Role of MRI Defecography in Diagnosis of Obstructed Defecation Syndrome

Obstructed defecation syndrome (ODS) is a term used to describe the whole complex of mechanical and functional disorders leading to difficult or inadequate rectal emptying. obstructed defecation syndrome has prevalence rate 3.4 % in general population, In obstructed defecation feces do reach the rectum, but rectal emptying is extremely difficult. These patients have a feeling that defecation is blocked. Despite repetitive attempts, complete evacuation of rectal contents is not possible. The patients may also complain of prolonged and unsuccessful straining at stools, feelings of incomplete evacuation, digital removal of feces, and laxative abuse.

Constipation caused by obstructed defecation is of two basic types: functional and mechanical. The functional type includes anismus (pelvic floor dys-synergy), and descending perineal syndrome, whereas the mechanical type includes rectocele, enterocele, internal intussusception and overt rectal prolapse.

All of these conditions represent either a defect of pelvic support or abnormal function of the pelvic floor musculature. The etiology of ODS is controversial. It is presumed that in childbearing women damage to the innervation and soft tissues of the pelvis may occur as a direct consequence of vaginal childbirth. Trauma to the pelvic soft tissues can result in endopelvic fascial and pelvic support defects. Cumulative nerve damage from stretching of pelvic floor due to childbirth and activities that cause chronic and repetitive increases in intra-abdominal pressure such as obesity and chronic cough have been suggested to play a role in the development of symptomatic defects.

Although patients frequently complain of constipation, symptoms such as fruitless straining and incomplete evacuation are rather subjective and unreliable. Nevertheless, an international team of experts included these symptoms in the definition of constipation.This Rome III guidelines, for a patient to be labeled as suffering from functional constipation, which also includes obstructed defecation, following criteria should be present for at least 3 months:

1. Must include two or more of the following:
2. Straining during at least 25% of defecations,
3. Lumpy or hard stools in at least 25% of defecations,
4. Sensation of incomplete evacuation for at least 25% of defecations,
5. Sensation of ano-rectal obstruction/ blockage for at least 25% of defecations,
6. Manual maneuvers to facilitate at least 25% of defecations (e.g., digital evacuation, support of the pelvic floor),
7. Fewer than three defecations per week.
8. Loose stools are rarely present without the use of laxatives.
9. Insufficient criteria for irritable bowel syndrome. The same criteria define dys-synergic defecation as inappropriate contraction of the pelvic floor or less than 20% relaxation of basal resting sphincter pressure with adequate propulsive forces during attempted defecation .

On physical examination, the paradoxical contraction of the pelvic floor can be assessed by palpation of the puborectalis muscle while the patient is straining . Perineal descent >3 cm, mucous discharge or mucosal prolapse may also be seen when the patient is asked to strain for stools . However, most clinicians do not rely on palpation and advocate the use of specific tests to diagnose ODS . Electromyography (EMG) of the pelvic floor, the balloon expulsion test (BET), and defecography are the most frequently used tests. Other radiologic methods for the dynamic evaluation of the ODS include magnetic resonance imaging and ultrasonography, each of which has its advantages and limitations.

The development of fast Magnetic Resonance Imaging sequences provides a new alternative to study all pelvic visceral movements in a dynamic fashion. MR defecography has several important advantages over conventional defecography. Its non-ionic nature, multiplanar capacity, dynamic evaluation and good temporal resolution along with its high-resolution soft-tissue contrast makes it an ideal modality in the assessment of ODS patients. Imaging in the mid-sagittal plane allows evaluation of the anal canal, anorectal angle, levator muscle and hiatus and the vaginal disposition as well as their relationship to a consistent electronically designated pubo-coccygeal Line (PCL). Diagnostic parameters for pelvic dys-synergy include an indented impression of the pubococcygeus muscle on the rectum with excessive obtuse anorectal angulation accompanied by very prolonged rectal emptying on T2-weighted MR images.