Defecating Proctography

Defecating proctography (evacuation proctography) is a dynamic fluoroscopic study of the rectum and anal canal during rest, squeeze, straining and defecation.

A thick barium paste of a faeces like consistency (about 150 to 250 mL) is introduced into the rectum, the small bowel may be opacified with oral barium and the vagina with a tampon or gel in women, and the patient sits on a radiolucent commode in the lateral position to defecate in front of the fluoroscopy unit.

Images or video are recorded at rest, on squeeze, during straining and during evacuation, and the post-evacuation film shows the residue. Measurements of the anorectal angle, the position of the anorectal junction relative to the pubococcygeal line, the perineal descent, the pelvic floor movement and the time and completeness of evacuation are made.

The effective dose is about 1.5 to 3 mSv, mostly to the gonads, and it is kept low by pulsed fluoroscopy and shortened screening. MR defecography shows the soft tissue and pelvic organs without ionising radiation, and supine or upright magnets are used.

The test answers a functional anatomical question: why is evacuation difficult, or is there pelvic organ prolapse? Chronic constipation affects 10 to 15% of adults, and obstructed defecation is a cause in about a third. Pelvic floor disorders affect up to a quarter of parous women, and surgical planning depends on identifying the structural problem, which clinical examination misses in many patients.

Indications. Obstructed defecation, a sense of incomplete evacuation, straining, or the need for digitation (manual assistance by finger in the vagina or perineum). Suspected rectocele, enterocele, sigmoidocele, rectal intussusception or prolapse. Faecal incontinence with suspected prolapse or descent. Pelvic organ prolapse symptoms (vaginal bulge, pelvic pressure).

Before surgery for constipation or prolapse. Chronic anal or pelvic pain with suspected pelvic floor dyssynergia. Failure of conservative treatment for constipation. Not for the first investigation of constipation, which starts with history, examination, laxatives, a bowel diary, exclusion of cancer and endocrine causes.

Contraindications and precautions. Pregnancy: avoid. Severe anorectal pain, anal stenosis or active proctitis may prevent contrast insertion. Embarrassment and lack of privacy interfere with defecation, so the room should be private and the patient should be well informed, since a failed evacuation is the commonest cause of a non-diagnostic study. Latex allergy: use latex free applicators. Patients with faecal impaction need disimpaction first.

Preparation and technique. Usually no bowel preparation or only a small enema the day before (so the rectum is not empty or overfilled). Small bowel contrast is given 1 to 2 hours before. A paste of barium, thickener and water is placed in the rectum with a caulking gun, a vaginal gel or tampon and a bladder catheter contrast may be used in selected cases.

The patient sits on the commode and is asked to rest, squeeze, cough, strain and evacuate. The radiologist may place a marker at the anal verge and the pubococcygeal line is drawn from the lower pubic symphysis to the coccyx tip.

Normal appearances. At rest, the anorectal angle is about 90 to 110 degrees, and the anorectal junction lies 1 to 2 cm over the ischial level or pubococcygeal line. On squeeze, the angle narrows to about 70 to 90 degrees and the anorectal junction rises.

On straining and evacuation, the anorectal angle widens by about 15 to 20 degrees (to more than 110 degrees), the puborectalis relaxes, the anal canal opens, perineal descent is under 3 cm (up to 4 cm in women), and the rectum empties 90% or more of the barium within 30 seconds.

The pelvic organs (bladder, uterus, small bowel and sigmoid colon) stay on the upper side of the pubococcygeal line at rest and during straining.

Interpretation.

• Anterior rectocele: a bulge of the anterior rectal wall into the vagina; depth is measured from the expected line of the anterior anal wall: under 2 cm minor, 2 to 4 cm moderate, over 4 cm large. Barium trapping in the pouch after evacuation is more relevant than depth, and a rectocele is common in asymptomatic women.
• Enterocele and sigmoidocele: peritoneal sac with small bowel or sigmoid between the rectum and vagina; the sac descends to the lower side of the pubococcygeal line on straining and separates the rectum and vagina by more than 2 cm.
• Rectal intussusception and prolapse: infolding of the rectal wall into the rectal lumen or anal canal (intra-rectal, intra-anal), and external prolapse beyond the anal verge.
• Pelvic floor dyssynergia (anismus): failure of the anorectal angle to open (a paradoxical contraction or non-relaxation of the puborectalis), with a prominent puborectalis impression at the posterior rectal wall and incomplete evacuation. The finding is also seen in asymptomatic people and needs confirmation with anorectal manometry.
• Excessive perineal descent: descent of the anorectal junction more than 4 cm on the lower side of the pubococcygeal line on straining, associated with weak pelvic floor, nerve damage, faecal incontinence and chronic straining.
• Cystocele and uterine prolapse: bladder or uterine descent past the line (with bladder opacified).
• Megarectum: an enlarged rectum with a large capacity and incomplete evacuation.
• Solitary rectal ulcer and rectal mucosal prolapse: a thickened, nodular or ulcerated anterior rectal wall with intussusception.
• Retained barium after evacuation: more than 10% to 25% of the barium left in the rectum or in a rectocele indicates incomplete evacuation.
• Sphincter defect: not shown directly (endoanal ultrasound or MRI is used), but a patulous anal canal or fixed open sphincter on rest suggests injury.

Critical values and findings needing immediate action. No finding is an emergency in most patients. Findings needing prompt action include:

• Full thickness rectal prolapse (external) with incarceration, strangulation or ulceration: urgent surgical review.
• Large enterocele or sigmoidocele with bowel obstruction symptoms: surgical referral.
• Findings of a mass, ulcer or stricture of the rectum: urgent flexible sigmoidoscopy with biopsy.
• Severe pelvic organ prolapse with urinary retention or hydronephrosis: gynaecological or urological referral.

Limitations and artefacts.

• Failure to evacuate because of embarrassment, a poor paste consistency, a small rectal volume, or the unnatural setting makes dyssynergia appear falsely, and a normal study in a quiet room does not exclude a disorder at home.
• Many findings (a small rectocele, intussusception, perineal descent) occur in asymptomatic people, so results must be matched with symptoms, examination and physiology.
• The test does not show the pelvic floor muscle, sphincters or soft tissue; MR defecography and endoanal ultrasound do, and MR defecography shows all three compartments with no ionising radiation but is done in a supine position unless an open magnet is used.
• The paste differs from real stool, and the supine MR position underestimates prolapse.
• A rectocele may be filled by barium but is not the cause of symptoms in many women, and surgery for a rectocele alone often fails to cure constipation.
• Radiation dose to the gonads is not trivial in young women, so use is limited to cases in which surgical decisions depend on the result.
• Reproducibility of anorectal angle measurements is limited.

Special groups.

• Women, especially parous: rectocele, enterocele and prolapse are the commonest abnormalities, and multicompartment imaging with the bladder and vagina opacified is useful.
• Men: dyssynergia, megarectum and rectal prolapse are the main findings.
• Elderly: perineal descent and incontinence, with sphincter and nerve damage.
• Children: evacuation disorders and encopresis are managed clinically, and imaging is reserved for selected cases.
• Patients with spinal cord injury, spina bifida or multiple sclerosis: neurogenic bowel.
• Regions: pelvic organ prolapse with obstetric injury is common where deliveries are unsupervised, and obstetric fistula and rectovaginal fistula are important causes of faecal incontinence; MR defecography is rarely available, so the barium study is used.

Follow up tests. Anorectal manometry and balloon expulsion test for dyssynergia, colonic transit study with radio opaque markers, MR defecography, endoanal ultrasound for sphincter defects, flexible sigmoidoscopy or colonoscopy to exclude a mass, pudendal nerve and electromyography testing, thyroid function, calcium,

glucose and full blood count for secondary causes of constipation, and referral to colorectal surgery, urogynaecology, biofeedback therapy or pelvic floor physiotherapy.