Effect of Prolonged Vaginal Distention and Sphincter Transection on Physiologic Function of the External Anal Sphincter in an Animal Model Clifford Y. Wai, MD, Rodney T. Miller, MD, and R. Ann Word, MD OBJECTIVE: To estimate the effect of prolonged vaginal distention and anal sphincter transection on contractile properties of the external anal sphincter. METHODS: Young female virgin rats were randomly as- signed to four treatment groups (sham, vaginal distention, transection of anal sphincter plus repair, or combined distention and transection plus repair). After 3 weeks, the anal sphincter complex was analyzed for twitch tension, maximal tetanic force, fatigue, and maximal responses to electrical field stimulation. Each sphincter was serially cross- sectioned and stained with hematoxylin and eosin to eval- uate sphincter integrity. Statistical evaluation was per- formed using analysis of variance (Student-Newman-Keuls) and  2 analysis. RESULTS: The function of a transected and repaired anal sphincter is compromised significantly after 3 weeks. Vagi- nal distention alone did not alter function of the sphincter. Further, prolonged vaginal distention together with sphinc- ter transection did not result in statistically significant dif- ferences in sphincter function compared with transection alone. CONCLUSION: In an animal model, there is a major effect on external anal sphincter function in vitro 3 weeks after laceration with repair. Although prolonged vaginal distention had no significant effect alone and minimal adverse effects on the lacerated anal sphincter, laceration of the sphincter has major adverse effects on its morphol- ogy and function. (Obstet Gynecol 2008;111:332–40) LEVEL OF EVIDENCE: II A nal incontinence is a debilitating condition that has psychosocial and economic consequences. The rate of postpartum anal incontinence ranges from 6 –24%. 1– 4 Research indicates that direct injury of the anal sphinc- ter mechanism is associated with postpartum anal incon- tinence. 1–17 The exact mechanism is unclear. It is un- known whether prolonged vaginal distension by the fetal head alters the effect of anal sphincter injury on the continence mechanism, whether the muscular injury and defective repair process cause anal incontinence, or whether the circumstances leading to anal sphincter laceration during childbirth (ie, denervation and hyp- oxia caused by prolonged vaginal distention) contribute to this disorder. Finally, the effect of prolonged vaginal distention on healing of the surgically repaired anal sphincter complex is not defined. It is possible that neuronal injury occurs before episiotomy, and the com- bination of neuronal injury (or stretch) and fourth- degree laceration worsens anal sphincter function. In the current study, we used the rat as an animal model to begin to understand the effect of prolonged vaginal distention on function of the external anal sphincter and to estimate the effect of prolonged vaginal distention and anal sphincter laceration (with repair) on physiologic function of the external anal sphincter. MATERIALS AND METHODS All procedures were approved by the Institutional Animal Care and Use Committee at the University of From the Divisions of Urogynecology and Reconstructive Pelvic Surgery and Reproductive Endocrinology, Department of Obstetrics and Gynecology, University of Texas Southwestern Medical Center, Dallas, Texas; and ProPath Laboratories, Inc., Dallas, Texas. Supported by a research grant from The American Urogynecologic Society/June Allyson Foundation. Corresponding author: Clifford Y. Wai, MD, Department of Obstetrics and Gynecology, University of Texas Southwestern Medical Center, 5323 Harry Hines Boulevard, Dallas, Texas 75390-9032; e-mail: Clifford.Wai@UTSouthwestern. edu. Financial Disclosure The authors have no potential conflicts of interest to disclose. © 2008 by The American College of Obstetricians and Gynecologists. Published by Lippincott Williams & Wilkins. ISSN: 0029-7844/08 332 VOL. 111, NO. 2, PART 1, FEBRUARY 2008 OBSTETRICS & GYNECOLOGY