The seizure comprises an abrupt loss of consciousness (the absence) and cessation of all motor activity. Tone is usually preserved, and there is no fall. The patient is not in contact with the environment, is inaccessible, and often appears glazed or vacant. The attack ends as abruptly as it started, and previous activity is resumed as if nothing had happened. There is no confusion, but the patient is often unaware that an attack has occurred. Most absence seizures (> 80%) last less than 10 seconds. Other clinical phenomena including blinking, slight clonic movements of the trunk or limbs, alterations in tone and/or brief automatisms can occur particularly in longer attacks. The attacks can be repeated, sometimes hundreds of times a day, often cluster and are often worse when the patient is awakening or drifting off to sleep.
Absences may be precipitated by fatigue, drowsiness, relaxation, photic stimulation or hyperventilation. Typical absence seizures develop in childhood or adolescence and are encountered almost exclusively in the syndrome of idiopathic generalized epilepsy (see pp. 17-19). Variations from this typical form include the myoclonic absence, absence with perioral myoclonia or with eyelid myoclonia. Whether or not these are distinct entities is controversial (see pp. 17-18).
The EEG during a typical absence has a very striking pattern. A regular, symmetric and synchronous 3 Hz spike-wave paroxysm is the classic form, although in longer attacks and in older patients the paroxysms may not be entirely regular and frequencies vary between 2 and 4 Hz. The inter-ictal EEG has normal background activity and there may be intermittent short-lived bursts of spike-wave. These spike-wave paroxysms can frequently be induced by hyperventilation and less commonly by photic stimulation.
The features useful in differentiating a complex partial seizure and a typical absence are shown in Table 1.4.
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