THE ABSENCE EPILEPSIES

Four syndromes comprise the absence epilepsies. Each is classically as sociated 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 ele ctroencephalographic (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 continuu m 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 o f absence epilepsy is juvenile myoclonic epilepsy, characterized espec ially by myoclonic jerks in the morning; these attacks occasionally pr ogress 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 obser ved to a greater or lesser extent in all of these syndromes. This seiz ure is a curious event, and its causes are poorly explained by current knowledge of the fundamental mechanisms of the epilepsies. Although t he 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 seizu re. The evidence of a genetic predisposition for the absence epilepsie s is overwhelming. Although the nature of the genetic abnormality rema ins unclear, promising investigations may soon reveal the location and the nature of the genetic defeat.