Abstracts

Rationale: Objective: To establish that stringent analysis of semiology, scalp EEG and functional imaging data can guide as successful subsequent stereoelectroencephalography (SEEG) evaluation, to successfully localize the putative epileptogenic zone for resection in patients presenting with EEG features suggestive of a generalized epilepsy.Rationale: Surgical resection of putative epileptogenic zone(s) following scalp and intracranial EEG monitoring is widely accepted as effective treatment of non-lesional, medically refractory focal epilepsy. However, generalized spike wave discharges (particularly 3 - 5.5Hz) are most commonly a feature of genetic generalized epilepsies, in whom surgical resections have no role. Occasionally, focal epilepsies can present with ostensibly generalized EEG features because of rapid secondary bilateral synchrony. Failure to recognize this phenomenon may result in focal epilepsy patients being misdiagnosed as generalized epilepsy, thus eliminating the possibility of potentially curative options. We set out to identify features of patients presenting with generalized spike wave discharges resulting from a focal epilepsy, and their response to resective surgery.  Methods: Methods: A comprehensive surgical database was analyzed to identify all patients with generalized inter-ictal or ictal EEG features and no positive findings on MRI 3T scans. We identified 9 patients who met the above criteria. Their demographics, imaging, semiology, scalp EEG, and intracranial EEG data were compiled and analyzed.  Results: Results: We identified 9/370 (2.4%) patients (7 female and 2 male) aged between 20 - 42 years, who underwent resective surgery for focal epilepsy between 2012 - 2018. All patients presented with seizure semiologies consistent with focal epilepsy; four patients had hypermotor seizures and six had versive seizures. All patients had generalized, anterior predominant, 3 - 5.5 Hz spike wave discharges in the interictal state, apart from one patient with 1-2 Hz spike wave frequency, and none had localizable or lateralizable ictal EEG features. Two were previously diagnosed as generalized epilepsy. None had identifiable structural MRI abnormalities. PET showed bilateral findings on all but one patient who had decreased uptake in the left frontal region. All patients underwent bi-hemispheric SEEG implantations based on semiology and functional imaging (in one patient), where possible. Six patients had putative epileptogenic zones localized to the right frontal lobe (2 superior frontal gyrus, 2 prefrontal, 1 premotor, 1 both prefrontal and premotor) and three patients to the left frontal lobe (1 prefrontal, 2 superior frontal gyrus). Following resective surgery, at 6 month follow up, 78% (seven) patients were free of disabling seizures (Engel Ia and Ib), one patient had worthwhile seizure reduction (Engel IIIa), and one patient had no appreciable change post surgery (Engel IVb outcome).  Conclusions: Conclusion: Our study shows that a small subset of patients with focal epilepsy present with EEG features of generalized spike wave discharges, but are potentially treatable with resective surgery with promise of excellent outcomes. Careful analysis of semiology and functional imaging may make the case for SEEG evaluation, which in turn may accurately identify the putative epileptogenic zone. We emphasize that immediate secondary bilateral synchrony probably drives this apparently generalized EEG phenomenon, and electroencephalographers should take the whole patient picture into context, rather than rely mainly on scalp EEG features.  Funding: No funding