From the Guidelines
The best investigation for this patient is MRI (option B). This is based on the most recent and highest quality evidence from the American College of Radiology, which states that MRI of the anorectal region is the standard for imaging perianal fistula, especially those associated with complex tracts 1. The symptoms described, including intermittent colicky abdominal pain, loose stool, perianal discharge, and a tract connecting the perineum to the anal canal, are consistent with a perianal fistula, and MRI is the gold standard for evaluating this condition.
Key Benefits of MRI
- Provides excellent soft tissue resolution to accurately delineate the anatomy of the fistula tract
- Can identify any associated abscesses and determine the relationship of the fistula to the anal sphincter complex, which is crucial for surgical planning and management
- Has high sensitivity and specificity for the evaluation of perianal fistula, with a sensitivity ranging from 81% to 100% and specificity from 67% to 100% 1
- Non-invasive nature and superior ability to characterize soft tissue pathology make it the investigation of choice for suspected perianal fistulas
Comparison to Other Options
- Colonoscopy and sigmoidoscopy can help evaluate the intestinal mucosa for inflammatory bowel disease, but they cannot adequately visualize the fistula tract itself
- Barium enema is less sensitive and specific for perianal disease and has largely been replaced by cross-sectional imaging techniques like MRI
- The use of IV contrast with MRI facilitates visualization of the fistula tract and associated abscesses, and diffusion-weighted sequences can increase the conspicuity of fistulae 1
Overall, MRI is the best investigation for this patient due to its high sensitivity and specificity, excellent soft tissue resolution, and ability to provide crucial information for surgical planning and management.
From the Research
Best Investigation for Fistula-in-Ano
The patient's symptoms, including intermittent colicky abdominal pain, loose stool, perianal discharge, and a tract connecting the perineum to the anal canal, suggest a diagnosis of fistula-in-ano. The best investigation for this condition is:
- MRI (Magnetic Resonance Imaging), as it is the most accurate modality for evaluating fistula-in-ano and can demonstrate hidden areas of sepsis and secondary extensions 2, 3, 4, 5, 6
- MRI can accurately characterize the anatomic course of the fistula and associated infection, which is critical for effective surgical treatment 3, 6
- Other imaging modalities, such as CT fistulography, may also be useful, but MRI is generally considered the gold standard 2, 3
Comparison of Imaging Modalities
The following imaging modalities are not the best choice for evaluating fistula-in-ano:
- Colonoscopy: not specifically designed for evaluating fistula-in-ano
- Sigmoidoscopy: may not provide detailed information about the fistula tract and associated infection
- Barium enema: not as accurate as MRI for evaluating fistula-in-ano