Factors associated with severe obstetric anal sphincter injury following vaginal delivery

Obstetric anal sphincter injury (OASI) is a major complication of vaginal delivery. Tear severity can be categorized according to the structures involved. To date, little is known regarding risk factors for severe OASI tears. We aimed to evaluate the association between known risk factors for OASI and tear severity.

This was a retrospective cohort study, performed at a tertiary university hospital. The study population included women aged 18-50 years who underwent a vaginal delivery and were diagnosed with OASI. The study cohort was divided into two groups based on OASI severity according to OASI grade: low-grade (3A and 3B) and high-grade (3C and 4). A comparison was performed between the two groups regarding demographic, obstetric and labor related parameters. Quantitative independent variables were compared using Student's t or Mann-Whitney tests and presented as either mean ± standard deviation (SD) or median (interquartile range). The association between categorical variables was tested using either the Chi-squared test or Fisher's exact test, as appropriate. A multivariate regression model was applied to obtain adjusted odds ratios (aOR) for parameters found to be associated with high-grade OASI tear in the univariate model.

Two hundred and sixteen patients within the study group and 405 within the control group were included in the final analysis. Patients with high-grade OASI had increased mean parity (0.56 ± 1.0 vs. 0.39 ± 0.7, p = 0.04) and were less likely to be nulliparous (66.2% vs. 74%, p = 0.03). Operative vaginal delivery (37.0% vs. 28.8%, p = 0.03) was associated with an increased risk of high-grade OASI while artificial rupture of membranes (AROM) was more common among the low-grade group (50.3% vs. 41.2%, P = 0.03). Multivariate logistic regression analysis was conducted for the dependent parameters of high grade OASI. Operative vaginal delivery (OR 1.72, CI 1.16-2.55, p < 0.01) and higher parity (OR 1.23, CI 1.00-1.51, p = 0.04) were associated with high-grade OASI. AROM (OR 1.49, CI 1.04-2.14, p = 0.02) was more common among the low grade OASI tears.

Operative vaginal delivery and higher parity are factors associated with more severe OASI, while artificial rupture of membranes was related to lower tear severity.