Enterovesical Fistula Workup

• Urinalysis: Urinalysis usually shows a full field of WBCs, bacteria, and debris. A variant of the Bourne test (see Bourne test) using orally administered charcoal is also helpful. Charcoal in the urine is detected either visually or microscopically in the centrifuged urine of patients.^[33]
• Microbiology: Urine culture findings are typically interpreted as mixed flora, although the most common organism identified is Escherichia coli. In the setting of sepsis, attempts should be made to characterize the predominant organisms and to obtain sensitivities to guide further therapy. Recurrent UTIs with various organisms are consistent with, but not diagnostic of, enterovesical fistulae.
• Serum studies: The blood urea nitrogen (BUN), creatinine, and electrolytes should be assessed; findings are typically within the reference range. The results of CBC count are typically normal. Leukocytosis may be found in cases associated with focal areas of undrained abscess or development of florid cystitis or pyelonephritis. Anemia may be present with chronic disease and may be associated with malignancy.

CT scanning

CT scanning of the abdomen and pelvis is the most sensitive imaging test for detecting a colovesical fistula, and CT scanning should be included as part of the initial evaluation of suspected colovesical fistulae. CT scanning can demonstrate small amounts of air or contrast material in the bladder, localized thickening of the bladder wall, or an extraluminal gas-containing mass adjacent to the bladder. Three-dimensional reconstruction is useful when traditional axial and coronal images fail to demonstrate the anatomy in sufficient detail.^[34] The images below show a series of CT scans.

Preoperative CT scanning in 9 consecutive patients with colovesical fistulae secondary to diverticulitis was accurately used to predict the presence and location of fistulae in 8 patients and led to suspicion in 1 patient.^[35]

In another study, colovesical fistulae identified preoperatively with CT scanning in 12 patients were surgically confirmed in 11 of those patients. CT scanning was also used to exclude fistulae in 20 patients with uncomplicated acute diverticulitis.^[36]

Avoiding oral contrast ingestion and having the patient evacuate rectally administered barium can enhance the value of CT scanning in the process of fistula identification.^[37] CT scanning also plays an important role in preoperative surgical planning by demonstrating the extent and degree of pericolonic inflammation.

In another study, 3-dimensional CT scanning provided improved imaging of the anatomic relationships. Additionally, multidetector row CT urography is useful in identifying urinary tract abnormalities, including fistulae.^[38] More sophisticated CT imaging modalities, such as CT colonoscopy, have been reported in the literature, but no clinical trials demonstrating a clinical benefit to this modality over traditional CT scanning have been published to date.^[39]

Barium enema

Barium enema (BE) imaging is unreliable in revealing a fistula but is useful in differentiating diverticular disease from cancer. BE imaging can demonstrate the nature and extent of colonic disease. In a 1988 series, Woods et al used BE imaging to demonstrate fistulae in 42% of cases.^[40]

Radiography of centrifuged urine samples obtained immediately after a nondiagnostic BE, called the Bourne test, may enhance the yield of the BE. Barium detected in the urine sediment confirms the presence of a fistula. In one study, the Bourne test results were positive in 9 of 10 patients. In 7 of these patients, the Bourne test finding was the only evidence of an otherwise occult colovesical fistula.^[41]

Cystography

Cystography may demonstrate contrast outside the bladder but is less likely to demonstrate a fistula.

Radiographic signs have been described. The herald sign is a crescentic defect on the upper margin of the bladder that is visualized best in an oblique view. The herald sign represents a perivesical abscess. A "beehive on the bladder" sign is associated with the vesical end of the fistulous tract.^[42]

Because of the superiority of CT scanning as a tool for diagnosis and treatment planning, plain cystography is no longer used in the evaluation of fistulae. CT scanning with rectal contrast only is the best diagnostic imaging modality.

Ultrasonography

Ultrasonography of colovesical fistulae has been described. In some instances, the fistula is easily identified, with no additional maneuvers needed.^[43] Ultrasonographic examination of suspected fistulous sites has been enhanced with the technique of manual compression of the lower abdomen, which reveals an echogenic "beak sign" connecting the peristaltic bowel lumen and the urinary bladder.^[44] As with cystography, ultrasonography is rarely used for primary imaging of fistulae.

Magnetic resonance imaging

MRI can be used to identify enterovesical fistulae. In a study of 25 patients with Crohn disease, 16 patients had enterovesical, deep perineal, or cutaneous fistulae. One false-negative result occurred in a patient who had a colovesical fistula.^[45] Some authors recommend MRI evaluation in patients with Crohn disease given the presence of chronic inflammation and superior anatomic detail in relation to the anal sphincter. Another benefit is that this study does not expose the patient to additional radiation.^[46]

T1-weighted images delineate the extension of the fistula relative to sphincters and adjacent hollow viscera and show inflammatory changes in fat planes.

T2-weighted images show fluid collections within the fistula, localized fluid collections in extra-intestinal tissues, and inflammatory changes within muscles.

MRI may be useful in identifying deep perineal fistulae but is not generally used in the routine workup of colovesical fistulae. In a study of 22 patients who presented with symptoms suggestive of colovesical fistula, MRI was performed in conjunction with cystoscopy. Afterward, 19 of the patients underwent laparotomy and repair. They found that MRI correctly identified 18 cases of fistula. Fistula was ruled out in the remaining patient. This data showed MRI to be a highly sensitive and specific study for colovesical fistula.

Although MRI is an excellent study, the increasing image quality of CT scanning, together with the high cost and limited availability of MRI, limit the practical application MRI as a diagnostic study for enterovesical fistulae.^[47]

Cystoscopy

Cystoscopy can be a helpful component of the diagnostic evaluation. Prior to the advancement of radiological diagnostic techniques, cystoscopy was considered to be the most reliable method of diagnosis.^[31] The findings of this procedure can suggest the presence of a fistula, and cystoscopy can be used to evaluate for a possible malignancy.

Cystoscopy can be useful in paring down the list of differential diagnoses, and it enables the physician to obtain a biopsy of the fistula to check for malignancy. Localized erythema, papillary/bullous mucosal changes, and, occasionally, material oozing through an area are present in 80%-90% of diagnosed cases (see image below).

Inflammatory mucosal changes of edema and pseudopolyp formation have been termed the herald patch (see images below).^[48]

Cystoscopy is used to initially diagnose fistulae in 30%-50% of cases. Cystoscopy findings are used to confirm enterovesical fistulae in 60%-75% of patients.

The presence of a localized area of edema and congestion is a typical finding in the early stages of a fistula. Bullous edema and mucosal papillomatous hyperplasia surround a fistula as it matures. Often, the fistula opening is not identified. Fecal material or mucus may be observed in the bladder.^[48] An attempt may be made to catheterize the tract or inject contrast retrograde to confirm the presence of fistula using plain radiography or fluoroscopy. Lesions are most commonly observed on the dome of the bladder. A lesion on the left dome of the bladder is typically diverticular. A lesion on the right posterior wall or the right dome of the bladder is more likely associated with Crohn ileitis or an appendicovesical fistula.

Poppy seed test

The poppy seed test has recently proven to be a potentially helpful diagnostic tool. This test consists of administering 1.25 g of poppy seeds with 12 ounces of fluid or 6 ounces of yogurt to the patient. The urine is then collected for the next 48 hours and examined for poppy seeds.

In a recent trial, the accuracy of the poppy seed test was compared with CT scanning and nuclear cystography in 20 patients with surgically confirmed fistulae. The poppy seed test yielded a 100% detection rate, whereas CT scanning and nuclear cystography yielded rates of 70% and 80%, respectively. Because of the low cost of the test ($5.37 for the poppy seed test, $652.92 for CT scanning, $490.83 for nuclear cystography), this may serve as an excellent confirmatory test when fistula is suspected. An obvious problem with the poppy seed test is that it provides little detail as to the location and type of fistula present.^[49]

When large areas of inflammation are appreciated or when abscess is involved, possible ureteral involvement should be considered, especially in the setting of any hydronephrosis. Preoperative evaluation with retrograde pyelography or intravenous pyelography (IVP) helps to demonstrate the extent of involvement for surgical repair.^[50]

Colonoscopy

Colonoscopy, like BE, is not particularly valuable in detecting a fistula, but it is helpful in determining the nature of the bowel disease that caused the fistula and is typically part of the evaluation. Further, if malignancy is considered, colonoscopy should be performed preoperatively to allow for proper surgical planning.^{[48, 51]}

Laparoscopy/laparotomy

The use of laparoscopy has been described in diagnosing a pediatric patient with an appendicovesical fistula.^[13] Adult laparoscopy is commonly used for investigating abdominal pain in women and may become a more frequently used diagnostic tool in men. Exploratory laparotomy is used for diagnosis and therapy in all types of fistulae.

Histologic findings associated with a biopsy of fistulous sites are usually consistent with chronic inflammation. Even in the case of carcinoma, inflammation is the usual finding on the bladder side. In more advanced cases, mucin-producing adenocarcinoma may be identified. The differential diagnoses must include primary adenocarcinoma of the bladder or poorly differentiated urothelial carcinoma. The clinical scenario and laparotomy findings are usually helpful in determining the diagnosis.

Staging is appropriate when the etiology of the fistula is carcinoma. The staging of colorectal carcinoma is discussed in other eMedicine articles such as Colon, Adenocarcinoma, Rectal Carcinoma, and Colon Cancer, Adenocarcinoma.