What causes absence seizures?

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The etiology of idiopathic epilepsies with age-related onset is genetic. About 15-40% of patients with these epilepsies have a family history of epilepsy; overall concordance in monozygotic twins is 74%, with a 100% concordance during the peak age of phenotypic expression.^[9]Family members may have other forms of idiopathic or genetic epilepsy (eg, febrile convulsions, generalized tonic-clonic seizures).

The idiopathic generalized epilepsies are a group of primary generalized epilepsies with absence, myoclonic, and tonic-clonic seizures. Based on age of onset and seizure types, some can be grouped into well-recognized syndromes, such as childhood absence epilepsy, juvenile absence epilepsy, and juvenile myoclonic epilepsy.

However, patients with other syndromes, such as generalized epilepsy with febrile seizures plus (GEFS+), as well as patients who have childhood absence epilepsy that leads into juvenile myoclonic epilepsy, illustrate that these syndromes represent a genetically determined lower threshold to have seizures.

The idiopathic generalized epilepsies are best viewed as a spectrum of clinical syndromes^[10]with varied genetic causes that affect the function of ion channels.

Genetic studies have shown that these syndromes are channelopathies, but different gene mutations have been found in the same syndromes. Juvenile myoclonic epilepsy has been linked to chromosome 6,^{[11, 12]}with linkage to chromosome 6p12 in Mexican families.^[13]Mutations in the EFHC1 gene have been found in Mexican^{[14, 15]}and Italian families^[16]with juvenile myoclonic epilepsy, but not in a group of Dutch families.^[17]

Childhood absence epilepsy with generalized tonic-clonic seizure has been linked to chromosome 8q24 in a 5-generation family from Bombay, India.^[18]Childhood absence epilepsy with febrile seizures has been linked to the GABA(A) receptor γ2 subunit (GABRG2) on chromosome 5q3.1-33.1.^[19]

A mutation in the GABA(A) receptor gene GABRB3 was found in Mexican families with childhood absence epilepsy. Mutations showed hyperglycosylation in vitro, with reduced GABA-evoked current density from whole cells. Expression of this gene in the developing brain may help explain an age-related onset and remission in childhood absence epilepsy.^[20]

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Percentage of absence seizures with automatisms as a function of duration in seconds. (Data gathered from Penry et al, 1975.)
EEG of a typical absence seizure with 3-Hz spike-and-wave discharges.
Slow spike-and-wave discharges (2.5 Hz). This is an interictal pattern in a child with seizures and developmental delay.

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