Thermocoagulation of the Ictal Onset Zone Using SEEG (Thermo-SEEG)

Stereo-EEG (SEEG)-guided radiofrequency thermocoagulation (thermo-SEEG), is a stereotactic lesioning procedure in which the SEEG electrodes are used to deliver a radiofrequency current. This technique offers drug-resistant epileptic patients undergoing phase 2 investigations the possibility to benefit from lesions of the ictal onset zone without any additional invasive stereotactic procedure. A SEEG recording site can be considered eligible for thermo-SEEG if bipolar recordings through two adjacent contacts of the same electrode, located in cortical grey matter, show evidence of either spike-wave discharges or low-amplitude fast pattern at the onset of seizures. Electrical stimulations are systematically performed during video-SEEG recording sessions to avoid possible side-effects of a lesion. After 1-year follow-up, 45.6% of patients show greater than 50% improvement in terms of seizure frequency and 11.9% are seizure-free. Moreover, thermo-SEEG is safer than conventional surgery, with only a 3.2% rate of side-effects, which are almost always transient, including the expected ones.

Invasive EEG in Tuberous Sclerosis

This chapter reviews the application of intracranial EEG monitoring and cortical stimulation in the surgical treatment of tuberous sclerosis (TS) patients with uncontrolled epilepsy. It begins with a review of issues related to seizure localization and determination of epileptogenic tubers, followed by a review of surgical series in which intraoperative electrocorticography (ECoG) or extraoperative EEG monitoring with subdural or depth electrodes was utilized. Specific interictal and ictal EEG patterns suggesting intrinsic epileptogenicity of tubers are described, and similarities with focal cortical dysplasia are emphasized. The discussion is illustrated with examples of invasive EEG findings in patients with TS, and their relationship to the centre and rims of epileptogenic and non-epileptogenic tubers, and to perituberal and remote cortex. The chapter provides a comprehensive resource that will help epileptologists and neurophysiologists to decide on the need for invasive EEG, and the significance of findings, in TS patients with uncontrolled epilepsy.

The Irritative Zone and Seizure Onset Zone in Acute ECOG: The Quest for Relevant Epileptogenic Tissue

Acute intraoperative electrocorticography (ECoG) is a time-honoured technique to identify the relevant epileptogenic tissue (RET) and hence guide cortical resection to control medically refractory seizures. ECoG identifies the RET through careful analysis of pattern, morphology, frequency, and localization of interictal spikes recorded directly from the exposed cortical surface. Because the development and dissemination of chronic intracranial EEG recording techniques has put emphasis on ictal recordings (thus defining an ictal onset zone), acute ECoG is often considered unnecessary in surgical planning. The chapter describes limitations and advantages of acute ECoG to define the RET in comparison with more costly and risky procedures, particularly subdural grid and SEEG recording. Specifically, it shows how the integration of lesion type and sequentially recorded ECoG spikes during operation may provide a highly cost-effective approach to successful epilepsy surgery.

Physiological Activity and Artefacts in Epileptic Brain in Subdural EEG

‘Physiological activity and artefacts in epileptic brain in subdural EEG’ reviews intracranial appearances of physiological brain rhythms in each brain region, many of which are also seen on scalp EEG. The alpha rhythm has been described as originating from multiple occipital and extra-occipital cortical generators variously overlapping and influencing each other, probably under the relative control of a central pacemaker. Another more focal pattern has been described in intracranial EEG recordings in the calcarine region, with a third rhythm arising in midtemporal regions, not detectable in scalp EEG, with a frequency in the alpha or theta range. Lambda waves, sleep structures, and mu rhythms over motor cortex can also be detected on subdural electrodes. On a region-by-region basis, intracranial EEG appearances are summarized, including brain oscillations in hippocampus and motor cortex and their modifiers, as well as ongoing rhythms in cingulum. Common sources of physiological and non-physiological artefacts are reviewed.

Depth Electrodes: Approaches and Complications

Depth electrode placement is an invaluable technique in treating patients with refractory epilepsy. Like any neurosurgical operation, planning is the most important phase of the procedure. The seizure focus should first be grossly localized using data from scalp electrodes and seizure semiology. This gross localization will guide placement of invasive electrophysiological hardware. All electrode implantation methods rely on Talairach’s principles of stereotaxis. Traditional electrode implantation is performed with a stereotactic frame. Evolving techniques use frameless stereotaxy or neuroendoscopy for implantation. The most worrisome complication of electrode placement is electrode-associated intracranial haemorrhage. Electrode deviation is a much more common complication, which can be minimized by avoiding extreme insertion angles, minimizing intracranial electrode length, and maximizing entry point accuracy.

Technical and Practical Aspects of Invasive Recordings and Brain Stimulation

This chapter discusses the technical and practical issues involved in invasive recording and cortical stimulation mapping in patients with drug-resistant epilepsy. It reviews the way in which EEG signals are generated, circumstances when intracranial electrodes are needed, and how such electrodes operate. It also discusses the basic principles of cortical stimulation mapping and different methods of using intracranial electrodes for stimulation purposes, and relevant concepts involved in the process such as charge density and electrode surface area. It reviews different electrodes used for mapping including subdural surface electrodes and depth electrodes.

Simultaneous Macro- and Microrecordings

Invasive recordings from patients with epilepsy are increasingly employing new hybrid electrode technologies, which combine macro- and microelectrode electrophysiology, to explore the large scope of brain processes. These range from action potentials of single neurons to coordinated activity of neuronal ensembles to oscillations of widespread brain networks, all buried in oscillations of the extracellular local field potential sampled at various spatiotemporal scales. This chapter reviews various approaches to simultaneous micro- and macroscale electrophysiology, with current electrode designs and applications being discussed together with particular examples of epilepsy pathophysiology. The chapter highlights the advantages, limitations, and challenges for new technologies in the emerging era of human brain mapping projects. It is this technological advance that drives the progress of human intracranial electrophysiology and shifts attention to a vast scale of neuronal networks.

Dynamic Spectral Imaging: Online and Offline Functional Brain Mapping Using High-Frequency Activity [50–150 Hz] in SEEG

At the end of the twentieth century, a handful of research groups discovered that neural processing leaves a characteristic signature in intracranial EEG recordings: an increase of power in a broad frequency range above 50 Hz, dubbed ‘high-gamma’ of high-frequency activity ([50–150 Hz]). Since then, intracranial EEG research on human cognition has focused primarily on high-gamma activity to reveal the large-scale cortical dynamics of most major cognitive functions, not only offline in well-controlled paradigms, but also online, while patients freely interact with their environment. This chapter introduces that approach, including its recent extension to task-induced neural activity suppressions and functional connectivity mapping, and its clinical application to minimize cognitive deficits induced by epilepsy surgery.

Invasive Studies of Sensorimotor Cortex Epilepsy

Intracranial exploration of the sensorimotor cortex exemplifies the balance of preserving function while optimizing the surgical quest for seizure freedom. While surgical intervention in the sensorimotor region is certainly a ‘high-stakes’ undertaking, positive results can be obtained for carefully selected patients. In this chapter, the clinical features of sensorimotor epilepsy are reviewed and the anatomy of this critical functional region is described. The intrinsic rhythms that are commonly seen on intracranial sensorimotor recordings are also discussed. A description of sensorimotor mapping techniques and findings is followed by a discussion of special surgical considerations for patients with sensorimotor cortex epilepsy.

Subdural EEG in Posterior Cortex Epilepsy

Surgery for extratemporal epilepsy surgery provides seizure control or reduction, but its overall results are less rewarding than those for temporal lobe epilepsy. Strategies for planning extratemporal surgery are more challenging and complex. Completeness of resection of the epileptogenic zone (EZ) is the single most important predictor of postsurgical seizure outcome, and therefore precise spatial delimitation of the EZ is at least as important as its localization. Moreover, careful definition and mapping of the functional state of the cortex bordering the EZ is crucial to ensure more precise and less risky surgical procedures. Invasive studies play a central role in the presurgical evaluation of extratemporal pharmacoresistant epilepsies. This chapter describes the rationale supporting the use of subdural electrodes, their role and limitations in definition of the irritative and ictal onset zones, in functional mapping of the eloquent cortex, and especially in planning of the surgical strategy.