A perianal fistula is an abnormal tunnel that forms between the inside of the anal canal and the skin near the anus. It typically develops after an infection in one of the small glands just inside the anus, creating a passageway that drains pus, blood, or other fluid to the outside. Most perianal fistulas don’t heal on their own and require some form of treatment to close.
How a Fistula Forms
Inside the anal canal, there are eight to ten tiny glands arranged in a ring at a point called the dentate line. These glands can become blocked and infected, forming a pocket of pus called an abscess. In roughly 30 to 50 percent of anal abscesses, the infection eventually tunnels outward through the surrounding tissue and creates an opening on the skin near the anus. That tunnel is the fistula.
The tract always has two ends: an internal opening inside the anal canal and an external opening on the perianal skin. Some fistulas follow a short, straight path. Others curve through deeper layers of muscle tissue, making them more complex to treat. The path the tunnel takes, and how much sphincter muscle it crosses, determines how a surgeon will approach it.
Common Causes
The most frequent cause is a previous anal abscess that either drained on its own or was surgically drained but left behind a persistent tunnel. This is sometimes called the cryptoglandular theory: infection starts in the anal glands, spreads into the surrounding muscle layers, and carves out a tract that the body lines with tissue instead of closing off.
Crohn’s disease is the other major cause. About 35 to 50 percent of adults with Crohn’s disease develop a fistula at some point. In Crohn’s, chronic inflammation penetrates the full thickness of the bowel wall, and the resulting tissue breakdown can create tunnels between the intestine and nearby structures, including the perianal skin. Fistulas caused by Crohn’s tend to be more complex, more likely to recur, and harder to manage surgically.
Less common causes include prior surgery or radiation to the pelvic area, tuberculosis, sexually transmitted infections, and, rarely, cancer.
What It Looks and Feels Like
The most noticeable sign is a small hole or opening on the skin near your anus that intermittently leaks fluid. The drainage can include pus, blood, or stool, and it often has a noticeable smell. You may see staining on your underwear or feel moisture in the area throughout the day.
Pain is common and often described as intense or throbbing. It tends to worsen when you sit, cough, or have a bowel movement. The skin around the opening is frequently swollen, red, and tender to the touch, which are signs of ongoing infection beneath the surface. Some people also develop fever, pain while urinating, or difficulty controlling bowel movements, though these are less typical.
Symptoms often come and go. The external opening may temporarily seal over, trapping fluid inside and causing pressure and pain to build. When it opens again, the drainage returns and the pain eases. This cycle of sealing and reopening is a hallmark of fistulas and is one reason they rarely resolve without intervention.
Types of Perianal Fistulas
Fistulas are classified by the path they take through the sphincter muscles that control bowel continence. This matters because the surgical approach depends entirely on how much muscle the tract crosses.
- Intersphincteric: The most common type. The tunnel runs between the two layers of sphincter muscle and exits through the skin. These are generally the simplest to treat.
- Transsphincteric: The tunnel passes through both the internal and external sphincter muscles before reaching the skin. These involve more muscle and carry a higher risk of affecting continence if cut through surgically.
- Suprasphincteric: The tract loops up and over the top of the external sphincter before descending to the skin. Uncommon and more complex.
- Extrasphincteric: The tunnel bypasses the sphincter muscles entirely, connecting the rectum directly to the perianal skin. These are rare and usually result from trauma, surgery, or Crohn’s disease rather than a simple abscess.
How Fistulas Are Diagnosed
A doctor can often identify a fistula during a physical exam by spotting the external opening and feeling the tract beneath the skin. But the exam alone doesn’t reveal the full picture, especially when the tunnel takes a deep or branching path through the tissue.
MRI and endoanal ultrasound are the two main imaging tools used to map a fistula’s course. Both are highly accurate. In a study published in Gastroenterology comparing the two methods with examination under anesthesia, each approach achieved at least 85 percent accuracy on its own. When any two of the three methods were combined, accuracy reached 100 percent. Imaging is particularly important for complex or recurring fistulas, where knowing the exact relationship between the tract and the sphincter muscles determines whether a surgery is safe to perform.
Treatment Options
Perianal fistulas almost always require procedural treatment. The specific approach depends on whether the fistula is classified as simple (shallow, involving minimal muscle) or complex (crossing significant sphincter muscle, branching, or associated with Crohn’s disease).
Simple Fistulas
A fistulotomy is the standard treatment for straightforward fistulas. The surgeon opens the entire tunnel from the inside out, converting it from a closed tube into an open groove that heals from the bottom up. It is effective, but recovery takes time. The wound itself can take several months to fully heal, and common complications include bleeding, infection at the surgical site, and some degree of fecal incontinence, particularly if the fistula crosses a portion of the sphincter.
Complex Fistulas
When a fistula passes through a significant amount of sphincter muscle, cutting it open would risk permanent damage to bowel control. Surgeons use sphincter-sparing techniques instead.
One option is the LIFT procedure, which involves accessing the tract through the space between the two sphincter muscles and tying it off. A study with a median follow-up of 37 months found a success rate of 51 percent on the first attempt. Among patients whose LIFT procedure failed, 74 percent went on to have a second surgery, which succeeded 71 percent of the time.
An advancement flap involves pulling a piece of healthy tissue over the internal opening to seal it shut. In a large comparative study of patients with high transsphincteric fistulas, flap advancement healed the fistula in about 60 percent of cases at 12 weeks.
Biological fistula plugs, which are inserted into the tract to encourage it to close, showed a similar healing rate of roughly 59 percent at 12 weeks. Fibrin glue, a liquid sealant injected into the tract, was less effective at about 39 percent. Seton drains, flexible loops threaded through the tunnel to keep it open and draining, healed fistulas in about 33 percent of cases but are often used as a first step to control infection before a definitive repair.
Crohn’s-Related Fistulas
Fistulas caused by Crohn’s disease are treated differently because the underlying inflammation must be controlled alongside the fistula itself. Biologic medications that suppress the specific inflammatory molecules driving tissue destruction are a cornerstone of treatment. Surgery is still used but tends to be more conservative, often starting with a seton drain to manage infection while medication works to quiet the inflammation fueling the fistula.
Recovery and What to Expect
Recovery timelines vary significantly depending on the procedure. A straightforward fistulotomy may keep you off work for a week or two, but the open wound left behind can take several months to close completely. During that time, you’ll likely need to soak the area in warm baths and keep the wound clean to prevent reinfection.
Sphincter-sparing procedures generally involve less pain and a shorter initial recovery, but recurrence rates are higher. It’s not uncommon for people with complex fistulas to need more than one procedure before the tract fully closes. The on-and-off nature of the condition, cycling between drainage and discomfort, is one of the most frustrating aspects for people living with it. Recurrence can happen months or even years after an apparently successful repair, so follow-up with a colorectal specialist is an ongoing part of management.