Introduction
The anal canal is at the end of the gastrointestinal tract. It is typically 2.4 cm to 4 cm in length. The anal sphincter includes a group of muscles at the end of the rectum and around the anus. These muscles control the passage of stools and hence have a role in maintaining continence. The anal sphincter comprises the external anal sphincter and the internal anal sphincter muscles.
What Is the Anatomy of the Anal Sphincter?
An adult rectum is about five inches long. The terminal portion of the rectum is the anal sphincter. The anal sphincter comprises two muscles, the external anal sphincter and the internal anal sphincter muscles. The internal anal sphincter is located inside the rectum. It is a continuation of the inner rectal muscle. The internal anal sphincter is about 5 mm in thickness and has muscular fibers arranged in a circular pattern. The external anal sphincter goes around the end of the anal canal. It is made up of skeletal muscles and divided into three sections, deep, superficial, and subcutaneous. The deep part is made up of circular muscle fibers. This is palpable on rectal examination and is called an anorectal ring. The superficial part comprises elliptical muscle fibers and is the only part of the sphincter with bony attachment. The subcutaneous part also has circular muscle fibers and lies below the end of the internal anal sphincter. The anal sphincter is supplied by the pudendal nerve. This nerve signals its branches to close the anal sphincter to prevent stool leakage. This is called anal sphincter innervation. The arterial supply is provided by the inferior rectal arteries. The anal canal is an area of portosystemic venous connection since it contains venous drainage to both the systemic and portal venous plexuses. The lymphatic drainage goes to the superficial inguinal group of lymph nodes.
What Is the Function of the Anal Sphincter?
The internal anal sphincters relax when the stool moves through the digestive tract. This, in turn, stimulates the external anal sphincter to contract. The process is called defecation reflux or the anal sphincter reflex. The external and internal sphincters work together in complementary ways to facilitate bowel movement. If a loss of muscle control of the anal sphincter occurs, it can result in fecal incontinence. Fecal incontinence is the leakage of stool without warning due to not being able to control bowel movements. The internal sphincter muscle is controlled by the autonomic nervous system and is a thin, involuntary muscle. It helps to keep the rectum closed when it is not ready for a bowel movement. The internal anal sphincter is programmed to stay closed always; this is why adults who are healthy do not leak stool while asleep. The external anal sphincter is a voluntary muscle. It helps to hold the bowels until a restroom is reached. Conscious activation of this muscle allows the stool to move out of the body. The actions of the external anal sphincter can be controlled to a great extent.
What Are the Diseases of the Anal Sphincter?
There can be many diseases, conditions, and injuries to the anal sphincter. They may include:
1. Anal Stenosis: This is not a common condition, but it can be related to inflammatory bowel disease (IBD), a condition where the lining of the digestive tract gets inflamed, causing persistent diarrhea and abdominal pain. A condition in which the internal anal sphincter becomes very narrow so that it is difficult to pass the bowels is called anal stenosis. There can be pain and bleeding. Anal stenosis can occur after surgery (for example, after surgery for hemorrhoid removal). It can also be caused as a result of an infection or due to the overuse of laxatives.
2. Anal Crohn’s Disease: Any part of the digestive system (from the mouth to the anus) can be affected by Crohn’s disease, and it can affect the anal sphincter as well. Studies show that about one-third of people with Crohn’s disease have complications in the perianal area (the region around the anus). Those with Crohn’s disease may have issues with the anal sphincter including:
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Abscess: A pus collection as a part of an infection is called an abscess.
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Fissure: A tear in the anal canal is called a fissure. It can be painful.
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Fistulae: An abnormal connection between two parts of the body like the anus and the skin is called a fistula.
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Swelling: Swelling can be found in the anal sphincter.
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Hematoma: A pool of blood that is collected around the anus.
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Ulcer: An unusual opening or sore in the lining of the muscle is called an ulcer.
3. Hemorrhoids: Hemorrhoids in the anal canal are also called piles. When the veins in the anus become swollen, the condition is called piles. Anyone can develop hemorrhoids, but pregnant women, obese people, those who have IBD, older people, or anyone who has chronic constipation or diarrhea is at a higher risk of the condition. Hemorrhoids can be internal or external, depending on the location. At times, the internal sphincter muscles trap the hemorrhoids and cause much pain. This condition is called strangulated hemorrhoids.
4. Fecal Incontinence: The leakage of stool without warning, the condition of not being able to control the bowel movements are referred to as fecal incontinence. This can be seen in people with IBD. This can be caused as a result of the flare-up of the disease, or due to damage to the anal sphincter muscles. It can be an embarrassing condition for the patient.
Conclusion
The anal sphincter comprises a group of muscles at the end of the rectum and around the anus. They include the internal anal sphincter and the external anal sphincter. Both the internal and external anal sphincter muscles are responsible for maintaining fecal incontinence. The pudendal nerve supplies the anal sphincter muscles, while the arterial supply is by the inferior rectal arteries. The pudendal nerve signals its branches to close the anal sphincter to prevent stool leakage. This is termed anal sphincter innervation.
