Which antiepileptic drug is most effective for absence seizures and acts primarily by blocking T-type calcium channels?

Explanation

## Ethosuximide: The Absence Seizure Drug **Key Point:** Ethosuximide is the drug of choice for absence seizures (petit mal) and is the only antiepileptic with a specific indication for this seizure type. ### Mechanism of Action Ethosuximide blocks **T-type (low-voltage-activated) calcium channels** in the thalamus, which are critical for the 3 Hz spike-and-wave discharges characteristic of absence seizures. ### Clinical Features - **Seizure type:** Absence seizures exclusively (not generalized tonic-clonic or focal seizures) - **Efficacy:** 60–80% seizure control in absence epilepsy - **Onset:** Rapid (within days to weeks) - **Side effects:** Nausea, anorexia, hiccups, rarely agranulocytosis (requires CBC monitoring) ### Why Ethosuximide Over Others? | Feature | Ethosuximide | Phenytoin | Levetiracetam | Lamotrigine | |---------|--------------|-----------|---------------|-------------| | **Absence seizures** | Excellent | Poor | Moderate | Moderate | | **Mechanism** | T-Ca²⁺ block | Na⁺ channel | SV2A binding | Na⁺ channel | | **Drug interactions** | Minimal | Many | Few | Many | | **Monitoring** | CBC | Levels, CBC | None | Rash risk | **High-Yield:** Ethosuximide does NOT work for generalized tonic-clonic seizures; if a patient with absence seizures also has GTC seizures, add valproate or lamotrigine, do not rely on ethosuximide alone. **Clinical Pearl:** The classic EEG finding in absence seizures is a **3 Hz spike-and-wave** pattern, which resolves with ethosuximide therapy.