The background EEG may be normal, or may show focal slowing over the cleft. In the presence of epileptic spasms or generalized seizure types, the background may show widespread slowing or hypsarrhythmia.
Focal interictal spikes, polyspikes or sharp waves may be seen, at the location of the cleft.
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.
Focal, rhythmic epileptiform discharges are seen, showing spatial correlation with the cleft.