Perianal Abscess and Fistula
Blockage of a mucus-secreting anal gland lets bacteria multiply into a walled-off abscess, and if that pus tracks a path to the skin before it is drained, the epithelialised tunnel it leaves behind becomes a persistent fistula-in-ano.
Definition
A perianal (anorectal) abscess is a collection of pus in the tissues around the anus, usually from infection of an anal gland. An anal fistula is an abnormal epithelialised track between the anal canal and the perianal skin, often a sequel of a previous abscess.
First principles
It starts with a blocked anal gland, not the skin
Small mucus-secreting glands sit in the intersphincteric space and drain into the anal crypts at the dentate line. When a duct becomes obstructed, infection can spread through the intersphincteric plane and form an anorectal abscess. This cryptoglandular model is the most plausible explanation for most idiopathic anal sepsis, although the exact aetiology is not certain in every case; the route of spread determines the abscess location and later fistula complexity.
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