Anal Sphincter Length as Determined by3-DimensionalEndoanal Ultrasound and Anal Manometry A Study in Healthy Nulliparous Women

Objectives The normal female external anal sphincter (EAS) is shorter anteriorly than laterally and posteriorly. Furthermore, the thickness of the very proximal part of the circular EAS is thinner than 50% of the lateral and posterior EAS thickness. The extent of these features is not fully explored. The aim of this prospective study was to assess the normal anal sphincter with 3-dimensional (3D) endoanal ultrasound (EAUS) and to relate 3D EAUS length measurements to sphincter length determined by anal manometry. Methods Healthy premenopausal nulliparous women underwent anal manometry and 3D EAUS examinations. Two experienced colorectal surgeons independently assessed all scans, blinded to any patient data. Results A total of 43 women were included. Four scans were incomplete and excluded from the analysis. Interobserver agreement was fair to very good for the various length measurements. The mean length from the distal border of the puborectal muscle to the very proximal border of the anterior EAS (the anterior gap) was 4.4 (95% confidence interval, 3.9-4.9) mm, whereas the length to the level where the anterior EAS thickness was at least 50% of the lateral and posterior EAS thickness was 7.2 (95% confidence interval, 6.5-7.9) mm. Manometric sphincter length at rest did not correlate with any 3D EAUS length measurements. Conclusions In the normal anterior female anal canal, the EAS is not present or appears with less than 50% of the thickness of the lateral and posterior EAS for the first 7.2 mm below the distal border of the puborectal muscle.