Abstracts

RATIONALE: Previous studies have shown that nonlinear analysis of intracranial activities (1) as well scalp EEG recordings (2) can identify a pre-ictal phase several minutes before partial seizures. These studies were carried out on time intervals of a few hours before the seizure. Here we utilize an anticipation method based on a measure of synchronization between several combinations of electrode pairs and we evaluate its sensibility and specificity over long-term continuous intracranial recordings.
METHODS: We analyzed continuous epochs of intracranial EEG, ranging from 6 to 15 days for 5 patients. The data were obtained from EEG-video recording sessions of patients suffering from intractable neocortical epilepsy. The instantaneous phase was extracted for consecutive windows of 5 seconds by the Hilbert transform. Each frequency range was analyzed to cover the whole part of the spectrum. The strength of synchrony between pairs of channels was statistically characterized by the Shannon entropy of the phase differences. For each subject, the detection threshold values were adjusted to avoid spurious fluctuations due to changes of normal interictal activity.
RESULTS: A decrease in synchrony was observed to be specific of the period before seizure onset. These changes were mostly observable in the alpha (8-12 Hz) and beta 1 band (12-20 Hz) and showed reproducible spatial patterns which were more pronounced over the surrounding regions containing the epileptogenic focus. The time course of the preictal changes allowed to anticipate 80% of the seizures with a mean anticipation time of 25 minutes. Some synchrony changes were induced in the interictal dynamics by transition between normal brain states but were below the anticipation threshold.
CONCLUSIONS: The present results suggest that the loss of synchronization is unique to the period prior to seizure and is sensitive enough to anticipate the ictal onset. We hypothesize that the preictal phase reflects the activity of a specific population of neurons that are not fully incorporated in normal synchronization processes and define a state of increased susceptibility for pathological synchronization which acts as a route to the seizure.
1) Martinerie J, Adam C, Le Van Quyen M, Baulac M, Clemenceau S, Renault B, Varela FJ. Epileptic seizures can be anticipated by non-linear analysis. Nature Medicine 1998; 4: 1173-1176.
2) Le Van Quyen M, Martinerie J, Navarro V, Boon P, D[scquote]Havé M., Adam C, Renault B, Varela F, Baulac M. Anticipation of epileptic seizures from standard EEG recordings. The Lancet 2001; 357: 183-188.