Anal Fissure (cont.)

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What causes anal fissures?

Anal fissures are caused by trauma to the anus and anal canal. The cause of the trauma usually is a bowel movement, and many people can remember the exact bowel movement during which their pain began. The fissure may be caused by a hard stool or repeated episodes of diarrhea. Occasionally, the insertion of a rectal thermometer, enema tip, endoscope, or ultrasound probe (for examining the prostate gland) can result in sufficient trauma to produce a fissure. During childbirth, trauma to the perineum (the skin between the posterior vagina and the anus) may cause a tear that extends into the anoderm.

The most common location for an anal fissure in both men and women (90% of all fissures) is the midline posteriorly in the anal canal, the part of the anus nearest the spine. Fissures are more common posteriorly because of the configuration of the muscle that surrounds the anus. This muscle complex, referred to as the external and internal anal sphincters, underlies and supports the anal canal. The sphincters are oval-shaped and are best supported at their sides and weakest posteriorly. When tears occur in the anoderm, therefore, they are more likely to be posterior. In women, there also is weak support for the anterior anal canal due to the presence of the vagina anterior to the anus. For this reason, 10% of fissures in women are anterior, while only 1% are anterior in men. At the lower end of fissures a tag of skin may form, called a sentinel pile.

When fissures occur in locations other than the midline posteriorly or anteriorly, they should raise the suspicion that a problem other than trauma is the cause. Other causes of fissures are anal cancer, Crohn's disease, leukemia as well as many infectious diseases including tuberculosis, viral infections (cytomegalovirus or herpes), syphilis, gonorrhea, Chlamydia , chancroid (Hemophilus ducreyi), and human immunodeficiency virus (HIV). Among patients with Crohn's disease, 4% will have an anal fissure as the first manifestation of their Crohn's disease, and half of all patients with Crohn's disease eventually will develop an anal ulceration that may look like a fissure.

Studies of the anal canal in patients with anal fissures consistently show that the muscles surrounding the anal canal are contracting too strongly (they are in spasm), thereby generating a pressure in the canal that is abnormally high. The two muscles that surround the anal canal are the external anal sphincter and the internal anal sphincter (already discussed). The external anal sphincter is a voluntary (striated) muscle, that is, it can be controlled consciously. Thus, when we need to have a bowel movement we can either tighten the external sphincter and prevent the bowel movement, or we can relax it and allow the bowel movement. On the other hand, the internal anal sphincter is an involuntary (smooth) muscle, that is, a muscle we cannot control. The internal sphincter is constantly contracted and normally prevents small amounts of stool from leaking from the rectum. When a substantial load of stool reaches the rectum, as it does just prior to a bowel movement, the internal anal sphincter relaxes automatically to let the stool pass (that is, unless the external anal sphincter is consciously tightened).

When an anal fissure is present, the internal anal sphincter is in spasm. In addition, after the sphincter finally does relax to allow a bowel movement to pass, instead of going back to its resting level of contraction and pressure, the internal anal sphincter contracts even more vigorously for a few seconds before it goes back to its elevated resting level of contraction. It is thought that the high resting pressure and the "overshoot" contraction of the internal anal sphincter following a bowel movement pull the edges of the fissure apart and prevent the fissure from healing.

The supply of blood to the anus and anal canal also may play a role in the poor healing of anal fissures. Anatomic and microscopic studies of the anal canal on cadavers found that in 85% of individuals that the posterior part of the anal canal (where most fissures occur) has less blood flowing to it than the other parts of the anal canal. Moreover, ultrasound studies that measure the flow of blood showed that the posterior anal canal had less than half of the blood flow of other parts of the canal. This relatively poor flow of blood may be a factor in preventing fissures from healing. It also is possible that the increased pressure in the anal canal due to spasm of the internal anal sphincter may compress the blood vessels of the anal canal and further reduce the flow of blood.

Medically Reviewed by a Doctor on 6/25/2014

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