Transvaginal three-dimensional ultrasound: a method of studying anal anatomy and function.

OBJECTIVES: To explore the possibility of using transvaginal 3D ultrasound as a method of assessing the compartments of the anal canal during rest, to determine the effect of squeeze, and to compare these findings with those obtained using the endoanal technique in the same women.
METHODS: Transvaginal 3D ultrasound was used in a cross-sectional study measuring the anal mucosa and sphincters during rest and squeeze in 20 nulligravida women, comparing the results with those of endoanal measurements. Measurements were also performed of the anal mucosa, anorectal curvature (ARC), and anovaginal angle (AVA) and distance (AVD).
RESULTS: Volume measurements of the anal mucosa were only possible using transvaginal ultrasound. The mean volume of the anal canal using the transvaginal technique was 7.93 (SD 2.26) and 7.34 (1.54) cm(3) during rest and squeeze, respectively (P = 0.082). The ARC became more acute (3.46 vs. 4.12°/mm, P = 0.031) during squeeze and AVA increased (31.7° vs. 35.8°, P = 0.010). The volume of the anal mucosa (3.12 cm(3)) did not change (P = 0.193), but was distorted during squeeze at the level of the puborectalis sling (P < 0.001 for upper cross-section and diameter). The anal canal was 1 cm longer (P < 0.001), the IAS volume larger (2.97 and 2.08 cm(3), P = 0.003), and the EAS volume smaller (1.85 and 3.96 cm(3), P < 0.001) using the 3D transvaginal technique compared with the endoanal measurements at rest, with similar differences observed on squeeze.
CONCLUSION: In contrast to the endoanal method, transvaginal 3D ultrasound provides a complete assessment of the anal canal, including the mucosa. This method seems suitable for functional studies such as in the present one, which shows that voluntary squeeze increases the angulation of the anal canal and distorts the bowel cross-section at the level of the puborectal muscle.