The background EEG may be normal, and is often so initially. Focal slowing may also be seen (temporal > frontal > occipital areas). In the presence of epileptic spasms or generalized seizure types, the background may show widespread slowing or hypsarrhythmia.
The interictal EEG is usually normal. Focal interictal spikes can be seen (in temporal > frontal > occipital areas).
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.
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 be normal, and abnormality may only be present in sleep.
Hypothalamic hamartomas are not accessible by scalp EEG, and ictal EEG may be normal or may show only subtle attenuation or rhythmic slowing.