Pain Perception – It’s All in the Genes

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Previous studies have shown that women experience more severe postoperative pain and require more narcotics than men in the early postoperative period. A study featured in the September issue of Anesthesiology investigates women’s pain perception and relief after Caesarean section and the impact of genetics on these outcomes.

The study, authored by Alex T. Sia, M.D., and colleagues at KK Women’s & Children’s Hospital in Singapore, evaluated 588 women who were injected with morphine in the spinal canal after delivering their children via Caesarean section.

Dr. Sia indicated that, “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.”

Previous studies have shown that genetic variability at position 118 of the human m-opioid receptor gene altered patients’ responses to intravenous morphine. This new study included women receiving spinal injections of morphine for post Caesarean analgesia.

Based on their blood samples, women participating in the study fell into one
of three genotype groups for the m-opioid receptor alleles: homozygous AA, homozygous GG, and heterozygous AG.

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 major finding was that the various genetic types correlated with a significant variability in morphine consumption after Cesarean section. 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.

The study also showed that individuals with the AA genotype had the highest risk of developing nausea (but not vomiting), despite lower consumption of PCA morphine.

From this research, it can be inferred 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. It could also be inferred that the greater analgesic sensitivity to morphine in the AA group is also related to a higher risk of developing nausea.

As a result, the study showed that various forms of the human opioid receptor has a significant effect on pain perception, analgesic requirement, and nausea for the first 24 hours after cesarean section.

That said, Dr. Sia noted that the findings likely have important implications in differences among women’s pain perception in general, pain after childbirth, and narcotic use after surgery. Furthermore, the study has 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.

For more information visit the Anesthesiology Web site at http://www. anesthesiology.org and the American Society of Anesthesiologists Web site at http://www.asahq.org.

Founded in 1905, the American Society of Anesthesiologists is an educational, research and scientific association with 43,000 members organized to raise and maintain the standards of the medical practice of anesthesiology and improve the care of the patient.

Editor’s Note: The reference to GG in paragraph nine in an earlier version of this release has been changed to AA. The sentence now reads “It could also be inferred that the greater analgesic sensitivity to morphine in the AA group is also related to a higher risk of developing nausea.”

Source: American Society of Anesthesiologists (ASA)

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