Epileptogenic focus localization through connectivity analysis of the intracranial EEG: A retrospective study in 2 patients

Some patients with refractory epilepsy can benefit from surgical removal of the brain region responsible for causing the epileptic seizures, i.e. the epileptogenic focus. In several cases the patient is admitted for Invasive Video Electroencephalography Monitoring (IVEM) to delineate the ictal onset zone based on intracranial electroencephalographic (IEEG) signals. However, in clinical practice this delineation is based on a visual identification strategy out of the IEEG signals. The purpose of this study is to identify the epileptogenic focus based on the concept of functional brain connectivity. The full frequency Adapted Directed Transfer Function (ffADTF) values were calculated for each seizure of the 2 investigated patients. The mean out-degree, i.e. the sum of outgoing connections, over all seizures for each patient was calculated out of the ffADTF-values. The epileptogenic focus was identified based on the mean out-degree. The localization of the hypothesized epileptogenic focus in the 2 patients was concordant with post-operative findings. We showed that connectivity patterns derived out of the ffADTF analysis can provide useful information about seizure propagation and may improve the accuracy of the presurgical evaluation in patients affected by refractory epilepsy.