Abstracts

Rationale: Intracranial EEG (iEEG) recordings are routinely used to localize epileptic networks in patients with drug-resistant epilepsy undergoing presurgical evaluation. It is known that there is an immunologic reaction to chronic iEEG implants resulting in changes in overall signal quality(Hanisch, 2002; Liu et al., 1999; Ludwig et al., 2006; Polikov et al., 2005; van Kuyck et al., 2007). However it is unknown whether the interictal and ictal intracranial electrographic biomarkers of the epileptic network that are currently used in clinic practice to determine what brain tissue is resected are stabilized within the typical time frame of intracranial implantation (1-3 weeks). To address the question regarding whether the epileptic network has stabilized during the current gold standard monitoring period we critically evaluated intracranial recordings obtained from chronically implanted iEEG macroelectrodes in dogs with naturally occurring epilepsy.Methods: Four dogs with were implanted with bilateral 8-contact subdural strip electrodes for chronic (range 45-479 days, average 326 days) iEEG recording from a wireless system(Davis et al., 2011). Epileptic bursts were extracted from the continuous iEEG record by thresholding the average line-length feature across all the channels. Burst similarities were determined by calculating the average probability that each pair of events displays time points with similar event dynamics (model described in Wulsin et al., 2013).Results: For the ~5 days after initial iEEG implantation the normalized burst similarity to future bursts is near 0 (figure 1, example in one dog). From ~day 5 after implantation until ~day 21 the normalized burst similarity to future bursts gradually increased. In all dogs the normalized burst similarity stabilized between days 20-25. Figure 1 shows the change in normalized burst similarity in one dog. Similar results were seen in all dogs in this experiment.Conclusions: Our analysis of long-term iEEG macroelectrode recordings from four dogs indicate that signal characteristics change significantly during the acute and subacute phase after electrode implantation during the typical time frame of iEEG implantation in humans. These findings are particularly pertinent in light of the expanding enthusiasm in interictal features such as high frequency oscillations (HFOs). In light of our findings, the reliability of features such as HFOs extracted within the first 4 weeks after implantation must be questioned, as the variability of the spectral properties may be too great during the acute and subacute phases to use such features for clinical decision-making. Preliminary results indicate bursts are similar to seizure onsets and that in addition to bursts stabilizing with time after implant the seizure onset network also changes and stabilizes. Future directions for this research include further investigation into how seizure onsets change with time after iEEG implantation. This works suggests that chronic monitoring during presurgical evaluation may more accurately define the epileptic network and lead to improved seizure freedom after surgery.