Graphic representation of the EEG during epileptic seizures

In seizures starting gradually, careful examination of the background is required to determine the time and region of onset. We propose a method for displaying EEG variables which characterize seizure onset: changes in amplitude and in average frequency compared to background. These variables are displayed topographically for each EEG channel so that 1 or 2 min of EEG may be viewed on a standard screen, thus representing pre-ictal and ictal periods. We evaluated the ability of this display to indicate seizure onset, by comparing the times and regions of seizure onset indicated by traditional EEG and by the display. The comparison was performed on 30 seizures from scalp recordings and 49 seizures from depth recordings. Seizures were selected for having a gradual onset with no or minimal artefact. Onset times from traditional EEG and the computer method, examined independently, coincided (within 3 sec) in 77% of scalp seizures and 63% of depth seizures. In 14% of seizures the computer display indicated an onset more than 3 sec earlier than visual examination; upon reexamination, however, this onset was thought to be possibly correct. This quantitative display, which in practice should be used with traditional EEG, may thus be useful for computer-assisted seizure interpretation and for a condensed representation of seizures.