Magnesium and Neuromuscular Block — FRCA Final MCQ

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ModerateObstetric AnaesthesiaMagnesium and Neuromuscular BlockFRCA Final

A 34-year-old primigravida at 38 weeks' gestation with pre-eclampsia is receiving an intravenous magnesium sulphate infusion at 1 g/hour for seizure prophylaxis. She requires a category 1 caesarean birth for persistent fetal bradycardia. If general anaesthesia includes a non-depolarising neuromuscular blocking agent, what effect is the magnesium sulphate expected to have?

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Correct answer: B — Prolongation of the block

Explanation lettering: E = shown as A · A = shown as E

B is correct. Parenteral magnesium potentiates non-depolarising neuromuscular blockade and prolongs its duration. Magnesium reduces calcium-dependent presynaptic acetylcholine release at the neuromuscular junction and also reduces postsynaptic neuromuscular excitability, increasing sensitivity to non-depolarising agents. Clinically, the dose should be titrated carefully and quantitative neuromuscular monitoring used to guide further dosing and confirm adequate recovery before extubation. Magnesium therefore does not cause resistance or shorten the block. Fasciculations are associated with initial depolarisation after a depolarising blocker such as suxamethonium, not with magnesium potentiation of a non-depolarising agent.

Reference: AS Kalceks. Magnesium Sulfate 50% w/v Solution for Injection/Infusion, Summary of Product Characteristics, section 4.5: Interactions—Muscle relaxants. eMC; updated 20 May 2025. https://www.medicines.org.uk/emc/product/13170/smpc