Overview of the Anal Sphincter

This Complicated Structure Is Actually Two Muscles

Anal Canal

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An anal sphincter is a group of muscles at the end of the rectum that surrounds the anus and controls the release of stool, thereby maintaining continence. There are two sphincter muscles: one is internal and one is external. The external muscle helps maintain continence and keep stool in the rectum. If there is a loss of muscle control in the sphincter, incontinence may occur. The inner muscle is not under voluntary control but rather is controlled by the autonomic nervous system. The external muscle can be voluntarily controlled (clenched and unclenched). The internal and the external muscles work together to eliminate stool from the body via a bowel movement.

Diseases and Conditions

Anal stenosis. Anal stenosis is not common, but it can be related to inflammatory bowel disease (IBD), and in Crohn's disease more so than ulcerative colitis. In anal stenosis, the anal sphincter becomes narrowed, to the point where it is difficult to have a bowel movement. Other symptoms include pain and bleeding. This condition could also occur after surgery (especially hemorrhoid removal) or be associated with laxative overuse or infections.

Anal Crohn's Disease. Because Crohn's disease can affect any part of the digestive tract from the mouth to the anus, it can affect the anal sphincter. It's estimated that as many as 1/3 of patients with Crohn's disease will have complications in the perianal area (the part of the body around the anus). People with Crohn's disease may develop complications in the anal sphincter, and this could include:

  • Abscesses: An abscess is an area of pus that collects after an infection. 
  • Fissures: A fissure is a tear in the anal canal that can be very painful.
  • Fistulae: A fistula is an abnormal channel between two parts of the body, such as the anus and the skin.
  • Swelling in the anal sphincter
  • Ulcers: An ulcer is a hole or sore in the lining of a structure, such as the anal muscles.

Hemorrhoids (Piles). A hemorrhoid is a vein around the anus that becomes swollen. Almost anyone can develop a hemorrhoid, and they are a special problem for pregnant women, people with inflammatory bowel disease (IBD), anyone who experiences chronic constipation or diarrhea, and people over the age of 50. 

Incontinence. Some people with IBD experience incontinence, or the involuntary release of stool from the rectum. This could happen because of a flare-up of the disease ​or could be a result of damage to the muscles of the anal sphincter. This can be very distressing for patients, and getting the inflammation from IBD under control is important to prevent the occurrence of fecal incontinence.

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