Assessment of normal anal sphincter anatomy using transanal ultrasonography in healthy Korean volunteers: a retrospective observational study

Background: To date, there have been no studies on the normal anatomic values of the anal sphincter in healthy Koreans. Therefore, this study aimed to determine the normal anatomic values of transanal ultrasonography (TAUS).Methods: The thickness of the external anal sphincter (EAS) and internal anal sphincter (IAS) was measured by TAUS from healthy Korean volunteers between September 2019 and August 2021.Results: Thirty-six volunteers with a median age of 37 years (range, 20–77 years) and a median body mass index (BMI) of 23.5 kg/m2 (range, 17.2–31.2 kg/m2) were examined. The median thickness of the EAS at 4 cm and 2 cm from the anal verge was 7.4 mm (range, 5.8–8.8 mm) and 6.5 mm (range, 5.6–8.0 mm), respectively. The median thickness of the IAS at 2 cm from the anal verge was 1.8 mm (range, 0.8-4.3 mm). There were no differences in sphincter muscle thickness between the sexes. However, the EAS tended to thicken as the BMI increased (EAS at 2 cm and 4 cm from the anal verge, Spearman rho=0.433, 0.363; p=0.008 and p=0.029, respectively).Conclusion: In healthy Korean, the median thickness of the IAS at 2 cm from the anal verge was 1.8 mm and the median thickness of the EAS at 2 cm and 4 cm from the anal verge was 6.5 mm and 7.4 mm respectively. There were no differences in anal sphincter thickness between sexes, but BMI was related to EAS thickness.