Abstracts

Rationale: Accurate localization of a seizure onset zone remains a critical goal when evaluating patients for epilepsy surgery. Surgery to remove the seizure onset zone often provides the greatest hope for seizure freedom for patients with drug-resistant focal epilepsy. Cross-frequency coupling between lower and higher frequency bands, originally thought to be an indicator of cognitive processing, is also found prominently during ictal events. Studies have shown that during seizures, it is possible to distinguish different areas, named the "ictal core" from the penumbra, using cross-frequency coupling. Differentiating the ictal core from areas of seizure propagation could give a more precise assessment of the seizure onset zone. Methods: Phase-locked high gamma index (PLHG) is computed from the relation between the high gamma band (80-150Hz) and the lower theta band (4-8Hz). We compared propagations maps using PLHG and matching pursuit based Gabor atom density (GAD), Higuchi fractal dimension (HFD) and line-length (LL) activity detectors. Records from 4 consecutive patients with intractable partial epilepsy undergoing presurgical evaluation with intracranial recordings since 2012 at the Johns Hopkins Epilepsy Center were selected for this study. Data was recorded at 10kHz using a Nihon Kohden™ system. EEG files were saved in MEF file format. Events analyzed included spontaneous sub-clinical seizures, and focal seizures with or without secondary generalization. Results: Line length is used as an indicator of ictal propagation and epileptiform activity. While all methods are able to show the ictal event onset, we use PLHG propagation maps to identify the ictal core as a subset of the ictal onset zone. In all four patients, PLHG is more specific and only shows activity in the ictal core while LL, GAD and HFD show higher levels also during propagation. Identification of the ictal core is here based on the seizure onset assessment made by physicians. Each method provides specific information regarding onset and propagation characteristics using respective algorithms. While GAD is especially sensitive in the complexity of high frequency content, LL is more sensitive to the maximum frequency of the signal envelope. PLHG is sensitive to the co-occurrence of the lower theta driving activity with high frequency components. Therefore, periods lacking one or the other show no increase in their PLHG index. Conclusions: Phase locked high gamma activity provides new insights into the propagation pattern of focal seizure onset that can help assess the true ictal core of the seizure onset zone. Correlation of the ictal core identification may ultimately provide superior information compared to clinical electro-corticography interpretation, and improve clinical outcomes using cross-frequency coupling.