Management of Fistula with Discharge Located Posterior to Anal Sphincter
MRI is the recommended initial management approach for a fistula with discharge located posterior to the anal sphincter, as it provides the most accurate anatomical characterization needed before surgical intervention. 1
Diagnostic Approach
MRI is superior to other diagnostic methods for several key reasons:
- Provides detailed anatomical information about the fistula tract, including primary and secondary tracts, internal opening location, and relationship to sphincter muscles 1
- Correctly classifies primary fistula tracts in up to 90% of cases compared to surgical findings 1
- Accurately identifies secondary extensions, abscesses, supralevator extension, and horseshoe ramifications 1, 2
- Essential for determining the course of the fistula in relation to the anal sphincter and levator plate (superficial, intersphincteric, trans-sphincteric, suprasphincteric, extrasphincteric) 3
Comparison with Other Options
- Fistulogram (Option A): Less accurate than MRI for defining complex fistula anatomy and may miss secondary tracts or abscesses 1
- Fistulotomy and marsupialization (Option B): Should not be performed without proper imaging assessment first, especially for trans-sphincteric fistulas, as this could lead to sphincter damage and incontinence 1
- Lateral internal sphincterotomy (Option C): Inappropriate for fistula management as it is primarily indicated for anal fissures, not fistulas 1
Clinical Significance of MRI Assessment
MRI contributes significantly to surgical management in 33.8% of patients with anal fistulas, with even higher significance in complex fistulas 2. This is particularly important for:
- Fistulas with external openings more than 2 cm from the anal canal 2
- Recurrent fistulas 2
- Horseshoe fistulas 2, 4
- Complex fistulas such as RIFIL (Roof of Ischiorectal Fossa Inside Levator-ani muscle) fistulas, which have a higher failure rate (30.6%) compared to non-RIFIL fistulas (7.2%) 4
Treatment Planning Based on MRI Findings
After MRI assessment, treatment should be guided by the fistula classification:
- For simple subcutaneous fistulas not involving sphincter muscle: fistulotomy can be performed safely 1
- For trans-sphincteric fistulas involving sphincter muscles: a loose draining seton should be placed rather than immediate fistulotomy to preserve sphincter function 1
- For high transsphincteric, suprasphincteric, or extrasphincteric fistulas: more complex surgical approaches are needed to avoid incontinence 3, 1
Important Considerations
- Posterior fistulas are more common (90% of anal fistulas) and require careful assessment as improper management could lead to incontinence 1
- The presence of proctitis should be assessed, as it is highly relevant for fistula management and prognosis 3
- Examination under anesthesia (EUA) has an important role in diagnosis and classification and allows immediate therapeutic intervention such as abscess drainage and/or seton placement, but should be guided by MRI findings 3
MRI provides the most comprehensive assessment of fistula anatomy and associated pathology, making it the optimal first step in management to guide appropriate surgical intervention and reduce recurrence rates.