Anal Fistula

An anal fistula is an abnormal small tunnel that forms between the inside of the anal canal and the skin outside — most often as a consequence of a previous perianal abscess. It causes a different symptom pattern from a hemorrhoid, and unlike a hemorrhoid, it essentially never resolves on its own without treatment.

Key takeaways

A fistula most often develops after an abscess has been drained (or has drained on its own), when the channel it created doesn’t fully close.
The hallmark symptom is persistent or recurring drainage — pus, blood, or sometimes stool — from an opening in the skin near the anus, often with intermittent swelling and pain.
Fistulas don’t heal on their own and require a procedure — there’s no home care or topical treatment that resolves the underlying tunnel.
Recurrent fistulas, or ones with an unusual pattern, can be a sign of an underlying condition like Crohn’s disease and may need broader evaluation.

How it develops

Most anal fistulas start with a perianal abscess (see Anal abscess: a surgical emergency) — a blocked, infected gland inside the anal canal. Whether the abscess drains on its own or is drained by a clinician, the channel it created between the gland and the skin surface sometimes doesn’t close completely, leaving a persistent tunnel: the fistula.

Less commonly, fistulas arise from other causes, including Crohn’s disease, prior surgery or trauma in the area, radiation treatment, certain infections, or (rarely) tuberculosis or other unusual causes. A fistula that develops without a clear preceding abscess, or one that’s recurrent or unusually complex, is more often investigated for one of these underlying causes.

Symptoms

Persistent or recurring drainage from a small opening in the skin near the anus — pus, blood, or occasionally stool
Irritation or itching of the skin around the opening from ongoing drainage
Pain and swelling that comes and goes, sometimes worsening if the external opening temporarily closes and pressure builds up again (functioning like a mini-abscess flare)
Occasionally, a small visible opening or “pit” in the skin

This pattern — an opening that periodically drains and reswells — is fairly distinctive and different from typical hemorrhoid symptoms, which don’t usually involve visible drainage from a discrete opening in the skin.

Why this doesn’t heal on its own

A fistula is a tract lined with tissue that keeps the channel open, similar in concept to a pierced earring hole staying open once healed around the piercing. This lining prevents the tunnel from closing permanently on its own, even between flares when things seem quiet. Topical treatments, hemorrhoid creams, or antibiotics alone don’t close this tract — treatment requires addressing the tunnel itself.

Treatment

Treatment depends on the fistula’s path relative to the anal sphincter muscles, since preserving continence is a central consideration. Options a colorectal surgeon may discuss include:

Fistulotomy — opening the tract to allow it to heal from the inside out, generally used for simpler fistulas that don’t involve much of the sphincter muscle
Seton placement — a thread-like material placed through the tract, either to allow continued drainage while healing occurs, or as a staged part of more complex treatment
More complex sphincter-preserving procedures — used for fistulas that pass through a significant amount of sphincter muscle, where a straightforward fistulotomy could risk continence

The right approach depends on the specific anatomy of the fistula, which is typically assessed through exam and sometimes imaging (such as MRI) before a treatment plan is made.

When a fistula suggests something more than a one-off abscess

If fistulas are recurrent, multiple, or unusually complex, or occur alongside diarrhea, abdominal pain, or other digestive symptoms, this combination is a reason to be evaluated for Crohn’s disease — see Crohn’s disease and perianal symptoms.

When to see a doctor

Any persistent or recurring drainage from an opening in the skin near the anus
Recurring swelling and pain in the same spot after a previous abscess was treated
Any of these symptoms alongside diarrhea, abdominal pain, or other digestive symptoms

Dealing with a new, painful swelling rather than ongoing drainage? See Anal abscess: a surgical emergency — that’s the more urgent, earlier-stage version of this same underlying process.

This page is for general education. Fistula anatomy varies considerably between individuals, and treatment decisions require an in-person evaluation by a colorectal surgeon, often with imaging.