Anal incontinence can have a significant impact on quality of life. Obstetric Anal Sphincter Injuries (OASIS) are a major aetiological factor for anal incontinence. Although episiotomy is one of the most commonly performed obstetric procedures, current evidence on the impact of episiotomy on the incidence of OASIS is conflicting (1). The current recommendation from the Royal College of Obstetricians and Gynaecologysts (RCOG) is to consider a mediolateral episiotomy routinely for instrumental deliveries, but not for spontaneous vaginal deliveries (2). The aim of this study was to estimate the incidence of OASIS over a ten-year period and to evaluate the impact of episiotomy on the incidence of OASIS in different methods of delivery.

A retrospective cohort study was undertaken. Data on 50,997 deliveries were reviewed from the maternity database of two maternity units within an NHS Trust between 1st of January 2008 and 31st of December 2017. A total of 29,954 live singleton vaginal deliveries of more than 37 weeks’ gestation were included in the study. Vaginal births were categorised into two groups – those with OASIS and those without. OASIS were classified according to the RCOG classification. Descriptive statistics were applied to describe the cohort. Logistic regression was used to derive adjusted odds ratios (aOR), along with 95% Confidence Intervals (CIs), to estimate the effect of episiotomy on the incidence of OASIS. The effect of episiotomy was estimated for the entire cohort, regardless of the method of delivery, plus for each method of delivery separately. For each, episiotomy was compared against no episiotomy (reference category). The odds ratio were adjusted for all other maternal and birth characteristics. Traditional statistical hypothesis testing with a critical level of significance of 0.05 (5%) was used.

Of the 29,954 women, 1,291 (4.3%) sustained OASIS. The women who had OASIS were more likely to be of Asian ethnicity than those that did not (25.8% vs 12.3%), more likely to have had a ventouse delivery (10.1% vs 6.1%), forceps delivery (27.4% vs 9.7%), and an episiotomy (37.5% vs 17.0%), as shown in Table 1. 
In 2008 the incidence of OASIS was 3.47% and increased to 5.37% in 2017. The rate of episiotomy rose from 15.30% in 2008 to 18.20% in 2017. Overall, episiotomy was performed in 1375 (5.5%) spontaneous vaginal births, 2007 (68.6%) ventouse and 1984 (94.4%) forceps assisted deliveries. Temporal trends of severe perineal tears, use of episiotomy, and instrumental deliveries are shown in Figure 1. 
Of the women who have had episiotomy, 8.2% (n=250) sustained OASIS, compared to 91.8% (n=2790) who did not experience OASIS. Overall, the use of episiotomy was associated with a significantly reduced risk of OASIS (aOR = 0.71; 95% CI: 0.53 to 0.95; P=0.002). Each method of delivery was subsequently considered separately. The routine use of episiotomy in forceps delivery was also associated with a significantly reduced risk of severe perineal tears (aOR = 0.45; 95% CI: 0.26 to 0.80; P=0.006). However, when episiotomy was performed in spontaneous vaginal and ventouse assisted births no statistically significant effect was observed (aOR = 0.84; 95% CI: 0.55 to 1.29; P=0.427); (aOR = 0.77; 95% CI: 0.46 to 1.30; P=0.336).
Regarding the known risk factors for OASIS, there were several temporal changes in their prevalence. The rates of primiparous women increased from 21.2% in 2008 to 39.3% in 2017. Asian ethnic background, a known risk factor for OASIS, rose from 12.3% in 2008 to 14.1% in 2017. The incidence of shoulder dystocia decreased from 2.1% to 1.7% over the study period. There were no significant changes in the mean birthweight over time.

The observed increase in the incidence of OASIS is consistent with previously published studies (3). This finding may be attributed to increased awareness and improved clinical diagnosis and introduction of systematic classification of severe perineal tears by the RCOG. Moreover, certain risk factors may have contributed to the time trends in the incidence of OASIS including increased rates of primiparity, Asian ethnicity and incidence of forceps deliveries. 
Our findings indicate that episiotomy performed during forceps delivery is associated with a significantly lower incidence of OASIS signifying a protective effect. However, this observation did not feature in ventouse deliveries. Restrictive or selective use of episiotomy in these cases may be advisable. 
By identifying risk factors for OASIS, the selective use episiotomy in high-risk women (primiparas, Asian, etc.) may result in superior perineal outcomes and may be more restricted in low-risk cases.

Overall, episiotomy was found to be associated with a lower risk of OASIS. Routine use of episiotomy in forceps assisted deliveries was associated with a lower risk of OASIS, whilst this effect was not observed in spontaneous vaginal deliveries and vacuum assisted births. Further prospective studies may provide more robust evidence on the effect of episiotomy in the prevention of OASIS.

Jiang H, Qian X, Carroli G, Garner P. Selective versus routine use of episiotomy for vaginal birth. Cochrane Database of Systematic Reviews. 2017
The Royal College of Obstetrician and Gynaecologists. The Management of Third- and Fourth-Degree Perineal Tears (Green-top Guideline No. 29). 2015
Doumouchtsis S, Fahmay Y, Sedgwick P, Durnea C. A comparative study of obstetric anal sphincter injuries in vaginal deliveries of twins and singleton pregnancies. Neurourology and Urodynamics. 2018;37(8):2717-2723