absence seizure

**Clinical features:** Sudden behavioral arrest/staring with unresponsiveness lasting seconds (typically <20 s), sometimes with eyelid fluttering, lip smacking, or automatisms. Abrupt onset and termination; NO aura and NO postictal confusion (contrast with focal impaired-awareness seizures). Can occur dozens to hundreds of times daily; often presents as "daydreaming" or declining school performance. Provoked by hyperventilation (classic bedside/EEG activation test).

**Pathophysiology:** Abnormal thalamocortical circuit oscillations. T-type (low-threshold) voltage-gated Ca2+ channels in thalamic neurons generate rhythmic bursting, producing synchronous bilateral cortical discharge. GABA-B-mediated hyperpolarization de-inactivates T-type channels, sustaining the cycle.

**Diagnosis:** EEG shows generalized, bilateral, synchronous 3-Hz (3/sec) spike-and-wave discharges with normal background. Hyperventilation for 3 to 5 minutes provokes events.

**Treatment:** - First-line: ethosuximide (blocks thalamic T-type Ca2+ channels) for absence seizures alone. Adverse effects: GI upset, headache, lethargy, Stevens-Johnson syndrome. - Valproate: first-line if patient also has generalized tonic-clonic seizures (e.g., juvenile myoclonic epilepsy or absence with GTC). Adverse effects: hepatotoxicity, pancreatitis, teratogenicity (neural tube defects), tremor, weight gain. - Alternative: lamotrigine. - AVOID carbamazepine, phenytoin, gabapentin, and tiagabine, which can worsen absence seizures.

**Prognosis/associations:** Childhood absence epilepsy usually remits by adolescence, and most children outgrow it. A minority develop generalized tonic-clonic seizures. Normal development and neuro exam are typical.

**Differential/high-yield distinctions:** - Focal impaired-awareness (complex partial) seizure: longer (1-2 min), has aura, automatisms, postictal confusion, and a focal EEG abnormality. - Daydreaming: can be interrupted by touch or stimulation; absence seizures cannot. - Atypical absence (Lennox-Gastaut): slower (<2.5 Hz) spike-and-wave, longer duration, gradual onset/offset, developmental delay, and multiple seizure types.

**Board pearl:** A school-age child with staring spells, no postictal state, and 3-Hz spike-and-wave on EEG that is provoked by hyperventilation → ethosuximide.