The absence epilepsies.

Four syndromes comprise the absence epilepsies. Each is classically associated with the absence seizure, although other syndromes also have absence attacks as part of their repertoire. The most common syndrome is childhood absence epilepsy; it usually occurs in the age range of 6-7 years. The absence seizures may occur many times daily, and the electroencephalographic (EEG) characteristics are the most typical of the absence epilepsies. The second form of absence epilepsies is juvenile absence epilepsy; it begins near puberty and may represent a continuum from the childhood form. Myoclonic seizures are more common than in the childhood form, and the spike-wave discharges in the EEG are often faster than that seen in childhood absence epilepsy. The third form of absence epilepsy is juvenile myoclonic epilepsy, characterized especially by myoclonic jerks in the morning; these attacks occasionally progress to generalized tonic-clonic seizures. The final form of absence epilepsy is epilepsy with myoclonic absences, a rare disorder with a specific form of absence seizures. The absence seizure itself is observed to a greater or lesser extent in all of these syndromes. This seizure is a curious event, and its causes are poorly explained by current knowledge of the fundamental mechanisms of the epilepsies. Although the etiology of the absence seizure at a biochemical level is unknown, some studies suggest that certain low-threshold calcium ion currents (T currents), which are partially controlled by GABA-B mechanisms, may activate burst firing of thalamic neurons, initiating an absence seizure. The evidence of a genetic predisposition for the absence epilepsies is overwhelming.(ABSTRACT TRUNCATED AT 250 WORDS)