Management of Posterior Anal Fistula with Discharge
For a fistula with discharge located posterior to the anal sphincter, MRI is the recommended initial diagnostic approach to accurately characterize the fistula tract and guide appropriate surgical management. 1
Diagnostic Approach
Initial Imaging
- MRI is the preferred imaging modality for several reasons:
- Provides superior visualization of fistula tracts, especially complex ones
- Accurately identifies the relationship between the fistula and the sphincter complex
- Helps detect secondary extensions and abscesses
- Shows higher accuracy in evaluating complex fistulae compared to other modalities 1
Why MRI over other options:
- Fistulogram is limited in providing complete anatomical information and has been largely replaced by cross-sectional imaging techniques 2
- CT with contrast can be useful but is less sensitive than MRI for soft tissue delineation of fistula tracts 1
- Endoanal ultrasound (not listed in options) has lower accuracy than MRI for complex fistulae 1
Treatment Algorithm Based on MRI Findings
1. For Simple Posterior Fistulas (not involving sphincter muscle)
- Fistulotomy is appropriate only if the fistula is subcutaneous and does not involve the sphincter muscle 1
- This avoids the risk of incontinence while providing definitive treatment
2. For Trans-sphincteric Fistulas
- Fistulotomy with marsupialisation is NOT recommended as the primary approach if the fistula involves significant sphincter muscle, due to high risk of incontinence 1
- Instead, a sphincter-preserving approach is preferred:
- Placement of a loose draining seton to control infection 1
- Consider staged procedures or sphincter-sparing techniques
3. For Complex Fistulas
- Complex fistulas (supralevator, suprasphincteric, extrasphincteric) require specialized management 3
- Lateral internal sphincterotomy is NOT appropriate for fistula management as it's primarily used for anal fissures 4
Important Clinical Considerations
Pitfalls to Avoid
- Performing fistulotomy without adequate imaging assessment of sphincter involvement
- Probing fistula tracts blindly, which can create false passages or iatrogenic complications 1
- Assuming simple anatomy without proper imaging, especially for recurrent or complex fistulas
- Premature surgical intervention before controlling active infection
Special Circumstances
- If active abscess is present, incision and drainage should be performed first 1
- Antibiotics are indicated only if there is surrounding soft tissue infection, sepsis, or immunocompromise 1
- For recurrent fistulas, more extensive evaluation may be needed before definitive treatment 5
MRI provides the most comprehensive assessment of the fistula anatomy, which is crucial for planning the appropriate surgical approach that balances effective treatment with preservation of sphincter function and prevention of incontinence.