The background EEG may be normal, or may show focal or widespread slowing. In the presence of epileptic spasms the background may show hypsarrhythmia.
Focal interictal spikes or polyspikes may be seen, referrable to the location of the heterotopia.
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 may be seen with focal seizures, showing spatial correlation with the heterotopia. Ictal onset may occur in the heterotopic nodule itself, or in overlying cortex. Heterotopia 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.