Human opioid μ-receptor A118G polymorphism may protect against central pruritus by epidural morphine for post-cesarean analgesia.

Background: Intrathecal or epidural morphine used for post-operative analgesia frequently induces central type pruritus. The purpose of this study was to investigate the association between the severity of central type pruritus induced by epidural morphine for post-cesarean analgesia and the A118G polymorphism of the human μ-opioid receptor gene (OPRM1).

Methods: Pregnant women (212) received pure epidural morphine (2 mg) twice per day for post-cesarean analgesia. Blood samples were collected and sequenced with high-resolution melting analysis to detect three different genotypes of OPRM1 (AA, AG and GG). We interviewed all candidates 24 h post-operatively to record the clinical phenotype with subjective complaints and objective observations.

Results: The genotyping revealed that 99 women (46.7%) were AA, 88 (41.5%) were AG and 25 (11.8%) were GG. Sixty-two of 212 women suffered from significant pruritus (29.2%), and 150 of 212 women had non-significant pruritus (70.8%). In genotype AA, 33 patients (53.2%) experienced significant pruritus, 26 (41.9%) in genotype AG and 3 (4.8%) in genotype GG. The G allele was a statistically independent protective factor for individuals developing pruritus, and the multivariate-adjusted odds ratio was 0.27. There was a trend for progressively decreasing severity scores among the three groups, with the lowest severity score (0.72) for pruritus in the GG group.

Conclusions: The incidence of significant pruritus in the recessive type (GG) was significantly lower compared with the dominant types (AA+AG). The recessive G allele in the A118G polymorphism may have protective effects against significant pruritus after epidural morphine for post-cesarean analgesia.