Description

• Absence seizures are a generalized epileptic seizure type, characterized by brief lapses of awareness.
• Committee on Classification and Terminology of the International League Against Epilepsy divides epilepsy into 4 groups (1):
  • Electroclinical syndromes: Identified by age at onset
  • Constellations: Distinctive constellations on the basis of specific lesions (e.g, mesial temporal lobe epilepsy with hippocampal sclerosis)
  • Structural/Metabolic epilepsies: Secondary to structural or metabolic lesions or conditions
  • Epilepsies of unknown cause: ≥1/3 of all epilepsy
• Epilepsy syndromes further classified by etiology (2):
  • Genetic: A known or genetic defect in which seizures is the core symptom of the disorder
  • Structural/Metabolic: An increased risk of developing epilepsy due to a distinct structural or metabolic condition
  • Unknown: The nature of the underlying cause is unknown.
• Typical absence:
  • Associated with pediatric idiopathic generalized epilepsy syndromes, namely, childhood absence epilepsy
  • Formerly called petit mal seizures
  • Abrupt-onset behavioral arrest, loss of awareness, and blank staring, sometimes with mild upward eye deviation, repetitive blinking
  • May include automatisms, tonic or atonic features, eyelid or facial clonus, autonomic features
  • Lasts 5–30 seconds
  • Immediate return to normal consciousness
• Atypical absence:
  • Associated with symptomatic generalized epilepsy syndromes, such as Lennox-Gastaut
  • Onset and offset less abrupt than typical absence seizures
  • Lasts 10–45 seconds
  • Impairment of consciousness often incomplete with continued purposeful activity
  • Postictal confusion sometimes occurs.
  • Associated clinical features more pronounced and frequent than in typical absence; atonia most common
• Absence with special features:
  • Myoclonic absence:
    • Rhythmic clonic jerking at 2–4 Hz which last 5–10 seconds
    • Unlike myoclonic seizures with no impairment of consciousness, brief lapses of awareness are characteristic of myoclonic absence.
  • Eyelid myoclonia:
    • Rhythmic clonic jerking of the eyelids at 5–6 Hz which last 3–5 seconds
    • Eyelid myoclonia with absence associated with an impairment of conciousness
• Childhood absence epilepsy (CAE):
  • Also known as pyknolepsy
  • Typical absence seizures are the only seizure type in 90% of children.
  • 10% develop additional generalized tonic–clonic seizures.
  • Seizures last ~10 seconds and often occur hundreds of times per day.
  • Onset ages 4–10 years, with peak at ages 5–7 years (3)
  • Normal neurologic state and development
  • Spontaneous remission occurs in 65–70% of patients during adolescence.
• Juvenile absence epilepsy (JAE):
  • Typical absence seizures are the main seizure type.
  • Seizures last longer than in CAE and occur usually less than once a day (4).
  • Onset ages 9–16 years, with peak at ages 10–13 years
  • Generalized tonic–clonic seizures occur in most patients, often in the first 1–2 hours after awakening.
  • Seizures often persist into adulthood.

Epidemiology

Incidence
6–8/100,000 per year

Prevalence
5–50/100,000

Risk Factors

• 70–85% concordance occurs in monozygotic twins; 82% share EEG features.
• 33% concordance among 1st-degree relatives
• 15–45% have a family history of epilepsy.
• Female > Male (3:2–2:1)
• Complex multifactorial inheritance
• For childhood absence, genes/loci implicated include 6q, 8q24, and 5q14 (4).
• Mutations of GABA_A receptor and voltage-gated Ca²⁺ channel are implicated.

Pathophysiology

• Corticoreticular theory implicates abnormal activity in thalamocortical circuits.
• Thalamic reticular nucleus is responsible for both normal sleep spindles and pathologic slow-wave discharges; contains inhibitory GABAergic neurons.
• These neurons affect low-threshold calcium currents.
• These circuits can fire in oscillatory/rhythmic fashion:
  • Normally, activation of GABA_A receptors causes 10-Hz oscillations in sleep spindle frequency.
  • If GABA_B receptors are strongly activated, oscillation frequency will be 3–4 Hz, similar to spike-and-wave typical absence seizure frequency.

Commonly Associated Conditions

• 3–8% of CAE cases evolve into juvenile myoclonic epilepsy.
• Associated with cognitive/learning problems