An anal fistula is a small tunnel that forms between the inside of the anal canal and the skin near the anus. It develops when an infection creates a pathway from an internal gland to an external opening, leaving a chronic tract that drains pus, blood, or stool. About 1 in 10,000 people develop one each year, and most cases trace back to an anal abscess that didn’t fully heal.
How an Anal Fistula Forms
The anal canal contains small glands that can become blocked and infected, forming an abscess (a pocket of pus). When that abscess drains or is surgically opened, the body sometimes heals the surface but leaves behind a deeper tunnel connecting the internal infection site to the skin. This tunnel is the fistula. An abscess represents the acute infection; a fistula represents the chronic aftermath.
Roughly 30 to 40 percent of people who are treated for an anorectal abscess go on to develop a fistula. The tract keeps itself open because it’s lined with tissue that resists closing on its own, and it continues to collect drainage from the gland where the infection started.
Common Causes and Risk Factors
The vast majority of anal fistulas begin with a blocked anal gland that becomes infected. But several other conditions can also lead to fistula formation:
- Crohn’s disease is one of the most well-known causes outside of abscess. The chronic inflammation it creates in the digestive tract makes fistulas more likely and harder to treat.
- Previous anal surgery or repeated abscesses increase the risk of a tract forming.
- Radiation therapy to the pelvic area, trauma, or infections like tuberculosis can also trigger fistula development, though these are less common.
What It Looks and Feels Like
An anal fistula typically looks like a small hole in the skin near the anus. It may ooze pus, blood, or stool, particularly when touched or during a bowel movement, and the drainage often has a noticeable smell. The skin around the opening is frequently red and swollen.
The most common symptoms are:
- Throbbing anal pain that worsens with sitting, coughing, or having a bowel movement
- Persistent drainage from a spot near the anus, which may stain underwear
- Redness and swelling around the anus, signaling active infection under the skin
Less commonly, people experience fever, pain while urinating, or difficulty controlling bowel movements. The pain often follows a cycle: pressure builds as drainage accumulates, then temporarily improves when the fistula drains, then builds again.
Types of Anal Fistulas
Fistulas are classified by how deep the tunnel goes through the muscles that control the anus (the sphincter muscles). This matters because the deeper the tract, the more complex the surgery and the higher the risk to bowel control.
The most common type is an intersphincteric fistula, which accounts for about 70 percent of all cases. It travels between the two layers of sphincter muscle and exits through the skin near the anus. These are generally considered simple fistulas and are the most straightforward to treat.
A transsphincteric fistula passes through both sphincter muscles before reaching the skin. Because it involves more muscle tissue, repairing it requires more careful surgical planning. Suprasphincteric and extrasphincteric fistulas are rarer and involve higher, more complex tracts that loop above or completely outside the sphincter muscles.
How It’s Diagnosed
Most anal fistulas can be identified through a physical exam. Your doctor will look for the external opening and may be able to feel the tract beneath the skin. Routine imaging isn’t necessary for straightforward cases.
For recurrent, complex, or hard-to-find fistulas, imaging helps map the full path of the tunnel before surgery. MRI with contrast dye offers the highest accuracy, with both sensitivity and specificity above 90 percent for most fistula types. Endoanal ultrasound (an ultrasound probe placed in the anal canal) also performs well for common fistula types, detecting intersphincteric fistulas with 89 percent sensitivity and transsphincteric fistulas with 96 percent sensitivity. For deeper, rarer fistula types, both methods become less reliable.
Surgical Treatment Options
Anal fistulas almost always require surgery. They rarely close on their own because the tract’s lining prevents natural healing.
For simple fistulas, the standard procedure is a fistulotomy. The surgeon cuts open the entire tunnel, converting it from a closed tube into an open groove that heals from the bottom up. This procedure has an exceptionally high success rate, with one large study of 353 patients reporting 100 percent healing after one or two operations. It’s considered the gold standard when the fistula involves only a small amount of sphincter muscle.
Complex fistulas that pass through a significant portion of the sphincter require techniques that avoid cutting through muscle. The LIFT procedure works by accessing the tract through the space between the two sphincter muscles and tying it off from the inside. A systematic review of over 1,300 patients found a pooled success rate of about 76 percent.
Another option for complex cases is a seton, which is a thin thread or rubber band looped through the fistula tract. A draining seton stays loose and keeps the tunnel open so infection can drain rather than forming new abscesses. This is particularly useful for people with Crohn’s disease, where the goal is often long-term management rather than a single cure. In a two-stage approach, a draining seton is placed first to clear infection and encourage scarring around the muscle, then the surgeon performs a fistulotomy once the tissue has stabilized.
Advancement flaps, where a piece of tissue is pulled over the internal opening to seal it, offer another sphincter-sparing option with an average success rate around 81 percent.
Recovery After Surgery
Recovery depends on the type of procedure, but even a straightforward fistulotomy takes longer than most people expect. Daily activities are typically disrupted for about two weeks, and full recovery can take several months. During that time, you’ll likely need to avoid strenuous lower-body exercise like squats or heavy lifting.
The wound from a fistulotomy is intentionally left open to heal from the inside out, which means you’ll manage an open wound with regular sitz baths and dressing changes. Pain is usually manageable with over-the-counter medication and tends to improve steadily after the first week.
Risks to Bowel Control
The biggest concern with any fistula surgery is damage to the sphincter muscles, which can affect your ability to control gas or stool. This is precisely why the type and location of the fistula dictates which surgery is appropriate.
For simple fistulotomy involving minimal muscle, the risk of significant incontinence is low. But studies report that some degree of continence disturbance after fistulotomy can reach as high as 58 percent when accounting for all severities, including minor issues like occasional difficulty controlling gas. The risk increases with factors like prior anal surgeries, the amount of muscle involved, and, in women, a history of childbirth-related injury. For cutting setons specifically, minor incontinence occurs in 10 to 20 percent of patients, with major incontinence in up to 10 percent.
This tradeoff between cure rate and continence risk is the central decision in fistula treatment. Procedures that cut through muscle heal more reliably but carry more risk. Sphincter-sparing procedures protect continence but have higher recurrence rates. Your surgeon will weigh your specific anatomy, fistula complexity, and baseline sphincter function to recommend the best approach.