A fistula-in-ano is a medical phrase derived from Latin and used to describe an abnormal opening at the end of the anal canal, which is the canal used to eliminate byproducts of digestion from the body. The abnormal opening resembles a tube and is called a fistula. It connects to the perianal skin — the region surrounding the junction of the anal canal and the skin at the anal sphincter muscle. Most fistulas in this region are caused by an abscess and are characterized by redness, swelling, soreness, pain, and frequently with fever. A fistula-in-ano occurs more than twice as often in men than in women.
This type of fistula generally occurs very close to the sphincter area, and arises from internal glands in this area that can become blocked — similar to the process causing skin acne. These glands can then become infected, producing abscesses. The infection can increase, causing the abscesses to progress into the muscle wall which develop channels, or tracts, out to the skin.
It has been clinically noted that about 70% of all fistulae occur from this type of abscess. Moreover, although abscesses can be treated by a simple surgical procedure called incision and drainage (I&D), once the abscess has reached the stage of a fistula, more extensive surgery is necessary. It has also been noted that even though rectal abscesses have undergone an I&D, there is still a large chance of developing a recurring, or chronic, fistula-in-ano, which would require more intensive surgery.
Other terms used to describe a fistula-in-ano are: anal fistula, anorectal fistula, perianal fistula, or rectal fistula. All of these terms refer to an abnormal connection between the lining of the anal canal and the skin surface. Since the anal canal is only about 1-1.5 inches (2.5-4.0 cm) long, this immediate area is relatively small. Fistulae tend to be concentrated around the anal sphincter due to the high number of glands in this area that lubricate the sphincter muscles. There are a lot of tissue folds in this area which could also contribute to developing anal fissures by drying out, and then becoming infected, with subsequent abscess development.
Many reasons have been given as possible causes of developing a fistula-in-ano. Although the large majority have been found to be caused by abscesses, some have been known to occur spontaneously. Other more complex, and rarer, rectal fistulae have been found to be caused by injuries, radiation used to treat cancer, complications of Crohn’s disease, or by sexually transmitted disease (STD) — a fistula-in-ano can even be caused by infection following surgery performed for a different reason.