An anal fissure is essentially a tear or split in the anoderm at the dentate line. 80-90% of these are located in the posterior midline. Anterior midline locations are far less common, occurring in 5-10% of all cases. Symptoms of anal fissures often arise soon after the passage of a hard bowel movement through the anal canal. This is believed to cause stretching and subsequent tearing of the anoderm. Occasionally, this may be associated with a bout of severe diarrhoea.
The anterior and posterior midlines are the weakest areas of the anal canal. It is at these locations that the external sphincter fibers decussate. In addition, the midline locations are areas where the skin is tethered more adherently to the sphincter complex. During the process of defecation, partial eversion of the anal canal leads to more tension arising in these areas. Coupled with the less adequate anodermal blood flow here, ischemia occurs and results in an ulcer being formed.
Once a fissure has formed, a cycle of repeated injury results. The resulting internal anal sphincter spasm tends to hold the edges of the ulcer apart, thereby preventing wound healing. Instead, further tearing of the mucosa tends to result with each bowel movement.
Only 2-5% of fissures are located in other locations and these should be viewed with suspicion. Common aetiologies of these fissures include Crohn's disease, ulcerative colitis and chronic infective processes such as anal tuberculosis, HIV infection and cancer. Patients who present with such atypical fissures should be referred to a colorectal surgeon for further evaluation. Biopsies of these fissures are mandatory.