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anal fistula / fistula-in-ano


  • a fistula with its internal opening in the anal canal and its external opening in the skin near the anus
  • they originate from the anal glands which are located between the internal and external anal sphincter, and which drain into the anal canal
  • blockage of these glands can result in an perianal abscess which can extend to the anal skin and result in a fistula tract
  • if the fistula tract seals over, a further abscess may form
  • fistulae can be a cause of ano-rectal pain and can cause distress due to the chronic drainage of pus and the risk of recurrent abscess formation and a source for systemic haematogenous spread infections
  • most patients prefer to have elective surgical repair


  • quite rare, affecting only 1 in 5000 people
  • males have 2-6x prevalence compared with females
  • most common in 30's and 40's
  • increased risk with Crohn's disease

5 main anatomic types

  • Extrasphincteric fistulae
    • begin at the rectum or sigmoid colon and proceed downward, through the levator ani muscle
  • Suprasphincteric fistulae
    • begin between the internal and external sphincter muscles, extend above and cross the puborectalis muscle, proceed downward between the puborectalis and levator ani muscles, and open an inch or more away from the anus
  • Transphincteric fistulae
    • begin between the internal and external sphincter muscles or behind the anus, cross the external sphincter muscle and open an inch or more away from the anus
  • Intersphincteric fistulae
    • begin between the internal and external sphincter muscles, pass through the internal sphincter muscle, and open very close to the anus
  • Submucosal fistulae
    • pass superficially beneath the submucosa and do not cross either sphincter muscle


source: Wikipedia

Clinical features

  • bloody or purulent perianal discharge
    • +/- malodour
    • pain or bleeding with defecation
  • pruritus ani +/- skin ulceration
  • there may be evidence of perianal abscess such as
    • pain
    • tenderness
    • swelling
    • fever


  • clinical exam may be diagnostic
  • may require endoanal ultrasound or rarely, MRI scan


  • if active infection or abscess then treat accordingly - surgical drainage may be required as well as antibiotics
  • a drainage seton may be placed to prevent pus build up
  • referral to surgeons for definitive Rx
anal_fistula.txt · Last modified: 2018/09/24 09:11 (external edit)