Epidural Analgesia and the Risk of Obstetric Anal Sphincter Injury

There are conflicting data regarding the association between epidural analgesia and obstetric anal sphincter injuries (OASIs). There are also few predictive models that document assessment for multicollinearity among risk factors or precisely identify effect modifiers and confounders.

The objective of this study was to evaluate the association between epidural analgesia and OASIs, using rigorous statistical methods to identify effect modifiers and confounders in a diverse, urban population.

This was a secondary analysis of a retrospective case-control study of women with singleton vaginal deliveries. Our primary analysis was to assess the association between epidural analgesia and OASIs, adjusting for modifiable and nonmodifiable risk factors. These risk factors were additionally assessed for effect modification and confounding effect.

Our sample consisted of 1,326 women who had a vaginal delivery. Among the women with OASIs, 218 women (75.4%) had an epidural, and among those without OASIs, 620 women (59.8%) had an epidural ( P  < 0.001). Epidural analgesia increased the odds of OASIs by 107% (odds ratio 2.07, 95% confidence interval [1.54 to 2.77]). Through further examination of the relationship between OASIs and epidurals, while considering multiple risk factors and addressing multicollinearity, effect modification, and confounding, we identified operative delivery and labor induction as confounding variables. Accounting for these factors mitigated the association between epidural use and OASIs (adjusted odds ratio 1.38, 95% confidence interval [1.00 to 1.91]).

Initially, we found epidural analgesia to be associated with OASIs; however, after performing additional statistical analysis and accounting for additional risk factors, this association was no longer statistically significant.