An anal fistula is not immediately life-threatening for most people, but it is a condition that won’t resolve on its own and carries real risks if left untreated. The danger depends on how complex the fistula is, whether infection spreads, and whether an underlying condition like Crohn’s disease is driving it. Even a simple fistula causes significant pain and recurring infection, and in rare cases, a neglected one can lead to serious complications including tissue destruction and, over many years, a small risk of cancer.
What Makes a Fistula Risky
An anal fistula is an abnormal tunnel between the inside of the anal canal and the skin near the anus. It usually forms after a perianal abscess (a pocket of infection) drains or bursts. The tunnel doesn’t close on its own because it’s constantly exposed to bacteria from stool, so the cycle of infection, swelling, and drainage tends to repeat.
The immediate danger is ongoing infection. Active infection under the skin around the anus, called cellulitis, causes intense throbbing pain, swelling, redness, and drainage of pus or blood. Fever, pain while urinating, and difficulty controlling bowel movements can also develop. These symptoms signal that infection is worsening, not improving.
In rare but serious cases, infection from an untreated abscess or fistula can spread deeper into surrounding tissue, causing a condition called necrotizing fasciitis, where soft tissue rapidly dies. Even more rarely, infection can reach the abdominal cavity or the space behind it. These are surgical emergencies. They’re uncommon, but they illustrate why ignoring persistent anal pain and swelling is a bad idea.
Simple vs. Complex Fistulas
Not all fistulas carry the same level of risk. Doctors classify them on a spectrum from simple to complex based on how deep the tunnel runs, how many branches it has, and whether it passes through the muscles that control bowel function (the sphincter muscles).
A simple fistula runs a short, straight path close to the surface and involves minimal sphincter muscle. These are more straightforward to treat and less likely to cause lasting problems. A complex fistula, on the other hand, may tunnel through or above the sphincter muscles, branch into multiple tracts, or be accompanied by an abscess. Complex fistulas are harder to treat, more likely to recur, and carry a higher risk of complications both from the disease itself and from surgery.
The distinction matters because complex fistulas sometimes require more aggressive procedures, and in the most severe cases, may eventually require a temporary or permanent stoma (a surgically created opening for stool to bypass the anus).
The Crohn’s Disease Connection
People with Crohn’s disease face a substantially higher fistula burden. Up to 50% of Crohn’s patients develop a fistula within 20 years of diagnosis, and roughly 75% of those fistulas are complex. The ongoing inflammation that characterizes Crohn’s damages the intestinal lining, creating the conditions for fistulas to form and making them harder to heal.
Treatment options for Crohn’s-related fistulas are limited and often provide only temporary relief. More than a third of these patients experience recurrence even with medical therapy. An estimated 40,000 people in the United States are living with complex Crohn’s-related anal fistulas at any given time, many of whom require repeated surgeries and aggressive medication regimens. If you have Crohn’s and develop anal pain or drainage, it’s especially important to have it evaluated promptly.
Cancer Risk From Chronic Fistulas
A long-standing, untreated fistula does carry a small risk of developing into cancer. The overall incidence of cancer arising from a perianal fistula is 0.3% to 0.7%. That’s low in absolute terms, but it’s not zero, and the risk increases the longer a fistula goes unmanaged. Chronic inflammation and repeated tissue damage over years or decades create an environment where abnormal cells are more likely to develop. This is one more reason not to simply live with a fistula indefinitely.
What Surgery Involves and Its Risks
Surgery is the primary treatment because fistulas don’t heal with antibiotics or other medications alone. The type of procedure depends on the fistula’s complexity.
For simple fistulas, the most common approach involves opening the tunnel and letting it heal from the inside out. This has high success rates, with recurrence around 5.9% for simple cases. For complex fistulas, surgeons may use sphincter-sparing techniques that aim to close the tunnel without cutting through the muscles that control continence. These include procedures that tie off the fistula tract or cover its internal opening with a tissue flap.
The tradeoff is real: sphincter-sparing procedures protect bowel control but have higher recurrence rates, sometimes exceeding 50%. Complex fistulas recur about 25.4% of the time overall. Meanwhile, procedures that cut through sphincter muscle are more definitive but carry an incontinence risk. Studies report incontinence rates ranging from about 6% to 20%, depending on the technique and the amount of muscle involved. Risk factors for post-surgical incontinence include being female, having had previous anorectal surgeries, and having a complex fistula.
This means you and your surgeon will likely weigh cure rates against continence risk, and the right choice depends on the specific anatomy of your fistula and your personal priorities.
How Fistulas Are Diagnosed
Diagnosis usually starts with a physical exam, but imaging is often needed to map the full path of the tunnel, especially before surgery. MRI and endoanal ultrasound (an ultrasound probe inserted into the anal canal) are the two main imaging tools. Both detect fistulas with about 87% sensitivity, meaning they catch the vast majority of cases. MRI tends to be better at ruling out fistulas when they’re not present, with higher specificity than ultrasound. Knowing the exact route of the fistula and any hidden branches is critical for planning surgery and reducing the chance of recurrence.
Warning Signs That Need Prompt Attention
If you have a known fistula or suspect one, certain symptoms suggest things are getting worse: increasing pain that becomes constant rather than intermittent, spreading redness or swelling around the anus, fever, or new drainage that includes foul-smelling pus. Difficulty controlling your bowels is another signal that the fistula may be affecting sphincter function. Any of these warrant prompt evaluation rather than a wait-and-see approach. Anal fistulas are highly treatable, but the longer they’re left alone, the more complex they tend to become.