Atrophy of the sphincters of continence in an experimental model

BACKGROUND: Pudendal nerve injury during childbirth may result in external anal sphincter (EAS) atrophy. Recently, balloon compression of the pelvic side wall has been shown to result in EAS atrophy in an experimental model. The aim of this study was to determine whether other sphincters of continence are similarly affected. METHOD: Sixteen adult female virgin Wistar rats (eight controls) were studied 4 weeks after surgery. Anal and urethral canals were dissected, snap frozen and sectioned using a cryostat (100 microm thickness). Masses of EAS, internal anal sphincter (IAS) and external urethral sphincter (EUS) were calculated stereologically and stained with succinate dehydrogenase histochemistry to differentiate striated from smooth muscle. Sphincter length was determined and total sphincter mass calculated. Data were analysed with an unpaired Student's t-test. RESULTS: Atrophy of EAS (30.9 +/- 1.7 to 21.3 +/- 1.7 mg/ kg, P = 0.001), IAS (98.1 +/- 11.3 to 67.1 +/- 5.0 mg/kg, P = 0.01) and EUS (9.6 +/- 0.98 to 7.4 +/- 0.8 mg/kg, P = 0.05) was found 4 weeks after the injury. CONCLUSION: In an experimental model of obstetric pudendal nerve injury, significant atrophy of striated and smooth muscle sphincters of continence occurs and may contribute to altered continence following vaginal childbirth.