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There is a significant difference between these two conditions, and they should not be considered equal. An abscess is a localized pocket of pus caused by bacterial infection, which can occur in any part of the body. When bacteria seep into the underlying tissues in the anal canal, an abscess may develop. Patients with conditions that reduce the body’s immunity, such as cancer or AIDS, are more likely to develop anal abscesses.
You should know that an abscess causes tenderness, swelling, and pain. These symptoms clear when the abscess is drained, but patient may also complain of fever, chills, and general weakness or fatigue.
A fistula, on the other hand, is a tiny channel or tract that develops in the presence of inflammation. In some cases a fistula develops because of an infection. It may or may not be associated with an abscess, but like abscesses, certain illnesses such as Crohn’s disease can help fistulas develop. The channel usually runs from the rectum to an opening in the skin around the anus, although sometimes the fistula opening develops elsewhere. For example, in women with Crohn’s disease or obstetric injuries, the fistula could open into the vagina or bladder, not just in the anal region. Since fistulas are infected channels, there is usually some drainage and most commonly draining fistula is not painful, but it can irritate the skin around it.
An abscess and fistula often occur together. In fact, if the opening of the fistula seals over before the fistula is cured, an abscess may develop behind it. Diagnosis of an abscess is usually made on examination of the affected area. If it is near the anus, there is always pain, and often redness and swelling associated with these conditions.
The physician will look for an opening in the skin, and try to determine the depth and direction of the channel or tract of the fistula. However, signs of fistula and abscess may not be present on the skin’s surface around the patient’s anus. In this case, the physician uses an instrument called an anoscope to see inside the patient’s anal canal and lower rectum. Whenever the physician finds an abscess, and especially a fistula, further tests are needed to be sure the patient does not have Crohn's disease. Blood tests, x-rays, and a colonoscopy are often required for a positive diagnosis.