Prevention versus treatment of intrathecal morphine-induced pruritus with ondansetron

Intrathecal morphine is used for post-cesarean analgesia, but pruritus is a common side effect. Ondansetron would be an attractive treatment because it prevents nausea, is non-sedative or has no anti-analgesic effect. We undertook a study to assess the efficacy of ondansetron for treatment or prophylaxis of intrathecal morphine-induced pruritus.

Healthy paturients undergoing cesarean delivery with intrathecal morphine 250μg and fentanyl 25μg were randomized to receive: prophylaxis (ondansetron 8mg at cord clamping, normal saline 4mL for treatment of pruritus in the post-anaesthesia care unit); treatment (normal saline 4mL at cord clamping, ondansetron 8mg as required in the post-anesthesia care unit) or control (normal saline 4mL in both). Visual analogue scale scores for pruritus, nausea and pain were recorded preoperatively, on arrival to, at 30, 60, and 120min and on discharge from the post-anesthesia care unit. The primary outcome was the peak pruritus score. ANOVA with Bonferroni correction or Fisher's exact test were used to analyze data; P<0.05 was considered significant.

The study was terminated early when interim analysis indicated no effect. Eighty-two of the intended 180 paturients completed the protocol (26 in control group, 32 in treatment group and 24 in prophylaxis). There were no differences in the rate or severity of pruritus at any assessment point, or the request for treatment. Pruritus was reduced after administration of treatment syringe.

Prophylactic ondansetron did not reduce pruritus when compared with placebo. The use of ondansetron as a treatment did not decrease the severity of pruritus when compared with placebo.