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2002 Selected Articles

Obstetric Anal Sphincter Lacerations: An Evidence-based Approach
Part 1: Anatomy and Risk Factors

Rebecca G. Rogers, MD; Dorothy N. Kammerer-Doak, MD; Maridee J. Spearman, MD

The prevention and repair of obstetric anal sphincter lacerations require more attention from obstetricians and gynecologists. Recent evidence reveals that traditional repair of anal sphincter lacerations following vaginal delivery is not as successful as had been assumed. In addition, the incidence of residual complications–including anal incontinence following laceration repair–is underestimated. Many physicians neglect to ask about anorectal symptoms after childbirth, and women may be reluctant to raise the subject of anal incontinence.^1-3 This two-part series reviews the impact of vaginal birth on posterior compartment function, with attention to prevention and repair of obstetric anal sphincter lacerations. Part 1 discusses the incidence of obstetric anal sphincter lacerations, the anatomy of the anal sphincter complex, and the risk factors for severe perineal trauma.

INCIDENCE

Anal incontinence occurs in approximately 0.5% to 13% of the US population, and is 6 to 8 times more common in women than men.⁴ The most common cause of anal incontinence is anal sphincter injury during childbirth. Such sphincter injuries may result in anal incontinence in up to 33% of affected women.⁵ Anal incontinence may immediately follow obstetric anal sphincter laceration despite primary repair, or occur some time after delivery.

Anal continence is maintained by a variety of mechanisms, including normal stool delivery and consistency, intact sensation and motor innervation, an intact anal sphincter complex, and a functioning puborectalis muscle (Table 1).^5,6 The true incidence of anal sphincter damage at the time of vaginal delivery may be higher than the number of observed injuries would suggest due to the presence of occult injury. For example, even continent women whose sphincters were disrupted by childbirth may develop anal incontinence later with the onset of neurologic changes or muscle atrophy as they age. Overt rectal sphincter injury is relatively rare (0% to 6.4%) in women who have not undergone episiotomy or operative vaginal delivery.^5,7-9 However, in studies using ultrasonography, the incidence of occult anal sphincter laceration ranges from 6.8% to 35% in nulliparous women, and 12.2% to 44% in multiparous women.^1,10 Furthermore, 13% of primiparous and 23% of multiparous women with anal sphincter injuries diagnosed ultrasonographically have symptoms of anal incontinence.^1-3

ANATOMY

Understanding the anatomy of the anal sphincter complex is essential to successful repair. The anorectal canal lies inferior to the perineal body, and is surrounded by a complex tube of muscle fibers that form the internal and external anal sphincters. The external anal sphincter (EAS) extends 2 cm up the anal canal, lying in close proximity to the puborectalis muscle. The EAS is responsible for the squeeze tone of the rectal canal and 10% to 20% of the resting tone. The EAS is comprised of striated muscle, and is under somatic (ie, voluntary) control.

The internal anal sphincter (IAS) provides the majority of the resting tone of the anus (80% to 90%). It is comprised of smooth muscle, overlapping and lying superior to the EAS. The IAS is rarely discussed in obstetric texts, but forms an important component of the fecal continence mechanism. The IAS can be identified as a glistening layer below the EAS and may be retracted laterally. It lies adjacent to the rectal mucosa, and is continuous with the longitudinal smooth muscle of the colon. Repair of the IAS restores the high-pressure zone of the anal canal. The entire anal sphincter complex extends for approximately 3 cm, with the IAS and EAS overlapping for about 1.7cm.¹¹ The puborectalis muscle lies close to the anal sphincter complex and runs from the pubic symphysis to the coccyx, with portions of the muscle inserting into the EAS. The anal sphincter complex is attached to the perineal body, which in turn is attached to the rectovaginal septum and the uterosacral ligaments. The perineal body is composed of multiple striated muscles and their connective tissues, including the bulbocavernosis and the transverse perineal muscles as well as fibers from the EAS. The posterior compartment is interconnected on multiple levels.

CLASSIFYING LACERATIONS

Obstetric lacerations are usually classified as mild (first or second degree) or severe (third or fourth degree) (Table 2). First-degree lacerations involve only the vaginal mucosa, while second-degree lacerations extend through the vaginal mucosa and fascia and the muscles of the perineum. Severe lacerations involve the anal sphincter complex. Third-degree lacerations extend into or through the EAS, with or without IAS involvement, and fourth-degree lacerations extend through the EAS, IAS, and rectal mucosa.

RISK FACTORS

Factors that are widely believed to increase the risk of third- and fourth-degree laceration have not been well studied in randomized clinical trials (Table 3). Episiotomy, performed in up to 62% of vaginal births in the United States, is one of the most common surgical procedures in the world. This innovation was originally introduced to protect the anal sphincter complex while ensuring clean-cut surfaces for postnatal repair. Episiotomy was also thought to result in less postpartum pain and fewer long-term sequelae for the parturient. However, like many interventions in medicine, it was adopted prior to the accumulation of evidence confirming its effectiveness or utility.⁵

A Cochrane review of six randomized trials compares "restrictive" with "liberal" use of episiotomy.¹² The incidence of episiotomy in the 4,850 subjects was 27% in the "restricted" group and 73% in the "liberal" group. The "restricted" group had fewer perineal lacerations, but an equivalent incidence of third- and fourth-degree lacerations when compared with the "liberal" group. The authors recommended restricted use of episiotomy because, while it did not reduce the incidence of third- or fourth-degree lacerations, it was associated with more perineal trauma. Additionally, the "restricted" group had fewer complications and less suturing than the "liberal" group. This meta-analysis does not substantiate the argument that episiotomy increases the risk of severe lacerations; however, it does show that liberal use of episiotomy confers no benefits and is associated with other complications.¹²

Multiple cohort studies also support restricted use of episiotomy. Anal sphincter laceration rates reported in the literature range from 0% to 6% in women who undergo spontaneous delivery without episiotomy, compared with 0% to 24% in women who undergo delivery with midline episiotomy.^5,7-9

Some studies report a decreased rate of sphincter lacerations with the use of mediolateral versus midline episiotomy. One randomized trial found that the incidence of anal sphincter laceration in 407 women was 24% with midline episiotomy and 9% with mediolateral episiotomy. In cohort studies, midline episiotomy increased the risk of anal sphincter laceration over mediolateral episiotomy (7.6% versus 1.1%).¹³ However, anal sphincter lacerations were more common in women who underwent any type of episiotomy than in those who did not.¹³

Figure. Anal Sphincter Complex Figure. Anal Sphincter Complex

Operative vaginal delivery is another purported risk factor for severe perineal laceration. One study of 845 women who delivered vaginally found an odds ratio of 6.5 (3.2 to 13.6) for anal sphincter laceration following operative delivery.⁹ Several randomized trials have examined whether forceps or vacuum delivery result in higher rates of third- or fourth-degree lacerations. Among 637 women randomized to forceps or vacuum delivery, anal sphincter lacerations occurred in 29% of the forceps group compared with 12% in the vacuum group (P<.001).¹⁴ A Cochrane review of six randomized trials including 2,582 women demonstrated that the relative risk of anal sphincter trauma for women undergoing vacuum delivery compared with forceps delivery was 0.41 (0.33 to 0.50), ie, a marked decrease in patients undergoing vacuum delivery.¹⁵

Cohort studies comparing forceps with vacuum delivery also reveal an increased incidence of anal sphincter lacerations with forceps deliveries. In a study of 8,603 women who delivered vaginally, 50 women sustained a third-degree laceration. Half of these women had forceps deliveries, while in the same cohort, none of the women who underwent vacuum delivery sustained a third-degree laceration. Incidence rates for third- or fourth-degree lacerations were 50% with forceps deliveries and 0% with vacuum deliveries.¹⁶ However, a possible confounder in this investigation was that most women with forceps deliveries also had episiotomies.¹⁶

Several cohort studies examine whether episiotomy at the time of operative vaginal delivery protects the anal sphincter complex. Among 2,832 women who underwent operative vaginal delivery, the addition of episiotomy increased the incidence of anal sphincter trauma when compared with operative vaginal delivery without episiotomy, with an odds ratio of 7.81 (5.9 to 10.3).⁸ In another trial of 444 women who underwent operative delivery, anal sphincter injury was more common in those who had an episiotomy, with a relative risk of 2.4 (1.7 to 3.5).⁷ These studies suggest restricting use of episiotomy at the time of operative vaginal delivery to reduce the risk of anal sphincter laceration.

Other possible risk factors, such as maternal position, fetal weight or position, and perineal massage or support have been reported to either increase or decrease the incidence of perineal trauma. However, these factors have not been well studied as to their impact on the incidence of third- and fourth-degree lacerations.

CONCLUSION

Injury to the anal sphincter complex during vaginal delivery may have significant anatomic and functional sequelae. As always, the best treatment is prevention. Avoiding liberal use of episiotomy and forceps delivery may decrease the incidence of anal sphincter lacerations. When severe lacerations do occur, understanding the anatomy of the anal sphincter complex can help to ensure appropriate and effective repair.

Rebecca G. Rogers, MD, is director, Division of Urogynecology and assistant professor of obstetrics and gynecology at the University of New Mexico, Albuquerque. Dorothy N. Kammerer-Doak, MD, is associate clinical professor, Department of Obstetrics and Gynecology, University of New Mexico and urogynecologist, Lovelace Health Systems, Albuquerque, NM. Maridee J. Spearman, MD, is a resident, Department of Obstetrics and Gynecology, University Hospital, Albuquerque, NM.

REFERENCES

1. Sultan AH, Kamm MA, Hudson CN, et al. Anal-sphincter disruption during vaginal delivery. N Engl J Med. 1993;329: 1905-1911.
2. Sultan AH, Kamm MA, Hudson CN, Bartram CI. Effect of pregnancy on anal sphincter morphology and function. Int J Colorect Dis. 1993;8:206-209.
3. Sultan AH, Kamm MA, Hudson CN, Bartram CI. Third-degree obstetric anal sphincter tears: risk factors and outcomes of primary repair. Br Med J. 1994;308:887-891.
4. Madoff RD, Williams JG, Caushaj PF. Fecal Incontinence. N Engl J Med. 1992;326:1002-1007.
5. Thacker SB, Banta HD. Benefits and risks of episiotomy: an interpretative review of the English language literature, 1860-1980. Obstet Gynecol Surv. 1982;38:322-338.
6. Haadem K, Ohrlander S, Lingman G. Long-term ailments due to anal sphincter rupture caused by delivery–a hidden problem. Eur J Obstet Gynecol Reprod Biol. 1988; 27:27-32.
7. Helwig JT, Thorp JM, Bowes WA. Does midline episiotomy increase the risk of third- and fourth-degree lacerations in operative vaginal deliveries? Obstet Gynecol. 1993;82: 276-279.
8. Combs CA, Robertson PA, Laros RK. Risk factors for third-degree and fourth-degree perineal lacerations in forceps and vacuum deliveries. Am J Obstet Gynecol. 1990;163:100-104.
9. Zetterstrom J, Lopez A, Anzen B, et al. Anal sphincter tears at vaginal delivery: risk factors and clinical outcome of primary repair. Obstet Gynecol. 1999;94:21-28.
10. Varma A, Gunn J, Gardiner A, et al. Obstetrical anal sphincter injury: prospective evaluation and incidence. Dis Colon Rectum. 1999;42:1537-1543.
11. Delancey JOL, Toglia MR, Perucchini D. Internal and external anal sphincter anatomy as it relates to midline obstetric lacerations. Obstet Gynecol. 1997;90:924-927.
12. Carroli G, Belizan J. Episiotomy for vaginal birth (Cochrane Review). In: The Cochrane Library. Oxford: Update Software; Issue 4, 2000.
13. Eason E, Labrecque M, Wells G, Feldman P. Preventing perineal trauma during childbirth: a systematic review. Obstet Gynecol. 2000;95:464-471.
14. Bofill J, Rust O, Schorr S, et al. A randomized prospective trial of the obstetric forceps versus the M-cup vacuum extractor. Am J Obstet Gynecol. 1996;175:1325-1330.
15. Johanson RB, Menon BKV. Vacuum extraction versus forceps for assisted vaginal delivery (Cochrane Review). In: The Cochrane Library. Oxford: Update Software; Issue 4, 2000.
16. Sultan AH, Kamm MA, Hudson CN, Bartrum CI. Third degree obstetric anal sphincter tears: risk factors and outcome of primary repairs. BMJ. 1994;306(887-891).

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