Which local anaesthetic agent is contraindicated in patients with a history of methemoglobinemia or in those at high risk for methemoglobinemia (e.g., neonates, patients on sulfonamides)?

Explanation

## Methemoglobinemia Risk with Local Anaesthetics ### Mechanism of Methemoglobinemia **Key Point:** Prilocaine is metabolized to o-toluidine, a metabolite that oxidizes haemoglobin Fe²⁺ to Fe³⁺, forming methemoglobin. This is a dose-dependent and metabolite-dependent adverse effect. ### High-Risk Populations Methemoglobinemia is particularly concerning in: - **Neonates and infants** (immature cytochrome P450 and reduced methemoglobin reductase activity) - **Patients on concurrent medications** (sulfonamides, nitrates, dapsone, phenazopyridine) - **Patients with G6PD deficiency** (reduced capacity to reduce methemoglobin) - **Patients with congenital methemoglobin reductase deficiency** ### Clinical Presentation **High-Yield:** Methemoglobinemia presents with: - Cyanosis (blue-grey discoloration) unresponsive to oxygen - Chocolate-brown colour of blood on laboratory examination - Dyspnoea, headache, fatigue (if severe) - Pulse oximetry typically reads ~85% regardless of true oxygenation ### Comparison of Local Anaesthetics and Methemoglobinemia Risk | Local Anaesthetic | Methemoglobinemia Risk | Metabolite | Notes | |-------------------|------------------------|-----------|-------| | **Prilocaine** | **HIGH** | o-toluidine | Contraindicated in neonates and high-risk patients | | **Lidocaine** | Low | Monoethylglycinexylidide | Safe in most populations | | **Bupivacaine** | Low | Pipecolylxylidide | Safe in most populations | | **Procaine** | Low | PABA | Allergic potential is main concern | | **Benzocaine** | **HIGH** | Aniline | Topical agent; high risk in neonates | ### Management of Methemoglobinemia **Clinical Pearl:** Methemoglobinemia induced by local anaesthetics is treated with **methylene blue** (1–2 mg/kg IV) in symptomatic patients or those with methemoglobin levels >30%. Ascorbic acid is an alternative for mild cases. **Mnemonic:** **PABA** — Prilocaine And Benzocaine Are problematic (for methemoglobinemia) [cite:KD Tripathi 8e Ch 12]