Abstracts

Rationale: Surgery for epilepsy is often guided by recordings of seizures on intracranial electrodes. Seizure onset location is usually used as a proxy for the epileptogenic zone which should be removed to eliminate seizures, though its localization is particularly difficult in MRI-negative cases. Moreover, it is not known whether broader area of seizure onset correlates with seizure outcome. In addition, while rapid seizure spread has been associated with poor outcome, metrics of how to measure seizure spread with prognostic accuracy are an area of ongoing research.

Methods: Intracranial recordings from N=19 patients with MRI-negative epilepsy who subsequently underwent an anterior temporal lobectomy were analyzed. All patients had greater than 1 year of follow up after surgery and Engel outcomes assigned to each. 3D reconstructions of the brains and seizure activity were created for qualitative and quantitative analyses. A Hilbert transform was performed to analyze Beta power (13 – 25Hz), which has previously been correlated with seizure activity in TLE for quantitative analyses. Ictal beta power was compared to baseline power for each electrode. The raw EEG was analyzed by an epileptologist and onset electrodes were assigned.

Results: For the 19 patients analyzed, 6 had Engel I outcome and 13 had Engel II or worse outcome at last follow up. On surgical pathology, 4 patients had cortical dysplasia, 9 had gliosis, 5 had hippocampal sclerosis and 1 had evidence of encephalitis. Patients who were Engel I seizure-free had on average fewer electrodes involved at seizure onset (Figure 1A) than patients whose seizures recurred (4.0 vs 8.6; P=0.0087). Number of electrodes involved in spread during the first 20s of seizures (Figure 1B) show a trend toward more electrodes involved in patients whose seizures recur (Figure 1C) vs patients who are Engel seizure free after surgery (Figure 1D).

Conclusions: These results suggest that a broader onset zone may directly correlate with poor postoperative outcome. Quantitative spatial and temporal metrics of seizure spread dynamics and their relationship to postoperative outcome hold promise as evidence-based tools in the guidance of epilepsy surgery including MRI-negative cases.

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