Could a mediolateral episiotomy prevent obstetric anal sphincter injury?

Objective: To analyse the significance of risk factors and the role of episiotomy in preventing obstetric anal sphincter injury at vaginal delivery.

Study design: This is a retrospective cross-sectional study in the Norfolk and Norwich University Hospital in the UK. All caesarean sections and non-vertex presentations were excluded, which resulted in a study population of 10,314 deliveries. Obstetric anal sphincter injury (OASI) was defined as third or fourth degree tears to the anal sphincter muscles, with or without a tear involving the anal mucosa. First a univariate analysis was done to identify factors that had a significant association with OASI. Factors included parity, age, gestation, labour induction method, duration of second stage, use of epidural analgesia, episiotomy, method of delivery, time and month of delivery, and birth weight. All factors were then combined in a multivariate logistic regression analysis. The multivariate analysis was then repeated including only factors that had a significant association with OASI in the univariate analysis. Adjusted odds ratios with 95% confidence intervals (CI) were calculated.

Results: The frequency of anal sphincter lacerations was 3.2%. There were statistically significant associations between an increased incidence of OASI and parity, birth weight, method of delivery and shoulder dystocia. Women giving birth without a mediolateral episiotomy were 1.4 times more likely to experience OASI (95% CI 1.021-1.983). Interestingly, the incidence of OASI has risen between 2005 and 2007.

Conclusion: Parity, age, birth weight, method of delivery and shoulder dystocia are strongly associated with obstetric anal sphincter injury. Mediolateral episiotomy appears to be protective against OASI but a randomised controlled trial would be needed to confirm this. The rising incidence of OASI after normal vaginal deliveries may be related to adoption of the hands off technique or increased identification of tears.