What is the management for a fistula with discharge located posterior to the anal sphincter?

Management of a Posterior Anal Fistula with Discharge

For a fistula with discharge located posterior to the anal sphincter, MRI (option D) is the recommended imaging modality to properly assess the fistula anatomy before surgical intervention.

Rationale for MRI as First-Line Management

MRI provides superior visualization of:

• The complete course of the fistula tract in relation to the anal sphincter
• The presence of secondary tracts or extensions
• Any associated abscesses
• The internal opening location
• The relationship to the levator ani muscle

According to guidelines, MRI is the preferred imaging technique for complex anal fistulas, particularly those with posterior location 1. MRI has higher accuracy in evaluating secondary extensions compared to other imaging modalities, especially in complex fistulas 1.

Diagnostic Algorithm for Posterior Anal Fistulas

1. Initial Assessment: Determine if the fistula is simple or complex

   • Simple: Superficial, low intersphincteric, or low transsphincteric
   • Complex: High transsphincteric, suprasphincteric, extrasphincteric, or with multiple tracts

2. For Posterior Fistulas:

   • Higher risk of being transsphincteric or complex
   • More likely to involve sphincter muscles
   • May have horseshoe extensions

3. Imaging Selection:

   • MRI: Gold standard for complex fistulas
   • CT: Less sensitive for soft tissue definition
   • Fistulogram: Limited value, does not show relationship to sphincters
   • Endoanal ultrasound: Operator-dependent, limited field of view

Why Other Options Are Less Appropriate

• Fistulogram (Option A): Limited value as it only shows the primary tract without demonstrating the relationship to the sphincter complex or identifying secondary tracts 1.

• Fistulotomy with marsupialization (Option B): Performing this procedure without proper imaging could lead to:

  • Incomplete treatment of complex tracts
  • Sphincter damage if the fistula is transsphincteric
  • Missed abscesses or secondary tracts
  • Higher recurrence rates

• Lateral internal sphincterotomy (Option C): This procedure is primarily indicated for anal fissures, not fistulas 2. Performing this for a fistula would be inappropriate and could lead to incontinence without addressing the fistula.

Post-MRI Management Considerations

After MRI assessment, the appropriate surgical approach depends on fistula classification:

1. Low fistula not involving sphincter muscle:

   • Fistulotomy with marsupialization may be appropriate 1

2. Fistula involving sphincter muscles:

   • Placement of a loose draining seton is recommended 1
   • Avoid immediate fistulotomy to prevent incontinence

3. Complex fistula with multiple tracts or supralevator extension:

   • Staged approach with initial seton placement
   • Consider specialized techniques like LIFT procedure or advancement flap

Important Considerations

• Recent studies show that proper preoperative MRI significantly reduces recurrence rates by identifying the full anatomy of the fistula 3
• MRI contribution to clinical evaluation was significant in 33.8% of patients with anal fistulas, with even higher significance for complex fistulas 3
• Fistulas with external openings more than 2 cm from the anal canal particularly benefit from MRI assessment 3
• Standardized MRI reporting templates improve surgical planning 4, 5

MRI provides crucial information that directly impacts surgical decision-making and outcomes for patients with posterior anal fistulas, making it the most appropriate initial management choice.