No abstract

Vagus nerve stimulation (VNS) is an adjunctive treatment for refractory epilepsy in patients who are unsuitable candidates for epilepsy surgery (Ben-Menachem 2002). Worldwide, more than 50 000 epilepsy patients have been treated with VNS. Several studies, including two large double-blind randomized clinical trials (Ben-Menachem et al. 1994;DeGiorgio et al. 2000), have confirmed the efficacy of VNS in different types of epilepsy. Seizure reduction as a result of VNS ranges from 25% to 55%, and varies considerably from patient to patient. In responders, VNS causes either a rapid or a delayed reduction in seizure frequency. However, a significant fraction (approximately one third) of patients do not respond to VNS. Because the mechanism of action of VNS in epilepsy is currently unknown, it is not clear which factors determine the patient's response to the treatment, nor what the most optimal stimulation parameters are.The vagus nerve is a mixed nerve consisting of 20% efferent (motor) and 80% afferent (sensory) fibers. The nucleus of the solitary tract receives the largest number of vagal afferents. The nucleus of the solitary tract in turn Received July 5, 2010; revised manuscript received January 18, 2011; accepted February 8, 2011.Address correspondence and reprint requests to Robrecht Raedt, Ghent University Hospital, De Pintelaan 185, 9000 Ghent, Belgium. E-mail: robrecht.raedt@ugent.be 1 These authors contributed equally to this work.

Conventional gel electrodes are widely used for biopotential measurements, despite important drawbacks such as skin irritation, long set-up time and uncomfortable removal. Recently introduced dry electrodes with rigid metal pins overcome most of these problems; however, their rigidity causes discomfort and pain. This paper presents dry electrodes offering high user comfort, since they are fabricated from EPDM rubber containing various additives for optimum conductivity, flexibility and ease of fabrication. The electrode impedance is measured on phantoms and human skin. After optimization of the polymer composition, the skin-electrode impedance is only ∼10 times larger than that of gel electrodes. Therefore, these electrodes are directly capable of recording strong biopotential signals such as ECG while for low-amplitude signals such as EEG, the electrodes need to be coupled with an active circuit. EEG recordings using active polymer electrodes connected to a clinical EEG system show very promising results: alpha waves can be clearly observed when subjects close their eyes, and correlation and coherence analyses reveal high similarity between dry and gel electrode signals. Moreover, all subjects reported that our polymer electrodes did not cause discomfort. Hence, the polymer-based dry electrodes are promising alternatives to either rigid dry electrodes or conventional gel electrodes.

SUMMARYPurpose: Fifteen percent to 25% of patients with refractory epilepsy require invasive video-electroencephalography (EEG) monitoring (IVEM) to precisely delineate the ictal-onset zone. This delineation based on the recorded intracranial EEG (iEEG) signals occurs visually by the epileptologist and is therefore prone to human mistakes. The purpose of this study is to investigate whether effective connectivity analysis of intracranially recorded EEG during seizures provides an objective method to localize the ictal-onset zone. Methods: In this study data were analyzed from eight patients who underwent IVEM at Ghent University Hospital in Belgium. All patients had a focal ictal onset and were seizure-free following resective surgery. The effective connectivity pattern was calculated during the first 20 s of ictal rhythmic iEEG activity. The outdegree, which is reflective of the number of outgoing connections, was calculated for each electrode contact for every single seizure during these 20 s. The seizure specific out-degrees were summed per patient to obtain the total out-degree. The electrode contact with the highest total out-degree was considered indicative of localization of the ictal-onset zone. This result was compared to the conclusion of the visual analysis of the epileptologist and the resected brain region segmented from postoperative magnetic resonance imaging (MRI). Key findings: In all eight patients the electrode contact with the highest total out-degree was among the contacts identified by the epileptologist as the ictal onset. This contact, that we named "the driver," always laid within the resected brain region. Furthermore, the patient-specific connectivity patterns were consistent over the majority of seizures. Significance: In this study we demonstrated the feasibility of correctly localizing the ictal-onset zone from iEEG recordings by using effective connectivity analysis during the first 20 s of ictal rhythmic iEEG activity.

Improvements in European epilepsy surgery over time are modest but significant, including higher surgical volume, shorter disease duration, and improved postsurgical seizure outcomes. Early referral for evaluation is required to continue on this encouraging trend.