Fistula-in-ano

Persistent communication between the anal canal and peri-anal skin following spontaneous or surgical drainage of a peri-anal abscess.

Risk factors

• Crohn disease
  • Multiple external openings - 'watering can perineum'
• Leukaemia
• Lymphogranuloma venereum
• Radiation proctitis
• Rectal foreign body
• AIDS
• Malignancy

Pathophysiology

• Chronic manifestation of a perianal abscess that ruptures or is drained - the tract can epithelialize and connect abscess with skin

Presentation:

• Intermittent anal pain - typical cyclical pattern of pain, followed by relief as drainage occurs
• Pruritis
• Purulent/mucoid/blood discharge, impaired anal hygiene, soiling.
• Previous episode perianal sepsis
• Should be able to see the external opening and perhaps feel a cord-like tract
• Internal opening may be palpable

Describing fistulae:

• Course of primary track between anal canal and skin (Parkes)
• The primary opening is generally at the level of the dentate line
• Presence of high blind tracks
• Any circumferential involvement
• Simple vs complex

Parkes classification (1976) essentially describes the relationship of the fistula to the external sphincter

1. Inter-sphincteric: Begins at dentate line and tracks along intersphincteric plane (goes through internal but not external sphincter)
2. Trans-sphincteric: Through both sphincters into the ischiorectal fossa, and terminates in buttock skin. Usually EO will be further away from the anal verge than for inter-sphincteric.
3. Supra-sphincteris: Originates in anal crypt, encircles entire sphincter, terminates in buttock
4. Extra-sphincteric: Originates high in anal canal - often due to iatrogenic injury, Crohn's, trauma

• Superficial is not part of Park's classification

Goodsall's rule:

• All fistula tracks with external openings within 3 cm of the anal verge and posterior to a line drawn through the ischial spines travel in a curvilinear fashion to the posterior midline.
• All tracks with external openings anterior to this line enter the anal canal in a radial fashion.
• Less true for women with anterior openings
• Less true for external openings >3cm from the anal verge anteriorly - these are often an anterior extension of a horseshoe abscess originating posteriorly
• Multiple secondary openings also points to a primary opening in the midline, along with suspicion for Crohn's/TB/hidradenitis/pilonidal disease

Management:

• EUA is essential. Identify fistula tract as per topic under 'colorectal operations'
• Of course, treatment is individualised to the fistula characteristics.
• Low-lying tracks that do not traverse sphincter can be laid open with fistulotomy.
• Seton fistulotomy is an option for tracts traversing the sphincter
  • Theoretically, should slowly cut through the striated muscle, producing fibrosis in its wake, preventing the muscle from 'springing open' and creating a 'gutter' through which soiling would happen if it was done at operation.

Crohn's disease

• Surgical treatment is not generally a productive path to pursue
• Treat sepsis with a loose seton and refer for medical management
• ACCENT II trial showed healing rates of 36% with infliximab vs 19% with placebo, therefore patients should be seen by gastro ASAP
• Best chance is to monitor the patient with serial EUA while medical management is escalated, then try to remove setons when everything is optimised and disease is quiescent