Genetic polymorphism (A118G) in the μ-opioid receptor has been reported to affect systemic opioid analgesia. However, reported pharmacogenetic effects on spinal opioid analgesia, particularly in labour, have been equivocal.Methods
We prospectively assessed effects of the μ-opioid receptor A118G single nucleotide polymorphism (SNP) on analgesia after 20 μg of spinal fentanyl. We studied two ethnically distinct hospital populations (Miami and Jerusalem). Independent variables were A118G, ethnicity, and hospital. Primary outcome was time from spinal analgesia until analgesic request. Secondary outcomes were pain and pruritus, assessed at repeated intervals until analgesia request.Results
One hundred and twenty-five nulliparous parturients in early labour were analysed. The allelic frequency of A118G was 14.8% (14.4% in Miami; 15.5% in Jerusalem). Time to analgesia request (SD) in Miami was 122 (44) min and in Jerusalem was 87 (32) min, P<0.001; Hispanic 123 (46) min vs Jew/Arab 87 (32) min, P<0.001; Black 121 (41) min vs Jew/Arab 87 (32) min, P=0.015. There was no significant effect of A118G. Survival analysis showed Miami > Jerusalem, P<0.001; Hispanics and Black > Jew/Arab, P<0.001; no effect of A118G. Within hospital groups, A118G had no effect on time to analgesic request; within genomic groups there was a significant difference between hospitals. The time-course for pruritus exactly paralleled the time-course for analgesia and was affected by hospital (P=0.006) and by ethnic group (P=0.03), but not by A118G.Conclusions
We found no significant effect for the A118G single nucleotide polymorphism (SNP) on analgesic duration after spinal fentanyl for labour. In contrast, ethnically distinct hospital population groups exerted a marked effect on the time-course of both analgesia and pruritus.