The background EEG is abnormal in all states. Either burst-suppression or multifocal epileptiform discharges with diffuse slowing are mandatory for this syndrome. A burst-suppression pattern composes high voltage bursts (150-300uV) of mixed spikes and sharp-waves, lasting 1-5 seconds, with an inter-burst interval of 3-10 seconds when there is marked suppression (<5uV) of the EEG.
The ictal EEG varies with the seizure type occurring, refer to individual seizure types for more information.