Abstracts

Rationale: Functional connectivity analyses have been identified as potentially helpful in delineating epileptogenic regions using resting state data, with the assumption that regions with dysfunctional connectivity correlate with areas of seizure onset and spread (Stufflebeam et al., 2011, Kramer et al. 2010). However, surgical decisions are currently made using visually based characterizations of intracranial EEG (iEEG) data. In this project, we attempted to compare iEEG connectivity measures of the visually identified seizure onset zone with all other implanted electrodes, with the goal of confirming and possibly uncovering other regions that may contribute to the initiation and spread of seizure activity. Methods: Intracranial EEG (iEEG) data was identified and extracted from a patient who had previously undergone three-stage epilepsy surgery. We computed the zero-lag phase-locking value (PLV) using previously described methods (Lemieux et al. 2011) for each electrode, using electrodes within the visually identified seizure-onset zone as reference. The phase-locking value was computed for signals between 4-50 Hz, for each pair of time series, using 50 ms time windows with a frequency resolution of 1.0 Hz. All EEG and data analysis was completed using BESA Research software (version 6.0, copyright 2012 Gmbh) and Microsoft Excel. Electrodes were color-coded for levels of connectivity to the seizure-onset zone, with red indicating the highest levels of connectivity (90-100% data points within the time x frequency analysis at significance). Results: The connectivity map for our test patient displays regions of high connectivity (>90%) to the visually identified seizure onset zone, in both the inferior frontal and inferior parietal regions. Blue and purple areas reflect lower connectivity (10-20%). Initial resection involved the frontal region, including the red colored electrodes, but the patient continued to have seizures arising from the inferior parietal region, necessitating a third stage resection, encompassing the red-colored electrodes in the inferior parietal region. Patient has been seizure-free for approximately 1 year post-surgery. Conclusions: Connectivity mapping using the visually identified seizure onset zone can potentially be a practical and powerful confirmatory technique for delineating areas of high functional connectivity with seizure onsets. In our example patient, resection of only one region with high functional connectivity did not stop the seizures, whereas resection of both highly connected regions was sufficient for making the patient seizure free. This technique can provide the opportunity for highlighting brain regions that escape visual identification of seizure onset, but are nonetheless significantly connected to seizure onset areas, and are likely part of the epileptogenic zone.