The background is normal. Occipital intermittent rhythmic delta activity (OIRDA) may be seen in a third of children with childhood absence epilepsy, at a frequency of 2.5-4 Hz, and may have a notched appearance.
CAUTION Focal slowing seen consistently in one area consider structural brain abnormality.
CAUTION Generalized slowing is not seen.
Generalized spike-and-wave, or fragments of generalized spike-and-wave are seen in the interictal EEG. These are brief (usually < 2 second) and most commonly seen in sleep.
CAUTION Although focal spikes (as fragments of generalized spike-and-wave) can occur, if they consistently arise in one area consider structural brain abnormality.
EEG abnormality and absence seizures are provoked by hyperventilation. If hyperventilation is poorly performed, generalized spike-and-wave may not be triggered.
Intermittent photic stimulation triggers generalized spike-and-wave in a small proportion of individuals.
EEG abnormality is enhanced by sleep deprivation and by sleep. Generalized spike-and-wave often becomes fragmented with sleep deprivation or in sleep. Fragmented generalized spike-and-wave can appear focal or multi-focal but usually is not consistently seen in one area. The morphology of the focal spike-and-wave typically appears similar to the generalized spike-and-wave.
CAUTION Where hyperventilation is performed well for three minutes and no generalized spike-and-wave is seen, childhood absence epilepsy is unlikely.
Regular 3 Hz generalized spike-and-wave occurs associated with absence seizures. Polyspike-and-wave can occur in the ictal EEG.
CAUTION Slow spike-and-wave (<2.5Hz) is exclusionary.
Example of 3Hz generalized spike-and-wave seen on the ictal EEG