Abstracts

Rationale: Deep brain stimulation has been used in an increasing frequency to treat refractory epilepsy. Many aspects of DBS still need further understanding, although DBS has been shown to be effective in reducing seizure frequency in well controlled randomized double blind studies. We have used the intraoperative EEG pattern obtained after high or low frequency as a potential biomarker for good outcome during chronic stimulation. In this paper, we explore the behavior of entropy during DBS procedures for epilepsy. Methods: Two adult patients with temporal lobe epilepsy who were submitted to unilateral or bilateral implantation of hippocampal electrodes were studied. One patient had left temporal lobe epilepsy, mesial temporal sclerosis, and no significant preoperative memory deficit (unilateral implant); the second one had bilateral temporal lobe epilepsy and right mesial temporal sclerosis (bilateral implant). Both patients were operated under general anesthesia. The anesthesia's level was monitored by BIS and entropy was monitored using specialized software (GE). After electrode positioning in the hippocampus, low frequency stimulation was performed using 4.0V, 300usec and 6Hz; high frequency stimulation was performed using 4.0V, 300usec and 130Hz. Results: Low frequency stimulation led to localized recruiting responses over the ipsilateral temporal neocortical projection. High frequency yielded no visible EEG changes. High frequency stimulation led to interictal spike disappearance. BIS showed no anestesia's level modification and no arousal during stimulation. Low frequency stimulation led to 30-40% increase in entropy; entropy at least double in value during high frequency stimulation during each of the three stimulation sessions, and returned to normal 30 minutes after stimulation was turned off. Conclusions: Seizures are associated with a sudden reduction of entropy. Entropy increase might be related to an anticonvulsant effect of hippocampal DBS and might represent a useful biomarker for good outcome in the future.