Obstetric Anal Sphincter Injury (OASI)

What is an Obstetric Anal Sphincter Injury?

An Obstetric Anal Sphincter Injury, or OASI, is a complication of vaginal childbirth where the muscles of the perineum and the anal sphincter (the muscles that control bowel movements) are torn. These injuries are also known as third- or fourth-degree perineal tears. While minor perineal tears are very common during childbirth, an OASI is a more severe injury that involves the muscles responsible for continence.

• Third-degree tear: The tear extends from the vagina to the anus and involves injury to the external and/or internal anal sphincter muscles.
• Fourth-degree tear: The tear is more extensive, going through the sphincter muscles and into the lining (mucosa) of the rectum.

Prompt and accurate diagnosis and repair of these injuries at the time of delivery are crucial for preserving long-term bowel control and quality of life.

Common Symptoms

In the immediate postpartum period, symptoms of an OASI can include:

• Significant pain, swelling, or bruising in the perineal area.
• Difficulty controlling gas or stool (fecal incontinence).
• A sense of urgency to have a bowel movement.
• Pain during sexual intercourse (dyspareunia).

If an OASI is not recognized or does not heal properly, long-term symptoms can develop, including:

• Chronic fecal incontinence (accidental bowel leakage) to gas, liquid, or solid stool.
• The development of a rectovaginal fistula (a tunnel between the rectum and vagina).
• Chronic perineal pain.

What causes an OASI?

An OASI is caused by the stretching and tearing of the perineum during vaginal childbirth. Certain factors can increase the risk of a severe tear:

• First vaginal birth: The perineal tissues are less elastic.
• Large baby: A baby weighing more than 4 kg (about 8.8 pounds).
• Forceps or vacuum delivery: The use of instruments to assist with delivery.
• Persistent occiput posterior position: When the baby is facing 'sunny-side up'.
• Shoulder dystocia: When the baby's shoulder gets stuck after the head is delivered.
• Midline episiotomy: A surgical cut made in the perineum to enlarge the vaginal opening. Mediolateral episiotomies are associated with a lower risk of OASI.

How is an OASI diagnosed?

An OASI is usually diagnosed immediately after childbirth through a careful visual and digital examination of the perineum, vagina, and rectum by the obstetrician or midwife.

If symptoms of incontinence persist after the initial repair and recovery period, further diagnostic tests may be performed by a colorectal surgeon or urogynecologist to assess the sphincter muscles:

• Endoanal Ultrasound: This is the gold standard imaging test. A small ultrasound probe is placed in the anal canal to create detailed images of the internal and external sphincter muscles to identify any persistent defects.
• Anorectal Manometry: A small, flexible tube with pressure sensors is placed in the rectum and anal canal to measure the strength and function of the sphincter muscles at rest and during squeeze.
• Pudendal Nerve Terminal Motor Latency (PNTML): This test measures the function of the pudendal nerve, which controls the external anal sphincter.

How is an OASI treated?

Initial Repair:
The standard treatment for an OASI is a surgical repair performed in the delivery room immediately after the injury occurs. The goal is to carefully re-approximate the torn layers of the rectal mucosa (in a fourth-degree tear), the internal anal sphincter, and the external anal sphincter. An overlapping repair of the external sphincter is often preferred to an end-to-end repair.

Managing Long-Term Complications:
If a woman continues to have fecal incontinence despite a good initial repair, treatment focuses on managing symptoms and improving sphincter function.

• Dietary management and fiber supplements: To optimize stool consistency.
• Pelvic floor physical therapy and biofeedback: To strengthen the pelvic floor muscles and improve coordination.
• Sacral Nerve Stimulation (SNS): A small, implantable device that stimulates the nerves controlling the sphincter muscles. It is highly effective for fecal incontinence.
• Injectable bulking agents: To help bulk up the anal canal tissue.
• Repeat Sphincter Repair (Sphincteroplasty): For women with a persistent, identifiable defect in the sphincter muscles on ultrasound, a delayed sphincteroplasty may be an option. This is a more complex operation performed months after the initial injury.

Self-Care after an OASI Repair

Proper care in the postpartum period is important for healing and comfort.

• Perineal hygiene: Keep the area clean using a peri-bottle with warm water after using the toilet. Pat the area dry gently.
• Sitz baths: Soaking in warm water can soothe discomfort and promote healing.
• Pain management: Use over-the-counter pain relievers as recommended by your doctor. Ice packs can also help with swelling.
• Stool softeners: It is very important to keep stools soft to avoid straining and putting pressure on the fresh repair.
• Pelvic floor exercises (Kegels): Once cleared by your doctor, gentle pelvic floor exercises can help regain muscle tone.

Can I have another vaginal delivery after an OASI?
This is a complex decision that should be made in discussion with your obstetrician and possibly a colorectal surgeon. If you have completely recovered and have no symptoms of incontinence, a subsequent vaginal delivery may be an option, although there is a risk of another tear. If you have persistent incontinence symptoms or a significant sphincter defect on ultrasound, an elective Cesarean section is often recommended to prevent further injury.

Is fecal incontinence permanent after an OASI?
Not necessarily. With a proper initial repair and appropriate postpartum care, many women regain full or near-full continence. For those with persistent symptoms, there are many effective treatments, from physical therapy to sacral nerve stimulation, that can significantly improve or restore bowel control.

Will the repair affect my sex life?
Pain with intercourse (dyspareunia) can be a problem after any perineal tear. It is important to wait until you are fully healed and have been cleared by your doctor before resuming sexual activity. If pain persists, talk to your doctor or a pelvic floor physical therapist, as there are treatments that can help.

• Kniery KR, Johnson EK, Steele SR. Operative considerations for rectovaginal fistulas. In: Steele SR, Hull TL, Hyman N, Maykel JA, Read TE, Whitlow CB, eds. The ASCRS Textbook of Colon and Rectal Surgery. 4th ed. Springer; 2022.
• American College of Obstetricians and Gynecologists. ACOG Practice Bulletin No. 197: Obstetric Anal Sphincter Injuries. Obstet Gynecol. 2018;132(3):e87-e96.
• Royal College of Obstetricians & Gynaecologists. The Management of Third- and Fourth-degree Perineal Tears. Green-top Guideline No. 29. Published June 2015. Accessed October 12, 2025. https://www.rcog.org.uk/en/guidelines-research-services/guidelines/gtg29/