Ano-rectal manometry measures pressures of the anal sphincter muscles. It also measures how well a person feels different sensations of fullness in the rectum.
A person prepares for the test by using a plain Bisacodyl or Saline enema (for example, Fleets brand) about two to four hours before the test. The enema can be purchased at the drugstore without a prescription. It is important not to eat anything, or drink liquids with caffeine, for about four hours prior to the test.
The person lies on his left side. A small, flexible tube, about the size of a thermometer, is inserted into the rectum. A tiny amount of water drips into the tube while it is connected to a machine that measures pressure.
During the test the nurse asks the person to squeeze, relax, and push. The anal sphincter muscle pressures are measured during each of these maneuvers. To squeeze, the person tightens the sphincter muscles as if trying to prevent anything from coming out. To push, the person strains down as if trying to have a bowel movement.
Ordinarily, the sphincter muscle tightens, and the anal canal pressures increase when the person tries to squeeze. When the person stops squeezing, the muscles should relax, and pressures return to baseline.
When the person pushes, as if having a bowel movement, the sphincter muscles should stay relaxed. This will cause the pressures to stay the same as during rest, or to decrease slightly. If the pressures increase during a push, this may be a sign that the sphincter muscles tighten when pushing. This tightness could contribute to constipation.
An anal manometry shows how strong the sphincter muscles are and if they relax as they should during rectal emptying.
Weak muscles that still tighten some can be strengthened with special exercise and treatments. Muscles that do not relax with a push can also be retrained. If the muscles are very weak, some constipation operations may be modified. If a person with colonic inertia has weak sphincters, then to prevent fecal incontinence, the colon may not be shortened as much.