Ultrasound imaging of the anal sphincter complex: a review

Endoanal Ultrasound is regarded as the gold standard for evaluating anal sphincter pathology in the investigation of anal incontinence. This article investigates and compares Endoanal US, Endovaginal US, and Transperineal US for the detection and measurement of IAS and EAS defects.  “2D EAUS generates cross-sectional images in the axial plane only and remains the mainstay of sphincter evaluation.”

“The aim of this review was, first, to critically evaluate the different ultrasound imaging modalities of the anal sphincter complex and, second, to analyze comparator studies between the three imaging modalities to determine the reproducibility of anal sphincter morphology and biometry among the three different methods (namely endoanal, endovaginal and transperineal).”

Endovaginal ultrasound TVUS was found to be useful in determining other causes of fecal incontinence (besides sphincter defects), such as rectal fistulae and abscesses. Although TVUS may be more readily accessible in some settings, interpretation of images requires more expertise, and clear images of the full length of the anal canal are not always obtainable.  

Transperineal ultrasound is generally a more “patient-acceptable” ultrasound technique. However, when compared with EAUS, some studies concluded that the user may have difficulties with poor visualization of the lateral border of the EAS and the fact that the anal mucosa and submucosa cannot be viewed as separate entities.  

A worthwhile read to understand the differences between the various ultrasound modalities and the pros and cons of each as they relate to identifying sphincter defects.