Abstracts

Rationale: Epilepsy surgery is an effective treatment for many patients with drug resistant epilepsy. Patients undergoing intracranial monitoring as part of their presurgical workup can experience a transient decrease in their seizure frequency. In rare cases, patients achieve permanent or years-long seizure freedom without subsequent surgery. A similar phenomenon has been documented after implantation of neuromodulation devices such as responsive neurostimulation (RNS). Herein, we describe a series of patients who achieved extended periods of seizure freedom following implantation of RNS, but prior to activation of stimulation.

Methods: We retrospectively reviewed electronic medical record charts of all patients who underwent RNS implantation at a level 4 epilepsy center in New York City. We identified patients who experienced the implantation effect, defined as clinical seizure freedom for a period of at least 4 months following RNS implantation and prior to the initiation of stimulation. If a patient met inclusion criteria, the following data were collected based on chart review: sex, duration of epilepsy, epilepsy type, epilepsy etiology, monthly seizure frequency prior to RNS, age at implantation, location of RNS leads, duration of seizure freedom without stimulation, the detection of an electrographic seizure on RNS Electrocorticography (ECoG) prior to the first clinically evident seizure and number of clinical seizures since RNS placement.

Results: A total of 183 patients underwent placement of the RNS device between July 2014 and April 2025. Fifteen patients (8%) experienced clinical seizure freedom for a period of at least 4 months following RNS implantation and prior to the initiation of stimulation. Among these 15 patients, 9 (60%) were male. Epilepsy type was focal in 10, generalized in 3, and combined focal and generalized in 2. Mean age at implantation was 28.3 years and mean epilepsy duration at time of implantation was 18.5 years. Epilepsy etiology was structural in 3, genetic in 3 and unknown in 9. Mean pre-implantation seizure frequency was 8.9 seizures per month. Mean length of follow up was 37.5 months. Mean period of seizure freedom after RNS placement but prior to stimulation was 15.9 months (range of 4 to 40 months). The mean reduction in monthly seizure frequency after RNS placement was 86% at most recent follow up. Four patients experienced an electrographic seizure on RNS ECoG prior to their first clinical seizure. In 8 patients (53%), there was at least one thalamic RNS depth electrode placed and in 6 of these 8 there were bilateral thalamic electrodes placed.

Conclusions: At our academic center, 8% of patients who received RNS experienced a period of seizure freedom lasting at least 4 months after implantation, even before stimulation was initiated. On average, this seizure-free interval lasted nearly 16 months. Many of these patients had thalamic electrodes placed, suggesting a potential role for thalamic targeting in mediating this implantation effect. These findings highlight a subset of patients who may benefit from the effects of RNS implantation alone, warranting further investigation into mechanisms and predictors of this phenomenon.

Funding: None.