Abstracts

Rationale: The primary objective in the presurgical evaluation of epileptic patients, especially the ones who are candidates for resective surgery, is to identify the brain region(s) responsible for generating the patient’s habitual seizures. The visual inspection of the electroencephalogram (EEG) is often subjective and unreliable, and therefore the clinical application of the gold rule for identifying the seizure focus by the region(s) that first electrographically exhibit(s) the seizure onset becomes difficult to impossible. Automated epileptogenic focus localization by mathematical analysis of the interictal EEG holds a theoretical and practical potential to contribute to this direction and therefore to a more accurate diagnosis and treatment of epilepsy. We will present the first algorithm that accomplishes this goal without any subjective electrode or EEG segments selection, or other shortcomings of previously reported algorithms Methods: Transfer Entropy (TE) is a recently proposed measure of the information flow between coupled linear or nonlinear systems. In the past, we have suggested improvements to this measure and its use in detecting the direction and strength of coupling in mathematical models. In this study, we show the application of the improved TE method to long (days), continuous, intracranial EEG recorded at two medical centers (University of Florida’s Shands Hospital and St. Joseph’s Barrow Neurological Institute) from four patients with focal temporal lobe epilepsy (TLE) for localization of their foci. The hypothesis we tested was that the epileptogenic focus acts as the driver of information (source) for the brain sites (sink) that will participate in a seizure activity, over extensive periods of time. All patients underwent successful ablative surgery of their clinically assessed foci. TE profiles per electrode site were estimated from successive, non-overlapping electrocorticographic and depth EEG segments of 10.24 seconds in duration (2048 points per segment at 200 Hz sampling rate) for days of EEG recording per patient. Statistical metaprocessing of the TE values, followed by statistical significance tests, correctly lateralized and localized the epileptogenic focus in all patients. Results: Based on surrogate statistical analysis of the TE profiles, the epileptogenic focus was successfully identified in each of the four patients as the electrode site or set of electrode sites that significantly and most often in the interictal periods drive the rest of the electrode sites. The thus estimated foci were in agreement with the clinically assessed sites of the epileptogenic focus in all patients analyzed. Conclusions: These robust findings suggest that a) the epileptogenic focus is dynamically active for long periods besides during seizures, b) it is possible to reliably localize the focus/foci by information analysis of the interictal EEG, and c) it may be possible to drastically reduce the currently high hospitalization burden associated with long-term EEG monitoring for focus localization (Supported by National Science Foundation Grant No. 0601740.)