Abstracts

Rationale: Two staged approach using intracranial EEG (iEEG) monitoring with subdural grid (SDG) electrodes is an established technique to identify the "epileptogenic zone" in patients with medically refractory focal epilepsy (MRE). The SDG placement is guided by several pre-surgical tests (MRI, PET scan, Neuropsychological testing, Wada test, ictal and interictal SPECT scan). Despite recent advances in non-invasive techniques, many remain difficult to lateralize with such methods. We use a minimally invasive stereotactic method to insert bilateral depth electrodes (stage 1) over wide areas to record stereo EEG (sEEG) for the lateralization of seizures. The results of sEEG guides subsequent placement of SDG (stage 2) which can further localize the epileptogenic zone for focal resection (stage 3). We present data of such three-staged approach for patients with MRE. Methods: We performed a retrospective review of all epilepsy surgeries between January 2012 and May 2016. All patients with lesional epilepsy (including hippocampal sclerosis) were excluded. Patients who had non-lesional MRE and underwent sEEG recording to lateralize seizure onset were selected for further review. The following information was analyzed; age, sex, duration of epilepsy, prior AED and other methods of treatment, ability of sEEG to localize or lateralize, surgical complications, and epilepsy surgery outcome. Results: Seven patients underwent bilateral minimally invasive depth electrode placement for sEEG monitoring to help lateralize seizure onset. Four patients were male and the average epilepsy duration was 16 years (st dev 11 years), average frequency of seizures was 6.4 per month (st dev 1.6), average number of current AEDs were 3, with average of 8.5 AEDs failure prior, with 3 also failing VNS therapy. Average number of depth electrodes inserted were 10.7 (st dev 2.9). None of the patients had complications from sEEG. Five of the patients (83.3%) were successfully lateralized and advanced to unilateral iEEG monitoring with SDG leading to focal resection. Both patients who did not advance to two stage surgery secondary to failure to localize despite capturing seizures remain intractable. Of the four patients who underwent cortical resection (1 is scheduled for surgery), 2 had Engel class I outcome at 2 years, one had Engel class IV initially but improved to Class I later, and one patient died from SUDEP (first seizure occurring 3 months after resection). Conclusions: In non-lesional, neocortical MRE patients with conflicting pre-surgical data on lateralization of seizure foci, a three-staged approach described above can be helpful in successful lateralization and later localization of epileptogenic zone. SEEG recording using multiple bilateral depth electrode placed with minimally invasive technique is safe. Such approach can lead to seizure free outcomes in individuals otherwise deemed unsuitable for resective epilepsy surgery. A larger series and longer follow-up is needed to validate this approach. Funding: None.