Chapter 21 — Obstetric Anal Sphincter Injury (OASI) — Fourth-Degree Perineal Tear with Cloaca Formation

Chapter 20 described anal incontinence in general. This chapter covers its single most common structural cause: injury to the anal sphincter during vaginal delivery. This injury is called obstetric anal sphincter injury, commonly shortened to OASI. This chapter explains OASI in full, including its most severe form, a fourth-degree tear leading to cloaca formation. Chapter 22 then covers secondary repair of a complete sphincter tear in full technical detail. This is for the patient who presents later, after the injury has already healed poorly.

21.1 What Is Obstetric Anal Sphincter Injury

Obstetric anal sphincter injury, or OASI, is a tear of the anal sphincter complex occurring during vaginal delivery. It is part of a wider spectrum of perineal tears, formally graded by degree, described in full in Section 21.3 below.

Most perineal tears do not involve the sphincter at all. A tear that does involve the sphincter carries a real risk of future anal incontinence. This risk applies if the tear is not recognised and repaired correctly at the time of delivery.

A cloaca is the most severe outcome of an unrecognised or poorly repaired sphincter injury. The wall separating the rectum from the vagina is lost, and the two form a single, common channel. This is described fully in Section 21.4.

21.2 Why It Happens

A first vaginal delivery carries a higher risk than subsequent deliveries, since the perineal tissues have not previously been stretched by childbirth.

A large baby increases the degree of stretching the perineum must withstand during delivery.

A prolonged second stage of labour, meaning a long period of active pushing, increases cumulative strain on the perineum.

An assisted delivery, using forceps or vacuum extraction, carries a higher risk than a spontaneous delivery.

An occipito-posterior position of the baby's head increases the mechanical difficulty of delivery. Shoulder dystocia does too. Both increase the risk of a significant tear.

A midline episiotomy, if performed, carries a higher risk of extending into the sphincter than a mediolateral episiotomy. This is because the midline runs directly toward the anal canal.

21.3 Classification

Perineal tears during childbirth are graded formally, by degree, based on the depth and structures involved.

First degree — injury to the perineal skin only.

Second degree — injury to the perineal muscles, but not the anal sphincter.

Third degree — injury involving the anal sphincter complex. This grade is further subdivided. Grade 3a is tearing of less than half the thickness of the external anal sphincter. Grade 3b is tearing of more than half the thickness of the external anal sphincter. Grade 3c is tearing of both the external and the internal anal sphincter.

Fourth degree — injury extending through the external anal sphincter, the internal anal sphincter, and the anal or rectal mucosa itself.

This classification directly guides both the urgency and the technical approach to repair, described in Section 21.9.

21.4 Cloaca Formation

A cloaca forms when the tissue separating the rectum from the vagina is lost entirely. This can happen because a fourth-degree tear was not recognised and repaired at the time of delivery. It can also happen because a repair that was performed has since broken down.

Instead of two separate openings, the rectum and vagina become confluent, forming one common channel. This causes severe faecal incontinence, since stool passes directly into the vagina, and causes significant distress and social isolation for the patient.

A cloaca needs specialised reconstructive repair, described in full in Chapter 22. A simple sphincter repair alone does not correct it. The missing rectovaginal septum, the tissue wall between the rectum and vagina, must also be reconstructed.

Operative Video — Illustrative Case

The following operative case is from the author's own practice. It illustrates the repair of a fourth-degree perineal tear of exactly this severity. Before repair, the patient had continuous soiling on a pad, from a cloaca-pattern presentation. Patient: Seema. Structures repaired: external anal sphincter, internal anal sphincter, and anorectal mucosa. Full continence was achieved postoperatively, with no infection, wound breakdown, or fistula.

Full case report, with operative technique and outcome: https://leafy-sundae- 4e6243.netlify.app/

Direct operative video (Patient: Seema): https://www.youtube.com/watch? v=yVjm7GPMOok&t=40s

21.5 Presentation

Most obstetric sphincter injuries are recognised at the time of delivery, during routine perineal inspection and repair. This is the ideal scenario, since immediate repair gives the best chance of a good functional outcome.

Some injuries are missed at the time of delivery. These are described as occult sphincter injury. They present later, sometimes weeks, months, or even years afterward, with symptoms of anal incontinence, described in Chapter 20. This delayed presentation is a well-recognised problem, and is one reason the examination described in Section 21.6 must be thorough at every delivery.

A patient presenting later with incontinence following childbirth should always be asked directly about her delivery history. Follow the approach described in Chapter 2. Ask specifically whether any tear was recognised and repaired at the time.

21.6 Examination

At the time of delivery, a thorough examination of the perineum must include a rectal examination. This is in addition to inspection of the vaginal and perineal tissues. A tear may appear, on vaginal inspection alone, to be no more than second degree. A rectal examination may then reveal that the sphincter itself has in fact been torn. This is precisely how an occult injury is missed.

At a later presentation, follow the sequence described in Chapter 3. Digital rectal examination, described in Chapter 5, should assess resting and squeeze tone. It should specifically feel for a palpable defect in the sphincter ring, most often located anteriorly.

Examination under anaesthesia may be needed for a full and accurate assessment. This applies to a patient with suspected cloaca formation. It also applies when the extent of a complex injury is not fully clear on examination in clinic.

21.7 Differential Diagnosis

The other causes of anal incontinence, described in Chapter 20, should still be considered in a woman presenting after childbirth. More than one contributing factor can coexist. A pudendal nerve injury from a difficult delivery, for example, can impair continence even in a patient whose sphincter muscle itself is structurally intact.

21.8 Investigations

Endoanal ultrasound, described in Chapter 20, confirms the presence, size, and position of a sphincter defect. It is particularly valuable in a patient presenting later with symptoms, rather than at the time of the original injury.

Anorectal manometry, described in Chapter 8, assesses resting and squeeze pressures, and helps quantify the severity of functional impairment.

21.9 Principles of Management

A recognised third or fourth degree tear should be repaired immediately, at the time of delivery. This is ideally done by a suitably trained obstetrician. Specialist colorectal input should be sought for a grade 3c or fourth degree injury, where available.

Repair proceeds in careful layers. Where the rectal or anal mucosa is torn, this is repaired first. The internal anal sphincter, if torn, is then repaired separately and specifically. It is easily

overlooked if attention is focused only on the more visible external sphincter. The external anal sphincter is repaired last, using either an end-to-end or an overlapping technique.

Antibiotic prophylaxis is given at the time of repair. Stool softeners or laxatives are prescribed afterward, specifically to prevent straining and constipation. Either of these can place damaging tension on a fresh repair.

Every patient with a recognised third or fourth degree tear should be followed up specifically to assess continence. Do not only check that the wound itself has healed. A wound can look healed on the surface while the sphincter repair beneath it has already failed.

21.10 Special Situations

Missed or occult injury, presenting later — assessed and managed as described in Chapter 20, with definitive secondary repair described fully in Chapter 22.

Cloaca formation — needs specialised, staged reconstructive repair, described fully in Chapter 22, rather than a routine sphincter repair alone.

Future pregnancy after a previous OASI — deserves a specific discussion about the mode of delivery for any subsequent pregnancy. A further vaginal delivery carries some risk of worsening an existing injury. This should be discussed jointly with the obstetric team, well before the next delivery. Take the patient's current symptoms and the severity of the original injury into account.

21.11 Complications

Wound infection or breakdown of the repair.

Persistent or recurrent anal incontinence, despite an apparently adequate repair.

A rectovaginal fistula, a smaller abnormal connection between the rectum and vagina, distinct from the wider, fully confluent channel seen in a true cloaca.

Dyspareunia, or pain during intercourse, related to scarring at the repair site.

21.12 Common Mistakes to Avoid

Performing only a vaginal inspection after delivery, without a rectal examination, and missing an occult sphincter injury as a result.

Repairing the external sphincter without specifically identifying and repairing a torn internal sphincter separately.

Delaying repair of a recognised tear, rather than proceeding promptly.

Failing to follow up specifically for continence symptoms, and relying only on visual wound healing to judge the outcome.

Not discussing mode of delivery for a future pregnancy in a patient with a history of significant OASI.

21.13 Recording and Follow-Up

Record the degree of tear precisely, using the classification in Section 21.3, together with the repair technique used. For example: "Third-degree tear, grade 3b, external and internal anal sphincter repaired in layers, antibiotic and laxative cover given. Review in 6 weeks to assess continence." Arrange follow-up specifically to assess continence at that visit, not only wound healing. Refer for endoanal ultrasound or specialist assessment if any symptoms are present.

21.14 Why This Chapter Matters

Obstetric anal sphincter injury is common, and its outcome depends heavily on whether it is recognised at the time it occurs. A rectal examination performed routinely after every vaginal delivery prevents the great majority of long-term consequences described in this chapter. So does a repair performed carefully, in the correct layers. The next chapter turns to the patient for whom this opportunity has already been missed. It covers secondary repair of a complete sphincter tear, presenting after the original injury has already healed poorly.