Incidence rate and predisposing risk factors

Incidence rate reporting is complex. The inclusion or exclusion of women with differing risk factors (for example, use of forceps or episiotomy) will alter the denominator population and influence the incidence rate of OASI reported (Abbott 2010). Also the rate is sometimes reported for the whole obstetric population (all primiparous and multiparous births, which will include women with previous OASI) (Hirayama 2012) and sometimes the rate is reported as a subset of the whole obstetric population, for example primiparous (Gurol‐Urganci 2013) or instrumental births only (de Leeuw 2007).

The incidence of OASI seems to be increasing (Gurol‐Urganci 2013; Laine 2009; McLeod 2003). Gurol‐Urganci 2013 reported that the OASI rate tripled in England between 2000 and 2012, (1.8% to 5.9%) and that improved recognition and standardised classification of perineal trauma and changes in second stage care practices such as the declining use of episiotomy, may be contributing to the increasing incidence.

The risk of recurrent OASI was found to be five‐fold higher than the risk for multiparous births with no history of OASI, the number of women included in this study were few and only two women (4.4% confidence interval (CI) 0.54 to 15.5) sustained a subsequent OASI (Harkin 2003). This finding however is supported by Edozien 2014 who reported a five‐fold increase in risk of recurrent OASI (adjusted odds ratio (OR) 5.5, 95% CI 5.2 to 5.9) using data from over 639,000 births. Dandolu 2005 examined 258,507 vaginal births and found that 18,888 (7.31%) women sustained a primary OAIS, 14,990 of these women went on to have a further vaginal birth and 864 (5.76%) sustained a recurrent OASI. Women with a primary fourth degree tear seemed at greater risk of OASI recurrence compared to women with a primary third degree tear (7.73% versus 4.69%) and overall, instrumental delivery accompanied by an episiotomy conveyed the greatest risk of OASI reoccurrence (17.7%). Episiotomy alone increased the risk of recurrent OASI by 50%. Jango 2012 reported a lower rate of primary OASI (7336/159,446, 4.6%) and higher rate of recurrent OASI (521/7336, 7.1%) compared to Dandolu 2005. Jango 2012 found increasing birth weight, ventouse (forceps was not performed in this population) and shoulder dystocia were associated with increased risk of a OASI but episiotomy use was not. The detrimental effect of episiotomy use in women with a previous OAS reported by Dandolu 2005 is opposite to the protective effect reported for episiotomy use during births of primiparous women or women without a history of OASI (de Leeuw 2007; Gurol‐Urganci 2013). The reasons for this difference are unclear, but may be due to increased risk of episiotomy extension from previous perineal scarring (Dandolu 2005).

Asian ethnicity seems to increase the risk of OASI, but this effect seems limited to South Asian women living outside Asia (Wheeler 2011). Increasing maternal age, primiparity, induction and augmentation of labour, length of second stage of labour (> two hours), forceps delivery, increased neonatal head circumference, occiput posterior position and birthweight greater than 4 kg are associated with an increased risk of primary and recurrent OASI (Dandolu 2005; Fizgerald 2007; Hirayama 2012; Kudish 2008; Payne 1999; Peleg 1999; Williams 2005).

Data regarding the incidence of primary and recurrent OASI and the influence of risk factors are limited and sometimes conflicting. The National Institute for Health and Care Excellence (NICE) intrapartum care guidance however, suggests the risk of recurrent severe trauma is similar to the risk of severe trauma at first vaginal birth, though this guidance is currently being updated (NICE 2007).

Adverse effects

OASI is associated with an increased risk of short‐ and long‐term morbidity which could seriously affect quality of life. Sequelae include: perineal pain (Macarthur 2004), dyspareunia (painful sexual intercourse) (Rathfisch 2010), defaecatory dysfunction, and urinary and faecal incontinence (Fenner 2003; MacArthur 1997; Richter 2006). Perineal pain is an immediate consequence that may adversely affect maternal and infant bonding, ability to breastfeed, increase the risk of urinary retention and dyspareunia (Barrett 2000; Buhling 2006), and could reduce well‐being and increase the risk of depression (Brown 2000).

Possibly the most distressing adverse effect of OASI is anal incontinence related to anal sphincter injury and pudendal nerve damage (Fynes 1998). Four per cent of women report faecal incontinence following vaginal birth (MacArthur 1997). The incidence of anal incontinence is related to the severity of the sphincter defect observed at follow‐up. For example, following clinically identified severe perineal trauma at the time of birth, 13% of women without identifiable sphincter defects on postnatal endo‐anal ultrasound (EAUS) reported anal incontinence, whereas 64% with internal and external defects on EAUS reported anal incontinence (Laine 2011). Incontinence rates may worsen with time and following subsequent births irrespective of degree of perineal trauma sustained (Baghestan 2012; Bek 1992). Various factors have been identified that may help determine the risk of anal incontinence following a subsequent birth; these include: age, parity, presence and severity of symptoms, EAUS‐identified injury and impaired sphincter function assessed by manometry.

Anal incontinence in the absence of identified OASI at the time of birth, may be in part due to pudendal nerve damage or unidentified anal sphincter damage. Sultan 1993 identified over 30% more anal sphincter injuries using EAUS compared with clinical examination alone. This difference may, however, be related to the experience of and/or technique used by the person undertaking the initial clinical examination rather than the superior or sensitivity of EAUS (Andrews 2006).

Identification of Injury

All women who have sustained perineal trauma should have a systematic examination of the vagina, perineum and rectum, including rectal examination before and after perineal repair by an experienced practitioner trained in the recognition and management of perineal tears (NICE 2007; RCOG 2007). Methods of OASI repair OASI have been examined in a separate Cochrane review (Fernando 2006). To our knowledge, there are no reviews examining suture materials for repair of OASI, the ideal clinician to perform the repair (obstetrician or colorectal surgeon), effectiveness of immediate versus delayed repair, prevention of OASI, or interventions for women in subsequent pregnancies following OASI to reduce the risk of recurrent injury and associated harms.

Description of the condition

NICE and the Royal College of Obstetricians and Gynaecologists guidelines (NICE 2007; RCOG 2007) recommend perineal or genital trauma caused by either tearing or episiotomy at birth should be defined as follows (described by Sultan 1999):

• first degree – injury to skin only;

• second degree – injury to the perineal muscles, but not the anal sphincter;

• third degree – injury to the perineum involving the anal sphincter complex:

• • 3a – less than 50% of external anal sphincter (EAS) thickness torn;

  • 3b – more than 50% of EAS thickness torn;

  • 3c – internal anal sphincter (IAS) torn;

• fourth degree – injury to the perineum involving the anal sphincter complex (external and internal anal sphincter) and anal epithelium.

OASI includes third and fourth‐degree perineal tears (RCOG 2007).

Description of the intervention

Antenatal interventions for women who have sustained a previous obstetric anal sphincter injury include: pelvic floor exercises that aim to strengthen the pelvic floor and have recently been found to reduce the risk of urinary incontinence (Stafne 2012); biofeedback training which uses computer‐generated feedback from rectal balloons to (a) improve patient awareness of the presence of faecal material in the rectum and (b) to co‐ordinate contraction of the external anal sphincter with relaxation of the internal sphincter and (c) improve the force of the muscle (Miner 1990; Norton 2012); or stimulation of the sacral nerves that control the lower part of the bowel and sphincters by inserting electrodes in the lower back and connecting them to a pulse generator (Mowatt 2007). Other antenatal interventions include: perineal massage or creams that aim to reduce the risk of perineal tearing.

Intrapartum interventions include: induction of labour to reduce the risk of macrosomia (infant birth weight greater than 4 kg) and subsequent risk of trauma; elective caesarean section to avoid vaginal and perineal trauma; vacuum extraction rather than forceps to reduce the risk of vaginal and perineal trauma; selective episiotomy to reduce the risk of severe perineal trauma; and different flexion techniques of the presenting fetal part to reduce the diameter and in doing so, reduce the risk of subsequent trauma.

How the intervention might work

Interventions may aim to improve the integrity of the anal sphincter (pelvic floor muscle exercises, electrical stimulation), avoid trauma (elective caesarean section) or reduce the risk of trauma (medio‐lateral episiotomy, vacuum and flexion techniques) to the perineum and anal sphincter and in doing so reduce the risk of adverse effects such as incontinence.

Why it is important to do this review

There are currently no systematic reviews, evidence‐based guidance on interventions or strategies for women in subsequent pregnancies following obstetric anal sphincter injury to prevent or reduce the risk of further damage/trauma to the anal sphincter complex. Guidance based on robust evidence would improve the care in subsequent pregnancies for women who have previously sustained a third‐degree tear and in doing so reduce the risk of morbidity and improve health.