• M Paech, B Sng, L Ng, E Nathan, A Sia, and B Carvalho.
• Pharmacology, Pharmacy and Anaesthesiology Unit, School of Medicine and Pharmacology, The University of Western Australia, Crawley, WA 6009, Australia Department of Anaesthesia and Pain Medicine, King Edward Memorial Hospital for Women, 374 Bagot Rd, Subiaco, WA 6008, Australia michael.paech@health.wa.gov.au.
• Br J Anaesth. 2015 Mar 1;114(3):469-76.

BackgroundIntrathecal morphine-induced pruritus is a very common side-effect that is difficult to prevent or treat. Central and peripheral mechanisms are believed to be involved. The aim of this study was to determine if a peripherally acting, μ-opioid antagonist would reduce morphine-induced pruritus.MethodsWe conducted a multicentre, randomized, blinded, placebo-controlled trial of women having elective Caesarean section under spinal anaesthesia with intrathecal morphine 100 μg. After delivery, participants received either subcutaneous methylnatrexone bromide 12 mg (MNTX group, n=69) or saline (placebo group, n=68). Pruritus, nausea, pain, analgesic use, and side-effects were assessed at 2, 4, 8, and 24 h. The primary outcome was the severity of pruritus (0-10 score).ResultsOne hundred and thirty-seven women completed the study, with five major protocol violations. There was no statistically significant difference between the MNTX and placebo groups for the median (IQR) pruritus AUC scores [24 (9-47) vs 36 (11-68), median difference 8.5, 95% confidence interval (CI) 0-20, P=0.09] or the worst pruritus score [3 (2-7) vs 5 (2-6), median difference 1, 95% CI 0-2, P=0.24]. The incidence of pruritus was 84% in the MNTX group and 88% in the placebo group (P=0.48). Analgesic and gastrointestinal outcomes did not significantly differ between the groups.ConclusionsA single dose of subcutaneous methylnaltrexone bromide 12 mg did not reduce the overall severity or incidence of pruritus. In this study, treatment with a peripherally acting μ-opioid antagonist was generally ineffective against intrathecal morphine-induced pruritus, but a small clinical effect cannot be excluded.Clinical Trial RegistrationAustralian New Zealand Clinical Trials Registry (ACTRN12611000345987).© The Author 2015. Published by Oxford University Press on behalf of the British Journal of Anaesthesia. All rights reserved. For Permissions, please email: journals.permissions@oup.com.

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