Fistula in Ano
A 50-year-old man with hypertension and type 2 diabetes reports two months of intermittent perianal discharge. Two months ago he had a painful perianal swelling that burst spontaneously, after which the pain settled but seepage of pus onto his underwear continued. Examination shows a small posterior perianal opening exuding pus. Temperature is 36.6 °C. Which of the following is the most likely diagnosis?
- A.Anal fissure
- B.Perianal fistulaCorrect
- C.Perianal abscess
- D.Thrombosed piles
Explanation
The sequence described is the classic natural history of a fistula in ano: an acute painful perianal swelling that discharges spontaneously, followed by relief of pain but persistent seepage from a residual cutaneous opening. A perianal fistula is a chronic epithelialised or granulation lined tract connecting the anal canal to the perianal skin. According to the cryptoglandular hypothesis it begins as infection of an anal gland at the dentate line, which forms an abscess that tracks to the skin; when that abscess drains, whether spontaneously or surgically, the tract fails to close in roughly one third of cases and becomes a fistula. The persistent internal opening at the dentate line continually seeds the tract, which is why it will not heal without surgery. Diabetes predisposes both to the initial abscess and to failure of healing. The absence of fever and the resolution of severe pain exclude an active undrained abscess, which would present with a tense tender swelling and systemic signs. An anal fissure causes severe pain on defecation with a midline tear and no purulent discharge. A thrombosed pile is an acutely painful bluish lump at the anal verge that resolves over days and does not discharge pus. Definitive treatment requires delineating the tract in relation to the sphincter complex before laying it open or using a sphincter preserving technique.
Why each option
- A.
- An anal fissure gives severe pain during and after defecation with bright red bleeding and a visible tear; it does not produce chronic purulent discharge.
- B.
- Correct. An abscess that burst and left a chronically discharging perianal opening is the classic evolution of a cryptoglandular fistula in ano.
- C.
- An undrained abscess would present with a tense, hot, tender fluctuant swelling and fever; this patient's pain has settled and he is afebrile at 36.6 °C.
- D.
- Thrombosed haemorrhoids cause a sudden painful bluish lump at the anal verge that resolves spontaneously, without a discharging opening.
Reference: ASCRS Clinical Practice Guideline for the Management of Anorectal Abscess, Fistula-in-Ano, and Rectovaginal Fistula, Diseases of the Colon and Rectum, 2022
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