Applied Evidence

Obstetric anal sphincter injury: How to avoid, how to repair: A literature review

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Practice recommendations
  • Avoiding obstetrical injury to the anal sphincter is the single biggest factor in preventing anal incontinence among women (A). Any form of instrument delivery has consistently been noted to increase the risk of obstetric anal sphincter injury and altered fecal continence by between 2- and 7-fold (A).
  • Routine episiotomy is not recommended (A). Episiotomy use should be restricted to situations where it directly facilitates an urgent delivery (A). A mediolateral incision, instead of a midline, should be considered for persons at otherwise high risk of obstetric anal sphincter injury (A).
  • The internal anal sphincter needs to be separately repaired if torn (A).
  • Women with injuries to the internal anal sphincter or rectal mucosa have a worse prognosis for future continence problems (A). All women, particularly those with risk factors for injury, should be surveyed for symptoms of anal incontinence at postpartum follow-up (C).

Do you routinely check with new first-time mothers at a postpartum visit about changes in anal continence? They are at particular risk for obstetric anal sphincter injury and could be too embarrassed to raise the issue.

Sphincter injury following labor is the most common cause of anal incontinence (including flatus) in women, which can severely diminish quality of life and lead to considerable personal and financial costs.1 Endoanal ultrasound can detect these injuries, even in the absence of clinically obvious damage to the anal sphincter (occult obstetric anal sphincter injury).2

In this article, we review measures to reduce the occurrence of obstetric anal sphincter injury, proper primary repair when it does occur, and appropriate long-term follow-up. Women with known obstetric anal sphincter injury must also be counseled about the risk of further damage during a future vaginal delivery.

Injury more common than symptoms would suggest

The conventional definitions of the 4 grades of perineal laceration in the US have been supplemented by more recent modifications included in a recent British Royal College of Obstetricians and Gynaecologists (RCOG) guideline (TABLE 1).3 The definition of third-degree laceration now reflects the various degrees of anal sphincter injury that may occur: partial (3a), full-thickness (3b), external anal sphincter injury, with or without injury to the internal anal sphincter (3c).

The incidence of clinical third- and fourth-degree lacerations varies widely; it is reported at between 0.5% and 3.0% in Europe and between 5.85% and 8.9% in the US.2,4-6 A landmark British paper from 1993 revealed that though only 3% had a clinical third- or fourth-degree perineal laceration, 35% of primiparous women (none of whom had any defect before delivery) had ultrasound evidence of varying degrees of anal sphincter defect at 6 weeks postpartum that persisted at 6 months.2 However, only about a third of these women had symptoms of bowel disturbance during the time of study.

These findings are supported by a meta-analysis in which 70% of women with a documented obstetric anal sphincter injury were asymptomatic.7 This meta-analysis concluded that clinical or occult obstetric anal sphincter injury occurs in 27% of primigravid women, and in 8.5% of multiparous women.

The long-term significance of occult obstetric anal sphincter injury and any relationship with geriatric fecal incontinence is unknown, although 71% of a sample of women with late-onset fecal incontinence were found to have ultrasound evidence of an anal sphincter defect thought to have occurred at a previous vaginal delivery.8 A recent English study9 reveals that when women were carefully re-examined after delivery by a skilled obstetrician looking specifically at the anal sphincter, the prevalence of clinically diagnosed third-degree lacerations rose sharply from the 11% initially diagnosed by the delivering physician or midwife to 24.5%. A subsequent endoanal ultrasound detected only an additional 1.2% (3 injuries, 2 of which were in the internal anal sphincter and therefore clinically undetectable). This strongly suggests that the vast majority of obstetric anal sphincter injuries can be detected clinically by a careful exam and that, when this is done, true occult injuries will be a rare finding.

Classification of perineal injury

First degreeInjury confined to vaginal mucosa
Second degreeInjury of vaginal mucosa and perineal muscles, but not the anal sphincter
Third degreeInjury to the perineum involving the anal sphincter complex (external and internal)
3a  <50% of external sphincter thickness is torn
3b  >50% of external sphincter thickness is torn
3c  Internal sphincter is torn
Fourth degreeInjury to external and internal sphincter and rectal mucosa/anal epithelium

Mechanisms of injury

Maintenance of fecal continence involves the coordinated action of several anatomical and physiological elements (FIGURE 1).10 An intact, innervated anal sphincter complex (both external and internal) is necessary. The sphincter complex can be damaged during childbirth in 3 ways.


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