Long-Term Intracranial EEG Lateralization of Epileptogenicity in Patients With Confirmed or Suspected Bilateral Mesial Temporal Lobe Onsets During Epilepsy Surgical Evaluation.

Purpose: The data resulting from epilepsy surgical evaluation are occasionally unclear in cases of mesial temporal lobe (MTL) epilepsy. Long-term intracranial EEG (iEEG) collected by the Responsive Neurostimulation (RNS) System may be an approach for capturing additional seizure data while treating patients with neurostimulation. We reviewed iEEG seizure lateralization and clinical outcomes in bilateral MTL patients at University of Utah.

Methods: Long-term RNS System iEEG seizure lateralization was compared with pre-RNS System lateralization obtained during surgical evaluation. Safety and clinical outcomes were extracted retrospectively from patient records.

Results: Twenty-six patients received an RNS System with bilateral MTL leads. Fifteen of the patients had adequate follow-up to report clinical outcomes ( > 1 year), and 25 patients had enough recorded data ( > 6 months) to perform iEEG analysis. Median percent reduction in clinical seizures at last follow-up was 58%, and 40% reported being seizure-free at last follow-up, for variable durations. The electrographic seizure lateralization (unilateral vs. bilateral) differed between surgical evaluation and long-term iEEG in 44% of our patients. In the subset of eight patients (32%) who had only unilateral seizures recorded during surgical evaluation, but were implanted with bilateral MTL leads based on bilateral interictal epileptiform discharges, 62% (5/8) had bilateral seizures recorded on long-term iEEG. Interestingly, in the 18 patients who had bilateral seizures recorded during surgical evaluation, 28% (5/18) were found to be unilateral on long-term iEEG.

Conclusions: Our data suggest that RNS System implantation in suspected bilateral MTL cases may be an option to assess a patient's true seizure lateralization on long-term iEEG. Responsive neuromodulation should be considered before resection or ablation in cases that have evaluation data suggesting bilaterality.