What is the role of EEG in the diagnosis of absence seizures?

Updated: Sep 25, 2018
  • Author: Scott Segan, MD; Chief Editor: Selim R Benbadis, MD  more...
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The only diagnostic test for absence seizures is the EEG.

Background activity is normal. In syndromes with frequent absence seizures, such as childhood absence epilepsy, a routine awake recording is often pathognomonic. Bursts of frontally predominant, generalized 3-Hz spike-and-wave complexes are seen during the seizures. [3] In syndromes with less frequent absence seizures (juvenile absence epilepsy or juvenile myoclonic epilepsy), an awake recording may be normal; a sleep or sleep-deprived recording may be needed.

Typical absence seizures have generalized 3-Hz spike-and-wave complexes (see image below).

EEG of a typical absence seizure with 3-Hz spike-a EEG of a typical absence seizure with 3-Hz spike-and-wave discharges.

The spike frequency is often faster at the onset, with a slight deceleration at the end. [26] They can range from 2.5-6 Hz, with the faster frequencies seen in syndromes with older age of onset.

Bursts of generalized polyspikes and waves (multiple spike-and-slow-wave complexes) may also be seen, [30] especially during sleep and in syndromes with older age of onset.

The onset and ending of these seizures are abrupt; no postictal EEG slowing is noted.

Hyperventilation often provokes these seizures and should be a routine part of all EEGs in children.

Photosensitivity may be present in idiopathic generalized epilepsies and is more often seen in juvenile myoclonic epilepsy and childhood absence epilepsy than juvenile absence epilepsy. [29]

EEG video monitoring demonstrates that clinical seizure manifestations may lag behind the start of ictal EEG activity; bursts lasting less than 3 seconds are usually clinically silent. During the absence seizure, rhythmic eye blinks and mild clonic jerks may be present. As a seizure progresses, automatisms may be seen. [32]

Clinical and EEG features may vary considerably in different children. [34]

Go to EEG Video Monitoring for procedural information on this topic.

EEG findings in atypical absence seizures

Atypical absence seizures are characterized by slow spike-and-wave paroxysms, classically 2.5 Hz (see the image below). The onset may be difficult to discern, and postictal EEG slowing may be noted.

Slow spike-and-wave discharges (2.5 Hz). This is a Slow spike-and-wave discharges (2.5 Hz). This is an interictal pattern in a child with seizures and developmental delay.

Background activity is often abnormal, reflecting the diffuse or multifocal underlying encephalopathy of symptomatic generalized epilepsy.

Generalized polyspike-and-wave complexes also may be present, and focal features may be observed.

The clinical correlation of generalized spike-and-wave complexes with clinical seizures is not as clear-cut as in typical absence seizures. Generalized slow spikes and waves may be present as an interictal pattern, as in Lennox-Gastaut syndrome.

EEG video monitoring can show a more varied alteration of consciousness than in typical absence seizures. If the patient has underlying mental retardation, discerning changes in mental status may be more difficult in atypical absence.

Changes in postural tone, most noticeably head nods, are common.

Ambulatory EEG monitoring over 24 hours may be useful to quantitate the number of seizures per day and their most likely times of occurrence.

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