A fistula is an abnormal tunnel that forms between two body parts that aren’t normally connected, such as two organs, or an organ and the skin. Fistulas develop when tissue breaks down due to infection, inflammation, injury, or pressure. The specific cause depends on where in the body the fistula forms, but the underlying mechanism is almost always the same: sustained damage destroys healthy tissue faster than the body can repair it, and a channel forms through the gap.
How a Fistula Forms
Healthy tissue acts as a barrier between organs, cavities, and the skin surface. When that tissue is damaged by chronic inflammation, infection, or loss of blood supply, it begins to break down. If the damage is deep enough, it creates a passage that tunnels from one space to another. The body then lines this passage with cells, essentially making the tunnel semi-permanent rather than allowing it to heal shut. That lined channel is a fistula.
In Crohn’s disease, researchers have mapped this process in detail. Destructive inflammation creates a defect in the intestinal lining. Cells that normally stay on the surface undergo a transformation that allows them to burrow into deeper layers of the gut wall. This process appears to be the body’s attempt at wound repair gone wrong, since normal healing mechanisms are impaired in Crohn’s patients. The burrowing cells activate enzymes that break down surrounding tissue, creating further damage and inflammation. Inflammatory signaling molecules then amplify their own production, feeding a cycle that drives the tunnel deeper until it reaches another organ or the skin surface.
Infection and Abscesses
Infection is one of the most common triggers for fistula formation, particularly around the anus. Small glands inside the anal canal can become blocked and infected, forming an abscess, which is a walled-off pocket of pus. Between 30% and 70% of patients with an anorectal abscess already have a fistula at the time of diagnosis. Among those who don’t, roughly 30% to 50% will develop one in the months or years after the abscess is drained. The abscess essentially carves a path through the tissue as it expands and eventually drains, leaving behind a persistent tunnel.
Infections elsewhere in the body follow a similar pattern. A deep abdominal abscess from a ruptured appendix, an infected surgical wound, or a pocket of infection near the bladder or intestines can all erode through tissue walls and create fistulas between organs.
Crohn’s Disease and Inflammatory Bowel Disease
Crohn’s disease is uniquely prone to causing fistulas because the inflammation it produces extends through the full thickness of the intestinal wall, unlike ulcerative colitis, which stays in the surface lining. This deep, transmural inflammation creates fissures that penetrate through layer after layer of the gut wall. The fissure can tunnel into the bladder, vagina, skin near the anus, or another loop of intestine.
The biology is self-reinforcing. Inflammatory molecules like TNF drive the process forward, and bacteria in the gut appear to contribute by triggering additional cellular changes that promote tissue invasion. Because the underlying disease is chronic, Crohn’s-related fistulas are notoriously difficult to heal and often recur.
Obstructed Labor
Obstetric fistulas remain a major global health problem, with up to 100,000 new cases occurring worldwide each year. They develop during prolonged, obstructed labor when the baby’s head becomes impacted against the mother’s pelvic bones. The soft tissues trapped between the skull and bone lose their blood supply. Hours of sustained pressure cause widespread ischemic injury, meaning the tissue is starved of oxygen and dies. After delivery, the dead tissue sloughs away, leaving an opening between the vagina and the bladder (vesicovaginal fistula) or the vagina and the rectum (rectovaginal fistula).
These fistulas cause continuous leaking of urine or stool and are devastating but largely preventable with access to timely obstetric care, including cesarean delivery when labor is not progressing. They occur almost exclusively in regions with limited healthcare infrastructure.
Surgical Complications
Surgery itself can accidentally create fistulas. In an 18-year review of over 800 iatrogenic (surgery-caused) genitourinary fistulas, four out of five developed following procedures related to obstetric complications: cesarean sections, repairs of a ruptured uterus, or hysterectomy performed for a ruptured uterus. Cesarean sections alone accounted for 57% of cases. Hysterectomy was the next most frequent cause at about 39%, split roughly evenly between hysterectomies for obstetric emergencies and those done for other gynecological reasons.
Any abdominal or pelvic surgery carries some risk. During the procedure, nearby organs can be nicked, burned, or have their blood supply disrupted. If the injury isn’t recognized and repaired, the damaged tissue breaks down in the days or weeks after surgery, forming a connection between structures that should remain separate.
Diverticulitis
Diverticulitis, the inflammation or infection of small pouches that form in the colon wall, causes fistulas in 4% to 20% of cases. The most common type is a colovesical fistula, which connects the colon to the bladder. It forms when an inflamed or infected diverticulum erodes through the colon wall and adheres to the bladder, eventually breaking through into it. The hallmark symptom is air or stool particles in the urine. Repeated bouts of inflammation make the surrounding tissue chronically inflamed and hardened, which complicates surgical repair.
Radiation Therapy and Cancer
Radiation treatment for pelvic cancers, particularly cervical cancer, can damage the tiny blood vessels in surrounding tissues. This vascular injury impairs the body’s ability to heal, sometimes for months or years after treatment ends. When a tumor shrinks rapidly under radiation, the combination of tissue loss and poor wound healing can leave a gap that becomes a fistula. Patients who smoke face additional risk because smoking further damages blood vessels and compromises healing.
Cancer itself can also cause fistulas when a tumor grows large enough to invade through the wall of one organ and into an adjacent one. In some cases, it is difficult to determine whether a fistula resulted from the tumor or the radiation used to treat it, since both mechanisms damage tissue in the same area.
Arteriovenous Fistulas
Not all fistulas involve hollow organs. Arteriovenous fistulas (AVFs) are abnormal connections between an artery and a vein, bypassing the tiny capillaries that normally sit between them. They can be congenital, traumatic, or deliberately created.
About 90% of traumatic AVFs result from penetrating injuries, most often gunshot wounds. These injuries are common where an artery and vein run close together, particularly near long bones. AVFs can also occur as a complication of medical procedures, most frequently from needle puncture of the femoral artery and vein during cardiac catheterization. Placement of central venous lines has been linked to fistulas in the neck and chest.
Congenital AVFs are rare and not well understood. They can occur in the lungs, brain, liver, or heart and may not cause symptoms until adulthood. Some are associated with connective tissue disorders or genetic conditions. A low-flow fistula present at birth can gradually enlarge over decades and become a high-flow connection that strains the heart.
Surgeons also create AVFs intentionally in patients who need long-term kidney dialysis. These are made by connecting an artery to a vein in the arm, typically at the wrist or upper arm, to create a durable access point for the dialysis machine.
How Fistulas Are Diagnosed
Clinical examination can identify many fistulas, but imaging is often needed to determine how deep and complex the tunnel is. MRI is the preferred tool for evaluating perianal fistulas because it shows soft tissue detail well. It can reveal the fistula’s path, how much of the surrounding muscle is involved, and whether there are hidden branches or abscesses. Endoscopic ultrasound is another option but has lower specificity (about 43%) compared to MRI (about 69%).
Fistulas are classified as simple or complex based on their anatomy. Simple fistulas are superficial or low-lying, involve less than 30% of the sphincter muscle, and typically have a single opening. Complex fistulas involve more of the muscle, extend higher, branch into multiple tracts, or are associated with radiation injury or cancer. This classification directly guides treatment decisions and gives an indication of how likely the fistula is to heal.