A 28-year-old man with newly diagnosed absence seizures is started on antiepileptic therapy. Which of the following is the most common first-line drug of choice for absence seizures in India?

Explanation

## First-Line Drug of Choice for Absence Seizures **Key Point:** **Ethosuximide is the drug of choice for typical (pure) absence seizures**, as established by international guidelines (AAN, ILAE) and standard Indian pharmacology textbooks including KD Tripathi and Ghai's Pediatrics. ### Mechanism and Comparative Efficacy | Drug | Mechanism | Efficacy in Absence | First-Line Status | Notes | |------|-----------|-------------------|-------------------|-------| | Ethosuximide | T-type Ca²⁺ channel blocker (thalamic) | Highly selective (95–100%) | **Yes — Drug of Choice** | Narrow spectrum; ideal for pure absence | | Valproic acid | GABA ↑, Na⁺ channel block, T-type Ca²⁺ block | Broad spectrum (90–95%) | Yes — preferred if mixed seizures | Covers absence + GTCS + myoclonic | | Lamotrigine | Na⁺ channel block, glutamate ↓ | Moderate (70–80%) | Second-line | Slower titration; rash risk | | Phenytoin | Na⁺ channel block | Poor / worsens absence | **Contraindicated** | May paradoxically worsen absence seizures | ### Why Ethosuximide is the Drug of Choice **Clinical Pearl:** Per KD Tripathi (*Essentials of Medical Pharmacology*, 8th ed.) and standard NEET PG references: - Ethosuximide selectively blocks **T-type (low-threshold) calcium channels** in thalamic neurons, interrupting the 3 Hz spike-and-wave discharge characteristic of absence seizures. - It has the **best efficacy-to-side-effect ratio** for pure absence seizures. - The landmark **CHILDHOOD Absence Epilepsy (CAE) trial (NEJM 2010)** confirmed ethosuximide as superior to valproate and lamotrigine for attention and tolerability in pure absence epilepsy. ### When Valproic Acid is Preferred Valproic acid is chosen over ethosuximide when: 1. The patient has **mixed seizure types** (absence + generalized tonic-clonic or myoclonic) 2. Ethosuximide is unavailable (a practical consideration in some settings) 3. Juvenile myoclonic epilepsy (JME) is the underlying diagnosis **High-Yield:** The question stem specifies **newly diagnosed absence seizures** without mention of comorbid GTCS — this is the classic scenario where ethosuximide is the textbook drug of choice. ### Why Phenytoin is Wrong Phenytoin acts on Na⁺ channels and is effective for focal and tonic-clonic seizures but **does not block T-type Ca²⁺ channels** and may **worsen absence seizures** — it is contraindicated in absence epilepsy. ### Dosing Reference - **Ethosuximide:** 15–20 mg/kg/day; therapeutic level 40–100 µg/mL - **Valproic acid:** 15–30 mg/kg/day; therapeutic level 50–100 µg/mL **Mnemonic:** **ETHOsuximide = ETHOlogically pure choice for pure Absence** (T-type Ca²⁺ block → thalamic 3 Hz spike-wave suppression) **Warning:** Phenytoin is contraindicated as monotherapy for absence epilepsy and may paradoxically increase seizure frequency.