Abstracts

Rationale: The data resulting from surgical evaluation is occasionally unclear in cases of mesial temporal lobe (MTL) epilepsy. If all data suggests unilateral MTL seizures, resective or ablative surgery is often recommended given that the seizure freedom rate is close to 60%^1,2. If a patient has bilateral MTL seizures, surgery is not a viable option due to the risk of memory deficits³ and neuromodulation may be the preferred treatment. However, the evaluation data collected is sometimes discordant and the epilepsy monitoring unit (EMU) stay may be too brief to capture stereotypical seizure lateralization^4,5. In these cases, intracranial electroencephalography (iEEG) collected by the RNS may be an approach for capturing additional seizure data while treating the patient with responsive stimulation. The iEEG data can also inform a possible future surgery⁶. We reviewed the RNS seizure lateralization and clinical outcomes in bilateral MTL patients from our center.

Methods: All patients received the RNS System with bilateral MTL depth leads at University of Utah. Demographics, surgical evaluation and clinical seizure count data were obtained retrospectively from charts. In patients with > 6 months of follow-up, change in clinical seizure frequency was calculated by comparing baseline to last follow-up frequencies. All iEEG records were reviewed to determine the time to record bilateral seizures and the laterality of the seizures.

Results: 20 patients received RNS with bilateral MTL leads over a 3-year period (2018 - 2021) with a median follow-up of 22 mos (mean: 22, range: 2-54). Demographics, evaluation details, rationale for RNS, seizure lateralization and clinical outcomes are in Table 1. 95% (19/20) had intracranial monitoring prior to RNS. 2 patients had previously failed epilepsy surgery and 5 had been implanted with VNS. 85% (17/20) of the patients had adequate follow-up to report outcomes, 47% (8/17) of those patients were responders (>50% reduction). This is consistent with RNS pivotal trial data⁷. Seizures were captured on long-term iEEG for all of our patients. 65% (13/20) had bilateral seizures recorded in a median of 20 days (mean: 51, range: 1-268). In a subset of 40% (8/20) that had only unilateral seizures recorded during surgical evaluation but were implanted with bilateral MTL depths because of bilateral interictal epileptiform discharges, 63% (5/8) had bilateral seizures recorded on long-term iEEG. Interestingly, 3 patients that had bilateral seizures recorded during surgical evaluation were found to be unilateral on long-term iEEG.

Conclusions: The RNS System allowed us to gather long-term bilateral iEEG data in our MTL patients which differed from our surgical evaluation seizure lateralization in 40% of our patients.

References:
1.Wiebe, S. et al. NEJM, 2001
2.Wu, C. et al. Epilepsia, 2019
3.Scoville, W. Neuropsy Clin Neurosci, 2000
4.Jehi, L. et al. JNS, 2010
5.King-Stephens, D. et al. Epilepsia, 2015
6.Hirsch, L. et al. Epilepsia, 2020
7.Morrell, M. et al. Neurology, 2011

Funding: Please list any funding that was received in support of this abstract.: None.