Although scalp EEG is a very useful tool for presurgical evaluation in
epilepsy, the 10-20 system of
electrodes in many cases fails to accurately localize the source of the epileptic
seizures. One suggested
solution to this problem is to use additional
electrodes. Sphenoidal
electrodes especially have been suggested to be helpful in identifying the irritative and seizure onset zones in patients with
temporal lobe epilepsy. However, the value of these
electrodes has been debated, and in many
epilepsy centers they are not used. In this study, we investigate the impact of sphenoidal
electrodes by comparing the results of EEG source localization with and without sphenoidal recordings. We retrospectively selected patients with
temporal lobe epilepsy based on their clinical semiology and electrophysiologic data. For each patient, a prototype spike was used as a template for an automatic pattern search to find similar activities. The identified spikes were then averaged and analyzed by fitting a dipole to the data. The recordings from sphenoidal
electrodes were then excluded and the analysis was repeated. It was found that in more than half of the patients inclusion of sphenoidal
electrodes resulted in a shift of more than 2 cm in the location of the fitted dipole, and in some cases moved the dipole from the frontal lobe or the insula to the temporal lobe. Our results suggest that sphenoidal
electrodes are helpful in the analysis of the EEG recordings of patients suspected of having
temporal lobe epilepsy.