Background

The background EEG shows focal slowing over the abnormal hemisphere. In the presence of epileptic spasms or generalized seizure types, the background may show widespread slowing or hypsarrhythmia.

Interictal

Focal interictal spikes or polyspikes may be seen over the abnormal hemisphere.

Multi-focal independent spikes or generalized spike-and-wave (usually < 3 Hz) can be seen in those with generalized seizure types.

Hypsarrhythmia can be seen in those with epileptic spasms.

Activation

EEG abnormality is enhanced by sleep deprivation and in sleep. Obtaining a sleep EEG is important in infants at risk of epileptic spasms, as the awake EEG may not show typical abnormality associated with epileptic spasms, this abnormality may only be present in sleep.

Ictal

Focal rhythmic epileptiform discharges are seen with focal seizures arising in the hemimegalencephalic hemisphere.

If epileptic spasms or generalized seizures are present, the ictal EEG associated with these seizure types may be seen.