Abstracts

The validity of chronic and intraoperative ECoG for guiding resective epilepsy surgery, prediction of epileptic seizures (ES), and preventing and terminating ES by direct brain stimulation (BS) remains uncertain. We suggest that the analysis of ECoG coherence before and during the developing ES may shed light on the cortical dynamics specific to the preictal state, primary and dependent epileptic foci (EF). Previous studies (Chkhenkeli, 1978-2002) demonstrated that the effectiveness of BS to prevent or stop the ES depends on the BS parameters and on ES stage.
One hundred and eighty intractable epilepsy patients were examined with stereotactically implanted intracerebral electrodes (150 patients) or subdural grids (30 patients), with video/ECoG recording, and direct BS of the EF. ECoG inter-electrode coherence was calculated for all possible subdural electrode pairs for each of five frequency bands.
The data demonstrated that the highly synchronized interictal raw ECoG activity consistent with a high level of low frequency band coherence does not always predict the time and place of the seizure onset. Coherence during the interictal state was disrupted, reduced and reorganized in the 5-20 sec before the ES. In cases of multifocal epilepsy, this reorganization of functional connectivity allows us to identify the initiating EF even if it does not reveal itself with overt epileptic activity, and predict the development of focal ES. Appearance of these multiple high coherence areas in the different frequency bands suggests the mutual independence of such foci. A high level of focal beta and gamma band coherence in the initiating EF during the preictal stage of the developing ES is replaced by low frequency coherence. During the advanced stage of ES, both types of coherence, especially in beta and gamma bands, progressively decrease and disappear. There was a discrepancy between the [ldquo]hidden[rdquo] intracortical relationships of the brain areas participating in the ES development and raw EEG or ECoG, suggesting synchronized activity of diverse areas.
The data demonstrate that determination of the cortical EF of high frequency coherence may predict the time and locus of focal ES development. The existence of two types of coherence: a [ldquo]conditioning[rdquo] preictal high frequency focal coherence in the initiating EF and a [ldquo]maintaining[rdquo] low frequency coherence observed at the beginning of ES with its progressive decrease suggests that BS parameters for the prevention or termination of an ES may have to be different. Presurgical and intrasurgical evaluation of these events may be helpful for resective surgery or for the prevention of ES development with therapeutic BS.
[Supported by: NIH grant NS40514]