Anal sphincter imaging: better done at rest or on pelvic floor muscle contraction?

Exo-anal ultrasound imaging of the anal sphincter is usually undertaken on pelvic floor muscle contraction (PFMC) as this seems to enhance tissue discrimination. Some women are unable to achieve a satisfactory PFMC, and in this situation, the sphincter is assessed at rest. We aimed to determine whether sphincter imaging at rest is inferior to imaging on PFMC.

We analysed 441 women in this retrospective study. All underwent a standardised interview, including St Mark's incontinence score, clinical examination and 4D trans-labial ultrasound (TLUS). On analysing volume data, tomographic imaging was used to obtain a standardised set of slices at rest and on PFMC to evaluate external anal sphincter (EAS) and internal anal sphincter (IAS) trauma as described previously.

When assessments obtained from volumes acquired at rest and on PFMC were tested against measures of anal incontinence (AI), all associations between the diagnosis of significant anal sphincter defects and AI were no stronger when imaging was performed on PFMC. On cross-tabulation, the percentage agreement for significant defects of the EAS and IAS at rest and on PFMC was 96.5% and 98.9% respectively, if discrepancy by one slice was allowed.

Exo-anal tomographic imaging of sphincter defects at rest seems sufficiently valid for clinical use and may not be inferior to sphincter assessment on pelvic floor muscle contraction.