The EEG background can be normal at onset, however diffuse slowing of the background occurs with time. If focal slow activity is present, this may migrate from one brain region to another.
Multifocal discharges appear later in all cases.
EEG abnormality is enhanced by sleep deprivation and by sleep.
The ictal EEG (the distribution of focal ictal patterns) should correlate with clinical features; and there should be involvement of multiple independent cortical regions randomly but consecutively in the same single seizure or status epilepticus event. Ictal discharges are typically rhythmic alpha or theta activities.