Abstracts

Rationale: Drug-resistant epilepsy is often evaluated by intracranial EEG (icEEG) to localize the epileptic focus (1). Risks associated with icEEG are time-dependent, so elucidating the surrogate marker of the epileptogenic zone (EZ), the seizure onset zone, quickly is imperative (2). Recent studies showed marked differences in SPEPs of physiological and pathological cortical tissue (3). Our study examines a novel SPEP-based metric, which is brief compared to recording seizures, suggesting it has significant clinical potential.

Methods: We developed a metric, the connectivity index (CI), based on the number of evoked responses weighted by their distance from the stimulation site (Figure 1). This measure acted as a marker of post-synaptic excitability strength. The goal of the pilot matched case-control study was to compare the metric following EZ vs. non-EZ stimulations—validating it for prospective applications. Existing data were taken from a single institution and analyzed retrospectively. CI was compared to standard of care by correlating values to outcomes and assessing predictive strength. Volumetric analysis was also carried out using quantitative EEG and cortical reconstructions. Volume data for both CI and standard of care were compared with surgical resections via resection ratios. 

Results: Evoked responses from 3019 electrodes in 22 cases were analyzed. The CI of the EZ was higher than for control contacts (median normalized CIs, 0.92 vs. 0.18, p=0.0002) and evoked responses of EZ were seen at further distance than control (median normalized distance, 0.96 vs. 0.62, p=0.0005). A higher non-EZ CI was observed in multifocal or poor outcome cases compared to localized onsets (median CIs, 0.5 vs. 0.12, p = 0.04). Resection ratios of standard EZ and abnormally evoked pathologic 1 to 4 Hz volumes were higher in cases with good outcomes (median ratios in good outcome, 0.52 vs. 0.71 and p0.12 vs. p0.04).

Conclusions: The results indicate that CI may serve as a reliable EZ marker while reducing the time and risk to patients in comparison with the standard of care and is being tested further in our current prospective study.
_x000D_ References:_x000D_ 1. Rosenow F, Luders H. Presurgical evaluation of epilepsy. Brain. 2001;124(Pt 9):1683-1700. doi: 10.1093/brain/124.9.1683_x000D_ 2. Fong JS, Jehi L, Najm I, Prayson RA, Busch R, Bingaman W. Seizure outcome and its predictors after temporal lobe epilepsy surgery in patients with normal MRI. Epilepsia. 2011;52(8):1393-1401. doi: 10.1111/j.1528-1167.2011.03091.x_x000D_ 3. Iwasaki M, Enatsu R, Matsumoto R, Novak E, Thankappen B, Piao Z, O'Connor T, Horning K, Bingaman W, Nair D. Accentuated cortico-cortical evoked potentials in neocortical epilepsy in areas of ictal onset. Epileptic Disord. 2010;12(4):292-302. doi: 10.1684/epd.2010.0334 _x000D_
Funding: No funding was utilized for the conduction of research or the generation of this abstract.