Endoanal ultrasound — sphincter and perianal assessment
The anal canal from the puborectalis sling to the subcutaneous external sphincter in three levels — upper, mid and lower canal — with the internal and external sphincters assessed circumferentially and any track followed to its internal opening.
Typical, not policy
Timings, volumes and delays here are representative values drawn from published guidance. Scanner generation, injector, cardiac output and local preference all move them. Confirm against your department's own protocol before you rely on a number.
When to use it
- Faecal incontinence, to identify and quantify an internal or external sphincter defect, including obstetric anal sphincter injury.
- Perianal sepsis and fistula-in-ano, to define the primary track and the internal opening where MRI is unavailable, unnecessary or not tolerated.
- Assessment before and after sphincter repair.
- Local staging of a low rectal or anal lesion where the depth of the layers of the wall is the question.
Technique
- A rotating high-frequency endoprobe gives a 360-degree image; defects are reported by clock face position and by the craniocaudal extent over which they persist.
- Hydrogen peroxide instilled into the external opening opacifies a fistula track and materially improves identification of the internal opening.
- Three-dimensional acquisition allows reconstruction in the coronal plane, which is where the length of a sphincter defect is best appreciated.
Where it goes wrong
- The field of view is only a few centimetres: supralevator extension, deep ischioanal or horseshoe collections and secondary tracks are readily missed, and MRI remains the reference standard for complex or recurrent fistulising disease.
- The probe distends and can distort the anal canal, which exaggerates a thin sphincter and can close a small track.
- Active sepsis makes the study painful and often incomplete — a limited endoanal study should escalate to MRI rather than be reported as negative.
- Normal anatomical variation in the anterior sphincter of a woman is repeatedly over-called as a defect.
Clinical questions that reach this study
Contrast
None
Acquisition
- Preparation
- A small rectal enema is used in some units for rectal work and is not needed for sphincter assessment. Anal pain from active sepsis may make probe insertion intolerable — where that is likely, MRI is the better first request.