Anal fistulas are classified into four general types:
Intersphincteric: through the dentate line to the anal verge, tracking along the intersphincteric plane, ending in the perianal skin
Transsphincteric: through the external sphincter into the ischiorectal fossa, encompassing a portion of the internal and external sphincter, ending in the skin overlying buttocks
Suprasphincteric: through the anal crypt and encircling the entire sphincter, ending in the ischiorectal fossa
Extrasphincteric: starting high in the anal canal, encompassing the entire sphincter and ending in the skin overlying the buttocks
Because most abscesses develop between the sphincters, the intersphincteric fistula is the most common type, accounting for 50%-80% of all cryptoglandular fistulas. An intersphincteric fistula-in-ano is caused by a perianal abscess. In its most common course, it begins at the dentate line, then tracks to the space between the internal and external anal sphincters (the intersphincteric space) via the internal sphincter, and finally terminates in the perineum or perianal skin. Other possible tracts include high tract to pelvis or lower rectum, high blind tract, or no perineal opening.
Learn more about anal fistulas.
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Cite this: Richard H. Sinert. Fast Five Quiz: Anal Fissures and Fistulas - Medscape - Sep 15, 2022.