Predicting obstetric anal sphincter injuries in a modern obstetric population

Background: Perineal lacerations are common at the time of vaginal delivery and may predispose patients to long-term pelvic floor disorders, such as urinary incontinence and pelvic organ prolapse. Obstetric anal sphincter injuries, which are the most severe form of perineal lacerations, result in disruption of the anal sphincter and, in some cases, the rectal mucosa during vaginal delivery. Long-term morbidity, including pain, pelvic floor disorders, fecal incontinence, and predisposition to recurrent injury at subsequent delivery may result. Despite several studies that have reported risk factors for obstetric anal sphincter injuries, no accurate risk prediction models have been developed.

Objective: The purpose of this study was to identify risk factors and develop prediction models for perineal lacerations and obstetric anal sphincter injuries.

Study design: This was a nested case control study within a retrospective cohort of consecutive term vaginal deliveries at 1 tertiary care facility from 2004-2008. Cases were patients with any perineal laceration that had been sustained during vaginal delivery; control subjects had no lacerations of any severity. Secondary analyses investigated obstetric anal sphincter injury (3rd- to 4(th)-degree laceration) vs no obstetric anal sphincter injury (0 to 2(nd)-degree laceration). Baseline characteristics were compared between groups with the use of the chi-square and Student t test. Adjusted odds ratios and 95% confidence intervals were calculated with the use of multivariable logistic regression. Prediction models were created and model performance was estimated with receiver-operator characteristic curve analysis. Receiver-operator characteristic curves were validated internally with the use of the bootstrap method to correct for bias within the model.

Results: Of the 5569 term vaginal deliveries that were recorded during the study period, complete laceration data were available in 5524 deliveries. There were 3382 perineal lacerations and 249 (4.5%) obstetric anal sphincter injuries. After adjusted analysis, significant predictors for laceration included nulliparity, non-black race, longer second stage, nonsmoking status, higher infant birthweight, and operative delivery. Private health insurance, labor induction, pushing duration, and regional anesthesia were not statistically significant in adjusted analyses. Significant risk factors for obstetric anal sphincter injury were similar to predictors for any laceration; nulliparity and operative vaginal delivery had the highest predictive value. Area under the curve for the predictive ability of the models was 0.70 for overall perineal laceration, and 0.83 for obstetric anal sphincter injury. When limited to primiparous patients, 1996 term vaginal deliveries were recorded. One hundred ninety-two women sustained an obstetric anal sphincter injury; 1796 women did not. After adjusted analysis, significant predictors for laceration included non-black race, age, obesity, and nonsmoking status. In secondary analyses, significant predictors for obstetric anal sphincter injury included non-black race, nonsmoking status, longer duration of pushing, operative vaginal delivery, and infant birthweight. Area under the curve for the predictive ability of the models was 0.60 for any laceration and 0.77 for obstetric anal sphincter injury.

Conclusions: Significant risk factors for sustaining any laceration and obstetric anal sphincter injury during vaginal deliveries were identified. These results will help identify clinically at-risk patients and assist providers in counseling patients about modifications to decrease these risks.