What are the types of fistula in ano?

Types of Fistula-in-Ano

Fistula-in-ano is classified using the Parks anatomical classification system, which describes five main types based on their relationship to the anal sphincter complex: superficial, intersphincteric, transsphincteric, suprasphincteric, and extrasphincteric. 1

Anatomical Classification (Parks System)

The Parks classification uses the external sphincter as the central reference point and categorizes fistulas as follows: 1

• Intersphincteric fistulas: Tract runs between the internal and external anal sphincters. These represent the most common type, accounting for approximately 54% of all anal fistulas. 1, 2 Can be further subdivided into low or high variants. 1

• Transsphincteric fistulas: Tract crosses through the external anal sphincter muscle into the ischioanal fossa. 1 These account for approximately 21% of cases. 2 Subdivided into:

  • Low transsphincteric: crosses the lower third of the external sphincter 1
  • High transsphincteric: crosses the upper two-thirds (mainly puborectal muscle and upper external sphincter) 1

• Suprasphincteric fistulas: Tract passes upward in the intersphincteric plane above the puborectalis muscle, then tracks laterally and downward into the ischioanal fossa. 1 These represent approximately 3% of cases. 2

• Extrasphincteric fistulas: Tract originates outside the anal canal and sphincter complex, passing directly from the rectum to the perineal skin through the ischioanal fossa. 1 These account for approximately 3% of cases. 2

• Superficial fistulas: Tract involves only the distal anal canal without involving the anal sphincters. 1 These represent approximately 16% of cases. 2

Clinical Classification (Simple vs. Complex)

In clinical practice, a widely used empiric classification divides fistulas into simple and complex categories: 1

Simple Fistulas

A simple fistula has ALL of the following characteristics: 1

• Low anatomical origin (superficial, low intersphincteric, or low transsphincteric)
• Single external opening
• No pain or fluctuation suggesting perianal abscess
• No rectovaginal involvement
• No anorectal stricture
• No active rectal inflammation (though presence of rectal Crohn's disease complicates management) 1

Complex Fistulas

A complex fistula has ONE OR MORE of the following: 1

• High anatomical origin (high intersphincteric, high transsphincteric, suprasphincteric, or extrasphincteric)
• Multiple external openings
• Associated pain or fluctuation suggesting abscess
• Rectovaginal involvement 1
• Associated anorectal stricture
• Active rectal disease on endoscopy 1

Special Variants

• Horseshoe fistulas: Semilunar region of sepsis spreading horizontally on either side of an internal opening, involving two or more adjacent quadrants in the ischioanal, intersphincteric, or supralevator compartments. 1

• Rectovaginal/anovaginal fistulas: Tract connecting the rectum or anus to the vagina, appearing as a hypoechoic tract on ultrasound or high-signal/enhancing structure on MRI. 1

Clinical Pitfalls

The distinction between simple and complex fistulas is critical because it determines surgical approach and predicts outcomes. Simple fistulas achieve healing rates above 95% with fistulotomy, while complex fistulas require sphincter-saving techniques with healing rates of 60-90%. 3

Additional diagnostic evaluation with examination under anesthesia (EUA), endoscopic ultrasound, or pelvic MRI may be required to accurately classify fistulas, as clinical examination alone may be insufficient for complex cases. 1 EUA performed by an experienced colorectal surgeon is approximately 90% accurate in detecting and classifying perianal fistulas. 1

In patients with Crohn's disease, perianal fistulas occur in 13-38% of cases in referral centers, with the highest frequency (92%) in those with colonic disease and rectal involvement. 1 These require individualized assessment considering rectal disease activity, fistula complexity, and sphincter function. 4