Fistula in ano

What it is

A fistula is an abnormal communication between two epithelial-lined organs. In fistula in ano the tract runs out to the perianal skin, and it is named by where it goes among the sphincters. The simple split is into low and high, and the two accounts draw the line differently for the trans-sphincteric fistula. One puts the whole of it low: low fistulas are the inter-sphincteric one, which passes between the sphincters, and the trans-sphincteric one, which passes via both sphincters, while high fistulas are the supra-sphincteric one, which passes above the external sphincter, and the extra-sphincteric one, which passes more above the external sphincter. The other cuts the trans-sphincteric in two: the low fistula is intersphincteric or low trans-sphincteric, and the high fistula is high trans-sphincteric, suprasphincteric or extrasphincteric. Which of the two is followed decides what surgery is possible, because fistulotomy is done for a low fistula and not for a high one. The Parks classification names the same four in more detail. Intersphincteric: the fistula is confined to the intersphincteric plane. Trans-sphincteric: it traverses the external sphincter, communicating with the ischiorectal fossa. Suprasphincteric: it extends cephalad over the external sphincter and perforates the levator ani. Extrasphincteric: it extends from the rectum to the perianal skin, external to the sphincter apparatus.