Abstracts

Rationale: Localization of the epileptogenic focus from interictal periods remains a difficult problem to address in patients with focal epilepsy. Due to the involved ambiguities in assessment of the seizure onset zone (SOZ), patient discomfort and overly cost at the EMU, development of alternative robust and efficient ways to localize the focus has been more than desirable. We will present results from a new promising methodology for focus localization that we recently developed and applied to our first set of patients with temporal and frontal lobe epilepsy during their first hours of recovery from general anaesthesia following the implantation of the EEG electrodes and before tapering of their AEDs.   Methods: The first set of 5 patients with focal epilepsy were recruited at the U. Alabama Birmingham site and consented for participation in our joint new NSF sponsored and IRB approved study on “Probing and Understanding the Brain: Micro and Macro Dynamics of Seizure and Memory Networks”. The implantation of depth electrodes was image-guided, robot-assisted (ROSA) and under general anesthesia using Propofol. Post-implantation patients were extubated and transferred to neuro ICU. The depth electrodes were connected to the Natus Xltek EEG machine and 120 EEG channel recording at 2KHz was initiated (Π period). We employed the Generalized Partial Direct Coherence (GPDC), a measure of directed causal interactions between brain sites over low (LFB: 0.1-50Hz) and high (HFB: 70-110Hz) frequency bands and a step of 0.1 Hz. The GPDC values were generated from a 7th order, 120-dimensional multivariate autoregressive model fitted to 100 sec successive and non-overlapping EEG segments for 1 hour in the Π period. We then estimated the percentage of time (POT) that each site showed the maximum cumulative incoming GPDC values from the other sites (effective inflow of information) [1].  Results: The results of this analysis per electrode site, over low and high frequency bands, from one of our patients with left temporal epilepsy are shown in Figure 1.  The clinically assessed focus in this patient in the HFB (right panel) coincided with the one from our analysis (left posterior hippocampus). In the remaining 4 patients with temporal and frontal lobe epilepsy, brain sites in the assessed SOZ also coincided with the largest POT in the HFB during period Π (see Table 1).  Conclusions: The above results suggest that it is possible to accurately localize the focus early on in the post-surgical electrode implantation period Π in patients with focal epilepsy from: a) short (just 1 hour) periods before tapering of AEDs, and b) the high frequency EEG band. Finally, the performance of focus localization analysis may be explained by compartmentation of the brain networks under anesthesia [2].  References:                                                            [1] I. Vlachos, B. Krishnan, D. Treiman, K. Tsakalis, D. Kugiumtzis, L.D. Iasemidis, IEEE Trans. Biomed. Eng. (in press)[2] L.D. Lewis et al., PNAS, E3377–E3386, Nov. 5, 2012. Funding: This study was supported by NSF grant OIA 1632891.