The background EEG may show focal or diffuse slowing slowing.
CAUTION The EEG background can be normal early after onset of epileptic spasms or abnormality may only be seen in sleep or on wakening.
The background and interictal EEG may be highly disorganized with high voltage irregular slow waves intermixed with multifocal spikes and polyspikes, this is termed 'hypsarrhythmia'. Some patients may have focal or multifocal discharges without hypsarrhythmia.
Example of hypsarrhythmia
EEG abnormality is enhanced in drowsiness, in sleep and on wakening.
Epileptic spasms are most commonly accompanied by a high voltage triphasic sharp or slow wave followed by low amplitude fast activity and voltage attenuation.