abscess: localised pocket of pus caused by bacterial infection.

fistula: abnormal communication between the anorectum and the perianal skin. Complication of an acute or chronic perianal abscess.

ischiorectal most common but all misnamed perianal

Aetiology

• 10% infected fissure/perianal haematoma
• 90% Infection of one of the anal glands that drain into the anal canal at the level of the dentate line.
• polymicrobial

Causative organisms

• Bacteroides
• E coli
• Enterococcus

Goodsalls rule: used to predict fistula track

• anterior fistulas will have direct track
• posterior fistulas will have curved track
• exception: anterior fistulas >3cm from anus may be curved and open into posterior midline of anal canal