Obstetrical Anal Sphincter Injuries and Sphincteroplasty
Bhumy Dave Heliker
Afiba Arthur
EPIDEMIOLOGY
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
PATHOPHYSIOLOGY
Understanding the anatomy of the anal sphincter complex is necessary for the diagnosis and management of injuries. The anal sphincter complex consists of the EAS and IAS.
The EAS is a striated muscle and under voluntary control. The innervation comes from the inferior rectal nerve, a branch of the pudendal nerve. It is responsible for the majority of the squeeze pressure. The EAS extends to the most distal portion of the anus, whereas the IAS does not extend fully to the distal anus; thus, the very distal portion of the sphincter complex consists of the EAS alone. It is also important to understand the relationship between the EAS and other skeletal muscles of the pelvic floor. The EAS works in conjunction with one of the levator ani muscles, the puborectalis muscle, to provide anal continence. The puborectalis muscle plays a critical role in the formation of the anorectal angle and thereby contributes to the continence mechanism by helping keep the proximal anal canal closed.20 Figure 58.1 shows the relationship between the puborectalis muscle, IAS, and EAS. In addition, 3D ultrasound shows that the EAS extends into the right and left transverse perineal muscles.20 This means that injuries to the transverse perineal muscles or cutting an episiotomy could also compromise the full functioning of the EAS. Figure 58.2 shows the external anatomy of the pelvic floor muscles and the relation to the anal sphincter.
TABLE 58.1 Classification of Perineal Lacerations | ||||||||||||||||||||||||
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The IAS arises from the distal thickening of the circular smooth muscles of the anal canal, and it is under the control of the autonomic nervous system.1 It is responsible for about 80% of the resting tone of the anal sphincter complex.1 Disruption of the EAS, the IAS, or the rectal mucosa can compromise the fecal continence mechanism.
DIAGNOSIS
OASIS should be diagnosed at the time of delivery but is occasionally diagnosed on exam remote from delivery or with an endoanal ultrasound.
 FIGURE 58.1 Sagittal view of the anal sphincter in relation to other structures of the female pelvic floor. (Reprinted from Jones HW, Rok JA. Te Linde’s operative gynecology, 11th ed. Philadelphia: Wolters Kluwer, 2015.) |
Making the diagnosis at the time of vaginal delivery is critical for ensuring immediate repair and reducing the morbidity to the patient. It is estimated that about a third of anal sphincter injuries are missed at the time of delivery, so it is important for obstetricians and midwives to be trained on how to properly identify these injuries.21 This requires a careful examination and a rectal exam to help determine the integrity of the anal sphincter and more subtle mucosal injuries (e.g., proximal to the sphincter complex). If there is any doubt about the degree of the laceration, it is better to make a diagnosis of a higher degree laceration than a lower degree injury to ensure that an adequate repair and appropriate care is provided to the patient.22
Examination of the integrity of the sphincter is performed by placing the dominant index finger in the anus and the thumb of the same hand in the vagina (Fig. 58.3).22 The two fingers are then pinched together in a side-to-side rolling motion.22 A defect can be palpated along the EAS if there is an injury because the muscles retract to the sides. Furthermore, a half speculum can be used to retract the anterior vagina and expose the length of the posterior wall. For identification of pinpoint lacerations, one may inject gel with methylene blue into the rectum, apply gentle pressure along the rectovaginal septum, and look for extravasation through the vagina (Fig. 58.4).
For sphincter injuries are not diagnosed at the time of delivery, an endoanal ultrasound can be performed to help locate the injury, especially for patients who have symptoms of anal incontinence. Figure 58.5 demonstrates an endoanal ultrasound image of an intact anal sphincter complex juxtaposed with an image of an EAS defect from 10 to 2 o’clock.23 Women with IAS injury compared to those who have EAS injury alone on endoanal ultrasound are more likely to have fecal incontinence.24 Of note, 24% of women diagnosed with occult OASIS on ultrasound were found to have no evidence of injury on surgical exploration.24 This suggests that universal ultrasound for all women with second-degree lacerations may result in a false-positive rate; further investigation about the role of universal ultrasound in asymptomatic women is needed. Currently, endoanal ultrasounds in women with second-degree perineal lacerations should be reserved for women with symptoms of anal incontinence or physical exam findings raising suspicion of occult OASIS.
MANAGEMENT
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
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
 FIGURE 58.2 Muscles of the female perineum. (From Toglia MR. Repair of perineal and other lacerations associated with childbirth. Accessed July 2, 2019. https://www.uptodate.com/contents /repair-of-perineal-and-other-lacerations-associated-with-childbirth. Copyright © 2019 UpToDate, Inc. and its affiliates. All rights reserved.) |
 FIGURE 58.3 Illustration of how to examine for anal sphincter defects. |
 FIGURE 58.4 Injection of dye to detect anal sphincter defect. |
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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