Why is an intersphincteric fistula more likely than a pelvi-rectal fistula in a patient with a rectal fistula without an external perianal opening and a pelvic collection?

Why Intersphincteric Fistulas Are More Likely Than Pelvi-Rectal Fistulas in Rectal Fistulas Without External Openings

Intersphincteric fistulas are significantly more likely than pelvi-rectal fistulas in patients with rectal fistulas without external perianal openings and pelvic collections because most anal fistulas originate from infected intersphincteric anal glands. 1

Pathophysiology of Rectal Fistulas

Origin of Most Anal Fistulas

• Anorectal abscesses primarily result from infection of the intersphincteric anal glands located at the dentate line 1
• When these glands become obstructed or infected, they form an abscess in the intersphincteric space first 1
• This cryptoglandular theory explains why the primary opening is almost always found at the dentate line 2

Progression of Intersphincteric Infections

1. Initial infection: Begins in the intersphincteric anal glands
2. Abscess formation: Creates an intersphincteric abscess
3. Extension patterns:
   • Downward extension: Creates typical perianal abscess with external opening
   • Upward extension: Creates high intersphincteric abscess without external opening 3
   • Lateral extension: May rupture through external sphincter into ischiorectal/ischioanal spaces 1
   • Cephalad extension: Results in high intramuscular, perirectal, or supralevator abscess 1

Clinical Presentation of High Intersphincteric Fistulas

Characteristic Presentation

• No external swelling, induration, or opening 2
• High extension with palpable mass or induration above the levator ani 2
• Primary opening almost always found at the dentate line 2
• Often associated with high intersphincteric and/or supralevator abscess 3

Diagnostic Challenges

• High intersphincteric fistulas are often not recognized clinically because they lack the usual visible signs 2
• Approximately 7% of anal abscesses and fistulas are of the high intermuscular (intersphincteric) type 2
• These fistulas are frequently misdiagnosed or inadequately treated due to their atypical presentation 2

Imaging Findings

MRI Findings

• MRI with intravenous contrast is the preferred imaging method for detecting fistulous tracts 4
• High intersphincteric fistulas typically show:
  • Tract in the intersphincteric plane extending upward to or above the level of the puborectal muscle 3
  • Associated high intersphincteric abscess and/or supralevator abscess 3
  • No external opening in most cases 3

Other Imaging Modalities

• CT with IV contrast has lower spatial resolution but can identify abscesses with rim enhancement 1
• Endoanal ultrasound shows lower accuracy for high fistulas compared to intersphincteric and transsphincteric tracts (67% vs. 88-93%) 1

Anatomical Considerations

Pelvi-Rectal Fistulas

• Pelvi-rectal (extrasphincteric) fistulas are much rarer
• These typically result from:
  • Pelvic disease extending to the rectum (e.g., diverticulitis, Crohn's disease)
  • Trauma or surgical complications
  • Malignancy
• Not part of the typical cryptoglandular disease progression 1

Parks Classification System

• Intersphincteric: Between internal and external sphincter (most common)
• Transsphincteric: Crosses both sphincters
• Suprasphincteric: Tracks up and over the top of the puborectalis
• Extrasphincteric (pelvi-rectal): Bypasses the sphincter complex entirely (least common) 4

Clinical Implications

Diagnostic Approach

• High suspicion is needed for prompt diagnosis 2
• Three key steps for diagnosis:
  1. Look for primary opening at the dentate line
  2. Pass a cannula from this opening into the cavity or induration
  3. Divide the circular muscle and internal sphincter until the upper end of the tract is reached 2

Treatment Considerations

• Flap repair with adequate drainage of associated abscesses is successful for high intersphincteric fistulas 3
• Fistulas with supralevator extension may require multiple procedures 3
• Simple fistulotomy risks incontinence in high fistulas 5

Pitfalls to Avoid

• Not performing adequate imaging before intervention
• Failing to identify the primary opening at the dentate line
• Treating only the abscess without addressing the fistula tract
• Not recognizing that most high fistulas without external openings are intersphincteric in origin 2

In summary, the cryptoglandular origin of most anal fistulas, combined with the typical upward extension pattern in the intersphincteric space without external manifestation, makes intersphincteric fistulas much more likely than pelvi-rectal fistulas in patients with rectal fistulas without external perianal openings and pelvic collections.