Abstracts

Rationale: Interictal spike activity is considered as one of the hallmarks of epilepsy. We analyze various parameters of interictal spike components - amplitude, duration and slope in order to define more precisely the localization of the epileptogenic area and their implication in the clinical outcomes. Methods: Three 10-minute interictal intracranial EEG (IC-EEG) samples (one sample per 24-hours, roughly at the same time of the day and during awake, resting phase, and >6 hrs following a seizure) from patients with refractory, non-lesional localization-related epilepsy undergoing 2-stage surgery were analyzed. An automatic spike detection software (Stellate Systems) was used to identify the individual spikes. Each file was manually corrected by a single EEGer to remove artifacts and manually mark the spikes missed by the software. Each electrode was labeled as “seizure onset” (EEG change at beginning of seizure); “seizure spread” (EEG involvement within 10 seconds of onset) or “neither”. Seizure onset and spread together were defined as epileptogenic zone. Spike frequency and quantitative measurements were obtained with automated analysis in Matlab (MathWorks, Detroit). Each spike was divided into two half waves with midpoint being the peak. To calculate the amplitude, duration and slope of each halfwave, 80 milliseconds of data were searched on either side of the peak to find the lowest value in that range, which was designated the spike's trough. Distance to the peak value was used to calculate duration while the difference in voltage was used to calculate amplitude (preceding trough to peak are defined as amp1 and dur1 while peak to succeeding trough are amp2 and dur2). Slope was calculated as amplitude/duration for each half wave (see figure 1). Statistical analysis: To deal with issues of the gross non-normality of the spike variables, tests used were the Spearman correlation coefficient between ranks, and the Kruskal-Wallace test for rank order differences across a categorical variable (i.e., nonparametric equivalent of one-way ANOVA and t-test). Results: A total of 598 electrodes in 7 patients were analyzed. The Spearman correlations between seizure onset zone (onset, spread, neither) and automatic spike detection variables were significant for number of spikes (r = -.22, p < .01), duration 1 (r = .13, p < .01), and slope 1 (r = -.12, p < .01). The Kruskal-Wallis tests were significant for seizure onset by number or spikes (χ2 = 28.84, p < .01; onset and spread were significantly different than neither), duration 1 (χ2 = 9.11, p < .01; onset and spread were significantly different than neither), and slope 1 (χ2 = 9.84, p < .01; onset was significantly different than neither). Conclusions: Quantitative interictal spike parameters suggest that the initial phase of the spike correlates better with the epileptogenic zone with shorter duration and steeper slope, perhaps due to more synchronized neuronal population generating the spikes.