A fistula (a term derived from the Latin word for pipe) is an abnormal connection between 2 epithelialized surfaces that usually involves the gut and another hollow organ, such as the bladder, urethra, vagina, or other regions of the gastrointestinal (GI) tract. Fistulas may also form between the gut and the skin or between the gut and an abscess cavity. Rarely, fistulas arise between a vessel and the gut, resulting in profound GI bleeding, which is a surgical emergency. (See Etiology.)
Most GI fistulas (75-85%) occur as a complication of abdominal surgery. However, 15-25% of fistulas evolve spontaneously and are usually the result of intra-abdominal inflammation or infection. Regardless of their cause, fistulas have a tremendous impact on patients and society. Increased morbidity and mortality rates, greater health care costs for diagnosis and treatment, prolonged hospital stays, and delayed return to work are just a few direct consequences of this condition. (See Etiology and Prognosis.)
Fistulas were formerly associated with considerable mortality rates. In the decades following the 1960s, however, the introduction of intensive care units (ICUs) and parenteral nutrition lowered the mortality rate to approximately 20%; however, prolonged hospital stays and the high cost of medical and surgical care remained unchanged. In addition, the frequency of fistula formation has not decreased, because of advanced and complicated disease, complex surgical techniques, and an aging population. 
Several classification systems for fistulas exist, none of which are used exclusively. The 3 most commonly used classification systems are based on anatomic, physiologic (output volume), and etiologic characteristics.  Used in combination, these classifications can help to provide an integrated understanding and optimal management scheme for the fistula. (See Etiology, Treatment, and Medication.)
Anatomically, the fistulas are named according to their participating anatomic components, and they can be divided into internal and external fistulas. Internal fistulas connect the GI tract with another internal organ, the peritoneal space, the retroperitoneal space, the thorax, or a blood vessel. External fistulas, which commonly occur postoperatively, are abnormal connections between the GI tract and the skin. (See Presentation.)
Risk factors for intestinal fistulas include the following:
Surgical procedures to treat cancer, inflammatory bowel disease (IBD), lysis of adhesions, or peptic ulcer disease
IBD - Such as Crohn disease and ulcerative colitis
Malignancy - Especially gynecologic and pancreatic
Perforation of duodenal ulcers
Abdominal trauma - Such as gunshot wounds, stabbing (sharp trauma), or motor vehicle accident (blunt trauma)
Aortic aneurysm, infected aortic graft, or previous abdominal aortic surgery
Contrary to common belief, fistulas do not necessarily develop as a consequence of downstream stenosis of the intestine. (See the image below.)
Gastric fistulas are iatrogenic in most cases (85%). The other cases are usually a consequence of irradiation, malignancy, inflammation, and ischemia. Anastomotic leak after a gastric resection for cancer, peptic ulcer disease, or bariatric surgery can lead to leakage of intestinal or gastric juices, which initiates a cascade of events: localized infection, abscess formation, and, possibly, abscess and fistula formation.
Small bowel fistulas
Nearly 80% of small bowel fistulas result from complications of abdominal surgery. These fistulas may occur from disruption of the anastomotic suture line, inadvertent iatrogenic enterotomy, or small bowel injury at the time of closure. Inadequate blood flow from devascularization or tension at the anastomotic suture lines, anastomosis of diseased bowel, or perianastomotic abscess may compromise the integrity of surgical anastomoses.
Fistulas in Crohn disease
Crohn disease, malignancy, peptic ulcer disease, and pancreatitis spontaneously cause 10-15% of small bowel fistulas. In patients with Crohn disease, fistulas arise from aphthous ulcers that progress to deep transmural fissures and inflammation, subsequently leading to adherence of the bowel to adjacent structures that eventually penetrate other structures.  Microperforation with abscess formation leads to subsequent macroperforation into the adjacent organ or skin, resulting in fistula formation.
Crohn fistulas are more often internal and less commonly external (to the skin). Ileosigmoid fistulas, usually a complication of a diseased terminal ileum that invades the sigmoid colon, are the most common type of fistula between two loops of bowel. Enteroenteric, gastrocolic, duodenocolic, enterovesical, rectovaginal, and perianal fistulas are other potential complications of Crohn disease.  Perianal fistulas are the most common external fistulas in patients with Crohn disease. (See the image below.)
Colonic fistulas are primarily a consequence of intra-abdominal inflammation but can also occur after surgical intervention for an inflammatory condition.  IBD, diverticulitis, malignancy, and appendicitis (especially with the presence of an appendiceal abscess requiring percutaneous drainage) are the most common inflammatory conditions that lead to colonic fistulas.
Aortoenteric fistulas most commonly occur secondarily, usually after the surgical placement of a graft. Aortoenteric fistulas can develop in the following ways:
A suture line, most commonly the proximal one, can communicate with the intestinal tract
A suture line pseudoaneurysm can erode into adjacent bowel
Erosions can occur in the graft close to the suture line, resulting in the midportion of the graft eroding into adjacent bowel; conversely, primary aortoenteric fistulas almost always result from erosion of the aneurysmal or infected aorta into surrounding areas, most commonly the bowel
Occurrence in the United States
In developed countries, Crohn disease is the most common cause of spontaneous fistula formation. In their lifetime, as many as 40% of patients with Crohn disease develop a fistula, most often an external or a perianal one.
The incidence of fistula formation in patients with diverticulitis is much lower. Fistula formation complicates diverticulitis in 1-12% of patients. Colovesical fistulas in men and colovaginal fistulas in women are the most common types of fistulas in this population.
Fistulas can complicate radiation therapy weeks to years after treatment. Radiation therapy for malignancy is associated with fistula formation in approximately 5-10% of patients.  Notably, surgery and anastomosis in previously irradiated tissue increases the risk of anastomotic leak and, subsequently, fistula formation.
Internationally, the frequency of various types of fistulas may vary in correlation with their prevalence in different populations.
For example, the prevalence of fistulas secondary to Crohn disease may be less prevalent in Africa primarily because the disease is less prevalent in that population. However, the prevalence of obstetric fistulas may be higher in developing countries because of obstructed labor (including malpresentation and cephalopelvic disproportion) and lack of prompt access to emergency obstetric care. Accurate prevalence rates of obstetric fistulas are unavailable, likely because of inaccurate reporting of the medical condition and the stigma of its associated symptoms.
Race-, sex-, and age-related demographics
Racial differences in patients with fistulas generally parallel those of the underlying disease or condition that predisposed persons in a specific racial population to developing fistulas. For example, since Crohn disease is more common in whites, patients with Crohn disease who develop fistulas are more likely to be white.
With regard to sex-related prevalences, colovesical fistulas are more common in men and in women who have undergone a hysterectomy. Colovaginal fistulas, of course, occur only in women. Otherwise, fistulas are equally prevalent in males and females.
As with race, age parallels the etiology or underlying condition that predisposes patients to develop fistulas.
The prognosis is based on the etiology of the fistula, as well as the comorbidities of the patient. Pain, wound management, abscess formation, local infection, nutritional deficiencies, and recurrent septic states are just a few of the physical consequences of intestinal fistulas.
Patients with fistulas most likely present with much more than physical discomfort and pain. The stigmas of malodorous fistula drainage, malnutrition, and emotional distress also cause significant psychological consequences. In addition, patients with postoperative fistulas have the added distress of lengthy hospital stays, associated morbidity, a delay in returning to work, and restricted social activities. Considerable mortality is associated with fistulas, primarily from sepsis.
In one study, in patients who developed fistulas after pancreaticoduodenectomy, specific factors were associated with increased mortality. These factors included fistula site, underlying disease, low hospital volume, the surgeon's experience, high intraoperative blood loss, and complications. 
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