The background EEG may be normal or show focal slowing over the location of the stroke. 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 or spike-and-wave over the location of the stroke.
Multi-focal independent spikes or generalized spike-and-wave (usually < 3 Hz) can be seen in young patients who evolve to have 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 slowing of the background may be accentuated during hyperventilation in underlying cerebrovascular disease.
CAUTION hyperventilation causes cerebral vasoconstriction, which may, in certain circumstances, impair critical cerebral perfusion and cause neurological symptoms. Hyperventilation is best avoided in the presence of intracranial vascular malformations or impaired cerebral perfusion.
The presence of focal, rhythmic epileptiform discharges is the most characteristic feature of the scalp EEG in patients with focal seizures, showing spatial correlation with the location of the stroke.