Seizure Disorder, Absence

Seizure Disorder, Absence is a topic covered in the 5-Minute Clinical Consult.

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Basics

Description

  • Absence seizures are a type of generalized nonmotor seizure characterized by a brief lapse of awareness.
  • Absence seizure types according to the International League Against Epilepsy (ILAE) 2017 classification include:
    • Typical
      • Abrupt onset and offset of behavioral arrest, loss of awareness, and blank staring, sometimes with eyelid movements, eye opening, or oral automatisms (e.g., lip smacking)
      • Typically occurs at 3 Hz
      • Lasts 5 to 30 seconds
      • Immediate return to normal consciousness with no aura or postictal phase
      • Associated with childhood absence epilepsy (CAE), juvenile absence epilepsy (JAE), and juvenile myoclonic epilepsy (JME)
    • Atypical
      • Onset and offset less abrupt than typical absence seizures and often with loss of muscle tone or subtle myoclonic jerks
      • Typically occurs at <2.5 Hz
      • Lasts 10 to 45 seconds
      • Impairment of consciousness often incomplete with continued purposeful activity, albeit done more slowly
      • Brief postictal confusion can sometimes occur.
      • Associated with Lennox-Gastaut syndrome and Dravet syndrome
    • Myoclonic
      • Abrupt onset and offset of staring and loss of awareness with continuous rhythmic jerks of shoulders, arms, legs, head, or perioral muscles
      • Typically occurs at 2.5 to 4.5 Hz
      • Lasts 10 to 60 seconds
      • Impairment of consciousness varies from complete loss of awareness to retained awareness.
      • Associated with epilepsy with myoclonic absences, learning disability, and behavioral problems
    • Eyelid myoclonia
      • Abrupt onset and offset of repetitive, rhythmic jerks of the eyelids with simultaneous upward deviation of the eyeballs and extension of the head
      • Typically occurs at 4 to 6 Hz
      • Lasts <6 seconds
      • Impairment of consciousness often incomplete with awareness mostly retained
      • Associated with epilepsy with eyelid myoclonus

Epidemiology

  • Incidence: 6 to 8/100,000 per year
  • Prevalence: 5 to 50/100,000
  • Predominant age of onset: 1 to 8 years
  • Predominant gender: female > male (2:1) with male predominance in myoclonic absence seizure

Etiology and Pathophysiology

  • Etiology is mainly genetic with complex, multifactorial inheritance; however, may be secondary to a variety of congenital or acquired brain disorders such as hypoxia–ischemia, trauma, CNS infection, cortical malformations, or inborn errors of metabolism
  • Absences are triggered in the thalamus when γ-aminobutyric acid (GABA)-mediated activity induces prolonged hyperpolarization and activates T-type (“low-threshold”) calcium channels, resulting in sustained-burst firing of these neurons causing absence seizures.

Genetics
  • 70–85% concordance occurs in monozygotic twins; 82% share EEG features.
  • 33% concordance among first-degree relatives
  • 15–45% have a family history of epilepsy.
  • Mutations of GABA-A/B receptors, calcium or chloride channels
  • Mutations of SLC21A, which encodes GLUT1

ALERT
Onset of absence seizures <4 years, consider GLUT1 deficiency syndrome

Commonly Associated Conditions

  • Difficulties in visual attention and visuospatial skills, verbal learning and memory, fine motor skills, executive functions, and reduced language abilities
  • Elevated rates of behavioral and psychiatric comorbidities including ADHD, anxiety, depression, social isolation, and low self-esteem

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