Acute vagus nerve stimulation does not suppress spike and wave discharges in genetic absence epilepsy rats from Strasbourg.

We evaluated the efficacy of vagus nerve stimulation (VNS) in Genetic Absence Epilepsy Rats from Strasbourg (GAERS), a validated model for absence epilepsy. In the first experiment, we investigated whether VNS applied at seizure onset can interrupt spike and wave discharges (SWD). In the second experiment, we investigated whether SWD are suppressed or shortened in duration when VNS is applied several hours per day. Both control and VNS groups underwent EEG and VNS electrode implantation. For the first experiment, a randomized crossover design was used. Stimuli (amplitude: 3 V; frequency: 30 Hz; pulse duration: 500 micros) were given when an SWD occurred on the EEG. The experiment was repeated the next day. In the second experiment, treated animals were stimulated (amplitude: 1.5 mA; frequency: 30 Hz; pulse duration: 500 micros; on/off time cycle: 30 s / 5 min) for 3h per day, during five consecutive days. In the first experiment, the duration of the SWD was increased on day 1, (P < 0.05). There was no difference in SWD duration on Day 2. In the second experiment, no significant differences could be found in number, duration and EEG frequency of SWD. VNS applied at the onset of an SWD can prolong the duration of SWD in GAERS. As a 5-day stimulation protocol had no effect, long-term VNS might be necessary to affect SWD.

Ictal source localization in presurgical patients with refractory epilepsy.

Source localization of epileptic foci using ictal spatiotemporal dipole modeling (ISDM) yields reliable anatomic information in presurgical candidates. It requires substantial resources from EEG and neuroimaging laboratories. The profile and number of patients who may benefit from it are currently unknown. The purpose of this study is to demonstrate the clinical usefulness of source localization in a prospectively analyzed series. One hundred patients (51 male and 49 female patients) with mean age of 31 years (range, 2 to 63 years) and mean duration of refractory epilepsy of 20 years (range, 1 to 49 years) were enrolled consecutively in a presurgical protocol. Ictal EEG was available in 93 patients. ISDM was performed when suitable ictal EEG files were available. The clinical applicability of ISDM was examined in three patients groups: 37 patients in whom ictal EEG recording and MRI were congruent (group I), 30 patients in whom results were not completely congruent but not incongruent (group II), and 26 patients in whom the results were incongruent (group III). ISDM could be performed in 31 of 100 patients: 11 in group I, 8 in group II, and 12 in group III. ISDM influenced decision making in none of the patients in group I, in 4 of 8 patients in group II, and in 10 of 12 patients in group III. Typically, the results of ISDM directed avoiding intracranial EEG recordings in what appeared to be unsuitable candidates for resection by clearly confirming the incongruency between ictal EEG and MRI findings. In this series of 100 presurgical candidates, ictal source localization could be performed in 31% of patients. In 14% of patients, it proved to be a key element in the surgical decision process.

Comparison of performance of spherical and realistic head models in dipole localization from noisy EEG.

The performance of a three-shell spherical head model versus the performance of a realistic head model is investigated when solving the inverse problem with a single dipole, in the presence of noise. This is evaluated by calculating the average dipole location error for 1000 noisy scalp potential sets, originating from the same test dipole and having the same noise level. The average location errors are obtained utilizing a local linearization, which is validated with a Monte-Carlo simulation. When the difference between the average location error utilizing a spherical and a realistic head model, represented by deltaR, is large for a large number of test dipoles, then it is worth using the more computationally demanding realistic head model. However, if deltaR is small for a large number of test dipoles, then it does not matter which model is used. For 27 electrodes, an electroencephalogram (EEG) epoch of one time sample and spatially white Gaussian noise, we found that the importance of the realistic head model over the spherical head model reduces by increasing the noise level. We further found that increasing the number of scalp electrodes from 27 to 44 has limited impact on the importance of the realistic head model over the spherical head model in EEG dipole source analysis. By increasing the number of time samples to six, the performance of the realistic head model in the inverse calculation gains importance compared with the three-shell spherical head model. Finally, we used spatially and temporally correlated background EEG instead of Gaussian noise. The advantage of the realistic head model over the spherical head model is reduced when applying correlated noise compared to Gaussian noise.