Brain stimulation for epilepsy--local and remote modulation of network excitability.

BackgroundThe use of Deep Brain Stimulation (DBS) as a potential therapy for treatment resistant epilepsy remains an area of active clinical investigation. We recently reported the first chronic evaluation of an implantable, clinical-grade system that permits concurrent stimulation and recording, in a large animal (ovine) model developed to study DBS for epilepsy.ObjectiveIn this study we extended this work to compare the effects of remote (anterior thalamic) and direct (hippocampal) stimulation on local field potential (LFP) activity and network excitability, and to assess closed-loop stimulation within this neural network.MethodsFollowing anesthesia and 1.5T MRI acquisition, unilateral anterior thalamic and hippocampal DBS leads were implanted in three subjects using a frameless stereotactic system. Chronic, awake recordings of evoked potentials (EPs) and LFPs in response to thalamic and hippocampal stimulation were collected with the implanted device and analyzed off-line.ResultsConsistent with earlier reports, thalamic DBS and direct stimulation of the hippocampus produced parameter-dependent effects on hippocampal activity. LFP suppression could be reliably induced with specific stimulation parameters, and was shown to reflect a state of reduced network excitability, as measured by effects on hippocampal EP amplitudes and after-discharge thresholds. Real-time modulation of network excitability via the implanted device was demonstrated using hippocampal theta-band power level as a control signal for closed-loop stimulation.ConclusionsThe results presented provide evidence of network excitability changes induced by stimulation that could underlie the clinical effects that have been reported with both thalamic and direct cortical stimulation.