Abstracts

Rationale: Brain-responsive neurostimulation with the RNS® System provides a treatment option for patient with refractory left mesial temporal lobe epilepsy who are not candidates for surgical resection.  Most institutions will place RNS electrodes into the hippocampus (depth electrode) and over the temporal neocortex (subtemporal strip). Our alternative strategy utilizes an orthogonal approach to individually target entorhinal cortex and hippocampus (depth electrode).   Our methodology offers a unique way to differentially sample these two distinct structures for inter-ictal activity and seizure initiation.  In addition, this approach affords us the possibility of studying hippocampus and entorhinal cortex individually with respect to cognition as recent studies had argued that these areas have specific roles in processes such theta oscillation and memory encoding (Buzsaki et al, Nature Neuroscience 2015; Vass et al, Neuron 2016).The aims of our study are to determine whether there is a significant difference in epileptiform activity between entorhinal cortex and hippocampus and how this activity changes over time with RNS stimulation.  We also compared clinical outcomes in our patients with those reported in neurostimulation literature for refractory mesial lobe epilepsy. Methods: Ten consecutive temporal patients treated with the RNS System at University of California, Los Angeles (UCLA) were analyzed.   Out of these, four had electrodes implanted unilaterally and all four had electrodes in the left temporal lobe.  All of these patients had invasive monitoring prior to RNS implantation which lead to the implantation of the depth RNS electrodes into the hippocampus and the entorhinal cortex.  Changes in detection patterns over time were studied and individual ECoGs were compared to data obtained during invasive monitoring prior to implantation.  Patient reported seizure outcomes were compared to those reported in the literature. Results: Four consecutive patients with refractory left temporal epilepsy were implanted with hippocampal (Hip) and entorhinal cortex (EC) depth electrodes at UCLA. Three of these patients had the implant for 16, 29 and 33 months respectively, and all had a 75% reduction in seizure frequency and severity. One of the four patients had an implant for 6 months and had reported a 65% reduction in seizures at the last clinic visit.  These findings are significantly better with regards to seizure reduction as compared to published controls implanted with hippocampal depth and subtemporal strip which showed a 42-55% in 2 years and 70% reduction in 6 years.  In addition, our data indicates that interictal and ictal activity, as picked up by individual Hip and EC electrodes occurred simultaneously only part of the time; more often it was picked up by only one of the two electrodes.   This finding was surprising as data obtained during invasive monitoring showed simultaneous seizure onset in both electrodes. Conclusions: Our findings suggest that implantation of the entorhinal cortex in addition to the hippocampus provides to an additional benefit in terms of reducing seizure frequency and severity.  It is also likely that stimulation of the entorhinal cortex will lead to improvement in memory and cognition but additional studies are needed to answer this question Funding: None