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Park's classification of anal fistula
Under Park's classification, which is the commonest type of fistula-in-ano?
- correctA. Intersphincteric
- B. Suprasphincteric
- C. Transsphincteric
- D. Extrasphincteric
Explanation
Key fact
Intersphincteric fistulae account for about 70%, because almost all anal fistulae begin as an abscess in an intersphincteric anal gland.
Intersphincteric. The tract runs in the plane between the internal and external sphincters and opens near the anal verge.
Why
Nearly all anal fistulae start in an infected anal gland in the intersphincteric space.
Pus takes the path of least resistance, which is straight down that plane.
It emerges close to the anal margin, giving a short simple tract.
About 70% stop there, making this the commonest type.
Park's classification is by the relation of the tract to the external sphincter.
Park's four types
| Type | Course | Approximate share |
|---|---|---|
| Intersphincteric | Between the sphincters, down to the anal margin | About 70% |
| Transsphincteric | Crosses the external sphincter into the ischioanal fossa | About 25% |
| Suprasphincteric | Up over puborectalis, then down through the ischioanal fossa | About 4% |
| Extrasphincteric | Through the levator, outside the sphincter complex | About 1% |
The classification is by how much sphincter the tract crosses
Why not the others
- B. Suprasphincteric — is uncommon and arises from a supralevator abscess.
- C. Transsphincteric — is the second commonest at about a quarter, and follows an ischioanal abscess.
- D. Extrasphincteric — is rarest and usually secondary - to Crohn's disease, trauma, pelvic sepsis or a foreign body - rather than cryptoglandular.
Remember
Goodsall's rule: an external opening in front of a transverse anal line usually tracks radially to the anal canal; one behind it curves to the posterior midline.
Go deeper
The whole point of the classification is continence. The more external sphincter a tract crosses, the more dangerous laying it open becomes.
Low simple fistulae are laid open; higher ones need a seton, an advancement flap, or a sphincter-sparing procedure.
Multiple or recurrent fistulae, especially with perianal skin tags, should prompt a search for Crohn's disease.
Bailey & Love's Short Practice of Surgery, 28th ed. (CRC Press, 2023); Sabiston Textbook of Surgery, 22nd ed. (Elsevier, 2025).