Obstetric anal sphincter injuries (OASIS) occur in 2-6% of vaginal births, but the rate is significantly higher in operative vaginal birth compared to unassisted vaginal birth. There is a two- to three-fold increased risk with vacuum assisted births and four to eight-fold increased risk with forceps births.¹

The focus of guidelines and recommendations for the management of OASIS has evolved considerably over the past decade. Contemporary management extends beyond successful surgical repair to encompass prevention, standardised diagnosis, multidisciplinary follow-up, pelvic floor rehabilitation, and shared decision-making regarding future pregnancies.^1-4

Prevention of OASIS

A major shift in practice has been the emphasis on preventing OASIS through evidence-based intrapartum care. Current guidelines, such as the Women’s Healthcare Australasia (WHA) Perineal Protection Bundle, recommend mediolateral episiotomy at an angle of approximately 60° when clinically indicated, manual perineal protection during crowning, and the use of warm perineal compresses during the second stage of labour.^1–5 These strategies have been associated with reduced rates of severe perineal trauma and are now incorporated into many maternity unit prevention programs.^2-5

Episiotomy

The first documented description of a perineal incision to facilitate childbirth is generally attributed to the Irish obstetrician Sir Fielding Ould in 1742 – although, without appropriate anaesthesia and antiseptic techniques, the procedure was rarely performed at that time.⁶ During the 19^th century, the introduction of ether anaesthesia and antisepsis made episiotomy a more feasible intervention during difficult vaginal births.⁷

The routine use of episiotomy expanded rapidly during the early 20^th century. Many obstetricians advocated prophylactic episiotomy, arguing that a controlled surgical incision would prevent spontaneous perineal tears, preserve pelvic floor function, reduce urinary and faecal incontinence, and minimise neonatal trauma.^8,9 By the 1970s, routine episiotomy had become standard practice in many maternity units, with rates exceeding 60–90% among primiparous women in some countries.⁸

Fig. 1. 1951 Episiotomy scissors: A patent illustration of episiotomy scissors designed by Fred Haufrect in 1951.

Current guidelines suggest that episiotomy should be performed selectively, based on clinical indications. Routine episiotomy does not reduce severe perineal trauma, pelvic floor dysfunction, urinary incontinence, or neonatal morbidity. Instead, it is associated with increased maternal pain, blood loss, wound complications, and extended recovery.¹⁰ A Cochrane review demonstrated that a selective approach to episiotomy resulted in better maternal outcomes without compromising neonatal safety.¹⁰ Recent data would suggest the rate in Australia is similar to the UK, at 23% of vaginal births in 2023.¹¹ In the US, however, the rate has rapidly reduced over the last two decades, from more than 30% to less than 5%.¹²

When cutting an episiotomy, the angle is key, with evidence suggesting that the risk of concurrent OASIS is higher if the healed angle is less than 40° or more than 60° from the midline.¹ Most guidelines encourage aiming for an incision 60° from the midline when the perineum is distended at crowning, which typically would correspond with a post-delivery angle of about 45°.

The EPISCISSORS-60® were developed in the UK by Professor Robert Freeman and his team in the early 2010s and can be helpful to illustrate the appropriate angle to cut an episiotomy when teaching junior staff how to perform the procedure safely and accurately. Some maternity units encourage the routine use of these scissors for all clinicians. A 2026 systematic review reported a reduction in overall OASIS rates associated with EPISCISSORS-60®, while concluding that further adequately powered randomised controlled trials are required.¹³

Fig. 2. EPISCISSORS-60.

Recognition and Classification

Perineal tears are classified according to the structures involved:¹

• First-degree: Injury to vaginal mucosa and perineal skin only.
• Second-degree: Injury involving perineal muscles without anal sphincter involvement.
• Third-degree: Injury involving the external and/or internal anal sphincter.
  • 3a: Less than 50% of external anal sphincter (EAS) thickness torn.
  • 3b: More than 50% of EAS thickness torn.
  • 3c: Both EAS and internal anal sphincter (IAS) torn.
• Fourth-degree: Injury extending through both sphincters into the anorectal mucosa.

Fig. 3. OASIS Classification.

Current recommendations emphasise systematic examination of the perineum following every vaginal birth, including visual inspection, vaginal examination, and digital rectal examination.^1,7 These assessments should always take place with proper informed consent from the woman, which involves ensuring adequate analgesia and explaining the reasons for the examination and detailing what will occur. It is especially important (but sometimes challenging) to identify injuries involving the IAS.

The IAS lies immediately deep to the EAS and is a smooth, pale pink, or pearly tissue. If separated, the IAS often retracts slightly and may curl inward. Injuries to the IAS are associated with poorer long-term continence outcomes if not repaired appropriately.^1,4 A buttonhole tear is a rare injury where there is a defect in the rectal mucosa, potentially with the sphincters intact. Such an injury can only be reliably identified with a careful vaginal and digital rectal examination for all birthing women. Failure to identify these tears can result in a rectovaginal fistula.

Surgical Repair

Current guidelines recommend that OASIS repair be undertaken by clinicians with appropriate training or under direct supervision in an operating theatre with regional or general anaesthesia.^1,4 Increasingly, babies and support people will go to theatre with the woman if that is her preference. In certain situations, repairing an OASIS on the labour ward may be appropriate if there is adequate analgesia, lighting, and equipment.

The IAS should be repaired separately when identifiable, while either end-to-end approximation or overlap repair of the external anal sphincter (EAS) is considered appropriate depending on the nature of the injury and surgeon expertise.^1,4 End-to-end approximation is generally recommended for partial-thickness tears (grade 3a and some 3b injuries). An overlap repair is often preferred for complete-thickness EAS injuries.

Current evidence suggests that overlap and end-to-end repair produce comparable long-term continence outcomes when performed appropriately by experienced clinicians.¹ Delayed absorbable synthetic sutures are recommended for sphincter repair.

Typical choices include:

• 3-0 polyglactin (Vicryl) for anorectal mucosa.
• 2-0 polyglactin or polydioxanone (PDS) for anal sphincter repair.

Current evidence demonstrates similar clinical outcomes with both materials.

Fig. 4. Overlap technique (images on the left) and end-to-end technique (images on the right). Source: https://pmc.ncbi.nlm.nih.gov/articles/PMC8347477/figure/jcm-10-03261-f004/.

Postoperative Care

Postoperative management of OASIS has become increasingly standardised. Current recommendations include prophylactic broad-spectrum antibiotics, often administered as a single course of IV Cefazolin and Metronidazole at the time of repair. Guidelines and local hospital policies vary on whether a postoperative course of oral antibiotics are prescribed. Other recommendations include postoperative laxatives or stool softeners, adequate analgesia, and debriefing at the time and again prior to discharge with clear written discharge instructions regarding bowel care and wound management.^1,3,4 These interventions promote healing while reducing patient discomfort and complications such as wound infection and dehiscence.

Pelvic Floor Rehabilitation

There is growing evidence supporting early pelvic floor muscle training following OASIS. Consequently, referral to specialist pelvic floor physiotherapy is now recommended as part of routine care rather than only for women who develop symptoms.² Early rehabilitation may improve continence, pelvic floor muscle strength, sexual function, and overall quality of life.

Follow-up and Future Pregnancy

Women should receive specialist follow-up between six and 12 weeks postpartum to assess wound healing, bowel symptoms, pain, continence, and psychological wellbeing.^1,3,4 Many larger centres have dedicated OASIS clinics, while smaller units or private care providers follow these women up in general clinics. Approximately 60–80% of women remain asymptomatic 12 months after repair.¹ However, continence may deteriorate over time due to ageing, menopause, and subsequent vaginal births. Some women may experience ongoing faecal or flatal incontinence, chronic perineal pain, and dyspareunia

Women with persistent symptoms should be offered further assessment, including endoanal ultrasound and anal manometry where available, to guide management and counselling regarding future pregnancies.⁵ Some units also offer these investigations for asymptomatic women to assist with decision-making about the mode of birth in a subsequent pregnancy. Current recommendations support shared decision-making, taking into account symptoms, imaging findings, and the woman’s preferences rather than recommending elective caesarean birth for all women with previous OASIS.^3,4 If given the opportunity, most women elect to have a vaginal birth in their next pregnancy, with a recurrence rate generally quoted between 5-10%.^14-16

The psychological and social impact of OASIS for some women is profound. Anxiety, depression, post-traumatic stress symptoms, sexual dysfunction, altered body image, social isolation, general reduced quality of life, and fear of future childbirth are consistently reported.^17,18 Clinicians need to provide compassionate, individualised care and be mindful that emotional recovery may take longer than physical recovery. Caring for these women is best done in a multidisciplinary team, including support people and primary care providers.