The background EEG may be normal, or may show focal slowing over the focal cortical dysplasia. In the presence of epileptic spasms or generalized seizure types, the background may show widespread slowing or hypsarrhythmia.
The interictal EEG may be normal or may show focal interictal spikes, polyspikes or spike-and-wave in the location of the focal cortical dysplasia.
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.
The presence of focal, rhythmic epileptiform discharges is the most characteristic feature of the scalp EEG in patients with focal cortical dysplasia, showing spatial correlation with the focal cortical dysplasia. Focal cortical dysplasias that are not accessible by scalp EEG may have poorly localized ictal change on EEG, with patterns that may include subtle attenuation, or only rhythmic slowing.