Normal Values of Anal Sphincter Biometry by 4-Dimensional Translabial Ultrasound: A Retrospective Study of Pregnant Women in Their Third Trimester.

OBJECTIVES: Exoanal 4-dimensional translabial ultrasound (TLUS) is increasingly used to image the anal sphincter. The aim of this study was to define the limits of normality for assessment of external and internal anal sphincters with TLUS.

METHODS: This study was a retrospective analysis using data sets of nulliparous women seen antenatally. All women had a 4-dimensional TLUS examination at a mean gestational age ± SD of 36 ± 0.7 (range, 32.9-37.3) weeks. Anal sphincter biometry, including external anal sphincter (EAS) length and thickness, EAS proximal rotational asymmetry, and internal anal sphincter thickness, was assessed blinded against other data.

RESULTS: A test-retest series showed good repeatability (intraclass correlation coefficients, 0.619-0.849) of all parameters. The mean age of the women (n = 111) was 30.9 (range, 18.8-40.5) years. None reported anal incontinence. On tomographic imaging, none showed anal sphincter defects. The mean EAS length was 17.5 (range, 8.4-34.8) mm, being shorter dorsally at 16.4 (range, 7.0-32.7) mm versus 18.7 (range, 7.5-36.9) mm ventrally (P < .001). The ventral EAS reached farther cranially by 0.8 ± 2.5 (range, -4.8-5.1) mm on average. The mean EAS thickness was 3.4 (range, 2.0-5.8) mm, being thicker dorsally than ventrally (P < .001).

CONCLUSIONS: Anal sphincter biometry can be assessed with good repeatability by TLUS. The EAS seems longer ventrally. Asymmetry of the EAS could result in a false-positive diagnosis of defects in women in whom the ventral EAS terminates more caudally than its dorsal aspect, which seems uncommon. Hence, the likelihood of a false-positive diagnosis of substantial defects of the EAS using the published method seems low.