What is the best radiological study to diagnose a colovesical fistula in an adult patient with a history of abdominal conditions or surgeries?

Best Radiological Study for Colovesical Fistula

CT pelvis with intravenous contrast is the best initial radiological study for diagnosing colovesical fistula, with a diagnostic sensitivity of 76.5% for fistula detection and 94.1% for defining the underlying etiology. 1, 2

Primary Imaging Recommendation

The American College of Radiology explicitly recommends MRI pelvis without and with IV contrast OR CT pelvis with IV contrast as equivalent first-line imaging options for rectovesicular (colovesical) fistula. 1 However, in real-world clinical practice, CT is more widely available and faster to obtain, making it the practical first choice.

CT Protocol Specifics

• Use IV contrast to enhance visualization of the fistulous tract and identify the underlying etiology (diverticulitis, malignancy, inflammatory bowel disease). 2
• Add CT cystography (bladder opacification with water-soluble contrast) for improved detection and presurgical planning, as this can be performed either:
  • Retrograde (direct bladder instillation), or
  • Antegrade (delayed imaging after IV contrast administration). 1, 2
• Look for diagnostic CT findings: enhancing tract(s) extending from colon to bladder wall, focal bladder wall thickening, and intravesical gas. 2

When to Choose MRI Instead

MRI pelvis with and without IV gadolinium contrast should be selected when:

• Superior soft tissue resolution is needed for complex fistula anatomy or multiple tracts. 3
• Active inflammation assessment is required. 1, 3
• Radiation exposure is a concern (younger patients, need for multiple follow-up studies). 3
• CT findings are equivocal. 3

MRI demonstrated 94.7% accuracy (18 of 19 patients) in identifying colovesical fistulas and defining etiology in one research series. 4

Alternative Diagnostic Modalities (Lower Sensitivity)

Cystography

• Older literature reports 90% detection rate for enterovesicular fistulas, but only 11-30% sensitivity in other series. 1, 5, 6
• Use only if CT/MRI unavailable or findings equivocal. 3

Contrast Enema (Barium or Water-Soluble)

• Very poor sensitivity: only 20-50% detection rate for colovesical fistulas. 1, 5, 6
• Not recommended as first-line imaging. 2
• Water-soluble contrast preferred over barium to avoid peritoneal contamination and CT artifact. 1

Colonoscopy and Cystoscopy

• These are not radiological studies but are essential complementary procedures.
• Colonoscopy has 100% sensitivity for detecting underlying colonic malignancy (critical for determining treatment). 7
• Cystoscopy has only 8.5-10.2% sensitivity for directly visualizing the fistula itself. 7, 6

Clinical Pitfalls to Avoid

• Do not rely on contrast enema or cystography alone—these have significantly lower sensitivity than CT and will miss the majority of fistulas. 2, 7
• Do not skip contrast enhancement—unenhanced CT has reduced sensitivity for fistula detection. 2
• Do not forget to evaluate for malignancy—colonoscopy is mandatory even after imaging confirms the fistula, as 10-16% of colovesical fistulas are caused by colorectal cancer. 7, 5
• Do not use barium if perforation is suspected—water-soluble contrast prevents peritoneal contamination and subsequent CT interference. 1

Practical Diagnostic Algorithm

1. First-line: CT pelvis with IV contrast (add CT cystography if available). 1, 2
2. If CT equivocal or contraindicated: MRI pelvis with and without IV gadolinium. 1, 3
3. Always add: Colonoscopy to exclude malignancy as the underlying cause. 7, 5, 6
4. Consider cystoscopy: For surgical planning and to exclude bladder malignancy, despite low diagnostic yield for the fistula itself. 7, 5