The background EEG may be normal, or may show focal slowing or low voltage fast frequencies over the polymicrogyric cortex. In the presence of epileptic spasms or generalized seizures types, the background may show widespread slowing or hypsarrhythmia.
Focal interictal spikes or polyspikes may be seen, at the location of the polymicrogyria.
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, showing spatial correlation with the location of the polymicrogyria. Polymicrogyria that is not accessible by scalp EEG may have poorly localized ictal change on EEG, with patterns that may include subtle attenuation, or only rhythmic slowing.