 BC). The earliest documentation of trepanation for epilepsy is found in the writings of the Hippocratic Corpus and consisted mainly of just skull surgery. The transition from skull surgery to brain surgery took place in the middle of the nineteenth century when the insight of epilepsy as a cortical disorder of the brain emerged. This led to the start of modern epilepsy surgery. The pioneer countries in which epilepsy surgery was performed in Europe were the UK, Germany, and The Netherlands. Neurosurgical forerunners like Sir Victor Horsley, William Macewen, Fedor Krause, and Otfrid Foerster started with “modern” epilepsy surgery. Initially, epilepsy surgery was mainly done with the purpose to resect traumatic lesions or large surface tumours. In the course of the twentieth century, this changed to highly specialized microscopic navigation-guided surgery to resect lesional and non-lesional epileptogenic cortex. The development of epilepsy surgery in Southern Europe, which has not been described until now, will be elaborated in this manuscript. To summarize, in this paper, we provide (1) a detailed description of the evolution of European epilepsy surgery with special emphasis on the pioneer countries; (2) novel, never published information about the development of epilepsy surgery in Southern Europe; and (3) we review the historical dichotomy of invasive electrode implantation strategy (Anglo-Saxon surface electrodes versus French-Italian stereoencephalography (SEEG) model).

Kernel Electrical Source Imaging � spatial source localization from ECoG and SEEG recordings

Event Abstract Back to Event Kernel Electrical Source Imaging – spatial source localization from ECoG and SEEG recordings Daniel K. Wójcik1* and Hanuma Chaitanya Chintaluri1 1 Nencki Institute of Experimental Biology, Dept. of Neurophysiology, Poland In epilepsy patients with pharmacologically intractable epileptic seizures surgical treatment may be the only solution. Often, noninvasive methods do not sufficiently localize epileptogenic foci and invasive methods of presurgical evaluation are necessary. These may include recordings of extracellular electric potential with depth electrodes, which is called stereoencephalography (SEEG), or with subdural electrodes placed directly over the cortex (electrocorticography, ECoG). Given these recordings one needs to estimate the spatial location of the sources in the brain that are to be lesioned. Improving the precision of localization of these sources from ECoG and SEEG recordings, which is termed electrical source imaging, is a major challenge of the field [1]. Here we propose a new method, kernel Electrical Source Imaging (kESI), which takes into account realistic brain morphology and spatial variations in brain conductivity. The method can localize multiple sources, and is flexible to arbitrary electrode positions and so it can be used effectively for a specific patient's case. The core of the method is in the construction of kernel functions requiring computation of the potentials generated in the brain by numerous basis functions covering the probed volume, which is an extension of our previous work [2]. To show the proof-of-concept we generated dipolar ground truth data inside a simplified spherical brain model with uniform conductivity. We assumed the electrodes on the surface of the sphere and inside the spherical volume emulating ECoG and SEEG style recordings respectively. The potentials generated at these electrodes were computed using Finite Element Methods (FEM) in FEniCS software, the mesh was generated in gmsh. In kESI, this FEM model was used to compute the potentials generated by the basis functions, and hence obtain the reconstructed sources. We could show how different distributions of electrodes affect the quality of reconstruction. This may lead to a procedure for prescribing optimal distributions of electrodes depending on available prior knowledge (e.g. dysfunction of specific brain structures) and clinical resources (availability of specific electrodes, etc). Acknowledgements Research funded from grants FP7-PEOPLE-2010-ITN 264872 NAMASEN, 2948/7.PR/2013/2.

Surgery for Heterotopia: A Second Look

Electroclinical, MRI, and Neuropathological Study of 10 Patients with Nodular Heterotopia, with Surgical Outcomes Tassi L, Colombo N, Cossu M, Mai R, Francione S, Lo Russo G, Galli C, Bramerio M, Battaglia G, Garbelli R, Meroni A, Spreafico R Brain 2005;128:321–337 We present the results of a retrospective study on 10 patients operated on for intractable epilepsy associated with nodular heterotopia, as identified by high-resolution MRI. Seven patients had unilateral heterotopia, one patient had symmetrical bilateral heterotopia, and two patients had asymmetrical bilateral heterotopia. By stereoelectroencephalogram (SEEG; nine patients), interictal activity within nodules was similar in all cases, and ictal activity never started from nodules alone but from the overlying cortex or simultaneously in nodules and cortex. Excellent outcomes (Engel class Ia, 1987) were achieved in the seven patients with unilateral heterotopia, showing that surgery can be highly beneficial in such cases when the epileptogenic zone is carefully located before surgery by MRI and particularly SEEG. For the bilateral cases, surgical outcomes were Engel IIa (one patient) or Engel IIIa (two patients). Histologic/immunohistochemical studies of resected specimens showed that all nodules had similar microscopic organization, even though their extent and location varied markedly. The overlying cortex was dysplastic in nine patients, but of normal thickness. We suggest that nodule formation may be the result of a dual mechanism: ( 1 ) failure of a stop signal in the germinal periventricular region leading to cell overproduction; and ( 2 ) early transformation of radial glial cells into astrocytes, resulting in defective neuronal migration. The intrinsic interictal epileptiform activity of nodules may be due to an impaired intranodular GABAergic system. The Role of Periventricular Nodular Heterotopia in Epileptogenesis Aghakhani Y, Kinay D, Gotman J, Soualmi L, Andermann F, Olivier A, Dubeau F Brain 2005;128:641–651 A temporal resection in patients with periventricular nodular heterotopia (PNH) and intractable focal seizures yields poor results. To define the role of heterotopic grey matter tissue in epileptogenesis and to improve outcome, we performed stereoencephalography (SEEG) recordings in eight patients with uni- or bilateral PNH and intractable focal epilepsy. The SEEG studies aimed to evaluate the most epileptogenic areas and included the allo- and neocortex and at least one nodule of grey matter. Interictal spiking activity was found in ectopic grey matter in three patients, in the cortex overlying the nodules in five, and in the mesial temporal structures in all. At least one heterotopion was involved at seizure onset in six patients, synchronous with the overlying neocortex or ipsilateral hippocampus. Two patients had their seizures originating in the mesial temporal structures only. Six patients had surgery, and the resected areas included the seizure onset, with follow-up from 1 to 8 years. An amygdalohippocampectomy was performed in two (Engel classes Id and III), an amygdalohippocampectomy plus removal of an adjacent heterotopion in two (class Ia), and a resection of two contiguous nodules plus a small rim of overlying occipital cortex in one patient (class Id). One patient with bilateral PNH had three adjacent nodules resected and an ipsilateral amygdalohippocampectomy, resulting in a reduction of the number of seizures by 25 to 50%. The best predictor of surgical outcome is the presence of a focal epileptic generator; this generator may or may not include the PNH. Invasive recording is required in patients with PNH; it improves localization and is the key to better outcome.

The role of periventricular nodular heterotopia in epileptogenesis

A temporal resection in patients with periventricular nodular heterotopia (PNH) and intractable focal seizures yields poor results. To define the role of heterotopic grey matter tissue in epileptogenesis and to improve outcome, we performed stereoencephalography (SEEG) recordings in eight patients with uni- or bilateral PNH and intractable focal epilepsy. The SEEG studies aimed to evaluate the most epileptogenic areas and included the allo- and neocortex and at least one nodule of grey matter. Interictal spiking activity was found in ectopic grey matter in three patients, in the cortex overlying the nodules in five and in the mesial temporal structures in all. At least one heterotopion was involved at seizure onset in six patients, synchronous with the overlying neocortex or ipsilateral hippocampus. Two patients had their seizures originating in the mesial temporal structures only. Six patients had surgery and the resected areas included the seizure onset, with follow-up from 1 to 8 years. An amygdalo-hippocampectomy was performed in two (Engel class Id and III), an amygdalo-hippocampectomy plus removal of an adjacent heterotopion in two (class Ia), and a resection of two contiguous nodules plus a small rim of overlying occipital cortex in one patient (class Id). One patient with bilateral PNH had three adjacent nodules resected and an ipsilateral amygdalo-hippocampectomy resulting in a reduction of the number of seizures by 25-50%. The best predictor of surgical outcome is the presence of a focal epileptic generator; this generator may or may not include the PNH. Invasive recording is required in patients with PNH; it improves localization and is the key to better outcome.

Firing patterns of human limbic neurons during stereoencephalography (SEEG) and clinical temporal lobe seizures

Comparisons of the patterns of neuronal firing and stereoencephalography (SEEG) recorded from the same microelectrodes chronically implanted in the human limbic system were made in order to study neuronal electrogenesis at onset and during propagation of focal partial complex seizures. Alert or sleeping patients were monitored during spontaneous subclinical seizures (no alterations in consciousness detectable), during auras reported by the patients as typical, and during clinical seizures with loss of consciousness, movements and post-ictal confusion. During subclinical SEEG seizures (ipsilateral, normal consciousness), few neurons increased firing (estimated at only 7%) either at the focus or at propagated sites. During auras, with altered consciousness, there were relatively few neurons that increased firing, with the estimate about 14% or twice as many as during a subclinical seizure. During the onset of a clinical seizure that involved loss of consciousness, movements and post-ictal confusion, many neurons were recruited into increased firing, with an estimate of approximately 36%. During this increased electrogenesis, neurons fired briefly in association with high-frequency local SEEG; however, the bursts were shorter than the SEEG seizure pattern. Apparently, other local neurons were recruited to fire in bursts to sustain sufficient axonal driving for widespread propagation of the seizure. When the focal SEEG slowed, the units stopped firing, which suggested that the ‘focal’ seizure need not be sustained for more than several seconds because propagated seizure activity was self-sustaining at distant structures. The data lead to the conclusion that SEEG seizures can be generated focally by synchronous firing of fewer than 10% of neurons in the ‘epileptic pool’. However, when greater percentages of neurons are recruited in the ‘epileptic focus’ there is greater propagation to widespread sites, especially contralaterally, which will produce clinical partial complex seizures.