PassFinals
Colorectal Surgery

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.

In a nutshell

A blocked cryptoglandular anal gland becomes infected and forms an abscess; where the pus tracks (perianal, ischiorectal, supralevator) determines the clinical picture. If the tract persists after drainage it becomes a fistula-in-ano, and how much sphincter it crosses determines whether it can be laid open or needs a seton or other sphincter-preserving technique.

Classic presentation

Acute throbbing perianal pain with a tender fluctuant swelling and fever (abscess), or chronic intermittent perianal discharge with a history of a previously drained abscess (fistula).

Key points

  • Perianal sepsis begins as a blocked, infected anal gland in the intersphincteric space, not as a primary skin infection.
  • The abscess subtype (perianal, ischiorectal, supralevator) reflects the anatomical plane the pus has tracked into.
  • A fistula is the epithelialised tract left behind when an abscess drains without the internal opening being addressed.
  • Goodsall's rule predicts the internal opening from the external one based on the natural cryptoglandular drainage pathway.
  • Antibiotics do not replace incision and drainage of an abscess, but are added for cellulitis, systemic sepsis, diabetes, or immunosuppression.
  • Surgical choice depends on how much sphincter the tract crosses: low tracts are laid open, high or complex tracts need a seton, LIFT, advancement flap or fibrin glue/plug to protect continence.
  • Recurrent, multiple, or complex fistulae should prompt investigation for Crohn's disease with MRI and colonoscopy, and treatment of the underlying disease alongside surgical drainage.

First-line investigation

Examination under anaesthesia, with MRI pelvis or endoanal ultrasound to map a fistula tract relative to the sphincters, particularly if complex, recurrent, or Crohn's-associated.

Management

Drain the abscess

  • Arrange urgent surgical drainage of an anorectal abscess; antibiotics do not replace source control and are added when there is systemic infection, spreading cellulitis, severe/deep infection or significant immunosuppression.1,2,4

Map complex anatomy

  • Use examination under anaesthesia and pelvic MRI for high, complex, recurrent, occult or Crohn's-associated fistulae; do not force a probe or delay acute drainage for imaging.1,3

Match surgery to sphincter risk

  • Fistulotomy is for selected simple low tracts; high or complex tracts need drainage and a continence-preserving strategy such as a loose seton, LIFT or advancement flap through an experienced colorectal service.1,5

Treat Crohn's-associated disease jointly

  • Drain sepsis and coordinate colorectal and gastroenterology care; use the NICE Crohn's pathway for specialist biologic decisions and avoid definitive sphincter division in active complex disease.3,1,4

Protect continence and detect recurrence

  • Follow healing, discharge, recurrent pain, continence and systemic symptoms; investigate a non-healing wound or recurrent abscess for an underlying fistula or Crohn's disease.2,1

Exam traps

  • Antibiotics alone do not treat a perianal abscess: a walled-off collection needs incision and drainage regardless of antibiotic response, and antibiotics are reserved for cellulitis, systemic sepsis, diabetes or immunosuppression.
  • A fistula that keeps 'reabscessing' is not treatment failure of the abscess drainage, but the natural history of an untreated tract with an intact internal opening.
  • Do not lay open a high fistula tract without imaging: dividing too much sphincter risks incontinence, which is why setons and sphincter-preserving techniques exist.
  • Multiple or unusually sited fistulae in a young patient should trigger consideration of Crohn's disease, not be assumed cryptoglandular by default, and fistulotomy should be approached cautiously in active Crohn's disease.

Illustrations

Cryptoglandular origin of perianal sepsisDiagram of an anal gland at the dentate line becoming obstructed and infected, with the resulting abscess tracking through the intersphincteric space.PassFinals · original
Classification of perianal abscess by locationCross-sectional illustration showing perianal, ischiorectal and supralevator abscess locations relative to the anal sphincters.PassFinals · original
Goodsall's rule for fistula tractsDiagram of the anus showing anterior openings tracking radially inward and posterior openings curving to a posterior midline internal opening.PassFinals · original

Key sources

  1. ACPGBI: The treatment of anal fistula, second Position Statement (2018) (UK colorectal-surgery position statement covering anal-sepsis aetiology, investigation, complexity, fistulotomy, setons, sphincter preservation and Crohn's-associated disease; evidence quality is variable and treatment must be individualised.)
  2. NHS: Anal fistula (NHS information on typical causes, referral, assessment, surgery, setons and continence risk.)
  3. NICE NG129: Crohn's disease: management (Current NICE Crohn's pathway used for assessment of suspected Crohn's disease and treatment of active fistulising Crohn's disease after conventional therapy, including specialist biologic decisions.)
  4. BNF online: current antibacterial and biologic prescribing (Use the current BNF and local antimicrobial, gastroenterology and surgical protocols for antibiotic choice, dosing, renal adjustment, biologic screening, contraindications, interactions, duration and monitoring; no fixed regimen is reproduced here.)
  5. NHS: Anal fistula treatment (NHS treatment information describing fistulotomy, seton, advancement flap, LIFT and selected non-cutting options, with emphasis on matching surgery to sphincter involvement and continence risk.)

This page is exam revision material, not medical advice, and must not be used for patient care. Always check drug doses against the BNF and current guidance. Full disclaimer.