Endoanal ultrasound

Abstract 

Endoanal ultrasound is an essential investigation in the management of faecal incontinence. To accurately image the anal canal in two dimensions, a 10-MHz rotating transducer encased within a water-filled, hard sonolucent, parallel-walled plastic cone is used. In three-dimensional ultrasound the transducer moves slowly down a rigid probe positioned within the anal canal, a predetermined number of two dimensional images are digitized and converted to a single three-dimensional image. Endoanal ultrasonography produces an image dependent on tissue characteristics: highly reflective tissues give a white hyperechoic image; poorly reflective tissues are hypoechoic and appear black. Endoanal ultrasound identifies at least four layers within the anal canal. The innermost layer is the bright hyperechoic subepithelium tissue and is surrounded by the hypoechoic internal anal sphincter. The longitudinal muscle is hyperechoic and is often difficult to distinguish from the external sphincter. The final layer is the external anal sphincter, which has mixed echogenicity. Changes in normal sphincter morphology are apparent with ageing and gender. Endoanal ultrasonography is a method for the morphological diagnosis of damage to the anal sphincter and has brought new insights into the pathogenesis of anorectal disorders. It can differentiate between those individuals with faecal incontinence of idiopathic origin and those with sphincter damage who may be suitable for surgical repair, especially following obstetric or postoperative trauma. Three-dimensional ultrasound is particularly useful for defining the anatomy of the fistula tract in anal sepsis and for local staging of anal canal tumours.

Keywords: anal canal, anus, diagnostic imaging, endosonography, faecal incontinence, imaging, perianal fistula, three dimensional, ultrasonography