Abstracts

Rationale: Presurgical workup occasionally reveals focal, concordant data in primary generalized epilepsy which may lead to intracranial video-EEG (IVEEG). If the IVEEG confirms the primary generalized nature of the epilepsy, no surgery is indicated. Recent research has indicated that cross-frequency coupling (CFC) may be a marker of epileptogenic cortex. We hypothesized that primary generalized epilepsy that mimics focal epilepsy expresses a distinct CFC signature when compared to true focal seizures.Methods: 150 children underwent IVEEG from 2004-2015 at The Hospital for Sick Children (Toronto, Canada). Only 4 out of 150 patients had EEG and semiology suggestive of absence-like seizures. In these 4 patients, scalp video-EEG, MEG, MRI, and IVEEG were reviewed. CFC was calculated for each electrode in each patient using ictal IVEEG data during absence-like seizures. Briefly, CFC measures the degree to which the amplitude of a faster brain rhythm is modulated by the phase of a slower rhythm.Results: The four patients with absence seizures were divided into two groups; two with atypical absence seizures (AA) and two with typical absence seizures (TA). One TA patient had a focal lesion on MRI suggestive of focal cortical dysplasia. MEG dipoles were localized in the unilateral frontal lobe in two TA patients and one AA patient, and were lateralized diffusely in one hemisphere in one AA patient. One AA patient and one TA patient had lateralized PET hypometabolism. Two AA patients had epileptic spasms, whereas two TA patients had only absence seizures during both scalp video-EEG and IVEEG. Two TA patients had generalized 3 Hz spike-and-wave complexes that were activated by hyperventilation. Two AA patients had generalized 1.5-2.5 Hz slow-spike-and-wave discharges that were not activated by hyperventilation. Two AA patients underwent multi-lobar resection based on the consistently focal distribution of ictal high frequency oscillations (HFO) on the IVEEG. One AA patient has been seizure free for 2 years. The other AA patient eventually underwent a functional hemispherectomy due to recurrence of periodic spasms and has now been seizure free for 4 years. Two TA patients did not undergo resective surgery due to inconsistent localization of seizure onset zone on IVEEG. CFC showed high HFO amplitude modulation that was significantly elevated in the resected region compared to non-resected tissue in two AA patients (p<0.05), whereas CFC showed scattered non-focal coupling in two TA patients.Conclusions: Our data indicate that CFC may be used to distinguish between primary generalized epilepsy mimicking focal epilepsy and surgically remediable focal epilepsy, Further, our data suggest that in TA patients, even in the presence of focal findings on presurgical evaluation, the presence of generalized 3 Hz spike-and-wave complex that is induced by hyperventilation suggests that IVEEG is not indicated.