Effects of pregnancy, parturition, and anal sphincter transection on function of the external anal sphincter in an animal model

OBJECTIVE:: To estimate the effects of pregnancy, parturition, and anal sphincter laceration (with repair) on external anal sphincter morphology and neurophysiology and to define the time course of these effects after injury. METHODS:: Within 4 hours of vaginal delivery, 80 rats underwent either sham or anal sphincter laceration with repair. After 3 days, 3 weeks, and 3 and 6 months (n=20 for each time point), animals were killed, and the anal sphincter complexes dissected and removed for neurophysiologic studies. Twitch tension, peak tetanic force, fatigue, and maximal electrical field-stimulated force generation were determined. Sphincters were then fixed and serially sectioned (5-micrometer thickness) at 100-micrometer intervals for histologic analysis. RESULTS:: Maximal electrical field-stimulated force generation, maximal tetanic contraction, and twitch tension were decreased in the external anal sphincter 3 days after anal sphincter laceration with repair compared with sham-operated parturient rats (3.3 g compared with 11.6 g, 4.5 g compared with 14.5 g, and 0.6 g compared with 2.0 g, respectively, all P<.02). Increased fatigability of the sphincter muscle was observed in all newly parturient rats-sham and anal sphincter laceration with repair; recovery occurred in the shams by 3 months. A gradual recovery occurred in all these neurophysiologic measures, with no significant differences between anal sphincter laceration with repair and shams by 6 months postpartum. CONCLUSION:: Repaired anal sphincter transection in periparturient animals results in short-term severe compromise of neurophysiologic function of the external anal sphincter. Over time, however, force generation recovers and approximates that of postpartum rats with intact anal sphincters.