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Background/Interictal EEG

The background EEG is typically abnormal with high voltage irregular slowing.

CAUTION The awake EEG may be normal early after onset of epileptic spasms. Abnormality may only be seen in sleep or on wakening.


The background EEG is often highly disorganized with high voltage irregular slow waves intermixed with multifocal spikes and polyspikes, this is termed 'hypsarrhythmia'. Asymmetry and other patterns of modified hypsarrhythmia occur in one third of cases.

Example of hypsarrhythmia

CAUTION Hypsarrhythmia may not be present early after the onset of epileptic spasms. The EEG may need to be repeated.


In the early period after onset of epileptic spasms the EEG may only be abnormal in sleep. EEG abnormality is enhanced by sleep and on wakening. REM sleep shows relative EEG normalization.


Epileptic spasms are most commonly accompanied by a high voltage generalized sharp or slow wave followed by low amplitude fast activity and generalized voltage attenuation. This EEG pattern may be seen in sleep with or without clinical seizures.

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