A fistula is an abnormal connection between two epithelized surfaces. Fistulas can form between any two hollow spaces including blood vessels, intestine, vagina, bladder, and skin. There are three different categories used to define a fistula, anatomic, physiologic, and etiologic. Anatomically, fistulas are subdivided into two categories, internal and external. Internal fistulas are connections between two internal structures. A few examples of an internal fistula would be enterocolic, ileosigmoid, and aortoenteric. Alternatively, external fistulas form connections between an internal structure and external structure. Examples of this would be enterocutaneous, enteroatmospheric, and rectovaginal fistulas. When categorized physiologically, the fistula is differentiated based on fluid output. Low-output fistulas drain less than 200 ml of fluid per day, high-output fistulas drain greater than 500 ml of fluid per day, and medium-output fistulas fall between the two. Etiology is the last way in which fistulas are categorized. Common etiologic categories are traumatic fistulas, surgical site fistulas, and fistulas associated with Crohn's disease. This article will specifically cover fistulas that fall under the anatomical category of enterocutaneous fistulas.
It is estimated that 80% of enterocutaneous fistulas are of iatrogenic origin secondary to surgery. Surgical complications, such as enterotomies or intestinal anastomotic dehiscence, are known to be at high risk for the development of an enterocutaneous fistula. Trauma, malignancy, and inflammatory bowel disease increase risk of fistula development postoperatively. The 20% of fistulas not associated with surgery are caused by systemic diseases such as Crohn’s disease, radiation enteritis, malignancies, trauma, or ischemia.
Enterocutaneous fistula mortality rates vary from 6% to 33%. Incidence is dependent on etiology. Infected pancreatic necrosis has an extremely high incidence of 50%. Trauma patients have a 2% to 25% incidence, and abdominal sepsis has a 20% to 25% incidence.
The pathophysiology of an enterocutaneous fistula is simple since it is nothing more than an aberrant connection between intestine and skin. Anything that causes a potential communication between the intestine and the epidermis can lead to the development of an enterocutaneous fistula. A constant stream of fluid traveling through this connection will keep the tract patent, and it will provide time for epithelial tissue to migrate into and cover the inner surface of the tract. Epithelialization of the tract will further stabilize the patency of the fistula. These factors contribute to the reasons why short, wide, high output fistulas are more prone to stabilize than long, narrow, low-output fistulas.
As previously mentioned, the most common cause of an enterocutaneous fistula is iatrogenic and occurs in the postoperative period. A history of trauma, inflammatory bowel disease, and oncologic surgery places patients at a high risk of developing a fistula.
The following scenario is an example of the events leading up to the development of an enterocutaneous fistula. A patient with a postoperative fever, leukocytosis, ileus, and abdominal tenderness is found to have a wound infection. The next step in treating this patient is to drain the abscess. However, one or two days after draining the abscess, enteric contents are observed in the wound. Finding enteric contents that are continually leaking into the wound establishes a diagnosis of an enterocutaneous fistula.
A helpful, commonly used acronym for remembering the factors that make fistula formation favorable and unlikely to spontaneously regress is “FRIEND.” The acronym is remembered easily with the mnemonic “the friends of the fistula.”
After stabilizing the patient, the next step is to evaluate the fistula. Ultrasound, CT scan, and fistulography are three imaging modalities that can be used to help characterize a fistula. Small bowel follow-through and endoscopy studies may also be helpful. Imaging is important for determining whether or not all of the fluid traveling through the fistula is coming out of the external opening. In some cases, fluid can be partially leaking into the abdomen and can then lead to the formation of an abscess. CT scan with oral contrast is considered the single best radiologic test since it can identify the tract, abdominal leaking, intra-abdominal abscesses, distal obstruction, and foreign bodies. Fistulography is used less often but can be useful when CT or ultrasound is unavailable or inconclusive. It is performed by injecting contrast into the external opening of the fistula and taking plain film radiographs of the area.
The first step in patient management is stabilization. Patients are at a high risk for electrolyte imbalances, sepsis, and malnutrition. Controlling all three of these factors is essential for survival. Electrolyte abnormalities and fluid balance need to be monitored closely because these patients can develop severe derangements quickly. Electrolyte losses vary depending on the location of the fistula in the gastrointestinal (GI) tract and the amount of output. Any deficiencies need to be replaced. In septic patients, a source needs to be identified and appropriately treated. Sepsis is documented as being responsible for two-thirds of mortality in these patients. Intra-abdominal abscesses are common and should be high on the differential as the source of sepsis. The Surviving Sepsis Campaign guidelines should be followed when treating these patients. Most patients will need parenteral nutrition, but a subset of patients may be able to tolerate an enteral elemental diet if the fistula is distal in the GI tract and the output from the fistula is not increased by starting feeds. Either way, adequate nutrition is a well-established, essential component to treat these patients properly. Another important variable to stabilize is the output from the fistula. The fluid needs to be properly contained as not to damage the surrounding skin and to increase odds of healing. Various methods of wound care can aid in preventing skin loss, minimizing pain, and allowing the patient to function on a daily basis. Such strategies are typically similar to ostomy bag appliances, but some will require a more customized plan for containing the fistula output.
A decision then needs to be made on how to treat the fistula itself. There are some cases in which immediate surgical correction may be appropriate, but the majority of fistulas are treated non-operatively. This is because 90% of fistulas close on their own within 5 weeks of medical management. Depending on the surgeon, 2 to 3 months of will be attempted before the surgical correction of a fistula is considered. This waiting period gives the fistula an appropriate amount of time to close spontaneously. It also decreases the morbidity and mortality of surgical correction. When initiating medical management, the factors mentioned in the previous section that promotes fistula development should be evaluated. All modifiable variables should be corrected to increased chances of spontaneous closure. Low-output fistulas are more likely to close than higher output fistulas. Longer fistula tract length is associated with a higher chance of closing.
The goal of medical management is to decrease fistula output and encourage spontaneous closure. Nasogastric tubes should be avoided. In high output fistulas, proton pump inhibitors (PPIs) and H2 blockers can be used to decrease gastric secretions. Antidiarrheals, such as loperamide, are also effective in reducing the output of high-output fistulas. Octreotide, a somatostatin analog, has been extensively studied for controlling fistula output. It has been shown to decrease output, increase spontaneous closure, and decrease hospital stay, but has never been shown to decrease mortality. If a fistula has over one liter per day of output, an octreotide trial can be attempted. After 72 hours if there is a significant reduction in volume, the medication can be continued.
If the fistula does not resolve with medical management, surgical management will then be considered. Operating on fistulas is fraught with difficulties, and there is a high risk for recurrence. Surgical approach may be difficult due to previous surgeries and adhesions. The bowel must be run carefully, and extreme care must be taken to not cause any accidental enterotomies during lysis of adhesion and bowel mobilization. As long as the bowel looks healthy, the best option is to excise the fistula tract and resect a small amount of associated bowel followed by an anastomosis to reestablish bowel continuity. To decrease recurrence rate, one must make sure to close the fascia where the fistula tract was traversing. As long as medical management, proper nutrition, and an appropriate waiting time precede the operation, permanent resolution of an enterocutaneous fistula occurs in 80% to 95% of cases.