The mechanical cause: the tear
A large, hard stool stretches the anoderm past its limit and tears it, almost always at the back midline, where the blood supply is weakest. Repeated diarrhea does the same by irritation and frequent wiping. In women, the pressure of delivery tears the front midline, which is why postpartum fissures are usually anterior.
The maintaining cause: sphincter spasm
Studies with anal manometry show that most people with chronic fissures have a higher resting pressure of the internal sphincter. High pressure means less blood flow to the back of the anal canal, so the tear cannot repair itself. This is the cause that medications target.
Risk factors
- Low-fiber diet, little water, sedentary life.
- Holding stool in because of pain or lack of time.
- Pregnancy and childbirth.
- Anal intercourse.
- Previous anal surgery.
- Opioid painkillers and other constipating medicines.
Rare causes that must be ruled out
Crohn's disease, ulcerative colitis, tuberculosis, syphilis, HIV, herpes, leukemia and anal cancer can all present as a non-healing fissure. Warning signs: fissure away from the midline, several fissures, large irregular ulcers, no improvement after 8 weeks of correct treatment. A colorectal surgeon can examine and, if needed, biopsy.
Frequently asked questions
Can stress cause an anal fissure?
Not directly, but stress changes bowel habits (constipation or diarrhea) and increases muscle tension, both of which favor fissures.