What diagnostic tests are used to evaluate an enterocutaneous fistula?

Diagnostic Evaluation of Enterocutaneous Fistula

CT pelvis with intravenous contrast is the first-line imaging test for evaluating enterocutaneous fistulas, with a diagnostic sensitivity of 76.5% for fistula detection and 94.1% for defining the underlying etiology. 1, 2

Initial Imaging Approach

CT with IV Contrast (Preferred First-Line)

• CT abdomen and pelvis with IV contrast should be obtained as the primary diagnostic test because it provides comprehensive cross-sectional information about the fistula tract, associated abscesses, and underlying pathology 1, 3
• IV contrast is essential to delineate rim-enhancement of fluid collections and assess bowel wall integrity 1
• Water-soluble oral or rectal contrast can be added to opacify the fistulous tract and confirm communication with the bowel, though this is not always necessary for diagnosis 1, 2
• Water-soluble contrast is strongly preferred over barium to avoid peritoneal contamination and CT artifact if spillage occurs 1, 2

MRI Pelvis with and without IV Gadolinium (Alternative or Complementary)

• MRI provides superior contrast resolution compared to CT and is particularly valuable when CT findings are equivocal 1
• IV gadolinium contrast is essential because active inflammation in fistulous tracts enhances avidly, and abscesses show rim-like enhancement 1
• MRI demonstrated 88% sensitivity and 100% specificity for fistula detection in postoperative patients 1
• Adding diffusion-weighted sequences increases conspicuity of fistulae to 100% sensitivity and helps discriminate between inflammatory mass and abscess (100% sensitivity, 90% specificity) 1
• A phased array body coil provides better visualization of fistula extent compared to endoanal coils 1

Secondary Diagnostic Modalities

Radiographic Fistulography

• Fistulography rapidly depicts the fistula anatomy and confirms communication with the bowel in real-time 3, 4
• Performed by cannulating the external opening with a small-gauge catheter (IV catheter, pediatric feeding tube, or lacrimal cannula) and injecting water-soluble contrast under fluoroscopy 1
• Sensitivity ranges from 16% to 89% depending on the series, with limitations including incomplete filling due to inflammatory debris 1
• Should be reserved for cases where cross-sectional imaging is equivocal or to provide additional anatomical detail for surgical planning 1, 3

Water-Soluble Contrast Enema

• Has poor sensitivity (33.3% for fistulas, 50% for sinus tracts) and should not be used as first-line imaging 1
• May be complementary to CT when performed in conjunction with cross-sectional imaging 1
• Can be used selectively in patients with abnormal clinical findings when combined with digital rectal examination and endoscopy 1

Plain Radiography

• Limited utility but may show free air, ectopic air, or bubbly lucencies suggesting abscess or fistula containing air 1
• Frequently inconclusive and requires additional imaging for confirmation 1
• Can help exclude bowel obstruction in postoperative patients 1

Clinical Classification and Assessment

Output-Based Classification

• Enterocutaneous fistulas should be classified by output volume: low (<200 mL/day), moderate (200-500 mL/day), or high (>500 mL/day) 1
• This classification guides fluid and electrolyte management strategies 1

Anatomical Classification

• Fistulas should be classified by the bowel segment of origin followed by the target organ (e.g., ileo-colonic, entero-cutaneous) 1
• MRI is the preferred tool for anatomical diagnosis with the best sensitivity and specificity 1

Management Implications from Imaging

Indications for Surgery

• Surgery is strongly recommended for fistulas associated with bowel stricture and/or abscess, or those causing diarrhea and/or malabsorption 1
• Surgery should be planned only after stabilization through four steps: fluid/electrolyte balance, sepsis control (antibiotics and percutaneous drainage when appropriate), nutritional improvement, and skin care 1

Common Pitfalls to Avoid

• Not using IV contrast during CT imaging can lead to missed diagnoses 1, 2
• Relying solely on contrast enema studies, which have significantly lower sensitivity than CT, results in missed fistulas 1, 2
• Performing fistulography without adequate cross-sectional imaging fails to visualize the anal sphincter complex, levator ani, and relationship to surrounding structures 1
• Using barium instead of water-soluble contrast risks peritoneal contamination and interferes with subsequent CT imaging 1, 2