Overview

Longitudinal tear of the anal canal; distal to the dentate line

Location: 90% of all anal fissures located at the posterior commissure (6 o'clock in the lithotomy position)

Potential causes of trauma:

• Chronic spasm/increased tone in the internal anal sphincter

• Low fiber intake

• Chronic constipation or diarrhea

• Anal sex

• Vaginal delivery

Location: may occur lateral or anterior to the posterior commissure

Underlying conditions:

• Previous anal surgery (e.g., possible stenosis of anal canal)

• Inflammatory bowel disease (IBD; e.g., Crohn disease)

• Granulomatous disease (e.g., tuberculosis)

• Infections (e.g., chlamydia, HIV)

• Malignancy (e.g., leukemia)

Overdistension or disease of the anal mucosa → laceration of the anoderm

• Spasm of the exposed internal anal sphincter leads to pulling along the laceration, which impairs healing and worsens the extent of laceration with each bowel movement.

• The resultant pain results in voluntary avoidance of defecation and constipation, which worsens distension of the anal mucosa.

The posterior commissure is believed to have a very poor blood supply, which predisposes it to ischemia (exacerbated by poor perfusion during increased anal pressure).