Normative anatomy of the anal sphincter detected with 3D-endoanal ultrasonography

Background: Understanding anal sphincter anatomy is crucial in management of anorectal disorders, including anorectal sepsis and fecal incontinence. Three-dimensional endoanal ultrasound (EAUS) provides better resolution of the tissue layers. Previous normative studies were obtained in Western populations.

Objective: We demonstrated the anal sphincter anatomy in normal East Asian subjects.

Methods: Forty-six healthy subjects without anorectal symptoms (M:F = 15:31, mean age ± SD = 47 ± 13 years) were enrolled. High-frequency (16 MHz) EAUS was performed with a mechanically rotated probe. Thickness and length of anal sphincter components were measured. Differences between sexes were assessed using a Student t test.

Results: We demonstrated 4 differentiable components: the anal sphincter; internal anal sphincter (IAS), subcutaneous external anal sphincter (Sc EAS), superficial external anal sphincter (Sp EAS), and puborectalis muscle (PRm). The mean length of anal sphincter components were obtained in mm (men vs. women), IAS (28.5 vs. 25.3, p = 0.03), Sc EAS (13.2 vs. 11.2, p = 0.005), Sp EAS (24.1 vs. 19.6, p = 0.0001), and PRm (12.4 vs. 12.2, p = 0.84). The anal canal was significantly longer in men (38.6 vs. 34.0, p = 0.007). The mean thickness for IAS (1.7 vs. 1.8, p = 0.095), Sc EAS (7.5 vs. 7.6, p = 0.587), Sp EAS (8.1 vs. 6.9, p = 0.001), and PRm (8.7 vs. 9.0, p = 0.605) were measured. The PRm was the thickest and the Sp EAS was the longest voluntary sphincter.

Conclusion: Normative details of anal sphincter components in an East Asian population are described. This data can be used for future consideration of diseased states.