Obstetrical Anal Sphincter Injuries and Sphincteroplasty

Obstetrical Anal Sphincter Injuries and Sphincteroplasty

Bhumy Dave Heliker

Afiba Arthur


Obstetrical anal sphincter injuries (OASIS) are thirdand fourth-degree lacerations that occur at the time of vaginal delivery. A third-degree perineal laceration is defined as a laceration which involves the external anal sphincter (EAS) with or without involvement of the internal anal sphincter (IAS) and a fourth-degree perineal laceration is defined as injury to EAS, IAS, and the anal mucosa.1 In 1999, Sultan2 published a subclassification of perineal lacerations that further delineates the extent of injury with third-degree lacerations (Table 58.1).

The prevalence of OASIS in the general population is between 0.25% and 6% and higher in primiparous women (1.4% to 16%).3,4 In multiparous women, it is estimated to have a prevalence of 0.4% to 2.7%.3 Significant research has been done to better understand the risk factors for OASIS. Primiparity, operative vaginal delivery with vacuum or forceps, and fetal weight greater than 4,000 g have been associated with more significant lacerations.3,4,5,6,7,8 Other factors include a midline episiotomy at the time of delivery, persistent occiput posterior position of the fetal head, and shoulder dystocia.8,9 Increasing maternal age has also been shown to increase the risk of OASIS during vaginal delivery.9,10 Rahmanou et al.10 developed a model where the odds ratio of developing pelvic floor trauma at the time of a vaginal delivery for each additional year older than age 18 years was 1.064. A long second stage of labor, specifically more than an hour, also increases the risk for a third- and fourth-degree perineal laceration.11,12 Other factors such as epidural, induction, and increased length of labor can increase the risk for OASIS.11,12 With regard to demographic characteristics, Meister et al.13 reported significant risk with obesity, nonsmoking status, and other races besides black. Given the significant number of risk factors that can predispose a woman to sustaining an obstetrical anal sphincter injury, there have been attempts to develop a risk scoring system. McPherson et al.14 created a scoring system to help practitioners determine OASIS risk with a sensitivity of 52.7% and specificity of 71%. Although this sensitivity and specificity are relatively low, this is a tool that clinicians can use to gather patient’s risk factors and implement the appropriate measures to help reduce the risk of an anal sphincter injury.

The reported risk of recurrent OASIS varies between 3.2% and 10.7% depending on the population that is being studied.3,15,16,17 An odds ratio of greater than 5 has been reported for recurrence.7,18,19 Forceps delivery, birth weight greater than 4,000 g, persistent occiput posterior position, and Asian race are risk factors for sustaining a recurrent OASIS in a subsequent pregnancy.1,11


Anal sphincter injury diagnosed at the time of vaginal delivery should be properly repaired. Immediate identification and repair at the time of delivery will help reduce morbidity to the patient. Figure 58.6 outlines the muscles that are injured in third- and fourth-degree lacerations. It is important that once the diagnosis is made, the patient be given adequate pain control via epidural or pudendal block. Furthermore, the obstetrician should
ensure there is adequate retraction, good lighting, and consider liberal irrigation with septic solutions such as dilute povidone-iodine. The patient should be moved to the operating room if better exposure and lighting is needed or adequate anesthesia cannot be achieved in the labor room. The muscles of the torn EAS may retract into ischioanal fat and may not be easily visible. Allis clamps should be used to delineate the sphincter edges and the capsule to facilitate identification during the repair.25 Retractors such as a Gelpi retractor can be helpful to improve visibility.25

In cases where an ob-gyn has limited or no experience with repair, a consultant should be called, or the patient should be transferred to a facility with the appropriate expertise, so she can have the best outcome possible. Delaying the repair for a short duration of time to allow for hospital transfer or a consultant to arrive has not been associated with worse outcomes. In a Swedish study where women who sustained anal sphincter injuries at the time of delivery were randomized to either immediate repair or delayed repair for 8 to 12 hours, there was no difference in anal incontinence between the two groups.26

Primary Repair of Obstetrical Anal Sphincter Injuries

Primary repair is done at the time of delivery. It is important to identify the various parts of the anal sphincter that are injured so they can be appropriately repaired. Patients who have sustained OASIS are at increased risk for anal incontinence; women who sustain a fourth-degree laceration are at about 10 times higher risk for anal incontinence compared to those with third-degree laceration injuries.27,28

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May 1, 2023 | Posted by in GYNECOLOGY | Comments Off on Obstetrical Anal Sphincter Injuries and Sphincteroplasty

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