Fistulas are abnormal connections between two body surfaces that would not normally be connected, and they are far more common than most people realize. The term covers a wide range of conditions, from anal fistulas that follow a routine abscess to obstetric fistulas affecting millions of women in low-resource countries, to surgically created fistulas that keep dialysis patients alive. How common any given type is depends entirely on which fistula you are talking about, and the risk factors vary just as widely.
Anal and Perianal Fistulas
The type of fistula most people encounter in everyday medical practice is the anal fistula, an abnormal tunnel that forms between the inside of the anal canal and the skin near the anus. These almost always start as anal abscesses. When a small gland inside the anus becomes blocked and infected, pus collects and forms an abscess. The fistula represents what happens when that infection becomes chronic rather than healing cleanly after the abscess drains.1PubMed. Anorectal abscess and fistula
The numbers are consistent across studies: roughly one in three people who have an anal abscess drained will go on to develop a fistula. A multicenter retrospective study found that about 34% of patients developed an anal fistula after abscess drainage over a median follow-up of about three years.2PubMed Central. Fistula development after anal abscess drainage—a multicentre retrospective cohort study A study from Bahrain reported a nearly identical rate of about 34%, with most fistulas appearing within the first six months.3Annals of Coloproctology. Incidence of anal fistula after pyogenic perianal abscess drainage in Kingdom of Bahrain Men develop anal fistulas far more often than women, and the Bahrain data showed that over 86% of fistulas after abscess drainage occurred in male patients.3Annals of Coloproctology. Incidence of anal fistula after pyogenic perianal abscess drainage in Kingdom of Bahrain
Several factors raise the odds of a fistula forming after an abscess. Having Crohn’s disease increases the risk roughly sixfold. A history of recurrent abscesses quadruples it. The location and type of abscess matter too, with deeper abscesses in the intersphincteric and ischioanal spaces carrying about two to three times the risk compared to more superficial ones. Even lab markers play a role: finding E. coli in the pus culture or having a very high C-reactive protein level at the time of surgery both correlate with higher fistula rates.2PubMed Central. Fistula development after anal abscess drainage—a multicentre retrospective cohort study Socioeconomic status has also been linked to fistula development, likely because access to timely care and wound management varies across income levels.3Annals of Coloproctology. Incidence of anal fistula after pyogenic perianal abscess drainage in Kingdom of Bahrain
One question patients often ask is whether antibiotics after abscess drainage can prevent a fistula from forming. A randomized trial tested this directly and found no significant difference: about 16% of patients given antibiotics developed a fistula compared with about 10% in the group that received no antibiotics. The difference was not statistically meaningful, suggesting that once the abscess has been drained, antibiotics do not change the trajectory toward fistula formation.4PubMed. Antibiotic Treatment Has No Influence on Anal Fistula Formation and Recurrent Perianal Abscess After Incision and Drainage of Cryptogenic Perianal Abscess
Crohn’s Disease as a Major Driver
Among all the conditions that predispose someone to fistulas, Crohn’s disease stands apart. About 40% of Crohn’s patients develop fistulas at some point during their disease, and even with modern biologic treatments, roughly a third of those experience recurrence.5PubMed Central. Pathogenesis of Fistulating Crohn’s Disease: A Review Fistulas in Crohn’s disease can form between the bowel and almost any adjacent structure: the skin, bladder, vagina, or other loops of intestine.
A U.S. population-based study estimated the prevalence of fistulizing Crohn’s disease at about 25 per 100,000 people, with an incidence of roughly 7 new cases per 100,000 person-years between 2017 and 2019.6PubMed Central. Prevalence, incidence, and treatment patterns of fistulizing Crohn disease: A US population-based cohort study That may sound modest, but among the subset of people who already have Crohn’s disease, fistula is a defining complication. The chronic inflammation in the bowel wall creates tunnels that burrow outward, and unlike a fistula born from a simple abscess, Crohn’s-related fistulas tend to be more complex, with multiple tracts and higher rates of treatment failure. This is why Crohn’s disease shows up as such a strong risk factor in abscess-drainage studies: the underlying disease keeps driving inflammation even after a surgical procedure addresses the immediate problem.
Obstetric Fistulas
Obstetric fistulas occupy a completely different medical landscape. These form when prolonged, obstructed labor compresses the soft tissue between the baby’s head and the mother’s pelvic bones so severely that the tissue dies, leaving a hole between the vagina and the bladder (vesicovaginal fistula) or the vagina and the rectum (rectovaginal fistula). The result is continuous urinary or fecal leakage, with devastating social consequences.
Several million cases are thought to exist across sub-Saharan Africa and South Asia.7PubMed. Preventing obstetric fistulas in low-resource countries: insights from a Haddon matrix A systematic review found that incidence estimates ranged widely, from essentially zero to about 4 cases per 1,000 deliveries, depending on the region and data quality. Prevalence estimates were even more variable and harder to pin down, ranging from zero to 81 per 1,000 women, though those higher numbers were judged more prone to bias.8PubMed Central. Obstetric fistula in low-resource countries: an under-valued and under-studied problem – systematic review of its incidence, prevalence, and association with stillbirth
The risk factors here are deeply tied to poverty and geography. Women from rural areas, mothers delivering at very young or very old ages, those with less education, and patients whose labor lasted more than twelve hours are all at higher risk. A study from Malawi found that women with insufficient healthcare access were about two and a half times more likely to develop an obstetric vesicovaginal fistula than those with adequate access.9PubMed Central. Healthcare Access as a Risk-Marker for Obstetric Vesicovaginal Fistula in Malawi In wealthy countries with routine access to cesarean sections, obstetric fistulas are vanishingly rare. The condition persists almost entirely because of gaps in emergency obstetric care.
Fistulas After Surgery
In high-income countries, the most common cause of vesicovaginal fistula is not childbirth but surgery, particularly hysterectomy. A large English NHS study covering over 340,000 hysterectomies found an overall fistula rate of about 1 in 788. The risk varied enormously depending on the type of surgery and why it was done. Radical hysterectomy for cervical cancer carried the highest risk, about 1 in 87, while vaginal hysterectomy for pelvic organ prolapse had the lowest, at roughly 1 in 3,861. Younger women undergoing total abdominal hysterectomy for conditions like endometriosis or fibroids faced higher odds than women over 50 having the same procedure.10PubMed. The risk of vesicovaginal and urethrovaginal fistula after hysterectomy performed in the English National Health Service
Beyond gynecologic surgery, gastrointestinal operations can also produce fistulas. Bowel anastomosis leaks, cancer resections, and operations involving inflamed tissue all carry some risk of abnormal connections forming during healing. Diverticular disease, for instance, is the most frequent cause of colovesical fistulas, which are abnormal passages between the colon and bladder. These are uncommon complications of diverticulitis, but when they occur they produce distinctive symptoms like passing gas or stool through the urine.11PubMed Central. Colovesical Fistula due to Sigmoid Diverticulitis
Radiation and Cancer Treatment
Pelvic radiation, used to treat cancers of the cervix, prostate, rectum, and bladder, can damage surrounding tissues enough to create fistulas weeks, months, or even years after treatment ends. The risk level depends heavily on the type of cancer and the radiation dose delivered to nearby organs.
For prostate cancer survivors, the risk is quite low. A meta-analysis of over 7,600 patients who received pelvic radiotherapy found a pooled fistula prevalence of just 0.2%.12Urology. Prevalence of Radiation-Induced Pelvic Fistulas in Prostate Cancer Survivors: A Systematic Review and Meta-Analysis Cervical cancer patients face higher rates. A study of 150 women treated with chemoradiation and brachytherapy found that about 9% developed a fistula. On detailed analysis, smoking was the strongest independent risk factor, increasing the odds roughly fivefold. Disease extending into the bladder and lack of MRI-guided brachytherapy also contributed.13PubMed Central. Risk factors and clinical outcomes of radiation-induced fistula after chemoradiation and image-guided brachytherapy for locally advanced cervical cancer
Certain medications amplify the risk. Bevacizumab, a drug used alongside radiation for some cancers, substantially increases fistula formation. A study of cervical cancer patients found a three-year fistula rate of 27% when bevacizumab was combined with radiotherapy, compared to just 3% with radiotherapy alone. Waiting at least ten months between radiation and bevacizumab appeared to reduce the risk.14International Journal of Gynecological Cancer. Use of bevacizumab before or after radiotherapy increases the risk of fistula formation in patients with cervical cancer That is a striking difference that oncologists now factor into treatment sequencing.
Dialysis Access Fistulas
Not all fistulas are unwanted. In kidney medicine, an arteriovenous fistula (AVF) is a surgically created connection between an artery and a vein, usually in the arm, designed to provide reliable access for hemodialysis. These are intentional fistulas, considered the gold standard for long-term dialysis access because they tend to last longer and cause fewer infections than other options. But creating a functional AVF is not always straightforward.
Early failure, meaning the fistula never matures enough to be usable, is a significant problem. A meta-analysis identified several predictors of early failure: the fistula being placed in a distal location (like the wrist rather than the upper arm), small artery and vein diameters, low albumin levels, female sex, diabetes, and low blood pressure.15PubMed Central. Predictors of early arteriovenous fistula failure in patients with end stage renal disease on hemodialysis: a systematic review and meta-analysis One large study reported an immediate failure rate of about 12%, and even among fistulas that initially worked, only about 63% were still functioning at one year.16PubMed Central. Outcomes and predictors of failure of arteriovenous fistulae for hemodialysis
Longer-term dysfunction is also common. A retrospective study found that the two-year survival rate for dialysis fistulas was about 81%. Sex, body weight, and blood phosphorus levels all independently predicted dysfunction, with male patients and leaner patients faring better.17PubMed Central. Risk factors for arteriovenous fistula dysfunction in hemodialysis patients: a retrospective study For dialysis patients, keeping a fistula working is an ongoing medical concern, not a one-time surgical event.
Unintentional arteriovenous fistulas can also form as a complication of catheter procedures. In one large review covering over 23,000 cardiac catheterizations, the rate of accidental fistula formation was just 0.017%, making it rare but not unheard of.18JAMA Surgery. Arteriovenous Fistulas After Cardiac Catheterization
Congenital Fistulas
Some fistulas are present at birth. The most well-known congenital fistula is a tracheoesophageal fistula (TEF), an abnormal connection between the windpipe and the esophagus. It almost always occurs alongside esophageal atresia, where the esophagus fails to form a continuous passage to the stomach. The incidence is roughly 1 in 3,000 to 5,000 live births.19PubMed Central. Esophageal atresia with tracheoesophageal fistula: two case reports A population-based study placed the rate of esophageal atresia with or without TEF at about 0.27 per 1,000 births.20PubMed. The co-occurrence of congenital diaphragmatic hernia, esophageal atresia/tracheoesophageal fistula, and lung hypoplasia
These require surgical repair in the first days of life. Without treatment, a baby with TEF cannot feed safely because milk can enter the lungs through the abnormal connection. The causes are not well understood in most cases, though congenital fistulas sometimes occur alongside other birth defects involving the heart, kidneys, or limbs, suggesting disrupted embryonic development during a critical window.
Oral and Dental Fistulas
The mouth has its own category of fistulas. An oroantral fistula (OAF) is an abnormal opening between the oral cavity and the maxillary sinus, the air-filled space behind the cheekbone. The most common cause is the extraction of upper molar or premolar teeth, because the bone separating the tooth roots from the sinus floor can be paper-thin.21PubMed Central. Oro-Antral Fistulas and their Management: Our Experience Conditions that make tooth extraction more difficult, such as long roots that project into the sinus, dental infections, or improper use of extraction instruments, increase the risk.22Journal of Dentomaxillofacial Radiology, and Surgery. Review Paper: The Prevalence, Etiology & Management of Oroantral Fistula
A ten-year retrospective study from Nigeria found oroantral perforations occurred in just 0.2% of all dental extractions. The strongest predictor was tooth location, with the upper first molar carrying about twice the odds. Age, sex, and even the surgeon’s experience level were not significant predictors in that dataset.23Iraqi Dental Journal. Incidence and risk factors of oroantral perforation following tooth extraction among Nigerian population in a tertiary hospital Small perforations typically heal on their own. Larger ones, or those persisting beyond about three weeks, generally need surgical closure.
Rare but Dangerous Fistulas
Some fistulas are rare enough to qualify as medical emergencies when they appear. An aortoenteric fistula, an abnormal connection between the aorta and the intestine, is one of the most feared. Primary aortoenteric fistulas, meaning those arising without prior surgery, have an estimated incidence of just 0.007 per million people per year, typically caused by atherosclerotic aortic aneurysms.24PubMed Central. Aortoenteric fistula secondary to an Inflammatory Abdominal Aortic Aneurysm Secondary aortoenteric fistulas, forming after aortic graft surgery or endovascular repair, are more common but still unusual, occurring in roughly 0.25% to 3.6% of patients who have had those procedures.25Journal of Vascular Surgery Cases, Innovations and Techniques. Aortoenteric fistula following endovascular abdominal aortic aneurysm repair These fistulas can cause massive gastrointestinal bleeding and are often fatal without emergency surgery.
Chylothorax, while not labeled a “fistula” in everyday clinical language, is functionally a lymphatic fistula: damage to the thoracic duct or its branches causes chylous fluid to leak into the chest cavity. This occurs in roughly 1.8% of cardiothoracic surgeries, making it an uncommon but recognized postoperative complication.26PubMed Central. Outcomes of Chylothorax Nonoperative Management After Cardiothoracic Surgery: A Systematic Review and Meta-Analysis
Hidradenitis Suppurativa and Fistulas
One often-overlooked connection is between hidradenitis suppurativa (HS), a chronic skin condition involving painful recurring abscesses in areas like the groin and buttocks, and perianal fistulas. HS that affects the perianal region can produce fistula tracts that burrow through tissue around the anus, and one study reported that 64% of patients with perianal hidradenitis suppurativa had anal fistula involvement.27International Journal of Surgery Case Reports. Efficacy of magnetic resonance imaging in the diagnosis of perianal hidradenitis suppurativa, complicated by anal fistulae This matters because HS-related fistulas are sometimes misdiagnosed as ordinary anal fistulas, leading to incomplete surgery and high recurrence rates. Deep-seated remnant fistula tracts that are not fully removed during surgery tend to regrow, making thorough imaging before any procedure essential in these patients.
Why Fistulas Come Back
Recurrence is one of the most frustrating aspects of fistula treatment, and it happens more often than many patients expect. A retrospective study of anal fistula repairs found an overall recurrence rate of about 12.5%, but that average conceals wide variation. Simple fistulas recurred in about 6% of cases, while complex fistulas recurred in over 25%. Procedures designed to spare the anal sphincter, like the LIFT technique or biologic plugs, had recurrence rates of 50% or higher.28PubMed Central. Predictors of recurrence and long-term patient reported outcomes following surgical repair of anal fistula, a retrospective analysis
The risk factors for recurrence fall into predictable categories. The anatomy of the fistula itself is the biggest factor: more complex tracts with multiple branches are harder to eliminate. Comorbidities like diabetes and a history of prior abscesses also increase recurrence risk. Inadequate preoperative imaging, failure to identify the internal opening of the fistula, and surgeon inexperience all contribute to incomplete treatment.29PubMed Central. Factors Increasing the Risk of Recurrence in Fistula-in-ano Even postoperative care matters: wound neglect in the weeks after surgery can allow a tract to re-form before the tissue has fully healed.
For Crohn’s patients, the recurrence picture is particularly grim. As noted earlier, about a third of Crohn’s patients who develop fistulas will see them return even with biologic therapy.5PubMed Central. Pathogenesis of Fistulating Crohn’s Disease: A Review The underlying inflammation that drives the disease means that even a technically perfect surgical repair is fighting against a body that keeps producing the conditions for new fistulas to form. Managing fistulas in Crohn’s disease usually requires a combination of medical therapy to suppress inflammation and carefully timed surgery, rather than surgery alone.
Why Some Fistulas Heal and Others Persist
One of the open questions in fistula biology is why some heal spontaneously while others become permanent tracts. Part of the answer involves epithelialization, the process by which the lining of the fistula tract gets replaced by epithelial cells, essentially making the tunnel a permanent structure. A study examining high transsphincteric fistulas found epithelialization of the tract in only about 22% to 25% of cases, suggesting that in most patients the tract remains a raw wound rather than a lined channel.30PubMed Central. Identification of epithelialization in high transsphincteric fistulas Ongoing inflammation appears to prevent the migration of the cells needed for tissue repair. When inflammation is controlled, the body has a better chance of closing the connection. When it persists, the tract remains open, collecting debris and infection, which in turn perpetuates the inflammation in a self-reinforcing cycle. This explains why anti-inflammatory treatment and infection control are so central to fistula management across nearly every organ system.