Rationale: The responsive neurostimulator (RNS) system is an FDA-approved treatment for focal-onset refractory epilepsy. Some patients who have already been implanted with the RNS system may undergo scalp EEG monitoring for various reasons including diagnosing new neurobehavioral events or further surgical evaluation. The purpose of this study was to characterize the correlation between intracranial recordings from the RNS system with simultaneous scalp EEG. Methods: This was a retrospective chart review of patients presenting to Stanford Hospital from March 2019 to March 2020. Patients who had continuous scalp EEG recording at some point after RNS implantation were included in the study. Data collected included age and sex, epilepsy type, presence of lesion, indication for scalp EEG monitoring, monitoring durations and findings, RNS electrode implantation date and electrode locations. Number of seizures observed on scalp EEG was compared to simultaneous long episodes during the same recording session. Conversely, seizures recorded by RNS were compared to scalp EEG findings. The presence of RNS artifact was noted. Timing of seizure onset intracranially was also compared to the first changes on scalp. Results: Eight patients with nine continuous EEG recordings (eight with video), totaling to 42 days of monitoring, were included in the study. The cohort included four females and four males. Age ranged from 26-75 years old (median 44 IQR 34-46). Five patients had lesion on MRI (63%). Localization of epilepsy included left temporal (n=2), right temporal (n=2), bitemporal (n=2), and right parietal (n=2). Year of RNS implant ranged from 2014-2019. Locations of RNS electrodes included bilateral mesial temporal depths (n=3), mesial temporal depth + cortical strip (n= 2). Median length of monitoring was 5 days (IQR 3-6). Four patients did not have seizures on scalp EEG. Of those with seizures on scalp EEG, number of seizures ranged from 2-14. An outlier patient had 10 seizures per hour on scalp EEG. Interrogation of long episodes during scalp EEG recording ranged from 6-17 per record (median 11.5, IQR 9.8-16). One patient had insular seizures on RNS, but scalp EEG was non-localizing. Electrographic onsets of seizures as recorded by RNS preceded onsets on scalp EEG. RNS stimulation artifact was noted in 6 records. Conclusions: We describe a cohort of patients with concurrent intracranial EEG based on the RNS system and scalp EEG recordings. A large portion of cases did not show clear electrographic changes on scalp EEG to correlate with RNS-recorded seizures. Furthermore, seizure-related changes on scalp EEG lagged those recorded intracranially. Our findings support the notion that scalp EEG can be limited for detecting seizures. Funding: Please list any funding that was received in support of this abstract.: None
