Certain imaging tests can be performed to determine the cause of constipation. But despite use of sophisticated modern tests many patients with chronic constipation, the cause can not be identified. The imaging techniques which are used in identifying the cause of chronic constipation are defecography (it is a dynamic barium enema including lateral views obtained during barium expulsion), proctography during defecation, scintigraphy, electromyography, magnetic resonance imaging (MRI) etc.
Defecography can abnormalities in many patients. The important findings that can be shown by defecography are the measured changes in rectoanal angle, anatomic defects of the rectum (like internal mucosal prolapse, and enteroceles or rectoceles) etc. But patients who can be treated by surgery are very few. The surgical patients include severe, whole-thickness intussusception with complete outlet obstruction and an extremely large rectocele which fills preferentially during attempts at defecation instead of expulsion of the barium. The most common cause of outlet obstruction is failure of the puborectalis muscle to relax and this can not be identified by defecography (it requires a dynamic study such as proctography for identification).
Proctography during defecation of stool can help in measuring perineal descent and the rectoanal angle during rest, squeezing, and straining. Confirmation of pelvic floor dysfunction can be done by lack of straightening of the rectoanal angle by at least 15° during defecation.
Neurologic tests like electromyography is generally used to identify the incontinence rather than of those with symptoms of constipation (obstructed defecation).
Scintigraphy is done by checking the expulsion of artificial stool which helps to measure perineal descent and the rectoanal angle during rest, squeezing, and straining. Scintigraphic expulsion quantitates the amount of artificial stool emptied during test and can find out the degree of constipation.
MRI is being used and developed as an alternative to provide more information about the structure and function of the pelvic floor, distal colorectum, and anal sphincters. This is used frequently these days to investigate patients with severe and chronic constipation.