A colonoscopy can cause a fistula, but it is exceptionally rare. The pathway is indirect: the colonoscope would first need to perforate the colon wall, and that perforation would then need to heal abnormally, forming a tunnel between the colon and a neighboring organ. Perforation itself occurs in only 0.019% to 0.66% of colonoscopies, and only a small fraction of those perforations ever develop into fistulas.
How a Colonoscopy Could Lead to a Fistula
A fistula is an abnormal tunnel that forms between two body cavities or organs. In the context of colonoscopy, the concern starts with perforation, a full-thickness hole through the colon wall. This can happen from direct pressure of the scope, from removing polyps, or from inflating the colon with air to improve visibility.
Most perforations are caught and treated quickly, either during the procedure or within hours. But if a perforation goes unrecognized, or if the tissue heals incompletely, the body can form a channel lined with tissue that connects the colon to a nearby structure. The colon sits close to the bladder, the vagina (in women), the small intestine, and even the pancreas, so a fistula can theoretically develop between the colon and any of these organs. One published case documented a fistula forming between the colon and the pancreatic tail after a routine screening colonoscopy, discovered three months later on a CT scan after the patient experienced recurring left-sided abdominal pain.
The key factor is that the lining of the abnormal tunnel becomes “epithelialized,” meaning the body treats it as a permanent passage rather than a wound that needs to close. Once that happens, continuous exposure to digestive secretions and the resulting inflammation make it very difficult for the connection to seal on its own.
Who Is at Higher Risk
The vast majority of people who undergo colonoscopy have no complications at all. But certain conditions make both perforation and fistula formation more likely.
Crohn’s disease is the most significant risk factor. Patients diagnosed at a younger age, those with disease affecting the colon (as opposed to only the small intestine), and male patients all face higher odds of developing fistulas. Smoking at the time of diagnosis is also associated with perianal fistulas specifically. In Crohn’s disease, the intestinal wall is already weakened by chronic inflammation, so even minor instrumentation can cause injury that heals abnormally.
Diverticulitis is another important risk factor. Diverticula are small pouches in the colon wall that can become inflamed and thin. A scope passing through a heavily affected segment can more easily cause a tear. Diverticulitis is also the most common cause of colovesical fistulas (colon-to-bladder connections) outside of colonoscopy, so patients with active or recent diverticulitis already have vulnerable tissue.
Previous pelvic surgery, radiation therapy to the pelvis, and active inflammation of any kind in the colon all weaken the tissue and raise the baseline risk of both perforation and poor healing afterward.
Symptoms to Watch For
A fistula does not announce itself the same way a perforation does. Perforation typically causes sudden, severe abdominal pain within hours of the procedure. A fistula, by contrast, develops over days to weeks as the abnormal channel matures, and symptoms depend entirely on which organ the colon has connected to.
A colovesical fistula (colon to bladder) produces some of the most distinctive symptoms. Pneumaturia, or air bubbles in the urine, is the most common sign, occurring in up to 70% of cases. Some patients also notice fecal matter in the urine, a symptom called fecaluria. Recurrent urinary tract infections that don’t respond well to antibiotics can also signal a fistula, because bacteria from the colon continuously seed the bladder.
A colovaginal fistula (colon to vagina) often presents as foul-smelling vaginal discharge, passage of gas or stool through the vagina, and recurrent vaginal infections. A fistula to the small intestine may cause diarrhea, malabsorption, or worsening abdominal pain. In the case of the colopancreatic fistula mentioned earlier, the patient’s symptoms were initially mistaken for enteritis or diverticulitis on multiple occasions before imaging revealed the true cause three months after the colonoscopy.
The important pattern is persistent or worsening symptoms in the weeks following a colonoscopy, particularly symptoms that don’t match any expected recovery timeline.
How a Fistula Is Diagnosed
No single test reliably catches every fistula. Most cases require a combination of imaging studies. CT scans and MRI both offer the advantage of identifying the underlying cause, the shape of the fistula tract, and its exact anatomical location. MRI is particularly useful for fistulas in the pelvis because it provides better soft tissue contrast.
Contrast studies, where dye is introduced into the colon or the suspected connected organ, can sometimes reveal the abnormal passage directly. In some cases, surgical exploration remains the definitive way to confirm a fistula. The diagnostic process often takes time because symptoms can be vague and overlap with more common post-procedure complaints.
Treatment Options
How a fistula is treated depends on its size, location, and the patient’s overall condition. Small fistulas in stable patients sometimes respond to conservative management: bowel rest (no eating for a period), intravenous fluids, and antibiotics to control infection. This approach buys time for the body to attempt healing on its own.
Endoscopic closure is an option when the defect is accessible and the colon is clean enough for the procedure. Small openings (under 1 cm) can be sealed with clips placed through the scope. Larger defects may require specialized over-the-scope clips or endoscopic suturing devices. These procedures work best when performed within hours of discovering the problem, though they can sometimes be used on established fistulas as well.
Surgery remains necessary for larger or more complex fistulas, for patients who are unstable, and for cases where endoscopic repair has failed. The specific surgical approach varies. A colovesical fistula, for example, typically requires removing the affected segment of colon and repairing the bladder. The colopancreatic fistula case resolved after endoscopic treatment of the tract, confirmed by follow-up imaging. Recovery from surgical repair generally takes several weeks, and some patients need a temporary colostomy bag while the repaired tissue heals.
Putting the Risk in Perspective
Colonoscopy is one of the most commonly performed procedures in medicine, and for good reason. It remains the most effective tool for detecting and preventing colorectal cancer. The perforation rate of 0.019% to 0.66% means that for every 1,000 colonoscopies performed, somewhere between zero and seven result in a perforation. The fraction of those that progress to a fistula is smaller still, making this complication genuinely rare.
That said, perforation is serious when it does happen. A comprehensive review found that when iatrogenic colonic perforation occurs, it carries a mortality rate of about 25% and a morbidity rate of about 40%, largely because leaking bowel contents can cause severe abdominal infection. These numbers reflect the worst-case outcomes of unmanaged or late-detected perforations, not the typical experience. Most perforations are caught early and treated successfully.
If you have Crohn’s disease, active diverticulitis, or a history of pelvic surgery, discussing your individual risk profile with your gastroenterologist before the procedure is reasonable. For most people, the cancer-prevention benefit of colonoscopy far outweighs the small chance of complications like perforation or fistula.