A fistula is an abnormal tunnel that forms between two body parts that aren’t normally connected, such as between the intestine and skin, the bladder and vagina, or an artery and a vein. Fistulas develop when tissue breaks down due to infection, inflammation, injury, or surgical complications. The specific cause depends on where in the body the fistula forms, but chronic inflammation and untreated infections are the two most common drivers.
Infection and Abscess Formation
The most common path to a fistula, particularly around the anus, starts with an abscess. Small glands inside the anal canal can become blocked and infected, forming a pocket of pus. When that abscess drains (either on its own or through a surgical procedure), the tunnel it carved through tissue sometimes refuses to heal, leaving behind a permanent tract. In one study of 68 patients who had perianal abscesses drained, nearly 46% went on to develop a fistula. That makes abscess progression one of the single biggest risk factors.
The reason these tunnels persist is that the lining of the tract can develop a thin layer of cells that prevents it from closing naturally. Without treatment, the fistula continues to drain fluid, pus, or stool intermittently, sometimes for months or years.
Crohn’s Disease and Inflammatory Bowel Disease
Crohn’s disease is one of the most well-known causes of fistulas because the inflammation it triggers doesn’t stay on the surface of the intestinal lining. It penetrates through the full thickness of the bowel wall. When that deep inflammation spreads outward, it can bore into nearby organs or skin, creating fistula tracts. Population-based estimates put the lifetime risk of developing a perianal fistula at 14% to 38% for people with Crohn’s disease.
The biological process behind this is specific. Chronic inflammation causes the cells lining the intestinal wall to change their behavior. They lose the ability to stick tightly together and instead become more mobile, almost like wound-healing cells that migrate in the wrong direction. This triggers a cascade: immune cells flood the area, inflammatory signaling ramps up, and enzymes begin breaking down the structural framework of the tissue. The result is a channel of transformed cells tunneling outward through the bowel wall. That channel becomes the fistula.
Fistulas caused by Crohn’s disease are notoriously difficult to treat because the underlying inflammation tends to recur, and the tissue in the area is already compromised.
Obstructed Childbirth
In low-income countries, prolonged or obstructed labor is the leading cause of a specific type called obstetric fistula. When labor stalls and the baby’s head presses against the mother’s pelvic tissues for an extended period, the sustained pressure cuts off blood flow. The tissue dies, and a hole forms between the vagina and the bladder (vesicovaginal fistula) or between the vagina and the rectum (rectovaginal fistula). The result is continuous, uncontrollable leaking of urine or stool.
An estimated 2 to 3 million women worldwide live with obstetric fistula, almost exclusively in sub-Saharan Africa and South Asia. Every case is preventable with timely access to emergency obstetric care, particularly cesarean delivery. In high-income countries, obstetric fistulas are rare precisely because that care is widely available.
Diverticulitis
Diverticulitis occurs when small pouches in the colon wall become inflamed or infected. Like Crohn’s disease, the inflammation can extend through the entire thickness of the colon. When a diverticulum ruptures and forms an abscess, that abscess can erode into a neighboring organ, creating a fistula. The most common type caused by diverticulitis is a colovesical fistula, which connects the colon to the bladder. Less commonly, the abscess erodes into a loop of small intestine, forming a coloenteric fistula.
Fistulas from diverticulitis tend to be associated with chronic or recurrent disease rather than a single acute episode. Some researchers believe that patients who develop fistulas represent a distinct, more aggressive pattern of diverticular disease marked by frequent recurrences and a higher rate of complications.
Radiation Therapy
Pelvic radiation, commonly used for cervical, rectal, and other gynecological cancers, damages blood vessels in the surrounding tissue. Over time, the tissue becomes scarred and fibrotic, losing its ability to heal normally. This slow deterioration can eventually cause a breakdown between adjacent organs, most often a rectovaginal fistula connecting the rectum and vagina.
The timeline is surprisingly delayed. In a study of patients treated with pelvic radiation for gynecological cancers, the median time from completing radiation to fistula development was 20 months, with some cases appearing as late as 20 years afterward. Nearly 80% of those patients had significant adhesions and inflammatory changes in the pelvis. These fistulas tend to develop gradually, without any triggering injury or surgery, and they are particularly challenging to repair because the surrounding tissue is already damaged and poorly supplied with blood.
Surgery and Physical Trauma
Any surgery in the pelvic or abdominal area carries a small risk of fistula formation. Procedures involving the bowel, bladder, uterus, or rectum can inadvertently create connections between organs if tissue is nicked, if a surgical site doesn’t heal properly, or if an infection develops at the operative site. Hysterectomies, bowel resections, and prostate surgeries are among the procedures most associated with this complication.
Penetrating injuries can also create fistulas. Stab wounds and gunshot wounds that pierce through areas where an artery and vein sit close together can form arteriovenous fistulas, abnormal connections between an artery and a vein that bypass the capillary network. Blood flows directly from the high-pressure artery into the vein, which can cause swelling, pain, and, if large enough, strain on the heart.
Arteriovenous Fistulas: A Different Category
Not all fistulas involve the digestive or reproductive tract. Arteriovenous (AV) fistulas connect arteries directly to veins and have their own set of causes. Some are congenital, meaning the blood vessels simply didn’t form correctly during fetal development. Others result from genetic conditions. Hereditary hemorrhagic telangiectasia, for example, causes abnormal blood vessel formation throughout the body, particularly in the lungs, and can lead to pulmonary AV fistulas.
Interestingly, AV fistulas are also created intentionally. For people with severe kidney disease who need regular dialysis, surgeons create an AV fistula in the forearm on purpose. Connecting an artery to a vein makes the vein enlarge and toughen, giving dialysis nurses a reliable access point for the large needles required during treatment. This is one of the few situations where a fistula is a planned, beneficial procedure.
How Fistulas Are Diagnosed
MRI is the gold standard for mapping a fistula’s path through the body. It provides excellent soft tissue contrast and can show the tract, any branching tunnels, the internal and external openings, and whether abscesses are present. This level of detail is critical for planning surgery because missing a secondary tract is one of the main reasons fistula repairs fail.
In some cases, a CT scan with contrast dye injected into the fistula tract (called CT fistulography) can provide additional detail about the location of internal openings. Ultrasound, particularly when performed through the rectum for anal fistulas, offers a more affordable option and can approach the accuracy of MRI in experienced hands, especially when enhanced with a small amount of hydrogen peroxide to highlight the tract. For Crohn’s-related fistulas specifically, European guidelines recommend contrast-enhanced pelvic MRI as the standard imaging approach, because it helps classify the fistula as simple or complex and directly guides treatment decisions.