Obstructed defecation refers to the situation where the rectum is full of stool, the desire to defecate is registered centrally in the brain but there is an inability to physically expel the rectal contents. The transit of the stool through the colon may be at normal speed but when the stool reaches the rectum there is failure to progress. Typically the patient is called to stool at least daily but only small amounts of stool are passed with a sense of incomplete evacuation.

•The mechanism of blockage

Failure to evacuate the rectum is commonly due to a mechanical blockage of the anal outlet.
Mechanical blockage of the anal outlet may occur in a number of ways. The muscles and
connective tissue supporting the rectum can become weakened and unsupportive. There will
then be a failure to “lift” and straighten the rectum and tension the fascial sheaths that surround
the bowel and separate the pelvic organs when defecation is attempted. The result may be a
baggy rectum which bulges into the vaginal space (rectocoele).

Often, no stool can be evacuated unless the patient manually lifts the perineum and places a finger into the vagina or the rectum to reduce the 'bulge'
The passage of odd shaped stool is commonly noted.