Abstracts

Rationale: Vagus nerve stimulation (VNS) is an approved treatment for the management of partial onset seizures that has been commercially available in Europe since 1994 and in the United States since 1997. Responsive vagus nerve stimulation (RVNS) is a heart rate algorithm driven stimulation approved in 2015 as an augmentation of regular stimulation. The outcomes of RVNS in post-release practice are limited by pre-release data. Therefore, outcome and performance data regarding the effects of stimulation threshold on RVNS activity are needed.  We performed an exploratory review of threshold’s effect on seizure detection and hypothesized based on that review that greater sensitivity (automated, earlier intervention) would increase detections prior to stimulation and applied that hypothesis to a larger, independent data collection. Methods: We reviewed the data from one author’s practice, collecting basic demographic information and files from interrogation of the RVNS devices of 45 patients with at least one RVNS data collection. Of the 45 patients, 33 had one or more magnet activation within the available RVNS detection window with a total evaluable timeframe of 7.8 patient years. Review of the relationship between detection threshold setting and the probability and latency for responsive autostimulations seen prior to a magnet activation cluster (determined to be a marker for seizure occurrence) were calculated.  We determined a positive relationship existed between these features and applied this hypothesis to a larger data set from 153 patients with a total of 25.3 patient years of detection data. Results: The initial query of our center data showed that lower thresholds improved the probability of detection as well as the mean latency of autostimulation prior to magnet activation.  Median pre-magnetic stimulation latencies for thresholds of 20%, 30%, 40%, and 50% were 37s, 96s, 94s, and 57s respectively (n=8, 3, 18, and 3 patients). The reduction in these median times suggested a significant shift at the lowest (most sensitive) threshold setting.  We then queried the larger data set with the hypothesis that we would see a linear relationship with a larger sample size. The relationship in the larger sample size between threshold setting and mean pre magnetic activation latency for thresholds of 20%, 30%, 40%, and 50% was 49s, 64s, 67s, and 66s respectively (n=42, 23, 56, and 6 patients).  Both relationships can be found in Graph 1. The total percentage of magnet events with a preceding autostimulation at thresholds of 20%, 30%, 40%, and 50% was 45%, 42%, 26%, and 22%. Thus, lower thresholds were associated with both higher sensitivity (percent of events detected) and lower mean latency. Conclusions: The relationship between RVNS threshold for detection and pre-magnetic activations suggests lower thresholds improve detection in a linear fashion down to most sensitive setting of 20%. We suggest this relationship suggests the potential value of adjusting thresholds to lowest level to maximize the detection and potential therapeutic benefit.  We hypothesize that lower thresholds could detect and treat more seizures, thus providing improved outcomes for patients with RVNS.   Funding: No funding was received for this work.