Authors :
Presenting Author: Vishal Pandya, MD – Medical College of Wisconsin
Christophe Jouny, PhD – Johns Hopkins University School of Medicine
Gregory Bergey, MD – Johns Hopkins University School of Medicine
Rationale:
The RNS System enables chronic intracranial EEG monitoring in patients with evidence of multifocal epilepsy. The inpatient epilepsy monitoring unit (EMU) is utilized as a standard diagnostic tool to determine uni or multifocality of seizures and potential surgical candidacy. This environment is inherently artificial as patients are often rapidly tapered off anti-seizure medications (ASMs) and stressed in a manner that is not reflective of the conditions under which they ordinarily experience seizures. This may lead to unmasking of foci that are not drug resistant or misleading predominance of focality.
The RNS system allows for long term outpatient monitoring in a typical environment. ASM adjustment is individualized. A report by Hirsch et al in 2022 found that long term RNS assessment identified unilateral predominance in a substantial number of patients thought to be poor surgical candidates due to the degree of multifocality found on EMU evaluation. The purpose of our study was to assess if the seizure laterality ratio in patients with bitemporal lobe epilepsy remains stable over time. Secondarily, we sought to determine if seizure clustering influences conclusions about laterality. This information may assist in determining patients who may be good surgical candidates. Methods:
Fourteen patients were identified from our center who were implanted with bilateral hippocampal depth electrodes utilizing the RNS system. Date of implant ranged from 6/2016 to 12/2022. All ictal events for the first year after implantation were reviewed for each patient. The time, date, and laterality of each seizure was recorded. Clustering was operationally defined as within 3 hours or 24 hours of a prior seizure. Laterality ratios with and without clustering were computed at 3, 6, 9, and 12 month timepoints both as separate epochs and in a cumulative fashion.
Results:
Two patients had exclusively right sided seizures and one patient recorded no seizures during the first year of monitoring. The remaining 11 patients had a mixture of left and right onset seizures Lateral predominance for each patient was estimated by the cumulative ratio at 12 months. Eight patients (72%) had their lateral predominance correctly identified within 3 months. Nine patients (81%) had their lateral predominance correctly identified within 6 months. Exclusion of seizure clusters did not change the number of patients with correctly assessed laterality at 3 or 6 months. Clustering did impact individual laterality ratios. For example, Patient 9 demonstrated nearly equivalent laterality when using all seizures but was clearly left side predominant when seizure clusters were excluded as right onset seizures tended to cluster more often.
Conclusions:
Our data suggests that the lateral predominance of seizures in bitemporal lobe epilepsy is largely stable over time. The effects of clustering on assessment of laterality need to be considered on an individual basis. The RNS system is a valuable tool for assessing focal seizure laterality in bi-temporal lobe epilepsy outside of the artificial environment of EMU and may impact future considerations of surgical candidacy.
Funding: No funding support was received in support of this abstract.