Anal Sphincter Function, Anatomy, and Complications

The anal sphincter is a group of muscles at the end of the rectum. The rectum is located at the last few inches of the large intestine (colon). The anal sphincter surrounds the anus and controls the release of stool, thereby maintaining continence. There are two muscles: the internal anal sphincter and the external anal sphincter.

The anal sphincter muscles may be damaged if you have trouble holding in stool and/or urine (or it leaks out) or if you have pain in your rectum.

This article will go over the anatomy of the anal sphincter and its function. You will also learn about conditions that affect the anal sphincter and what happens if the anal sphincter is damaged.

Anatomy of Anal Sphincters

The rectum of an adult is, on average, about 4.7 inches long. The lower part of the rectum is the anal sphincter.

The anal sphincter consists of two muscles:

• Internal anal sphincter: This is located inside the rectum.
• External anal sphincter: This goes around the outside of the end of the anal canal.

The pudendal nerve is a major nerve in the pelvis. It tells the muscles that control pelvic organs and genitals to move.

One of the nerve's jobs is to send messages to its branches that tell the anal sphincter to close and prevent stool and urine from leaking out. This is called anal sphincter innervation.

Function of Anal Sphincters

The anal sphincter reflex is a process that occurs when stool moving through the digestive tract triggers the internal anal sphincter to relax, which in turn makes the external anal sphincter contract. This response is also called the defecation reflex.

The internal anal sphincter and external anal sphincter work together to produce a bowel movement but in different ways.

If there is a loss of muscle control in the sphincter muscles, fecal incontinence may occur.

Diseases and Conditions

Several diseases, conditions, and injuries can affect the anal sphincter. Symptoms of these conditions can vary, but generally speaking, signs that you may have a problem with your anal sphincter can include:

• Pain and bleeding from the anus
• Trouble controlling bowel movements or gas
• An urgent feeling of needing to have a bowel movement
• Anal discharge

Anal Stenosis

Anal stenosis is not a common condition, but it can be related to inflammatory bowel disease (IBD). It is more common in Crohn's disease than in ulcerative colitis.

In anal stenosis, the internal anal sphincter becomes narrowed to the point where it is difficult to have a bowel movement. Other symptoms include pain and bleeding.

Anal stenosis can occur after surgery—especially hemorrhoid removal—or be associated with laxative overuse or infections.

This condition may be treated with stool softeners or fiber supplements. Surgery to correct the problem may also be necessary.

Anismus

Anismus is a functional disorder of the pelvic floor muscles, including the external anal sphincter. It is caused by a problem with the way these muscles contract and relax during defecation.

Symptoms of anismus include:

• Fewer than three bowel movements a week
• Excessive straining to have a bowel movement
• Hard, painful stools
• Stomach pain and bloating
• Anal pain
• The feeling that you haven't finished a bowel movement

Anismus is often treated with pelvic floor exercises. Injections of Botulinum toxin (Botox) can also help.

Anal Fissure

An anal fissure is a tear in the lining of the anus. Anal fissures often extend into the internal anal sphincter. They are caused by overstretching the anal canal, which can occur when passing a very hard or large bowel movement. 

Symptoms of an anal fissure include pain and/or a sensation of tearing during a bowel movement. There may be bright red blood in the toilet or on the toilet paper. These symptoms usually occur with every bowel movement. Because the damage can be made worse with each bowel movement, it is not uncommon for the condition to become chronic.

Anal fissures may be treated with topical medications, medications that help relax the sphincter muscle, or surgery. 

Anorectal Malformation

An anorectal malformation is a condition that occurs in around 1 out of every 5,000 births. This birth defect causes the anus to develop abnormally.

Anorectal malformations can be mild to severe and can affect the sphincter and/or other parts of the rectum and anus. Surgery is required to correct them.

Anal Crohn's Disease

Because Crohn's disease can affect any part of the digestive tract from the mouth to the anus, it can also affect the anal sphincter. It's estimated that as many as one-third 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 problems in the anal sphincter, including:

• Abscesses: An area of pus that collects after an infection 
• Fissures: A tear in the anal canal
• Fistulae: An abnormal channel between two parts of the body, such as the anus and the skin
• Swelling: The anal sphincter can develop swelling
• Ulcers: A hole or sore in the lining of a structure, such as the anal muscles

Anal Crohn's disease can be managed in a few different ways depending on the symptoms and complications. In many cases, antibiotics are prescribed to treat infection, and surgery is sometimes also necessary.

Hemorrhoids (Piles)

A hemorrhoid is a vein around the anus that becomes swollen. Almost anyone can develop hemorrhoids, and they are a particular problem for older adults, people who are pregnant, and people who have chronic diarrhea or constipation.

Hemorrhoids can be internal or external. Internal hemorrhoids are above the internal anal sphincter. External hemorrhoids are below the external anal sphincter.

Sometimes, the internal anal sphincter traps hemorrhoids. This condition is called strangulated hemorrhoids and can be very painful.

Hemorrhoids can usually be managed at home with topical medication and regular sitz baths. Surgery is often the best way to correct a strangulated hemorrhoid.

Fecal Incontinence

Some people with IBD have incontinence—the involuntary release of stool from the rectum. This can happen because of a flare-up of the disease ​or damage to the muscles of the anal sphincter.

A damaged anal sphincter might need to be fixed with surgery. Sometimes, a surgeon can just tighten the sphincter to fix the problem. In other cases, a surgery called an anal sphincterotomy can be done to remove the sphincter.

The amount of time this repair might last depends on how well surgery goes and whether there are any complications. One study found that anal sphincter repairs to help prevent fecal incontinence were still helping patients at least three years after they had the procedure.

Summary

The anal sphincter is a pair of muscles at the end of the rectum that controls the release of stool. When these muscles become damaged, you may have difficulty controlling your bowel movements.

Several conditions can affect the anal sphincter muscles, including anal fissure, anal Crohn's disease, hemorrhoids, fecal incontinence, and others. These conditions range from mild to severe. Some can be corrected with home care while others require surgery.