Dr. Alex T. Sia and colleagues at KK Women's and Children's Hospital in Singapore evaluated 588 women who were injected with morphine in the spinal canal after delivering their children through Caesarean section.
The women participating in the study fell into one of the three genotype groups-namely homozygous AA, homozygous GG, and heterozygous AG-based on their blood samples.
Pain scores, assessment of the severity of nausea and vomiting, the incidence of itching, and the total dose self-administered intravenous morphine were recorded for the first 24 postoperative hours across the study group.
The researchers found that the various genetic types correlated with a significant variability in morphine consumption after Caesarean section.
They revealed that women with the AA genotype consumed the least amount of patient-controlled analgesia (PCA) morphine, and had demonstrably lower pain scores than those in other genotypic groups.
It was also observed that the women with the AA genotype had the highest risk of developing nausea, but not vomiting, despite lower consumption of PCA morphine.
The researchers said that their observations suggested that the AA group had a greater sensitivity to the analgesic effect of morphine injected into the spinal canal, a greater sensitivity to PCA morphine in counteracting post-operative pain, or a combination of the two.
They said that it could also be inferred that the greater analgesic sensitivity to morphine in the AA group was related to a higher risk of developing nausea.
Dr. Sia said that the findings likely had important implications in differences among women's pain perception in general, pain after childbirth, and narcotic use after surgery.
The researcher further said that the study had additional relevance to understanding the importance of genetic variation in the transition from acute to chronic pain, the development of chronic pain after surgery, and their treatment.
"Information from this study could be the beginning of a systematic approach to develop a method of predicting pain threshold and morphine requirement for pain relief," the researchers said.
A research article on the study has been published in the journal Anesthesiology.