The EEG background is abnormal in all cases from onset of seizures, with generalized or focal slowing.
CAUTION If there is prominent bi-parietal rhythmic theta consider epilepsy with myoclonic-atonic seizures.
There may be focal or multifocal spike-and-wave or sharp-slow waves, with an anterior predominance. Slow (<2.5 Hz) spike-and-wave and paroxysmal fast activity (10 Hz or greater) in slow sleep are mandatory requirements. Periods of suppression of the EEG may occur.
CAUTION Fast spike-and-wave (3 Hz or higher) consider other epilepsy syndromes.
CAUTION Continuous spike-and-wave during slow sleep consider epileptic encephalopathy with continuous spike-and-wave during sleep or Landau Kleffner syndrome.
Hyperventilation (if adequate co-operation) may facilitate spike-and-wave and atypical absences. Slow (<2.5Hz) spike-and-wave and paroxysmal fast activity are prominent in slow sleep. Focal and multifocal abnormalities seen in the awake state become bisynchronous in sleep.
CAUTION If activation with intermittent photic stimulation consider epilepsy with myoclonic-atonic seizures.
For ictal EEG patterns, refer to individual seizure types.