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Intrathecal Opioid Pruritus — FRCA Final MCQ

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ModerateObstetric AnaesthesiaIntrathecal Opioid PruritusFRCA Final

A 30-year-old woman (ASA I, 58 kg) at 39 weeks gestation has an uncomplicated spinal anaesthetic for elective caesarean section. Diamorphine 300 mcg is added intrathecally. Post-operatively she develops pruritus affecting her whole body. What is the mechanism and first-line treatment?

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Correct answer: B — Local anaesthetic allergy – remove epidural

Intrathecal opioid-induced pruritus is extremely common (60-90% with neuraxial morphine/diamorphine) and is NOT caused by histamine release. The mechanism is central mu-opioid receptor activation, particularly in the trigeminal nucleus and superficial dorsal horn. First-line treatments: ondansetron 4 mg IV (5-HT3 antagonism reduces central itch), naloxone low-dose infusion (0.25-1 mcg/kg/hr – reverses pruritus without reversing analgesia), or nalbuphine 5 mg IV (kappa agonist/mu antagonist). Antihistamines are largely ineffective because the mechanism is not histaminergic.

Reference: OAA – 2024 – QRH: Neuraxial opioid side effects; RCOA – 2023 – Obstetric anaesthesia pharmacology