A fistula is an abnormal connection between two body surfaces that are not supposed to be linked. More specifically, it is a passage between two epithelial-lined surfaces, such as two sections of intestine, the bladder and the vagina, or the inside of the gut and the skin. Fistulas can form almost anywhere in the body, from the ear to the anus, and their causes range from childbirth injuries and chronic inflammatory disease to surgery, radiation, and even deliberate medical creation. The word covers a surprisingly wide variety of conditions, and understanding what a fistula actually is means understanding the many forms it can take.
What Exactly Makes Something a Fistula
At its core, a fistula is a tunnel-like tract connecting two surfaces that should be separate. A gastrointestinal fistula, for example, can link two portions of the intestine, connect the intestine to another organ like the bladder, or run from the intestine to the skin surface of the abdomen.1SpringerLink / European Journal of Trauma and Emergency Surgery. The surgical anatomy and etiology of gastrointestinal fistulas That basic pattern holds across body regions: there is always an opening on each end and a channel in between. The channel can be short and direct or long and winding, sometimes branching into multiple tracts.
This distinguishes a fistula from a few related conditions people sometimes confuse it with. An abscess is a walled-off pocket of infection or pus, not a through-and-through connection. A sinus tract has only one opening and ends in a blind pouch. A fistula has two openings, or at minimum connects two distinct body spaces. That distinction matters for treatment: closing off a fistula means dealing with both ends and the tract between them, which is part of why they can be so stubborn to heal.
How Fistulas Form
Fistulas develop through several different mechanisms depending on where they occur and what triggers them. The most studied pathway involves chronic inflammation. In Crohn’s disease, persistent inflammation that extends through the full thickness of the bowel wall can eventually punch a hole through to another organ or to the skin surface.2PubMed Central. Pathogenesis of Fistulating Crohn’s Disease: A Review Research suggests that epithelial cells lining the gut undergo a transformation that allows them to burrow into deeper tissue layers, causing localized damage and eventually forming a tube-like structure that becomes the fistula tract.3PubMed Central. Pathophysiology of fistula formation in Crohn’s disease This same process activates enzymes that break down surrounding tissue, creating more damage and more inflammation in a self-reinforcing cycle.
Infection is another common trigger. When bacteria invade tissues and form an abscess, the pressurized pocket of pus may find its way out along the path of least resistance, boring a channel between two surfaces. Trauma, whether from surgery, radiation treatment, or childbirth injury, can destroy tissue and create gaps that the body cannot close on its own. In some cases, cancer itself erodes through tissue boundaries. And occasionally, fistulas are present at birth because of abnormal development in the womb.
What keeps a fistula from healing once it forms is almost as important as what causes it. Ongoing inflammation prevents the normal wound-healing cells from organizing and doing their repair work. In one study of surgically opened fistula tracts around the anus, full lining of the tract by epithelial cells was found in only about a quarter of cases, suggesting that persistent inflammation stalls the healing process rather than allowing it to complete.4PubMed Central. Identification of epithelialization in high transsphincteric fistulas Both fistulas and the scarring that sometimes accompanies them appear to result from the same underlying problem: wound healing gone wrong in the context of chronic intestinal inflammation.5PubMed Central. The two sides of the coin: Similarities and differences in the pathomechanisms of fistulas and stricture formations in irritable bowel disease
Anal Fistulas
The type most people encounter, or at least hear about, is the anal fistula. These are tracts that run from the inside of the anal canal to the skin near the anus. They almost always start with an infection. Tiny glands inside the anal canal, located at a landmark called the dentate line, can become infected and develop into an abscess.6PubMed. Treatment of anal fistula and abscess When the abscess drains, either on its own or after surgical intervention, a fistula tract remains in roughly 40% of cases.7PubMed Central. Anorectal infection: abscess-fistula The presence of gut bacteria inside the abscess is a key factor in the progression from abscess to fistula.8PubMed. Pathogenesis and persistence of cryptoglandular anal fistula: a systematic review
Anal fistulas are classified by how much of the anal sphincter muscle they pass through, because that determines how difficult they are to treat without risking incontinence. A simple tract that runs between the two rings of sphincter muscle is more straightforward to manage than one that loops over or through the outer sphincter. Crohn’s disease is another major cause of anal fistulas, and those tend to be more complex and harder to treat than the infection-related kind.
Obstetric Fistulas
Obstetric fistulas are among the most devastating forms, and they remain a significant problem in parts of the world where access to emergency obstetric care is limited. These fistulas occur during prolonged, obstructed labor when the baby’s head presses against the mother’s pelvic soft tissues for an extended period. The sustained pressure cuts off blood flow to the tissue between the vagina and the bladder or rectum, causing that tissue to die and slough away. What remains is a hole, a fistula, through which urine or stool leaks continuously through the vagina.9PubMed Central. Overcoming phase 1 delays: the critical component of obstetric fistula prevention programs in resource-poor countries
The injury is not limited to the fistula itself. The prolonged compression creates a widespread pattern of tissue death and damage sometimes called the obstructed labor injury complex.10PubMed. Obstructed labor injury complex: obstetric fistula formation and the multifaceted morbidity of maternal birth trauma in the developing world Women who develop obstetric fistulas often also have nerve damage, difficulty walking, kidney problems, and chronic infections. The social consequences compound the physical ones: constant leaking of urine or feces leads to isolation, stigma, and sometimes abandonment by family and community.11PubMed Central. Obstetric fistula awareness in The Gambia, Côte d’Ivoire, Madagascar and Sierra Leone: A cross-sectional analysis of Demographic and Health Surveys data
Because the root cause is a delay in delivery, prevention comes down to timely access to caesarean section or other emergency interventions. The time interval between when labor becomes obstructed and when delivery finally happens is the critical window. Obstetric fistulas are virtually unheard of in settings with functioning emergency obstetric care, which is why they are sometimes described as a disease of poverty and neglect rather than an inevitable medical complication.
Fistulas Caused by Surgery, Radiation, and Cancer
Medical treatment itself can sometimes cause fistulas. A vesicovaginal fistula, the connection between the bladder and the vagina, is a well-known complication of pelvic surgery, particularly hysterectomy. It can also develop as a delayed effect of radiation therapy for cancers of the cervix, uterus, or other pelvic organs. In one documented case, a vesicovaginal fistula and kidney damage appeared well after radiation treatment for advanced cervical cancer, illustrating how radiation-induced tissue injury can quietly progress for months or years before a fistula opens up.12PubMed Central. Vesicovaginal Fistula and Hydronephrosis Post-radiation for Cervical Cancer: Challenges and Strategies in Management
Cancer itself can cause fistulas by growing through tissue boundaries, and so can treatment directed at the cancer. In gynecologic malignancies, a fistula may be caused by the tumor directly, by surgery to remove it, or by radiation therapy aimed at it, and sometimes by a combination of these factors.13PubMed. Fistulas in malignant gynecologic disease: etiology, imaging, and management Gastrointestinal fistulas can also develop after abdominal surgeries. When bowel connections do not heal properly, or when infection develops at a surgical site, a fistula may form between the gut and the skin of the abdomen. These enterocutaneous fistulas can drain large volumes of intestinal fluid, creating serious nutritional and fluid-balance problems.
Congenital Fistulas
Some fistulas are present from birth. The most clinically significant congenital fistula is the tracheoesophageal fistula, an abnormal connection between the windpipe and the food pipe. During early fetal development, the trachea and esophagus start as a single tube that normally separates into two distinct structures around the third to fourth week of pregnancy. When that separation process goes wrong, the result can be an esophagus that ends in a blind pouch, a direct communication between the trachea and esophagus, or both.14Early Human Development. Management of neonates with oesophageal atresia and tracheoesophageal fistula The current understanding is that disruption to the molecular signals that guide the separation of these two tubes during development leads to the formation of the abnormal connection.15Chest. Long-term Complications of Congenital Esophageal Atresia and/or Tracheoesophageal Fistula
Babies born with a tracheoesophageal fistula typically need surgical repair shortly after birth. Without it, feeding is impossible or dangerous because milk and saliva can pass directly into the lungs. Even after successful repair, long-term complications like swallowing difficulties and respiratory problems can persist into adulthood.
Fistulas in Unlikely Places
Fistulas are not limited to the digestive and reproductive tracts. A perilymphatic fistula is an abnormal opening between the middle ear and the fluid-filled inner ear. This type can result from trauma, such as a penetrating ear injury, and causes hearing loss, vertigo, and balance problems.16PubMed Central. Perilymphatic Fistula After Penetrating Ear Trauma It can also develop after sudden pressure changes, like those from diving or straining. Perilymphatic fistulas are notoriously difficult to diagnose because the symptoms overlap with many other inner-ear conditions and imaging does not always reveal them.
Fistulas can also form between arteries and veins, between the brain’s fluid spaces and the nose or ear (cerebrospinal fluid fistulas), and in various other anatomical configurations. The common thread is always the same: a connection that should not be there, linking two spaces that are normally separate.
When Fistulas Are Created on Purpose
Not all fistulas are disease. One of the most important deliberately created fistulas is the arteriovenous fistula, or AVF, used for kidney dialysis. People who need regular hemodialysis require reliable, high-flow access to their bloodstream, and a standard vein cannot handle the repeated needle sticks and high flow rates that dialysis demands. By surgically connecting an artery to a vein, usually in the forearm, doctors create a fistula that causes the vein to enlarge and thicken over time. This “matured” fistula becomes durable enough for long-term dialysis access. In some patients whose veins are too deep to reach easily with a needle, the thickened vein can be repositioned closer to the skin surface.17PubMed. Superficialization of the wrist native arteriovenous fistula for effective hemodialysis vascular access construction Newer techniques even allow these fistulas to be created through a catheter rather than open surgery, with maturation times averaging around two months.18PubMed. Percutaneous creation of an arteriovenous fistula for hemodialysis access
In veterinary research, fistulas are sometimes surgically created in the rumen of cattle, sheep, and goats. Because the rumen’s complex microbial ecosystem cannot be replicated in a laboratory, researchers install a permanent, plugged opening in the animal’s side that allows direct access to the rumen contents for study. This technique has been used for decades to advance understanding of digestive physiology and nutrient processing in livestock.19PubMed Central. Ruminal Fistulation and Cannulation: A Necessary Procedure for the Advancement of Biotechnological Research in Ruminants
How Fistulas Are Found and Mapped
Diagnosing a fistula often starts with the symptoms: persistent drainage from an unexpected opening, recurrent infections in the same spot, or the passage of urine, stool, or air through an abnormal route. The challenge is usually not recognizing that a fistula exists, but mapping its exact path. Knowing which structures the tract runs through determines whether surgery is safe and what approach to use.
For anal fistulas, two main imaging tools compete. Endoanal ultrasound, where a small probe is inserted into the anal canal, is good at detecting the most common fistula types that run between or through the sphincter muscles. A meta-analysis of the available studies found that three-dimensional endoanal ultrasound had high sensitivity for detecting those common patterns. However, for fistulas that arch over the top of the sphincter complex, MRI was the stronger tool.20PubMed. Diagnostic Performance of Three-Dimensional Endoanal Ultrasound Compared with MRI in Anal Fistula: A Systematic Review and Meta-analysis Contrast-enhanced MRI performed particularly well across all fistula classifications, with both sensitivity and specificity above 90% for most types. In practice, many surgeons use both techniques, relying on ultrasound for straightforward cases and adding MRI when a complex or high tract is suspected.21PubMed. Comparison of MRI and Endoanal Ultrasound in Assessing Intersphincteric, Transsphincteric, and Suprasphincteric Perianal Fistula
For fistulas elsewhere in the body, CT scans with contrast, fluoroscopic studies where dye is injected through the fistula opening, and specialized MRI protocols are the standard tools. The choice depends on the body region and what information the surgeon needs before operating.
Treatment and Why Fistulas Are Hard to Fix
Simple fistulas sometimes heal on their own, particularly low-output gastrointestinal fistulas when the underlying cause (like a post-surgical infection) resolves. But many fistulas need active treatment, and the approach depends heavily on the type, location, and complexity of the tract.
For anal fistulas, the simplest operation is a fistulotomy: the surgeon cuts open the entire tract, turning the tunnel into an open groove that heals from the bottom up. This works well for superficial fistulas but becomes risky when the tract passes through a significant portion of the sphincter muscle, because cutting through that muscle can cause incontinence. For those more complex cases, sphincter-preserving techniques have been developed. Two of the most studied are the LIFT procedure, which closes the tract where it passes between the sphincter muscles, and the advancement flap, which covers the internal opening with a flap of tissue. A meta-analysis comparing these two approaches found similar healing and recurrence rates, but the LIFT procedure was associated with shorter operating time and less pain afterward.22PubMed. Endorectal advancement flap compared to ligation of inter-sphincteric fistula tract in the treatment of complex anal fistulas: A meta-analysis of randomized clinical trials
A common preliminary step for complex anal fistulas is the placement of a seton, a thread or rubber band passed through the fistula tract and left in place for weeks or months. This allows infection to drain, inflammation to settle, and the tract to mature before definitive repair. In one series of patients treated with setons before their final operation, healing rates for sphincter-crossing fistulas reached about 86% at a median follow-up of nearly three years, with a true recurrence rate under 4%.23PubMed Central. Revisiting an ancient treatment for transphincteric fistula-in-ano The seton concept itself dates back to ancient medicine, making it one of the oldest surgical techniques still in regular use.
Enterocutaneous fistulas, the kind that drain intestinal contents through the abdominal wall, pose different challenges. When these produce large volumes of output, the fluid and nutrient losses can be life-threatening. An interdisciplinary team approach is considered essential, and patients with high-output fistulas often need intravenous nutrition to maintain their fluid balance and nutritional status while waiting to see if the fistula will close on its own or require surgery.24PubMed. Nutrition Management of the High-Output Fistulae Intravenous nutrition has historically been the primary method of supporting these patients, though feeding through the gut is used when the anatomy allows it.25PubMed Central. Use of parenteral nutrition in the management of enterocutaneous fistula
Emerging Approaches and Stem Cell Therapies
Because fistulas, particularly those associated with Crohn’s disease, can resist repeated surgical attempts at closure, there is active research into biological therapies. The idea is to address the dysfunctional healing process at a cellular level rather than relying solely on mechanical closure. The exact cause of fistula formation in Crohn’s disease is still not fully understood, but the contributing factors appear to include immune system problems, shifts in gut bacteria, and the cellular transformation process that allows cells to burrow through tissue.26PubMed Central. Fistula in Crohn’s disease: classification, pathogenesis, and treatment options
Stem cell therapy has shown promise in early studies. One recent case report described a woman with a complex anal fistula that had failed multiple surgeries. After surgical preparation of the tract, she received local injections of tiny particles derived from stem cells. Within two weeks of the second treatment, the fistula had fully closed. Her continence and quality of life scores improved dramatically over the following six months.27PubMed Central. Complete Healing After Umbilical Mesenchymal Stem Cell-Derived Exosome Therapy For a Refractory Complex Anal Fistula A single case report is far from proof that this works broadly, but it represents a direction the field is moving toward: combining careful surgery with biological agents that promote the kind of tissue repair fistulas resist on their own. Larger trials are underway, and if the results hold, this approach could change the landscape for patients with stubborn, recurrent fistulas who have run out of conventional surgical options.