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Evaluation and Management of Obstetric Anal Sphincter Injuries (OASIS)

Author: Camaryn Chrisman-Robbins, MD

Mentor: Eric A. Strand, MD
Editor: Brett Worley, MD

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All women having vaginal delivery should be closely examined for vaginal and perineal injuries and those with perineal laceration should have rectal examination to determine the extent of their injury prior to surgical repair.  Obstetric anal sphincter injuries (OASIS) encompass 3rd and 4th degree lacerations.  A fourth-degree laceration involves the perineal structures, the external anal sphincter (EAS), internal anal sphincter (IAS), and the rectal mucosa.  A rectal buttonhole tear involves the rectal mucosa with an intact sphincter and may be overlooked without a systematic rectal examination. Therefore, digital rectal exam should be performed if there is any concern that the anal sphincter or rectal mucosa is compromised. Failure to recognize and properly repair a fourth-degree laceration poses a risk of infection, wound breakdown, anal incontinence, and fistula formation.

Pooled analysis of data involving 15,366 women with OASIS from 22 studies reported an overall rate of 3rd and 4th degree lacerations of 2.4 %.  Operative vaginal delivery with and without episiotomy and midline episiotomy increased the risk of an OASIS laceration substantially.  The odds ratios (OR) with forceps delivery, vacuum-assisted delivery, and midline episiotomy were 5.50, 3.98, and 3.82 respectively.  When forceps were combined with midline episiotomy, the OR for a 3rd degree laceration was 5.65 and for a 4th degree laceration 10.55.  Other risk factors which increase the OR 2-3 times for OASIS injuries include increased fetal birth weight, primiparity, labor induction and augmentation, persistent occipito-posterior position, and epidural anesthesia.

Surgical repair of fourth-degree lacerations requires adequate lighting, adequate anesthesia, appropriate surgical instruments, copious irrigation and meticulous attention to hemostasis for optimal wound reapproximation and healing.  Consideration should be given to performing the repair in an operating room setting.

The rectal mucosa is repaired in a continuous non-locked suture using a monofilament suture such as polydioxanone (PDS®) or a braided suture such as polyglactin (Vicryl®).  Suture size of 3-0 should be used.

The IAS is identified separately as a white line just proximal to the EAS and should be reapproximated with interrupted 2-0 Vicryl or 3-0 PDS to reduce the risk of post repair anal incontinence.  To reapproximate the EAS, the separated ends must be isolated and grasped with Allis forceps as they tend to retract laterally and inferiorly. There is inadequate data as to whether end-to-end or overlapping reapproximation of the EAS decreases risk of long-term incontinence. This choice may be left to the surgeon’s preference.  Regardless of approach, either interrupted 2-0 Vicryl or 3-0 PDS is typically used.  Permanent suture should not be used on the EAS.

The remainder of the repair to rebuild the perineal body is performed as a routine 2nd degree laceration.  Rectal examination should be performed after completing repair of the external anal sphincter to ensure adequacy of sphincter repair and to ensure sutures were not passed through the rectal mucosa.  Evidence suggests that intrapartum antibiotics reduce OASIS wound complications such as infection and breakdown. A single dose antibiotic at the time of repair is reasonable.

Postoperatively, women should be counseled to maintain a low fiber, low residue diet and take stool softeners for at least 10 days.  Non-steroidal anti-inflammatory agents and acetaminophen are preferred analgesics; oral narcotics should be used with caution because of risk of constipation.  In addition, daily Sitz baths and pelvic floor physical therapy may aid in wound healing.  An appointment is advised one or two weeks after discharge with an obstetric provider to ensure there are no signs of wound infection, breakdown, or hematoma formation.

Women should be advised that the prognosis following EAS repair is good, with 60–80% of women having no symptoms of anal incontinence or significant discomfort at 12 months.  Most women who remain symptomatic describe incontinence of flatus or feces or fecal urgency.  After a successful repair, most women can deliver vaginally with a recurrent risk of OASIS between 4% and 8%.  Patients with persistent incontinence may be offered elective cesarean delivery. Risk of occurrence of OASIS may be reduced by controlled head delivery, use of mediolateral episiotomy if episiotomy is indicated, particularly for operative vaginal deliveries, and use of warm compress on the perineum and perineal massage during second stage of labor.

Further Reading:

American College of Obstetricians and Gynecologists, Committee on Practice Bulletins-Obstetrics. ACOG Practice Bulletin No. 198: Prevention and Management of Obstetric Lacerations at Vaginal Delivery. Obstet Gynecol. 2018 Sep;132(3):e87-e102. doi: 10.1097/AOG.0000000000002841.

Society of Obstetricians and Gynaecologists of Canada, Harvey MA, Pierce M, et al., Obstetrical Anal Sphincter Injuries (OASIS): Prevention, Recognition, and Repair. J Obstet Gynaecol Can. 2015 Dec;37(12):1131-48.

Initial approval January 2015; Revised November 2017, May 2019, January 2021; Minor Revision May 2022; Minor revision March 2024.


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