Abstracts

The aim of this study was to investigate whether Independent Component Analysis (ICA) can be applied in intracranial recordings to analyze interactions between the temporal lobes during temporal lobe seizures. ICA has already been used in scalp EEGs to isolate spikes from background and to help lateralize temporal lobe seizures Intracranial recordings of temporal lobe seizures from 8 patients with bitemporal implantation were analyzed. Scalp EEG recordings showed that the patients had seizures originating from both temporal lobes. The seizures of each patient were classified in four categories: unilateral right or left (UR or UL), and bilateral with right or left onset (BR or BL). ICA decomposes the EEG into as many statistically independent components as there are channels. ICA was applied to the seizures, and only components with ictal activity in the first 10 seconds of the seizure were studied. Correlation coefficients were calculated between these components and every channel of the original EEG, in the time interval preceding the appearance of bilateral activity. Components were classified as unilateral if their correlation was greater than 0.2 exclusively with channels in one hemisphere; if the correlation was greater than 0.2 with channels in both hemispheres, the component was labeled as bilateral. Nonparametric statistics were applied. 46 seizures were analyzed (11 UR, 13 UL, 12 BR and 10 BL). There were five patients with seizures beginning independently in both temporal lobes, two patients with right temporal seizures only, and one patient with left temporal seizures only. Even though the correlation was calculated when seizure activity was unilateral, the proportion of components that were bilateral was significantly lower (p=0.001) in seizures that stayed unilateral (22% for UR and 26% for UL) than in seizures that became bilateral (46% for BR and 48% for BL). There were no significant differences between right and left seizures. Bilateral seizures were significantly longer than unilateral seizures (p=0.000), but the duration of the seizure was not significantly correlated (p[gt]0.05) with the proportion of bilateral components. In patients with bitemporal epilepsy, more than 20% of the components extracted using ICA have a bitemporal distribution even at the time when the seizures are apparently unilateral. The proportion of bilateral components during the unilateral phase is significantly higher in seizures with subsequent contralateral spread. It therefore appears that minimal contralteral seizure activity is present even when the discharge appears unilateral, and that the more such contralateral activity is present, the more the seizure is likely to spread. (Supported by CIHR Grant # MOP 38079
Postdoctoral research grant from the Department of Education of the Basque Government.)