Shape and volume of internal anal sphincter showed by three-dimensional anorectal ultrasonography

Purpose: The aim of this work is to characterize the shape and the volume of the internal anal sphincter (IAS) in normal subjects by three-dimensional anorectal ultrasonography. Methods: Thirty-nine normal volunteer males (mean age 58.5 ± 18.7) and 25 females (mean age 59.4 ± 14.1) were submitted to anorectal ultrasonography. The tissue is defined by a semiautomatic procedure. Measurements of thickness, length and volume were assessed automatically. The software provides an average number of 57,600 thickness measurements, 360 length measurements for each zone (90 for each quadrant) and seven volume measurements (one for each anatomical area).The mean values of magnitudes were calculated for the entire volume in each quadrant and zone. Age and gender-related variations were analyzed. Results: In assessments of the whole tissue, only thickness was gender-related, with greater thickness for females (male thickness: 1.81 ± 0.47 mm, female thickness 2.16 ± 0.57 mm, P-value < 0.01).In the distal zone: thickness, length and volume were all larger in females (for male and female respectively: 1.83 ± 0.49 mm vs 2.34 ± 0.58 mm, P-value < 0.01, for the thickness; 10.87 ± 2.10 mm vs 12.18 ± 2.21 mm, P-value < 0.02 for the length and 1501 ± 605 mm3 vs 2169 ± 871 mm3, P-value < 0.01 for the volume). In the medial zone, only thickness was gender-related, with greater thickness in females (male thickness: 2.04 ± 0.60 mm, female thickness:2.44 ± 0.74 mm, P-value < 0.02).The only variation observed in the proximal zone concerned length, larger in males (respectively: 11.27 ± 2.84 mm vs 9.55 ± 2.43 mm, P-value < 0.02). The male population was significantly positively correlated with ageing for volume in the whole tissue (ρ = 0.32, P-value < 0.05), and for both thickness and volume in the medial zone (ρ = 0.33, P-value < 0.05 for thickness; ρ = 0.39, P-value < 0.02 for the volume). Conclusion: This new method is useful to understand both functional anal disorders and local damage which may affect only part of the muscle tissue.