The background EEG may be normal, or may show focal slowing over the cyst. Widespread slowing may be seen if a widespread insult occurred previously. 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 at the location of the cyst. Multifocal interictal discharges may be seen if a widespread insult occurred previously.
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 porencephalic cyst related seizures, showing spatial correlation with the cyst and its margins.