What is the difference between pelvic-rectal fistulas and intersphincteric fistulas?

Differences Between Pelvirectal Fistulas and Intersphincteric Fistulas

The main difference between pelvirectal fistulas and intersphincteric fistulas is their anatomical course: pelvirectal (extrasphincteric) fistulas pass directly from the rectum to the perineal skin through the ischioanal fossa without involving the sphincter complex, while intersphincteric fistulas run between the internal and external anal sphincters.

Anatomical Differences

Intersphincteric Fistulas

• Located between the internal and external sphincter muscles 1
• Run through the intersphincteric space 1
• Typically originate from an infected anal gland at the dentate line 2
• May extend upward in the intersphincteric plane, sometimes reaching the level of the puborectal muscle 3
• Often have no external opening when they extend high in the intersphincteric plane 3
• Frequently associated with intersphincteric abscesses 1, 3

Pelvirectal (Extrasphincteric) Fistulas

• Do not originate in the anal canal or sphincter complex 1
• Pass directly from the rectum to the perineal skin through the ischioanal fossa 1
• Bypass the sphincter complex entirely 1
• Often result from rectal disease rather than anal gland infection 1
• May be associated with Crohn's disease, diverticulitis, or malignancy 4

Clinical Significance

Diagnostic Approach

• MRI with intravenous contrast is the preferred imaging method for both types of fistulas 4
• Endoanal ultrasound can help distinguish between the two types 1
• Examination under anesthesia remains the gold standard for accurate classification 1, 4
• Proctosigmoidoscopy is essential to assess for concomitant rectal inflammation 4

Treatment Implications

• Intersphincteric fistulas:

  • Simple cases may be treated with fistulotomy 5
  • High intersphincteric fistulas may require flap repair to preserve sphincter function 3
  • When associated with abscesses, drainage is essential 3

• Pelvirectal/extrasphincteric fistulas:

  • Often require more complex surgical approaches 1
  • May need multidisciplinary management, especially if associated with Crohn's disease or malignancy 4
  • Higher risk of recurrence and complications 1

Special Considerations

Relation to Crohn's Disease

• Both types can occur in Crohn's disease, but with different implications 1
• Presence of proctitis significantly affects management and prognosis 1
• In Crohn's disease with colonic involvement and rectal disease, the prevalence of fistulizing anal disease can be as high as 92% 4

Complications

• Intersphincteric fistulas may be associated with high intersphincteric abscesses 3, 6
• Pelvirectal fistulas may be associated with pelvic abscesses and have higher morbidity 4
• Risk of fecal incontinence varies between the two types, with generally higher risk in extrasphincteric fistulas due to more extensive surgery needed 7

Pitfalls in Diagnosis and Management

• Failure to identify the internal opening can lead to misclassification 6
• High intersphincteric fistulas may be missed clinically as they often lack external openings 3, 6
• Not considering the underlying etiology can lead to inappropriate treatment 4
• Inadequate imaging may fail to detect extensions or associated abscesses 1

Understanding these anatomical and clinical differences is crucial for proper diagnosis and management, as the surgical approach and risk of complications differ significantly between these two types of fistulas.