Abstract
Obstetric anal sphincter injuries (OASI) occur in around 6.1% of first vaginal births and around 5–7% of subsequent vaginal births. The multidisciplinary OASI clinic plays a vital role in the postnatal, and subsequent antenatal, management of women with a history of OASI. This review will outline the recommended best practice for the outpatient postnatal management of women who have sustained a recent OASI and also the current guidance on mode of birth recommendations in a subsequent pregnancy after an OASI. It will explore the evidence supporting current practice recommendations to facilitate clinicians leading mode of birth discussions for women with a previous OASI.
Introduction
Childbirth-related perineal trauma (CRPT) occurs in around 85% of vaginal births and its severity is classified depending on the anatomical structures involved. A first degree tear involves the perineal skin only and a second degree tear involves the perineal muscles but not the anal sphincters ( Figure 1 ).
An obstetric anal sphincter injury (OASI) is the most severe type of childbirth-related perineal trauma and complicates 2.9% of all vaginal births in the UK with an incidence of 6.1% for those women having their first vaginal birth.
The Sultan classification is widely used to classify the severity of an OASI based on the anatomical structures involved ( Table 1 ). Isolated rectal injury is not a part of the spectrum of OASI; recurrence is probably unusual and much less predictable and outcomes are usually excellent with correct repair. It may be associated with fistula formation if missed or repaired inadequately.
| Classification | Structures involved |
|---|---|
| 3a | Less than 50% of the external anal sphincter (EAS) thickness torn |
| 3b | More than 50% of the EAS thickness torn |
| 3c | Both EAS and internal anal sphincter (IAS) torn |
| 4 | Anal sphincter complex and anal mucosa torn. |
| Isolated rectal (buttonhole) injury | The anal sphincters are intact but an isolated tear occurs in the anal epithelium or rectal mucosa. This is NOT an OASI. |
Between 2000 and 2012 the reported rate of OASIS for first vaginal births in England tripled from 1.8% to 5.9%. It is widely assumed that this increase was observed as a result of improved rates of detection of OASI and not due to an actual rise in the number of women sustaining a severe tear. Whilst these figures may not indicate a deterioration in quality of care it did clearly highlight more accurately the numbers of women who require good quality follow up to maximize their short and long term health outcomes.
Risk factors for obstetric anal sphincter injury
An OASI can occur spontaneously or as an extension of an episiotomy, a surgical incision on the perineum, during a vaginal birth. See Box 1 .
It is important to remember that assessment of risk factors does not predict all OASI – very many OASI occur in women with no obvious risk factors – and currently there are no validated models available to predict an individual woman’s chance of sustaining an OASI. Furthermore many are unavoidable. All vaginal births should therefore be carefully managed to reduce the chances of severe perineal tearing.
Incidence and risk factors for recurrence of obstetric anal sphincter injury (rOASI)
Subsequent vaginal birth after a previous OASI carries a 5–7% risk of recurrence nationally (rOASI); the risk of OASI in women with previously unaffected vaginal birth is 1.7% ( Table 2 ). It can be helpful to explain this risk to women in such a way that they understand that if they have previously sustained an OASI then they are no more likely to sustain one again; they are approximately three times more likely to sustain an OASI with their next vaginal birth than someone with a previously unaffected birth, and they do not benefit from the risk reduction seen in women who have never had an OASI before.
| First vaginal birth | 6.1% risk of occurrence |
| Vaginal birth after previous OASI | 5–7% risk of recurrence |
| Vaginal birth after no previous OASI | 1.7% risk of occurrence |
Whilst a subsequent unaffected vaginal birth is reassuring there is no formal statistical evidence available about the chance of sustaining a rOASI in any subsequent birth after that; the recurrence risk of 5–7% continues to be used for counselling conversations, but the authors use this as a reason for optimism.
Purpose of the OASI Clinic
Rising numbers of diagnosed OASI and an increasing understanding of the detrimental impact of anal sphincter injury on women’s physical and psychological health has led to a recent national drive to improve the availability and access to specialist perineal trauma services. The ‘OASIS syndrome’ is a term that has been coined to describe the effect of anal incontinence on quality of life (QOL). It is common to see effects on intimacy, relationships, and on parenting experiences. Women may feel unclean, isolated, or mutilated, and describe the taboo of discussion.
Whilst appropriate and skilled repair is vital, it is only part of the overall package of care needed; postnatal follow up and future pregnancy advice is equally important in affecting long term outcome. High patient satisfaction scores associated with specialist perineal clinics that discuss mode of birth are seen when the advice is evidence based and consistent.
The Royal College of Obstetricians and Gynaecologists’ (RCOG) Green Top Guideline No.29 provides guidance on the “gold standard” for OASI follow up that should be offered by NHS perineal trauma services. They recommend that a review at between 6 and 12 weeks postpartum, ideally by a clinician with a special interest in OASI, should be available to all women who have sustained obstetric anal sphincter injury. This review is not dependent on severity of anal sphincter injury, nor on the presence or absence of symptoms. If this routine follow up reveals any ongoing concerns of pain or incontinence associated with their anal sphincter injury then a referral to specialist gynaecology or colorectal services should be offered.
In a subsequent pregnancy all women should have a conversation about recommendations for mode of birth and this discussion must be recorded in their pregnancy notes. For those women who are symptomatic of anal sphincter dysfunction or have evidence of abnormal endoanal ultrasonography and/or manometry this discussion on mode of birth must include the option of a planned caesarean section. Anal sphincter defects and/or symptoms imply a probable loss of anal sphincter functional reserve; further injury may therefore be associated with deterioration in quality of life because of anal incontinence. Where there are no anatomical or functional deficits, further injury is likely to be associated with good functional outcomes.
The 2019 NHS long term plan set out to “ensure that women have access to multidisciplinary pelvic health clinics and pathways across England” and this ambition was further cemented by the Perinatal Pelvic Health Service (PPHS) service specification in 2022. The PPHS document recommends that pathways and protocols must be in place for those women who sustain an injury to the anal sphincter during birth.
There are various templates of the OASI Clinic seen across the UK; some led by a specialist midwife, some by a gynaecologist, some by a physiotherapist and others as a team approach. No one type of clinic appears superior to the other but the onus should be on collaboration and multidisciplinary working to ensure the best outcomes for those women recovering from an OASI. It is important not to underestimate the value and contribution of each profession for this patient group and seamless access to all required specialty services should be the norm. The PPHS service specification recommends a multidisciplinary model to promote informed decision making and to facilitate the development of personalized care plans for women with postnatal pelvic health concerns including recovery after an OASI. Whilst the general NHS guidance is that non-surgical options should always be considered before surgical treatment for pelvic health issues, cooperation between specialities – including colorectal, urology, urogynaecology and gynaecology – is imperative to secure timely access to surgical intervention if required.
Postnatal management
Good quality postnatal management following an OASI is important for both the physical and psychological recovery of women affected by this injury. It is vital to ensure that when women are discharged home following an OASI that they have information about the nature of their injury, what to expect during their recovery, and where to go to obtain any help they may need prior to their routine follow up appointment. Provision of comprehensive information, acknowledgment of the severity of the injury they have sustained, and validation of their associated feelings helps to empower them to seek professional help if they have any physical or psychological issues related to their anal sphincter injury.
Confirming a postnatal clinical history
When a woman presents at the postnatal clinic it is important to obtain a thorough history, confirm details of her injury, and manage her expectations about her recovery timeframes. The specific details about the exact classification of her injury and the method of repair used will help to provide context to any symptoms she may be experiencing, and images obtained on ultrasound. Quality of the immediate post operative debrief will vary across units as will an individual woman’s recollections, therefore a full explanation about the injury should be provided including some discussion around her individual risk factors for the injury she sustained. The Postnatal OASI Clinic provides an opportunity to see how well she is recovering from her injury and to ascertain if any treatment or support is required to optimize healing and long term pelvic health outcomes.
Postnatal investigations
The RCOG recommends that follow-up of women with OASI should be in a dedicated clinic with access to endoanal ultrasonography and anal manometry as this can aid decision making regarding future delivery. There is still much variation across the UK around what investigations, if any, are offered to women in an outpatient setting but with the development of local PPHS teams access to imaging is becoming more readily available. Currently, some units offer endoanal ultrasound to all women, some to only those women who sustained injuries of grades 3c and 4th, and some to those only with symptoms of anal incontinence. Some units will offer manometry but this is less widely available as a routine investigation and its necessity is less clear in this patient group.
Endoanal ultrasound (EAUS): EAUS is an ultrasound of the anal sphincters and can be easily undertaken in an outpatient setting as it requires no special considerations such as the need to fast, bowel prep or sedation. EAUS can be performed by any clinician (midwife, physiotherapist, doctor) provided they have had appropriate training and have been agreed as locally competent to obtain and interpret the images. EUAS involves the insertion of an ultrasound probe to an approximate depth of 5 cm into the ano-rectum and a 360 degree image is obtained of the full length and thickness of the anal sphincters and anal canal. The examination can be uncomfortable due to its nature but is rarely described by patients as painful and usually only takes a few minutes to complete. An EAUS obtains images of the anal epithelium, soft tissues, pubo-rectalis and ano-rectalis muscles, the internal anal sphincter (IAS) and the external anal sphincter (EAS) ( Figure 2 ). These images can be examined to look at their condition post injury and repair and identify any defects (“gap”) in the sphincter muscles where the primary surgical repair has unfortunately not maintained muscle integrity to the intended level ( Figure 3 ). There is known to be a strong correlation between sonographic anal sphincter defects following OASI and symptoms of anal sphincter dysfunction. In the postnatal setting the images are used alongside any symptoms a woman may be experiencing to guide care planning but EAUS images are most useful when planning mode of future birth in any subsequent pregnancy.
