Digestive Problems
Diseases: Colon and Rectum: Anal Stenosis
There are two muscles which make up the anal sphincters. There is an internal anal sphincter and an external anal sphincter.
The internal anal sphincter is a thin, white muscle wrapped around the anal canal. The internal sphincter contracts during rest and sleep, and keeps small amounts of liquid and gas from escaping unexpectedly. The internal anal sphincter is involuntary smooth muscle, like the muscles of your intestines. You cannot mentally control it.
The external anal sphincter is a thick, red voluntary muscle. It is wrapped around the internal anal sphincter muscle. It is the one you squeeze when you feel the urge to go to the bathroom but are not near one. Since it is voluntary muscle, like the muscles in your arms and legs, you can control it.
Anal stenosis means that the anal opening is too small, or that it does not relax or open properly. Scar tissue can cause anal stenosis. This may need to be treated with surgery that brings a flap of skin down to enlarge the anal opening.
In some people, the internal anal sphincter does not relax as it should. This makes the anal opening too small. If it causes severe constipation or painful and difficult rectal emptying, a tight internal anal sphincter can be treated with dilatation or with a surgical myotomy (cutting the muscle).
Adapted from Dr. Christopher Lahr's book "Shining Light on Constipation."