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The background EEG may be normal or there may be focal or diffuse slowing.


The interictal EEG abnormalities seen during wakefulness are similar to those seen in childhood epilepsy with centrotemporal spikes, but are usually more prominent.


The EEG abnormality is activated in sleep.


Seizures with fronto-parietal opercular features may be accompanied by a brief decrease in amplitude of the background EEG, followed by diffuse sharp wave discharges of increasing amplitude, predominantly in one centrotemporal region, followed by high amplitude slowing and then a return to the usual interictal EEG.

Negative myoclonus is accompanied by a high-amplitude spike followed by a slow wave, maximal in the contralateral hemisphere to the body part affected. If both arms are affected the EEG finding will be a bilaterally synchronous discharge. The patient is best assessed in the standing position with arms outstretched during simultaneous EEG and EMG recording.

Focal motor seizures may be accompanied by variable EEG findings that may include localized ictal patterns of repetitive discharges, diffuse or localized low voltage fast rhythms, bilateral discharges with amplitude asymmetry (secondary bilateral synchrony) or the EEG may not show clear surface ictal patterns or be obscured by muscle artifact.

Atypical absence seizures are accompanied by slow spike-and-wave.

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