Stereo-EEG Implantation Strategy

EEG Source Imaging in Presurgical Evaluations

SUMMARY

Presurgical evaluations to plan intracranial EEG implantations or surgical therapies at most epilepsy centers in the United States currently depend on the visual inspection of EEG traces. Such analysis is inadequate and does not exploit all the localizing information contained in scalp EEG. Various types of EEG source modeling or imaging can provide sublobar localization of spike and seizure sources in the brain, and the software to do this with typical long-term monitoring EEG data are available to all epilepsy centers. This article reviews the fundamentals of EEG voltage fields that are used in EEG source imaging, the strengths and weakness of dipole and current density source models, the clinical situations where EEG source imaging is most useful, and the particular strengths of EEG source imaging for various cortical areas where spike/seizure sources are likely.

Invasive Epilepsy Monitoring: The Switch from Subdural Electrodes to Stereoelectroencephalography

Stereoelectroencephalography (SEEG) has experienced an explosion in use due to a shifting understanding of epileptic networks and wider application of minimally invasive epilepsy surgery techniques. Both subdural electrode (SDE) monitoring and SEEG serve important roles in defining the epileptogenic zone, limiting functional deficits, and formulating the most effective surgical plan. Strengths of SEEG include the ability to sample difficult to reach, deep structures of the brain without a craniotomy and without disrupting the dura. SEEG is complementary to minimally invasive epilepsy treatment options and may reduce the treatment gap in patients who are hesitant about craniotomy and surgical resection. Understanding the strengths and limitations of SDE monitoring and SEEG allows epileptologists to choose the best modality of invasive monitoring for each patient living with drug-resistant seizures.

Safety and efficacy of bihemispheric sampling via transmidline stereoelectroencephalography

OBJECTIVE

Stereoelectroencephalography (SEEG) is designed to target distributed cortical networks responsible for electroclinical seizure syndrome and to enable localization of the site of seizure onset in patients with intractable epilepsy. When the preimplantation hypothesis invokes the bilateral mesial frontal lobes, sampling of several deep-seated cortical sites in both hemispheres is required. In this study, the authors have demonstrated the feasibility of sampling bihemispheric areas with intentional implantation of an SEEG electrode crossing the midline (SECM) for sampling the cortex on both sides of the interhemispheric fissure.

METHODS

An analysis of 231 consecutive SEEG procedures over 8 years was used to identify instances of bihemispheric sampling by using the transmidline SEEG technique.

RESULTS

The authors identified 53 SEEG cases, with a total of 126 electrodes that crossed the interhemispheric fissure; all were in the frontal lobes. Eighty-three electrodes targeted the cingulate gyrus (18 rostral, 43 anterior, and 22 middle), 31 targeted the posterior orbitofrontal region, 8 sampled the medial prefrontal cortex, and 4 targeted nodular heterotopia around the frontal horns. The ictal onset zone was localized to the frontal lobe in 16 cases. SECM isolated interictal and ictal activity in the contralateral hemisphere in 6 cases and independent bihemispheric seizure activity in 2 cases. No hemorrhagic or infectious complications were noted in any of these cases.

CONCLUSIONS

Based on this extensive experience of bihemispheric sampling, the authors concluded that this technique is safe and effective. In this series, SECM showed contralateral interictal and/or ictal epileptiform activity in 8 (15%) cases, and 9 (16%) cases (with unilateral implantation) had sufficient data to discard contralateral involvement, contributing to support of the epileptogenic network. SECM may reduce the number of electrodes used to sample bilateral mesial frontal or orbitofrontal cortices, and such an approach may lower the risk of hemorrhage and costs.

Stereo-electroencephalography (SEEG) in pediatric epilepsy: Utility in children with and without prior epilepsy surgery failure

BACKGROUND

When noninvasive modalities fail to adequately localize the seizure onset zone (SOZ) in children with medically refractory epilepsy, invasive interrogation with stereo-electroencephalography (SEEG) or subdural electrodes may be required. Our center utilizes SEEG for invasive monitoring in a carefully selected population of children, many of whom have seizures despite a prior surgical resection. We describe the cohort of patients who underwent SEEG in the first 5 years of its employment in our institution, almost half of which had a history of a failed epilepsy surgery.

METHODS

We retrospectively reviewed the records of the first 44 consecutive children who underwent SEEG at Nicklaus Children's Hospital (Miami, Florida), a large, level 4 epilepsy referral center. Patient demographic, clinical, radiographic, and electrophysiological information was collected prospectively. Student's t-test was used for sampling of means and analysis of variance (ANOVA) for evaluation of variance beyond 2 means; chi-square test of independence was used to assess the relationship between categorical variables.

RESULTS

There were 44 patients in this cohort, of whom 17 (38.6 %) were male. The mean age of seizure onset was 6.2 years. Twenty-one patients (47.7 %) had previously failed an epilepsy surgery. Patients with a history of prior epilepsy surgery failure were older at SEEG implantation (17.6 vs. 13.7 years; p = 0.043), were more likely to have SEEG for identification of resection margins (9 vs. 4; p = 0.034), and had fewer electrodes placed (5.9 vs. 7.5; p = 0.016). No difference was seen in complication rates between groups with only 3/297 electrodes placed associated with complications, all of which were minor. Post-SEEG, 29 (65.9 %) patients underwent focal resection, 7 patients had VNS insertion, 3 underwent RNS placement, and 5 had no further intervention. The majority of patients that underwent resection in both groups experienced an improvement in seizures (Engel class I-III), reported by 13/15 (86.7 %) in those naive to surgery and 10/14 (71.4 %) in those with prior surgical failure. Seizure-freedom was much lower in those with prior epilepsy surgery, seen in only 4/14 (28.6 %) versus 8/15 (53.3 %).

CONCLUSION

Our data supports current literature on SEEG as a safe and effective method of electrophysiological evaluation in children naive to surgery and adds that it is a safe technique in children with a history of failed epilepsy surgery. There was no difference in complication rates, which were <1 % in both groups. A favorable outcome was seen in the majority of patients in both groups; the seizure freedom rate, however, was much lower in those with prior epilepsy surgery.

Epilepsy Surgery: Monitoring and Novel Surgical Techniques

Drug-resistant epilepsy warrants referral to an epilepsy surgery center for consideration of alternative treatments including epilepsy surgery. Advances in technology now allow for minimally invasive neurophysiologic monitoring and surgical interventions, approaches that are attractive to families because large craniotomies and associated morbidity are avoided. This work reviews the presurgical evaluation process and discusses the use of invasive stereo-electroencephalography monitoring to localize seizure onset zones. Minimally invasive surgical techniques are described for the treatment of focal and generalized epilepsies. These approaches have expanded our capacity to palliate and cure epilepsy in the pediatric population.

Extracranial Interictal and Ictal EEG in sEEG Planning

Analysis of scalp electroencephalogram (EEG) findings is indispensable to investigation of epilepsy surgery candidates. Maxima of slowing and epileptiform spiking on interictal EEG reflect gross localization of core epileptogenic regions within a network. Important negative scalp EEG findings are those associated with deep foci. Ictal EEG is important in confirming concordance with interictal EEG and other ancillary data. Generalized interictal and ictal EEG findings may occur in epilepsies that are otherwise focal. Detailed individual analyses of scalp EEG features are prelude to a more global synthesis, whose coherence in suggesting plausible network hypothesis presage a subsequently successful scalp EEG evaluation.

Interpretation of the Intracranial Stereoelectroencephalography Signal

The intracranial electroencephalogram (iEEG) is essential in decision making for epilepsy surgery. Although localization of epileptogenic brain regions by means of iEEG has been the gold standard for surgical decision-making for more than 70 years, established guidelines for what constitutes genuine iEEG epileptic activity and what is normal brain activity are not available. This review provides a summary of the current state of knowledge and understanding on normal iEEG entities and variants, the effects of sleep on regional and lobar iEEG, iEEG patterns of interictal and ictal epileptic activity and their relation to well-described epileptogenic pathologies and surgical outcome.

Robotically Steered Needles: A Survey of Neurosurgical Applications and Technical Innovations

This paper surveys both the clinical applications and main technical innovations related to steered needles, with an emphasis on neurosurgery. Technical innovations generally center on curvilinear robots that can adopt a complex path that circumvents critical structures and eloquent brain tissue. These advances include several needle-steering approaches, which consist of tip-based, lengthwise, base motion-driven, and tissue-centered steering strategies. This paper also describes foundational mathematical models for steering, where potential fields, nonholonomic bicycle-like models, spring models, and stochastic approaches are cited. In addition, practical path planning systems are also addressed, where we cite uncertainty modeling in path planning, intraoperative soft tissue shift estimation through imaging scans acquired during the procedure, and simulation-based prediction. Neurosurgical scenarios tend to emphasize straight needles so far, and span deep-brain stimulation (DBS), stereoelectroencephalography (SEEG), intracerebral drug delivery (IDD), stereotactic brain biopsy (SBB), stereotactic needle aspiration for hematoma, cysts and abscesses, and brachytherapy as well as thermal ablation of brain tumors and seizure-generating regions. We emphasize therapeutic considerations and complications that have been documented in conjunction with these applications.

Stereoelectroencephalography in Pediatric Epilepsy Surgery

Stereoelectroencephalography (SEEG) is an invasive technique used during the surgical management of medically refractory epilepsy. The utility of SEEG rests in its ability to survey the three-dimensional organization of the epileptogenic zone as well as nearby eloquent cortices. Once concentrated to specialized centers in Europe and Canada, the SEEG methodology has gained worldwide popularity due to its favorable morbidity profile, superior coverage of deep structures, and ability to perform multilobar explorations without the need for craniotomy. This rapid shift in practice represents both a challenge and an opportunity for pediatric neurosurgeons familiar with the subdural grid approach. The purpose of this review is to discuss the indications, technique, and safety of long-term SEEG monitoring in children. In addition to reviewing the conceptual and technical points of the diagnostic evaluation, attention will also be given to SEEG-based interventions (e.g., radiofrequency thermo-coagulation).

A modern epilepsy surgery treatment algorithm: Incorporating traditional and emerging technologies

Epilepsy surgery has seen numerous technological advances in both diagnostic and therapeutic procedures in recent years. This has increased the number of patients who may be candidates for intervention and potential improvement in quality of life. However, the expansion of the field also necessitates a broader understanding of how to incorporate both traditional and emerging technologies into the care provided at comprehensive epilepsy centers. This review summarizes both old and new surgical procedures in epilepsy using an example algorithm. While treatment algorithms are inherently oversimplified, incomplete, and reflect personal bias, they provide a general framework that can be customized to each center and each patient, incorporating differences in provider opinion, patient preference, and the institutional availability of technologies. For instance, the use of minimally invasive stereotactic electroencephalography (SEEG) has increased dramatically over the past decade, but many cases still benefit from invasive recordings using subdural grids. Furthermore, although surgical resection remains the gold-standard treatment for focal mesial temporal or neocortical epilepsy, ablative procedures such as laser interstitial thermal therapy (LITT) or stereotactic radiosurgery (SRS) may be appropriate and avoid craniotomy in many cases. Furthermore, while palliative surgical procedures were once limited to disconnection surgeries, several neurostimulation treatments are now available to treat eloquent cortical, bitemporal, and even multifocal or generalized epilepsy syndromes. An updated perspective in epilepsy surgery will help guide surgical decision making and lay the groundwork for data collection needed in future studies and trials.

French guidelines on stereoelectroencephalography (SEEG)

Stereoelectroencephalography (SEEG) was designed and developed in the 1960s in France by J. Talairach and J. Bancaud. It is an invasive method of exploration for drug-resistant focal epilepsies, offering the advantage of a tridimensional and temporally precise study of the epileptic discharge. It allows anatomo-electrical correlations and tailored surgeries. Whereas this method has been used for decades by experts in a limited number of European centers, the last ten years have seen increasing worldwide spread of its use. Moreover in current practice, SEEG is not only a diagnostic tool but also offers a therapeutic option, i.e., thermocoagulation. In order to propose formal guidelines for best clinical practice in SEEG, a working party was formed, composed of experts from every French centre with a large SEEG experience (those performing more than 10 SEEG per year over at least a 5 year period). This group formulated recommendations, which were graded by all participants according to established methodology. The first part of this article summarizes these within the following topics: indications and limits of SEEG; planning and management of SEEG; surgical technique; electrophysiological technical procedures; interpretation of SEEG recordings; and SEEG-guided radio frequency thermocoagulation. In the second part, those different aspects are discussed in more detail by subgroups of experts, based on existing literature and their own experience. The aim of this work is to present a consensual French approach to SEEG, which could be used as a basic document for centers using this method, particularly those who are beginning SEEG practice. These guidelines are supported by the French Clinical Neurophysiology Society and the French chapter of the International League Against Epilepsy.