Obstetric anal sphincter injury (OASIS) is a serious complication of a vaginal delivery. High proportion of women, 40-59%, suffer from faecal incontinence (FI) after this type of injury.1-3 OASIS and FI have a negative impact on women's quality of life.4 The rate of elective caesarean section at second birth was much higher in women with OASIS at first birth compared with women without the injury (adjusted odds ratio 18.3, 95% CI 16.4-20.4).5 Primiparity has an increased risk of OASIS (odds ratio (OR) 2.39-8.34).6,7 Other factors are macrosomia and instrumental vaginal delivery.6,7 The role of episiotomy on OASIS has also been widely studied but there are controversial results.

There were around 500-7000 deliveries at Prince of Wales Hospital annually. Among them, 50-55% was nulliparous women. About 65 -70% of women had normal vaginal delivery and another 5-10% had instrumental delivery. There was a change from 'routine episiotomy' to a more restrictive use of episiotomy in normal vaginal delivery during the last 10 years, with a reduction of rate of episiotomy from 90% to about 50% (from Dept.'s internal audit). The objective of this study is to evaluate the prevalence of OASIS in the era of a reduction of episiotomy.

Obstetric anal sphincter injury (OASIS) is a serious complication of a vaginal delivery. High proportion of women, 40-59%, suffer from faecal incontinence (FI) after this type of injury.1-3 OASIS and FI have a negative impact on women's quality of life.4 The rate of elective caesarean section at second birth was much higher in women with OASIS at first birth compared with women without the injury (adjusted odds ratio 18.3, 95% CI 16.4-20.4).5 Primiparity has an increased risk of OASIS (odds ratio (OR) 2.39-8.34).6,7 Other factors are macrosomia and instrumental vaginal delivery.6,7 The role of episiotomy on OASIS has also been widely studied but there are controversial results.

Episiotomy is a surgical enlargement of the vaginal orifice by an incision to the perineum during the last part of the second stage of labour or delivery.8 It is accepted that episiotomy facilitates delivery, prevents perineal lacerations and undue stretching of the pelvic floor.9 It is also believed that an episiotomy is easier to repair and heals better than a tear.10 Mediolateral episiotomy is the most frequently used type of episiotomy in Hong Kong. Some obstetric units perform episiotomy more liberally on a routine basis; while others adopt a more restrictive policy. Cochrane review identified eight randomized controlled trials (RCT) comparing routine episiotomy and restrictive episiotomy. 9,10-17 The restrictive use of episiotomy shows a lower risk of severe perineal trauma (relative risk (RR) 0.67, 95% CI 0.49-0.91), posterior perineal trauma (RR 0.88, 95% CI 0.84-0.92), need for suturing perineal trauma (RR 0.71, 95% CI 0.61-0.81), and healing complications at seven days (RR 0.69, 95% CI 0.56-0.85).13 No difference is shown in the incidence of major outcomes such as severe vaginal and perineal trauma (when both vaginal and perineal trauma are counted) nor in pain, dyspareunia or urinary incontinence. However, the restrictive use of episiotomy has an increased risk of anterior perineal trauma (RR 1.84, 95% CI 1.61-2.10).13 Therefore, it is not advised to practice routine episiotomy.

From a large retrospective studies conducted in Caucasian countries, mediolateral episiotomy has been shown to protect women from OASIS (OR 0.21-0.54). 6,7,18 Besides, there was also evidence showing that Asian women delivered in Caucasian countries with a low episiotomy rate had a high risk of OASIS.19,20 Furthermore, ethnical difference in pelvic connective tissues have been reported.21 And pregnant Chinese women has been shown to smaller genital hiatus and less mobility of pelvic organs than pregnant Caucasian women.22 This may lead to a different outcome in perineal trauma in Asian women with or without episiotomy during vaginal delivery.

There were around 500-7000 deliveries at Prince of Wales Hospital annually. Among them, 50-55% was nulliparous women. About 65-70% of women had normal vaginal delivery and another 5-10% had instrumental delivery. There was a change from 'routine episiotomy' to a more restrictive use of episiotomy in normal vaginal delivery during the last 10 years, with a reduction of rate of episiotomy from 90% to about 50% (from Dept.'s internal audit). The objective of this study is to evaluate the prevalence of OASIS in the era of a reduction of episiotomy. This would help inform obstetricians and midwives if there is any change in rate of OASIS and the relationship with episiotomy. The information would be useful to counsel our patients.

The protocol of current study complies with Declaration of Helsinki.

The HA CMS OBSCIS is the electronic system that have collected the demographic, pregnancy, delivery and postnatal data of pregnant women. The data was entered by midwives and obstetricians during all consultations and admission of delivery of all pregnant women.

In this study, the following information will be retrieved from the CMS OBSCIS database for all delivery conducted from 2011 to 2021.

1. Demographic data: age, parity, body weight and height
2. Past obstetric history (if any): mode of delivery, birthweight and sex of infant, use of episiotomy
3. Delivery data: gestation at delivery, need of induction of labour, analgesics during labour, duration of labour, mode of delivery, use of episiotomy, any perineal tear, blood loss during delivery, need of transfusion, wound complication, duration of hospital stay; and infant's birthweight, sex and apgar score
4. For women with OASIS: the method of repair, the symptom of pelvic floor disorders, such as faecal or flatal incontinence, urinary incontinence, symptoms of prolapse after the delivery.

Inclusion Criteria:

• All deliveries conducted from 2011 to 2021 in Prince of Wales Hospital

Exclusion Criteria:

• Deliveries with Caesarean Section

Women who had delivered in our unit from 2011 to 2021

The target sample size will be about 67,000 women who had delivered in our unit from 2011 to 2021. Among them, it is expected that 75-80% had vaginal deliveries, either normal vaginal delivery or instrumental delivery. 53,600 will be recruited in study site.

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