The background EEG is abnormal in all states, with a suppression-burst pattern. High voltage bursts (150-300uV) of spikes or sharp and slow waves, lasting 1-3 seconds are seen with inter-burst intervals of 3-5 seconds.
The EEG may evolve to a hypsarrhythmia pattern in children who evolve to have West syndrome.
With tonic seizures there is diffuse attenuation of the suppression-burst background pattern with emergence of low-voltage, high frequency activity. Focal ictal rhythms may be seen if there are focal motor seizures.