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Surgical Technique

Obstetric anal sphincter injury: 7 critical questions about care

When and how you manage an injury determines the patient’s quality of life. Here are 7 issues to consider.

February 2008 · Vol. 20, No. 02


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The authors report no financial relationships relevant to this article.

CASE Large baby, extensive tear

A 28-year-old primigravida undergoes a forceps delivery with a midline episiotomy for failure to progress in the second stage of labor. At birth, the infant weighs 4 kg (8.8 lb), and the episiotomy extends to the anal verge. The resident who delivered the child is uncertain whether the anal sphincter is involved in the injury and asks a consultant to examine the perineum.

What should this examination entail?

The obstetrician is rarely culpable when a third- or fourth-degree obstetric anal sphincter injury (OASIS) occurs—but there is little excuse for letting one go undetected.

To minimize the risk of undiagnosed OASIS, a digital anorectal examination is warranted—before any suturing—in every woman who delivers vaginally. This practice can help you avoid missing isolated tears, such as “buttonhole” of the rectal mucosa, which can occur even when the anal sphincter remains intact (FIGURE 1), or a third- or fourth-degree tear that can sometimes be present behind apparently intact perineal skin (FIGURE 2).1

Clinical training of physicians and midwives also needs to improve.

Every labor room should have a protocol for management of anal sphincter injury2; this article describes detection, diagnosis, and management, focusing on seven critical questions.

Only a physician formally trained in primary anal sphincter repair (or under supervision) should repair OASIS.

FIGURE 1 Buttonhole tear

A buttonhole tear of the rectal mucosa (arrow) with an intact external anal sphincter (EAS) demonstrated during a digital rectal examination. SOURCE: Sultan AH3 (used with permission).

FIGURE 2 Injury obscured by intact skin

(A) Intact perineum on visual examination. (B) Anal sphincter trauma detected after rectal examination. SOURCE: Sultan AH, Kettle C1 (used with permission).

1. When (and how) should the torn perineum be examined?

The first requisite is informed consent for vaginal and rectal examination immediately after delivery. Also vital are adequate exposure of the perineum, good lighting, and, if necessary, sufficient analgesia to prevent pain-related restriction of the evaluation. It may be advisable to place the patient in the lithotomy position to improve exposure.

After visual examination of the perineum, part the labia and examine the vagina to establish the full extent of the tear. Always identify the apex of the vaginal laceration.

Next, perform a rectal examination to exclude injury to the anorectal mucosa and anal sphincter.3

Palpation is necessary to confirm OASIS

Insert the index finger into the anal canal and the thumb into the vagina and perform a pill-rolling motion to palpate the anal sphincter. If this technique is inconclusive, ask the woman to contract her anal sphincter with your fingers still in place. When the sphincter is disrupted, you feel a distinct gap anteriorly. If the perineal skin is intact, there may be an absence of puckering on the perianal skin over any underlying defect that may not be evident under regional or general anesthesia.

Because the external anal sphincter (EAS) is in a state of tonic contraction, the sphincter ends will retract when it is disrupted. These ends need to be grasped and retrieved at the time of repair.

Also identify the internal anal sphincter (IAS). It is a circular smooth muscle (FIGURE 3) that is paler in appearance (similar to the flesh of raw fish) than the striated EAS (similar to raw red meat).4 Under normal circumstances, the distal end of the IAS lies a few millimeters proximal to the distal end of the EAS (FIGURE 4). However, if the EAS is relaxed due to regional or general anesthesia, the distal end of the IAS will appear to be at a lower level. If the IAS or anal epithelium is torn, the EAS is, invariably, torn, too.

General or regional (spinal, epidural, caudal) anesthesia provides analgesia and muscle relaxation and enables proper evaluation of the full extent of the injury.

FIGURE 3 Grade 3b tear

Grade 3b tear with an intact internal anal sphincter (IAS). The external sphincter (EAS) is being grasped with Allis forceps. Note the difference in appearance of the paler IAS and darker EAS. SOURCE: Sultan AH, Kettle C1 (used with permission).

FIGURE 4 Classification of anal sphincter injury

First- and second-degree injuries are described below.

2. Is endoanal US helpful to detect OASIS?

Endoanal ultrasonography (US) to identify OASIS requires specific expertise, particularly in the immediate postpartum period, when the anal canal is lax (especially after an epidural). Ultimately, however, the diagnosis rests on clinical assessment and a rectal examination because, even if a defect is seen on US, it has to be clinically apparent to be repaired.

In a study by Faltin and colleagues, in which routine postpartum endoanal US was used as the gold standard for diagnosis of OASIS, five of 21 women had unnecessary intervention because the sonographic defect was not clinically visible despite exploration of the anal sphincter.5 As a result of this unnecessary exploration based on endoanal US, 20% of these women developed severe fecal incontinence. Therefore, we believe that OASIS is best detected clinically immediately after delivery, provided the physician performs a careful examination with palpation of the anal sphincter.6 In such a scenario, endoanal US is of limited value.

3. How is obstetric anal sphincter trauma classified?

To standardize the classification of perineal trauma, Sultan proposed the following system, which has been adopted by the Royal College of Obstetricians and Gynaecologists and internationally7-9:

First degree: Laceration of the vaginal epithelium or perineal skin only

Second degree: Involvement of the perineal muscles, but not the anal sphincter

Third degree: Disruption of the anal sphincter muscles (FIGURE 4):

  • 3a: Less than 50% thickness of the external sphincter is torn
  • 3b: More than 50% thickness of the external sphincter is torn
  • 3c: Internal sphincter is also torn

Fourth degree: A third-degree anal tear with disruption of the anal epithelium (FIGURE 4).

If there is any ambiguity about grading of the injury, the higher grade should be selected. For example, if there is uncertainty between grades 3a and 3b, the injury should be classified as Grade 3b.

4. Is an operating room necessary?

OASIS should be repaired in the operating theater, where there is access to good lighting, appropriate equipment, and aseptic conditions. In our unit, we have a specially prepared instrument tray containing:

  • a Weislander self-retaining retractor
  • 4 Allis tissue forceps
  • McIndoe scissors
  • tooth forceps
  • 4 artery forceps
  • stitch scissors
  • a needle holder.

In addition, deep retractors (e.g., Deavers) are useful when there are associated paravaginal tears.

5. What surgical technique is recommended?

Buttonhole injury

This type of injury can occur in the rectum without disrupting the anal sphincter or perineum. It is best repaired transvaginally using interrupted Vicryl (polyglactin) sutures.

To minimize the risk of persistent rectovaginal fistula, interpose a second layer of tissue between the rectum and vagina by approximating the rectovaginal fascia. A colostomy is rarely indicated unless a large tear extends above the pelvic floor or there is gross fecal contamination of the wound.

Fourth-degree tear

Repair torn anal epithelium with interrupted Vicryl 3-0 sutures, with the knots tied in the anal lumen. Proponents of this widely described technique argue that it reduces the quantity of foreign body (knots) within the tissue and lowers the risk of infection. Concern about a foreign body probably applies to the use of catgut, which dissolves by proteolysis, rather than to newer synthetic material such as Vicryl or Dexon (polyglycolic acid), which dissolves by hydrolysis.

Subcuticular repair of anal epithelium using a transvaginal approach has also been described and could be equally effective if the terminal knots are secure.10

Sphincter muscles

Repair these muscles using 3-0 polydioxanone (PDS) dyed sutures. Compared with braided sutures, monofilament sutures are believed to lessen the risk of infection, although a randomized controlled trial revealed no difference in suture-related morbidity between Vicryl and PDS at 6 weeks postpartum.11 Complete absorption of PDS takes longer than with Vicryl, with 50% tensile strength lasting more than 3 months, compared with 3 weeks for Vicryl.11 To minimize suture migration, cut suture ends short and ensure that they are covered by the overlying superficial perineal muscles.

Internal anal sphincter. Repair the IAS separately from the EAS. Grasp the ends of the torn muscle using Allis forceps and perform an end-to-end repair with interrupted or mattress 3-0 PDS sutures (FIGURE 5). Overlapping repair can be technically difficult.

There is some evidence that repair of an isolated IAS defect benefits patients with established anal incontinence.

External anal sphincter. Because the EAS is normally under tonic contraction, it tends to retract when torn. Therefore, repair requires identification and grasping of the torn ends using Allis tissue forceps (FIGURE 6).

When the EAS is only partially torn (Grade 3a and some cases of Grade 3b), perform an end-to end repair using 2 or 3 mattress sutures, similar to repair of IAS injury, instead of hemostatic “figure of eight” sutures.

For a full-thickness tear (some cases of Grade 3b or 3c, or Grade 4), overlapping repair may be preferable in experienced hands. The EAS may need to be mobilized by dissecting it free of the ischioanal fat laterally using a pair of McIndoe scissors. The torn ends of the EAS can then be overlapped in “double-breasted” fashion (FIGURE 7) using PDS 3-0 sutures. Proper overlap is possible only when the full length of the torn ends is identified.

Overlapping the ends of the sphincter allows for greater surface area of contact between muscle. In contrast, end-to-end repair can be performed without identifying the full length of the EAS and may give rise to incomplete apposition. Fernando and colleagues demonstrated that, in experienced hands, early primary overlap repair carries a lower risk of fecal urgency and anal incontinence than does immediate primary end-to-end repair.12,13

FIGURE 5 End-to-end repair

Internal anal sphincter (I) repair using mattress sutures, demonstrated on the latex Sultan model, used for training ( (E, external sphincter; A, anal epithelium). SOURCE: Sultan AH, Thakar R2 (used with permission).

FIGURE 6 Locating the external anal sphincter

The external sphincter (E), grasped with Allis forceps, is surrounded by the capsule (C) and lies medial to the ischioanal fat. SOURCE: Sultan AH, Thakar R2 (used with permission).

FIGURE 7 Overlapping sphincter repair

Repair of a fourth degree tear (demonstrated on the Sultan model) using the overlap repair technique on the external sphincter (E). The anal epithelium (A) and the internal sphincter (I) have also been repaired. SOURCE: Sultan AH, Thakar R2 (used with permission).

Perineal muscles

After repair of the sphincter, suture the perineal muscles to reconstruct the perineal body and provide support to the repaired anal sphincter. A short, deficient perineum would leave the anal sphincter more vulnerable to trauma during a subsequent vaginal delivery.

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