Which antiepileptic drug acts primarily by blocking voltage-gated sodium channels and is contraindicated in absence seizures?

Explanation

## Mechanism of Action **Key Point:** Phenytoin stabilizes the inactive state of voltage-gated Na⁺ channels, preventing repetitive neuronal firing. It is a first-generation antiepileptic with broad-spectrum activity against generalized tonic-clonic and partial seizures. ## Why Phenytoin is Contraindicated in Absence Seizures **High-Yield:** Absence seizures arise from abnormal thalamic T-type calcium channel activity (3 Hz spike-and-wave discharge). Phenytoin, which targets Na⁺ channels, does NOT suppress this mechanism and may paradoxically worsen absence seizures. ## Comparison of Antiepileptics by Mechanism | Drug | Primary Mechanism | Seizure Type (Effective) | Seizure Type (Contraindicated/Ineffective) | |------|-------------------|--------------------------|--------------------------------------------| | Phenytoin | Na⁺ channel block | GTC, partial | Absence | | Ethosuximide | T-type Ca²⁺ channel block | Absence | GTC | | Valproic acid | Na⁺ channel + GABA↑ | Broad-spectrum (all types) | None (safest for mixed seizures) | | Lamotrigine | Na⁺ channel + glutamate↓ | GTC, partial, absence | None (broad-spectrum) | **Clinical Pearl:** Phenytoin is a classic example of a drug that is effective for some seizure types but contraindicated in others—a frequent NEET PG trap. **Mnemonic:** **PHEN** = Phenytoin Halts Excitatory Neurons (via Na⁺ block) — but NOT thalamic T-channels. [cite:KD Tripathi 8e Ch 12]