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Obstetric anal sphincter injury
Background
Overview
Definition
Obstetric anal sphincter injuries are perineal lacerations that occur during vaginal delivery and involve damage to the anal sphincter complex, referred to as third- and fourth-degree perineal tears.
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Pathophysiology
Fetal descent and delivery exert significant pressure on the perineum and anal sphincter. During delivery, the advancing fetal head stretches the perineum and applies direct tensile and shear forces to the anal sphincter complex. These mechanical forces can exceed the tensile strength of the sphincter fibers, leading to partial or complete disruption. In some cases, the rectal mucosa may also be torn if the force is transmitted through the sphincter. Additionally, the use of instruments such as forceps or vacuum amplifies these forces, increasing the risk of sphincter damage by exerting localized pressure and traction during delivery.
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Epidemiology
The incidence of obstetric anal sphincter injuries is estimated at 4.9%, with a higher rate in primiparous women at 6% compared to 2% in multiparous women. Other reports suggest a significantly higher incidence, including occult injuries, ranging from 11.5% to 35.0% after first vaginal deliveries and 3.4% to 12.1% with subsequent deliveries.
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Risk factors
Risk factors for obstetric perineal lacerations include maternal factors such as nulliparity, Asian ethnicity, and prolonged (> 2 hours) second stage of labor, fetal factors such as high birth weight, shoulder dystocia, and persistent occiput posterior presentation, as well as procedural factors such as induction of labor, epidural anesthesia, operative vaginal delivery, and episiotomy.
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Disease course
In most cases, perineal lacerations are clearly identified during vaginal delivery; however, some tears may be missed at the time of delivery and detected later in the postpartum period. Patients with postpartum tears typically present with symptoms such as perineal pain, bleeding, discharge, and/or fecal incontinence.
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Prognosis and risk of recurrence
Potential complications of unrepaired or improperly repaired third- and fourth-degree obstetric perineal tears include wound infection, wound dehiscence, perineal and rectovaginal fistulas, and urinary and fecal incontinence. The risk of recurrent injury in subsequent pregnancies is 5.6% with a second vaginal delivery and 9.5% with a third vaginal delivery.
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Guidelines
Key sources
The following summarized guidelines for the evaluation and management of obstetric perineal trauma are prepared by our editorial team based on guidelines from the Society of Obstetricians and Gynaecologists of Canada (SOGC 2024), the American Academy of Family Physicians (AAFP 2021), the American College of Obstetricians and Gynecologists (ACOG 2018), and the Royal College of Obstetricians and Gynaecologists (RCOG 2015).
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Classification and risk stratification
Classification
As per SOGC 2024 guidelines:
Use the Sultan's classification to grade obstetrical perineal trauma to ensure consistent reporting:
Situation
Guidance
First degree
Laceration of vaginal epithelium or perineal skin only
Second degree
Tear of the perineal muscles not involving the anal sphincter
Third degree
Any involvement of the anal sphincter
3a. Involvement of the anal sphincter, < 50% of external sphincter thickness torn
3b. Involvement of the anal sphincter, > 50% of external sphincter thickness torn
3c. Internal anal sphincter torn
Fourth degree
Tear of the perineum involving anal sphincter complex and anal epithelium
B
Disclose the degree of perineal laceration to the patient and document it in the medical record.
E
Diagnostic investigations
Medical management
Analgesics: as per SOGC 2024 guidelines, administer NSAIDs and acetaminophen as first-line analgesics. Consider using opioids with caution and administer them in conjunction with a laxative to prevent constipation.
B
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Laxatives
Nonpharmacologic interventions
Therapeutic procedures
Perioperative care
Perioperative antibiotic prophylaxis: as per SOGC 2024 guidelines, administer a single dose of prophylactic intravenous antibiotics (a second-generation cephalosporin, such as cefotetan or cefoxitin, or clindamycin in patients with penicillin allergy) immediately following the repair of obstetrical anal sphincter injury to reduce postpartum wound complications.
B
Surgical interventions
Surgical repair, indications and setting of surgery
As per SOGC 2024 guidelines:
Perform repair of third- and fourth-degree anal sphincter injuries by experienced specialists.
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Delay repair of obstetrical anal sphincter injury for 8-12 hours if a more experienced care provider is required, as this does not adversely affect anal incontinence. B
Pay careful attention to voiding function in patients with an obstetrical anal sphincter injury, particularly when compounding risk factors are present.
B
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Surgical repair (technical considerations)
Surgical repair (postoperative examination)
Preventative measures
Perineal self-massage: as per AAFP 2021 guidelines, advise primiparous females to perform digital perineal self-massage starting at 35 weeks of gestation to reduce perineal lacerations during labor (number needed to treat of 15).
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Perineal massage
Warm perineal compresses
Perineal protection
Considerations for episiotomy
Warm perineal compress
Follow-up and surveillance
Follow-up: as per RCOG 2015 guidelines, reassess patients after obstetric anal sphincter repair at a convenient time, typically 6-12 weeks postpartum. Assess patients by clinicians with a special interest in obstetric anal sphincter injuries where possible.
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Management of subsequent pregnancies