Findings depend on the severity of the hypoxic-ischemic injury, but typically the background EEG shows widespread slowing with alteration in EEG amplitude during the acute injury. Later, the background may be normal, or show focal or widespread slowing.
The interictal EEG may be normal or may show focal or multi-focal interictal spikes or spike-and-wave over the location(s) of the brain injury.
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 young patients who evolve to have 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 following hypoxic-ischemic injury, as the awake EEG may be normal, and abnormality may only be present in sleep.
In the presence of focal seizures, focal rhythmic epileptiform discharges are seen, showing spatial correlation with the area of brain injury from which focal seizures arise.