Abstracts

Rationale: Focal seizures are considered to be focal because of their localized onset. However, our concept of the focus and even if there is a focus, is being increasingly questioned due to a variety of imaging and physiological data which suggests that there may be greater heterogeneity in activity both inside and outside of the traditionally defined seizure onset zone than expected. To further explore the behavior of areas outside of the presumptive focus we analyzed intracranial EEG data from patients with temporal lobe epilepsy. Methods: Continuous intracranial EEG monitoring was used to localize seizure onsets for patients with longstanding pharmaco-resistant epilepsy. In this study, we analyzed data from five such patients who were implanted with bilateral depth electrodes and also had at least one focal temporal lobe seizure. Fourier analysis was used to quantify changes in power over delta, theta, alpha, beta, gamma, and high gamma frequency bands in regions distant from the area of seizure onset. Seizure onset was determined by clinical electroencephalographers as the first electrographic change in particular channels. Results: We analyzed data from 24 seizures in 5 patients. In 9 of those seizures there was an increase in power in the theta and/or alpha band in channels widely separated from the seizure onset zone which preceded the electrographically determined seizure onset by anywhere between 10 - 165 sec. As an example, in one seizure which began in the right mesial temporal structures, there was increasing activity in the left posterior temporal and subfrontal cortex 66 sec before seizure onset. Conclusions: Our results suggest that despite being "focal" in origin, seizure initiation may involve network interactions among disparate brain regions. However, it is also possible that there are non-specific, non-ictal increases in brain activity across a number of cortical and subcortical structures, with only the pathological region likely to convert these changes in activity into epileptic seizures. Furthermore, these changes may only reflect state changes (although obvious changes from sleep to wake or visa versa were not present at the time of seizure onset). More fine-grained analysis of the temporal ordering of these changes in neuronal activity is needed to select between these or other possibilities and help extend and refine our understanding of focality in localization related epilepsy. This work was funded by grants from the NIH (Grant NS062092) and the Epilepsy Foundation (Grant 222178).