Purpose
To evaluate the functioning of the anal canal, usually to diagnose the cause of chronic constipation or fecal incontinence.
To confirm suspected cases of Hirschsprung's disease, in which a defect in the nerves supplying the colon causes chronic constipation.
To confirm the cause of fecal incontinence when it is suspected that surgery or disease has injured the nerves or muscles in the anus, or when diabetes may have resulted in impaired sensation in the rectum.
As treatment for fecal incontinence, to retrain anal muscles to contract more forcefully.

How it works
A probe inserted into the anal canal measures the pressure exerted by the sphincter muscles that ring the canal and, by relaxing and contracting, control bowel movements.

Preparation
You may be given an enema to clear any feces from the rectum.
Children and very anxious individuals may be given a sedative.
You disrobe and don a hospital gown.

Test procedure
You recline on an examining table while the doctor examines your rectum with a gloved finger.
The manometry probe, a thin tube of soft plastic or rigid metal, is inserted into your rectum about 4 inches and then slowly withdrawn halfway. As the probe is withdrawn, the transducer continuously records the pressure at different points. The measurements help locate anal sphincters, muscle rings that close the anus when they contract. Alternatively, the pressure may be measured with a balloon manometry system, a hollow metal cylinder to which three balloons are attached.

You are asked to squeeze the anus as forcefully as possible, while pressure in the sphincters is recorded.

A balloon at the tip of the probe is inflated, and you are asked to report whether you have a sensation that your rectum is full and you feel an urge to defecate.

After the test
You are free to return to previous activities.

Factors affecting results
Pressure in the rectum decreases somewhat with age.
On average, the pressure is slightly higher in men than in women.

Interpretation
Higher-than-normal pressure in the rectum may lead to constipation, while pressure that is too low may cause fecal incontinence. Measurements made at different points in the rectum help locate any problem in the functioning of the rectal muscles or nerves. Abnormal reflexes in the rectum may also signal disease. For example, internal sphincters should relax when the rectum is full. Failure to relax when the rectum is dilated with a balloon may indicate Hirschsprung's disease.