Abstracts

Rationale: Patients with normal MRI (non-lesional) and medically refractory extratemporal epilepsy make up a disproportionate number of patients with non-excellent outcomes from epilepsy surgery. We investigated the association between surgical outcome and seizure onset patterns using scalp and intracranial EEG (IEEG) analysis Methods: Forty one consecutive patients (1997-2008) with normal MRI and extratemporal epilepsy who underwent epilepsy surgery were identified. The demographics of patients at the time of surgery, lobar localization, duration of epilepsy, the presence of epilepsy risk factors, seizure semiology, seizure onset patterns, Ictal SPECT, SISCOM and Engel outcome score were abstracted from the Mayo Rochester Epilepsy Surgery Database. .Seizure onset was defined within the first second of a localized, sustained EEG pattern that was visually distinguished from background activity, and ultimately accompanied by objective or subjective clinical manifestations. Ictal onset patterns were characterized by the frequency, morphology and spatial distribution of the seizure discharge. Onset frequency was categorized into EEG frequency bands. Spatial extent was divided into focal or diffuse (involvement of more than one region on scalp EEG or more than five contacts on IEEG). Waveform at seizure onset was divided into three distinct types based on morphology. An Engel class I outcome (free of disabling seizure) was considered as favorable outcome. The duration of follow-up was 1-12 years. Postoperative seizure freedom was determined by Kaplan-Meyer survival analysis. Results: Thirty-six of the 41 patients ultimately had adequate information with minimum of 1 year follow up. Twenty nine (29/36, 80.5%) had frontal lobe epilepsy. The pathology was either nonspecific gliosis or cortical dysplasia (11 patients). A favorable surgical outcome was reported by 49% after 1 year and 35% after 4 years of follow up(Fig 1). The only prognostic factor was seizure onset pattern on either scalp or intracranial EEG. On the scalp EEG, an ictal onset pattern consisting of focal low voltage fast activity was a strong predictor of favorable surgical outcome (83% Vs 25%, P=0.02). In addition, the ictal onset rhythm of beta frequency (>12Hz)or higher was more likely to be associated with a favorable outcome (86%Vs.29%,P=0.03). Similarly, on the intracranial EEG, an ictal onset pattern of low voltage high frequency oscillation was more frequent in patients with good outcome (Fig 2). Finally, the restriction of ictal onset to a focal distribution (5 electrodes or less) was a strong predictor for a better outcome (P=0.03) Conclusions: A focal low voltage high-frequency activity at seizure onset on scalp or intracranial EEG may identify non-lesional extratemporal epilepsy patients who are likely to have a favorable outcome after surgery. Further studies are required to understand the pathophysiology underlying the favorable outcome in these patients