Prilocaine Methaemoglobinaemia — MFDS Part 1 MCQ

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ModeratePharmacologyPrilocaine MethaemoglobinaemiaMFDS Part 1

Within an hour of receiving a dental local anaesthetic, a patient develops slate-grey cyanosis. Pulse oximetry remains approximately 85% despite high-flow oxygen, arterial PaO2 is normal, and a blood sample appears chocolate-brown. Which local anaesthetic is most likely responsible?

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Correct answer: E — Prilocaine

Prilocaine is correct. Persistent cyanosis with a low pulse-oximetry reading despite oxygen, normal arterial PaO2 and chocolate-brown blood indicates acquired methaemoglobinaemia. Prilocaine is metabolised to o-toluidine derivatives, which oxidise haemoglobin iron and promote methaemoglobin formation. The UK SmPC specifically identifies this adverse effect and warns that it may occur at lower doses in patients with anaemia, haemoglobinopathy or concomitant oxidising medicines. The other options are amide local anaesthetics, but methaemoglobinaemia is not their characteristic dose-related toxicity; overdose more typically produces central nervous system and cardiovascular toxicity. Lidocaine has occasionally been implicated, but prilocaine is the strongest and classically examined association.

Reference: Aspen Pharma Trading Ltd. Prilocaine hydrochloride 1% Solution for Injection: Summary of Product Characteristics, sections 4.2, 4.4, 4.8 and 5.2; updated 14 July 2022. https://www.medicines.org.uk/emc/product/870/smpc