What is a significant defect of the anal sphincter on translabial ultrasound?

The anal sphincter is commonly evaluated with endoanal ultrasound. Recently, translabial ultrasound imaging has been proposed for sphincter imaging, with moderate to good correlation between the methods. An endosonographic defect is defined as one with a radial extension of > 30° in at least two-thirds of the length of the anal sphincter. This is equivalent to defining significant anal sphincter trauma on translabial tomographic ultrasound imaging (TUI) as a defect in at least four of six slices, a definition which has been validated. This study was designed to validate a residual defect angle of > 30° for the definition of significant anal sphincter trauma on translabial ultrasound.

This was a retrospective study involving 399 women attending a tertiary urogynecology unit in 2014. All underwent a standardized interview, including determination of St Mark's fecal incontinence score (SMIS), clinical examination and 3D/4D translabial ultrasound examination with the woman at rest and on pelvic floor muscle contraction (PFMC). External (EAS) and internal (IAS) anal sphincter defect angles were measured in individual TUI slices and associations with anal incontinence symptoms, bother score and SMIS were analyzed.

There were weak but significant correlations of anal incontinence symptoms, bother score and SMIS with EAS and IAS defect angle, measured on images acquired with the woman at rest and on PFMC. The predictive value of single-slice defect angle on TUI was low, and areas under the receiver-operating-characteristics curves were too low to determine a distinct cut-off value for defect angle.

Anal sphincter residual defects on single translabial TUI slices are weakly associated with measures of anal incontinence. Single-slice defect angle is too poor a predictor to allow validation of the 30° defect angle cut-off used in endoanal ultrasound. Larger studies in populations with a higher prevalence of anal incontinence are needed before we can disregard anal sphincter defects smaller than 30° on translabial ultrasound. Copyright © 2019 ISUOG. Published by John Wiley & Sons Ltd.