Abstracts

Rationale: In patients with temporal lobe or temporal 'plus' epilepsy with no lesions on MRI, the target for surgical resection is not always clear and outcomes have been poor. In these patients, a carefully planned intracranial EEG(iEEG) study is increasingly recognized as an essential component of the presurgical evaluation. In this study, we visually reviewed ictal and interictal patterns on iEEG studies performed in non-lesional patients diagnosed with temporal or temporal 'plus' epilepsy.Methods: Our retrospective study reviewed the adult epilepsy surgery database at the Medical College of Wisconsin from 2003 to 2013 and identified 38 patients with Video-EEG findings and/or semiology suggestive of temporal lobe or temporal plus epilepsy. All our patients had no corresponding lesions on MRI. The representative samples of iEEG recordings performed as part of the pre-surgical evaluation were visually reviewed. Ictal and interictal patterns including the presence of spikes and high frequency oscillations (HFOs) were identified. These features were correlated to good surgical outcomes (Engel I) and histopathologies. Sampling rates were 200 Hz in 8 patients, 500 Hz in 5 patients and 1000 Hz in 25 patients.Results: A total of 141 intracranial ictal events were reviewed with a mean of 3.71 ( 2.79) seizures per patient. The mean age of seizure onset was 15.94 ( 10.34) years and the mean epilepsy duration was 15.57 ( 10.29) years. Female/male gender ratio was 18/20. Left handedness was noted in 5 (13.15%) patients. Among our patients, 31(81.57%) were classified as temporal lobe epilepsy, and 7(18.42%) as temporal plus epilepsy. Interictal patterns: 21/38 (55.26%) patients had both interictal spikes and HFOs. Of these, 11/21 (52.38%) showed broad regional interictal HFOs and 9 of the 11 patients underwent surgery with Engel I outcomes in 7 (77.77%). In 13/38 (34.21%) patients, spikes were observed, but no HFOs. In 10/13 (76.92%) patients, broad regions of spiking were seen and included extratemporal areas or bilaterally. A total of 8 patients with broad spiking underwent resection with Engel I outcome in 4/8 (50%). Ictal patterns: 23/38(60.52%) patients had ictal HFOs followed by rhythmic spiking onset. Resection was performed in 20/23 patients and 16/20 (80%) patients had Engel I outcomes. Spiking onset of seizures was observed in 11/38 (28.94%) cases. Of the 11 patients, 8 underwent resection with Engel I outcomes in 6/8 (75%). Pathology: 15/24 (62.5%) of the patients who had HFOs at ictal onset or interictal and underwent resection showed cortical dysgenesis in the pathology result. In the sampling rate of 1000 Hz subgroup, 6/7(85.71%) showed cortical dysgenesis.Conclusions: In our study, ictal onset characterized by high frequency oscillations has high correlation with cortical dysgenesis in pathology. Limited sampling rate of less than 1000 Hz prior to 2008 decreased our ability to review visually discernible HFOs. Broad network of spiking interictals is correlated with lower rate of seizure freedom post resection.