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Anticonvulsant NMBA Resistance — FRCA Final MCQ

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HardGeneral AnaesthesiaAnticonvulsant NMBA ResistanceFRCA Final

A 72-year-old man with known epilepsy on phenytoin and carbamazepine undergoes general anaesthesia. He requires significantly higher doses of rocuronium to achieve adequate neuromuscular block, and the duration of block is shorter than expected. What is the mechanism?

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Correct answer: A — Chronic anticonvulsant therapy induces hepatic enzymes increasing rocuronium metabolism

Chronic anticonvulsant therapy (phenytoin, carbamazepine, phenobarbital) induces hepatic cytochrome P450 enzymes, increasing the metabolism and clearance of hepatically-eliminated drugs including rocuronium and vecuronium. This results in resistance to non-depolarising NMBAs (higher doses required, shorter duration). Additionally, there is evidence of upregulated acetylcholine receptors at the neuromuscular junction with chronic anticonvulsant use. Atracurium and cisatracurium (organ-independent elimination) are less affected and may be preferred.

Reference: RCOA – 2023 – Pharmacology of NMBAs: drug interactions; Soriano SG – 2017 – Neuromuscular block and anticonvulsants