The background EEG is normal.
Occipital spike or spike-and-wave is seen in the interictal EEG of the majority of patients with this syndrome, but may only occur in the sleep EEG. Centrotemporal, frontal or generalized spike-and-wave may co-exist (seen in 20% of cases).
CAUTION Epileptic encephalopathy with continuous spike-and-wave during sleep can occur as a co-morbid disorder consider a sleep EEG in cases that develop cognitive deterioration.
Fixation-off sensitivity (facilitation of epileptiform discharges with elimination of central vision) is seen in 20-90% of patients. Giant somatosensory spikes may occur. EEG abnormality is enhanced by sleep deprivation and by sleep. It is debated whether a response to intermittent photic stimulation can be seen in this syndrome or is exclusionary.
At ictal onset, there is a reduction in the usual background occipital spike or spike-and-wave with the sudden appearance of occipital faster rhythms with spikes of low amplitude. There may be slower spike-and-wave during oculo-clonic seizures or ictal blindness.