Seizure Disorder, Absence
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- 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
- Myoclonic absence:
- 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.
6–8/100,000 per year
- 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 GABAA receptor and voltage-gated Ca2+ channel are implicated.
- 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 GABAA receptors causes 10-Hz oscillations in sleep spindle frequency.
- If GABAB 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