Obstetric Anal Sphincter Injury

Obstetric anal sphincter injury (OASI) is a perineal tear involving the anal sphincter complex during vaginal delivery. It occurs in about 1 to 3 percent of vaginal births in primiparous women (rates are higher in some populations and where recognition is improved).

Classification: third degree tear is partial or complete disruption of the external anal sphincter (3a: less than 50 percent of external sphincter thickness torn; 3b: more than 50 percent torn; 3c: internal sphincter also torn); fourth degree tear is disruption of external and internal sphincters with the anorectal mucosa torn. Button hole tear (rectal mucosa torn with intact sphincter) is not an OASI but must be recognised.

Risk factors: first vaginal delivery, birth weight above 4 kg, shoulder dystocia, occipitoposterior position, instrumental delivery (forceps higher than ventouse), prolonged second stage, midline episiotomy, Asian ethnicity, and induction. Mediolateral episiotomy at 60 degrees from the midline at crowning reduces risk with instrumental delivery.

• Diagnosed at delivery by systematic perineal and rectal examination in every woman with a vaginal birth, with good lighting and analgesia.
• Later symptoms: faecal incontinence to flatus, liquid or solid stool, urgency, perineal pain and dyspareunia, infection and wound breakdown, rectovaginal fistula (faeces or flatus per vagina), and psychological distress.
• Missed injuries cause delayed faecal incontinence.
• About 30 to 40 percent have symptoms of anal incontinence at some point after repair; 10 to 20 percent have persistent symptoms.

• Visual and digital inspection of the perineum, vagina and rectum after every vaginal delivery, with the woman in lithotomy with adequate analgesia. A rectal examination should be done with one finger in the anus and thumb on the perineal skin (pinch roll) to assess external sphincter, and the rectum inspected for mucosal tears.
• Grading is made before repair by an experienced operator; if in doubt, grade higher.
• Endoanal ultrasound at 6 weeks to 3 months if symptoms persist or following repair when the injury is uncertain; anorectal manometry if incontinence persists.

Repair in theatre under regional or general anaesthesia, with good light, by an experienced operator, within the first few hours:

• Prophylactic antibiotics at repair: a single dose of cefuroxime 1.5 g plus metronidazole 500 mg intravenously (or co-amoxiclav 1.2 g intravenously).
• Fourth degree tear: repair anorectal mucosa with a continuous or interrupted 3-0 polyglactin suture, then internal anal sphincter (separately) with interrupted or mattress 3-0 polydioxanone, then external sphincter.
• External sphincter repair: end-to-end or overlap technique with 3-0 polydioxanone (PDS) or 2-0 polyglactin; use overlap for complete tears. Grade 3a tears are repaired end-to-end.
• Repair the vaginal wall and perineal muscles and skin in layers; carry out a rectal examination after repair to confirm no sutures have been placed through the rectal mucosa.
• Indwelling catheter for 12 to 24 hours.

Post-repair care:

• Laxative: lactulose 10 to 15 mL twice daily (or ispaghula husk 3.5 g sachet twice daily) for 10 to 14 days to avoid straining, plus a stool softener; avoid constipating opioids.
• Oral antibiotics: co-amoxiclav 625 mg every 8 hours for 5 to 7 days (or cefalexin 500 mg every 6 hours plus metronidazole 400 mg every 8 hours).
• Analgesia: paracetamol 1 g every 6 hours and ibuprofen 400 mg every 8 hours (avoid codeine based drugs where possible).
• Pelvic floor physiotherapy from 2 to 6 weeks.
• Follow up at 6 to 12 weeks in a perineal clinic with symptom assessment, examination, and endoanal ultrasound and manometry if symptomatic.
• Persistent incontinence: sphincteroplasty, sacral nerve stimulation, injectable bulking agents or other surgery. Rectovaginal fistula requires surgical repair after inflammation settles.

Next pregnancy: counsel on mode of delivery. Elective caesarean section is offered to women with persistent symptoms, abnormal manometry or ultrasound findings, or previous fourth degree tear; vaginal delivery is reasonable if asymptomatic with a normal examination, with a low threshold for mediolateral episiotomy and careful perineal protection. Recurrence risk is 5 to 7 percent.