Transperineal three-dimensional ultrasound in evaluating anal sphincter muscles

To evaluate whether transperineal three-dimensional (3D) ultrasound can be used to depict normal anal sphincter anatomy and to measure the thickness of muscle layers and the anteroposterior length of the levator hiatus.

The study included 22 normal nulliparous female volunteers. Transperineal 3D sonographic evaluation of the anal canal included assessment of sphincter shape, echogenicity, marginal definition and muscle thickness. Measurements of the thickness of the internal anal sphincter (IAS) and puborectalis muscle (PRM) were determined with the women at rest and during squeezing by two observers, and interobserver reliability was determined. The anteroposterior length of the levator hiatus at rest and during squeezing was measured.

The proximal end of the anal canal (towards the rectum) consisted of overlapping IAS and PRM, and the distal end (towards the anus) consisted of overlapping IAS and external anal sphincter (EAS). At the PRM level, the mean +/- SD IAS thickness was 2.3 +/- 0.5 mm at rest and 2.5 +/- 0.4 mm during squeezing, and at the mid-EAS level it was 2.9 +/- 0.5 mm at rest and 2.8 +/- 0.5 mm during squeezing. The PRM thickness was 6.5 +/- 1.0 mm at rest and 6.4 +/- 1.2 mm during squeezing. The difference in muscle thickness of the sphincter layers with the woman at rest and during squeezing was not significant. The anteroposterior length of the levator hiatus was 51.7 +/- 5.0 mm at rest and 47.4 +/- 4.1 mm during squeezing (P < 0.01).

Transperineal 3D ultrasound may be useful in evaluating the anatomy of the anal canal.