Endoanal ultrasound: what the scan involves and why it is done

Ultrasound uses high-frequency sound waves to make pictures of tissues. In an endoanal ultrasound, the scanner is a thin probe, called a transducer, that is placed a short way into the anal canal. Because it sits right beside the sphincter muscles, it produces much more detailed pictures of them than a scan from the outside could.

NHS trust patient leaflets describe the purpose in plain terms: the scan looks at your anal sphincter, the muscle that surrounds the back passage, to check it is complete and to assess its size and shape. The pictures are saved and reported by a physiologist, radiologist or sonographer and sent to the consultant who requested the test.

The leaflets read for this guide describe the scan as an assessment of the sphincter muscle, and the NICE guideline on faecal (bowel) incontinence places it in specialist assessment. NICE guideline CG49 recommends that people whose faecal incontinence continues after specialised conservative management be considered for specialist assessment, which includes anorectal physiology studies and endoanal ultrasound. If endoanal ultrasound is not available, the guideline says MRI, endovaginal ultrasound and perineal ultrasound should be considered.

The scan answers one specific question: is there a gap or thinning in the muscle ring, and where? One trust leaflet explains that the results go to your consultant, who decides whether further treatment is needed, so the scan is an input to a wider plan rather than a treatment in itself.

Leaflets from NHS trusts describe a similar routine. The person doing the scan explains the procedure and asks you to sign a consent form once they are sure you understand it. You are then asked to undress from the waist down or change into a gown, and to lie on an examination couch, usually on your left side. One leaflet says some patients lie on their tummy.

A fine probe covered with a clean protective cover and lubricating gel is placed gently into the back passage. The operator assesses the area and stores the images for later review. Staff may ask questions about your bowel habit first. Most leaflets suggest, but do not require, passing urine and opening your bowels before the scan so that you are more comfortable.

• Explanation and consent before the scan.

• Change into a gown and lie on your side on a couch.

• A small lubricated probe is placed in the back passage.

• Pictures are taken for a few minutes and the probe is removed.

• You can usually get dressed and leave straight away.

Leaflets describe the scan as comfortable for most people. One says you should feel very little, if any, discomfort. Another says some mild discomfort is possible and that you may feel embarrassed, that staff will maintain your dignity, and that you can stop the scan at any time. Staff are used to the test, and you can ask for a chaperone or to have no trainees present.

Ultrasound does not use ionising radiation, and the leaflets state there are no known side effects or associated risks. One leaflet says there is no required aftercare and that you can eat and drink as normal and leave the department straight afterwards.

The result is not usually given on the day. Leaflets explain that the scan is reviewed and reported afterwards and the report goes to the consultant who referred you. One trust says that consultant will contact you by telephone or letter about the result and the need for any further treatment; another says it will be discussed at your next clinic appointment. If you do not have a follow-up appointment, you may need to contact the referring clinic.

The report describes what the images show about the size, shape and completeness of the muscle. It is then read by your specialist alongside your symptoms and any other tests.

Endoanal ultrasound is often one of several tests of bowel and pelvic floor function. NICE guideline CG49 names anorectal physiology studies alongside endoanal ultrasound, and names MRI, endovaginal ultrasound and perineal ultrasound as alternatives where endoanal ultrasound is not available. The related pages linked below explain the other tests. Your specialist will choose the tests that match your symptoms.