An anal fistula is a small tunnel that forms between the inside of the anal canal and the skin near the anus. It typically develops after an anal abscess (a pocket of infection) drains or bursts, leaving behind a channel that won’t close on its own. Roughly one in three people who have an anal abscess will go on to develop a fistula, with studies placing the rate between 26% and 46%.
How an Anal Fistula Forms
The anal canal contains tiny glands that sit between the two rings of muscle (sphincters) controlling bowel movements. These glands normally drain into small pockets called crypts along the inner lining of the canal. When one of those glands becomes blocked, bacteria from the bowel can trigger an infection. The infection builds into an abscess, and as the abscess expands, it carves a path outward through the surrounding tissue toward the skin.
Once the abscess drains, either on its own or through surgery, the tunnel it created sometimes fails to seal shut. The inner opening stays connected to the outer opening on the skin, and this persistent channel is the fistula. Because the inner end still communicates with the bowel, the tunnel continues to collect bacteria and fluid, which is why it rarely heals without treatment.
Common Causes and Risk Factors
The vast majority of anal fistulas stem from infected anal glands. This is called the cryptoglandular origin, and it accounts for most cases in otherwise healthy adults. But several other conditions can also produce fistulas:
- Crohn’s disease is the most significant risk factor outside of a simple abscess. Between 20% and 30% of people with Crohn’s develop a perianal fistula at some point, and these fistulas tend to be more complex and harder to treat.
- Previous anal surgery or trauma can create abnormal connections in the tissue.
- Radiation therapy to the pelvic area can weaken tissue and promote fistula formation.
- Infections such as tuberculosis or sexually transmitted infections occasionally cause fistulas, though this is uncommon in most populations.
What an Anal Fistula Feels Like
The most noticeable sign is a small opening on the skin near the anus that persistently leaks. You might see pus, blood, or a small amount of stool on your underwear. The skin around the opening is often red and irritated. Pain tends to be worst when sitting or during bowel movements, and it can range from a dull ache to a sharp throb, especially if the tunnel becomes re-infected and a new abscess forms. Some people also develop a fever during flare-ups.
A pattern many people recognize is a cycle of swelling, pain, and then sudden relief when the fistula drains on its own, followed by the whole process repeating weeks or months later. That recurring cycle is a strong signal that a fistula, rather than a one-time abscess, is present.
How Fistulas Are Diagnosed
A surgeon can often identify a fistula during a physical exam by locating the external opening on the skin and feeling for the tract beneath the surface. The key diagnostic challenge is mapping the full path of the tunnel, because knowing exactly where it runs relative to the sphincter muscles determines what kind of surgery is safe.
MRI is the gold standard for mapping complex fistulas. It shows the tract in three dimensions and reveals hidden branches or secondary abscesses that aren’t obvious on exam. Ultrasound performed inside the anal canal is another option, particularly useful for fistulas that sit close to the sphincter muscles.
Surgeons sometimes use a clinical guideline called Goodsall’s rule to predict where the internal opening sits based on the location of the external opening. The rule is accurate about 75% of the time overall, and it works better for fistulas that open behind the anus (73% accuracy) than those opening in front (52% accuracy). It’s a useful starting point, but imaging and direct examination during surgery provide the definitive map.
Types of Anal Fistulas
Fistulas are classified by the path the tunnel takes through the sphincter muscles. This matters because the more muscle the tract crosses, the more carefully the surgeon has to plan the repair to avoid weakening bowel control.
- Intersphincteric: The tunnel runs between the two sphincter muscles without crossing the outer one. This is the most common type and generally the simplest to treat.
- Trans-sphincteric: The tract passes through both the inner and outer sphincter muscles. These are common and often require more advanced surgical techniques.
- Suprasphincteric: The tunnel travels upward between the sphincter muscles, then loops over the top of the outer sphincter before reaching the skin. This is less common and more complex.
- Extrasphincteric: The tract bypasses the sphincter muscles entirely, running from the rectum through the surrounding tissue to the skin. This rare type often signals an underlying condition like Crohn’s disease.
Surgical Treatment Options
Anal fistulas almost always require surgery. They don’t heal with antibiotics alone because the tunnel is lined with tissue that prevents it from closing. The choice of procedure depends on how much sphincter muscle the fistula crosses.
Fistulotomy
This is the most straightforward and most successful option. The surgeon cuts open the entire length of the tunnel, converting it from a tube into an open groove that heals from the bottom up. In a study of over 600 patients, fistulotomy achieved a healing rate of 98.6% with only a 1.4% recurrence rate. It works best for simple, low fistulas that involve minimal sphincter muscle. The trade-off is that it does involve cutting some muscle tissue, which is why it’s not suitable for tracts that pass through a large portion of the sphincter.
Seton Placement
When the fistula crosses a significant portion of the sphincter, surgeons often place a seton, a thin loop of surgical thread threaded through the tunnel and tied loosely. A loose seton keeps the tract open so it can drain continuously, preventing new abscesses from forming. It doesn’t cure the fistula on its own but controls symptoms and buys time while the surgeon plans a definitive repair. Tighter setons can be used to slowly cut through the muscle over weeks, allowing it to heal gradually as it’s divided, which reduces the risk of incontinence compared to cutting through all the muscle at once.
LIFT Procedure
The LIFT technique involves accessing the fistula tract through the space between the two sphincter muscles, tying off the tunnel, and removing the infected portion. Because no sphincter muscle is cut, it carries a lower risk of affecting bowel control. Early reports showed success rates of 80% to 88%, but in studies limited to complex, high fistulas, success dropped to between 42% and 60%. It’s generally reserved for trans-sphincteric fistulas where a fistulotomy would put too much muscle at risk.
Risks of Surgery
The primary concern with any fistula surgery is damage to the sphincter muscles leading to some degree of fecal incontinence. Reported rates vary widely, reaching up to 40% depending on the type of fistula and the procedure used. In one prospective study, 38% of patients reported some level of incontinence after surgery, up from 18% before the operation. However, the severity was low in most cases, and 65% of patients experienced no change in their continence at all. About 27% had worsening symptoms, while 9% actually improved.
This is exactly why classification matters so much. A simple intersphincteric fistula treated with fistulotomy carries a very different risk profile than a complex trans-sphincteric fistula requiring multiple procedures. Surgeons weigh the chance of recurrence against the chance of incontinence when recommending a specific approach.
Recovery After Fistula Surgery
After a fistulotomy, most people feel uncomfortable for the first few days and can expect their daily routine to be disrupted for about two weeks. Returning to work depends on the type of job and the complexity of the surgery. The wound itself, which is left open to heal from the inside out, can take several months to fully close.
During recovery, sitz baths (sitting in a few inches of warm water) help keep the area clean and ease discomfort. A high-fiber diet and plenty of water are important for keeping stools soft, since straining puts pressure on the healing wound. Most surgeons advise holding off on strenuous exercise until the wound has healed significantly, even if you feel well enough to resume it sooner.
For fistulas treated with a seton, recovery looks different. The thread stays in place for weeks or sometimes months, and you’ll have follow-up visits so the surgeon can assess healing and decide when to remove or tighten it. Living with a seton can be mildly uncomfortable, but most people manage normal activities around it while waiting for the next stage of treatment.