(See also Overview of the Anus and Rectum.)
An anorectal abscess is a pus-filled cavity caused by bacteria invading a mucus-secreting gland in the anus and rectum.
The rectum is the section of the digestive tract above the anus where stool is held before it passes out of the body through the anus. The anus is the opening at the end of the digestive tract where stool leaves the body.
An abscess may be deep in the rectum or close to the opening of the anus. An abscess develops when a mucus-secreting gland in the anus or rectum is blocked, and bacteria grow and multiply. Although the anus is an area that is rich in bacteria, infection generally does not occur because blood flow to the area is rich. When infection does occur, it usually is caused by a combination of different types of bacteria.
An abscess can cause substantial damage to nearby tissues and may rarely lead to loss of bowel control (fecal incontinence). People who have Crohn disease are at particular risk of abscesses. Sometimes, abscesses are a complication of diverticulitis or pelvic inflammatory disease.
A doctor can usually see an abscess if it is in the skin around the anus. When no external swelling or redness is seen, however, a doctor can make the diagnosis of anorectal abscess by examining the rectum with a gloved finger. A tender swelling in the rectum indicates an abscess. If the doctor suspects a deep abscess or Crohn disease, computed tomography (CT) can determine the extent and location.
For an abscess just under the skin, treatment consists of cutting into the abscess and draining the pus after a local anesthetic has been given to numb the area.
For a deeper abscess, the person is usually hospitalized, and the abscess is drained in the operating room after general anesthesia has been given.
Even with proper treatment a drained abscess may lead to the formation of an abnormal channel from the anus or rectum to the skin (called an anorectal fistula).
Antibiotics have limited value except for people who have a weakened immune system, diabetes, or an infection elsewhere in the body.