Fissure, fistula or abscess? How to tell them apart

What is the difference between an anal fissure, a fistula and an abscess?

Patients often use these three words as if they were the same thing. They are not. They happen in the same small area, but each one is a different kind of problem, and each one is treated differently.

• Anal fissure: a tear in the anoderm, the thin skin that lines the lower anal canal, usually in the back midline. It stays open because the internal sphincter goes into spasm. See What is an anal fissure? A surgeon explains it with a drawing of the anal canal for the anatomy.
• Anal abscess: an infection, a pocket of pus that forms next to the anus or deeper around the rectum, most often starting in one of the small glands of the anal canal.
• Anal fistula: a tunnel, a small abnormal channel that connects the inside of the anal canal with the skin outside. Most fistulas are what remains after an abscess has drained, on its own or by surgery.

How does the pain of a fissure differ from an abscess or fistula?

The rhythm of the pain is the most useful clue.

• Fissure: the pain is tied to the bowel movement. It is cutting while the stool passes and burning or throbbing for minutes to hours after. Between bowel movements, many people are almost comfortable.
• Abscess: the pain is there all the time, whether or not you go to the bathroom. It throbs, makes sitting difficult and gets worse every day as pus accumulates. A deep abscess may show little on the skin but cause deep rectal pain and fever.
• Fistula: the pain comes in cycles. Pressure and discomfort build up for a few days, then the opening drains and the pain eases. Many people describe a “pimple” near the anus that keeps coming back in the same place.

If your pain has changed from “only when I go” to “all day and getting worse”, take it seriously. That change is one of the most common ways an abscess announces itself.

What does the discharge tell you: blood or pus?

A fissure produces blood: small amounts, bright red, on the paper or streaked on the stool. Some people with a chronic fissure also notice a little moisture or a slight stain, but not frank pus.

Pus, a thick yellow, green or brownish fluid, sometimes with a bad smell, points to infection. If it appears suddenly after days of increasing pain and the pain eases right after, an abscess has probably burst on its own. That is relief, not resolution: the cavity is often still there and can become a fistula.

A small opening near the anus that stains underwear again and again with pus or bloody fluid, sometimes for months, is the classic picture of a fistula. Itching and irritated skin around the opening are common. Learn more about blood patterns in Anal fissure bleeding: how much is normal and when should you worry?.

What are the signs of an anal abscess, and why can it not wait?

An abscess is the one of the three that needs attention without delay. Pus under pressure does not resolve with creams, sitz baths or, in most cases, antibiotics alone. The standard treatment is to drain it, usually a short procedure under local or general anesthesia. Left alone, the infection can spread into deeper spaces around the rectum, and in people with diabetes or a weakened immune system it can become serious.

If you recognize these signs, see a doctor or go to an emergency department the same day. Do not wait for a cream to work.

What are the symptoms of an anal fistula?

A fistula rarely causes the intense pain of an abscess. Its signature is persistence:

• A small opening on the skin near the anus, sometimes with a little raised tissue around it.
• Intermittent leakage of pus, bloody fluid or, occasionally, a little stool or gas through that opening.
• Cycles of swelling and discomfort that ease when it drains.
• Irritated, itchy skin from the constant moisture.
• A history of an abscess in the same area, drained or burst.

A fistula does not close with creams, because it is a lined tract with an inner opening inside the anal canal. Treatment is surgical, and the right operation depends on how much sphincter muscle the tunnel crosses. That is why a colorectal surgeon usually maps it first, sometimes with an MRI or an endoanal ultrasound.

Can an anal fissure turn into a fistula or abscess?

In the typical fissure, no. A common midline fissure in a healthy person heals or becomes chronic, but it does not usually become infected. Fissures and abscesses do share a risk factor, though: the hard or very loose stools that injure the anal canal.

The picture changes in Crohn’s disease. Here fissures are often off the midline, deeper, less painful than expected and slow to heal, and they can evolve. In a study of 56 patients with Crohn’s disease and anal fissures, 26 % of the fissures that did not heal progressed to an abscess or fistula (Fleshner, 1995). A fissure off the midline, several fissures, or a fissure together with diarrhea, weight loss or abdominal pain are reasons to look for inflammatory bowel disease; see Anal fissure and inflammatory bowel disease: how is a Crohn’s fissure different? and Lateral or multiple anal fissures: when should you think of Crohn’s, HIV, syphilis, tuberculosis or cancer?.

Occasionally, a long-standing chronic fissure can become infected at its base and form a small, superficial track. When this happens, the surgeon treats both at once, so it is worth mentioning any discharge at your visit.

How does a doctor tell a fissure from a fistula or abscess?

Most of the time, by listening and looking. The history (pain with stools versus constant pain, blood versus pus, fever or not) points to the diagnosis, and a gentle inspection of the anal opening usually confirms it. A fissure is visible as a tear when the buttocks are separated; an abscess shows as a red, tender swelling; a fistula as a small external opening.

• If a fissure is very painful, the doctor may avoid an internal examination on the first visit.
• If a deep abscess is suspected without visible swelling, imaging such as a CT scan, an MRI or an ultrasound may be needed.
• Fistulas are often mapped with an MRI or an endoanal ultrasound before surgery.
• Atypical fissures may need a biopsy or tests for Crohn’s disease and infections.

If you are unsure what you have, start with Anal fissure symptoms: a 2-minute self-check, read our symptoms page, and see All guides for what to expect at the visit. You can also contact our team.

Where does FISURIN fit?

FISURIN is for fissures, not for abscesses or fistulas. Our surgeons at Clínica Proctológica de Lima prescribe it for confirmed fissures, especially chronic and post-operative ones: nifedipine to relax the internal sphincter and lidocaine to ease the pain of each bowel movement, in a water-based hydrogel with a written 6-week protocol. It does not treat infection, does not close a fistula tract and must never delay the drainage of an abscess. Today it is a magistral formula prepared under prescription in Peru and is not shipped abroad; FISURIN Hydrogel and FISURIN Relief are in preparation, with a waiting list. See what is inside FISURIN or how to get it where you live.