Effect of repeat acute injury on contractile function of the external anal sphincter in an animal model

Introduction and hypothesis: Our aim was to estimate the physiologic effects of early repeat transection and repair on the contractile properties of the external anal sphincter (EAS) in a rat model. Methods: Eighty young female rats underwent anal sphincter transection and repair. After 7 days, they were randomized to repeat sphincter transection (injury-injury, n = 40) or sham operation (injury-sham, n = 40). Thereafter, the anal sphincter complex was dissected, mounted, and analyzed for contractile function 7 days, 21 days, 3 months, or 6 months after the second operation. Contractile function was also determined in 40 age-matched unoperated controls (n = 10 for each time point). Statistical analysis was performed using analysis of variance (ANOVA) with Tukey-Kramer adjustment for multiple testing. P ≤ 0.05 was considered significant. Results: Although single injury (injury-sham) resulted in modest compromise of sphincter function, repeat injury (injury-injury) resulted in profound impairment of twitch tension, maximal tetanic responses, and maximal electrical-field stimulation (EFS) induced-force generation at 7 days. After single injury, parameters of contractile function returned to baseline uninjured levels by 21 days. In contrast, sphincter function remained reduced 21 days after repeat injury. Contractile function of sphincters from both injury-sham and injury-injury animals were no longer impaired at 3 and 6 months. Conclusion: In this animal model, repeat injury and repair of the EAS 7 days after the initial injury resulted in prolonged compromise of EAS function compared with single injury. Nevertheless, contractile function of the double-injured sphincter fully recovered with time, resulting in no long-term impairment.