Chapter 21: Childhood Absence Epilepsy and Myoclonic Absence Epilepsy

CHILDHOOD ABSENCE EPILEPSY (CAE)

EPIDEMIOLOGY

Childhood absence epilepsy is defined as frequent daily absence seizures (pyknolepsy) in normal school age children. It accounts for 8%–15% of all childhood epilepsies¹ with an annual incidence of 4.7–8.0 per 100,000 children between the ages of 1 and 15 years. The average age at presentation is 6 years (range 2–10 years) with previous febrile seizures reported in 11%.²

PATHOPHYSIOLOGY

This syndrome is likely due to complex inheritance involving several genes.³ High concordance rates for absence seizures in monozygotic twins confirm that genes play a major role. However, as the concordance rate is not 100%, nongenetic factors are also involved. Most of the genes known to be associated with CAE encode calcium channel and GABA receptor subunits. Some of these mutations may have major effect but do not explain the phenotypic heterogeneity within families nor are they found commonly in CAE. Mutations in SLC2A1, the gene encoding glucose transporter type 1 (GLUT1), have recently been reported in children with CAE.⁴ Most of these children also had paroxysmal exertional dyskinesia but this was subtle and not diagnosed until a molecular diagnosis was made. GLUT 1 deficiency should be considered in children with family histories suggestive of dominant inheritance of genetic generalized epilepsy (GGE)⁴ as this may have therapeutic implications particularly if the epilepsy is intractable. Microdeletions in 15q13.13 are found in 1%–2% of children with GGE and are felt to behave as a susceptibility component in a polygenic model where a combination of susceptibility alleles contribute to the phenotype in any one patient.³ The presence of this copy number variation increases a person's risk of developing GGE from 1 in 200 to 1 in 3.³

CLINICAL PRESENTATION

Seizures

Absence seizures are the only seizure at presentation^2,5 occurring up to thousands of times per day. They are enhanced by hyperventilation (HV) in over 90% of children with many clinicians suggesting this is essential to the diagnosis.² The clinical and EEG features of absence seizures are influenced by a number of factors which result in variation from seizure to seizure both between and within individual children. Some of these factors are inherent to the child, such as age, while others, such as state (awake, drowsy, asleep) or provocation (HV, intermittent photic stimulation (IPS)), reflect a changing environment in which the seizures occur.

The mean duration of the absence seizure in CAE is 10 seconds (range 1–44 seconds) with 75% lasting 4–20 seconds.² If response testing is performed most seizures reveal a clinical change.² Children usually arrest or alter their activity if performing a task.² Abnormal eyelid movements are common, with 3-Hz eyelid movements seen in 40% of seizures.² Eye opening occurs in 70% of seizures ...