Define Fistula-in-Ano. Describe its aetiology and classification.

The Concept

A fistula is an abnormal, epithelialised track connecting two epithelial surfaces. A fistula-in-ano connects the anal canal (an internal opening, usually at the dentate line) to the perianal skin (an external opening). It usually represents the chronic phase of an anorectal abscess — after the abscess drains, a persistent track remains, keeping the cycle of infection going. The central challenge in treatment is to cure the fistula while preserving the anal sphincter and continence.

Aetiology

The great majority are cryptoglandular (following an abscess); other causes are Crohn's disease, tuberculosis, malignancy and previous radiation, which should be suspected in atypical, multiple or recurrent fistulas.

Classification (parks)

The Parks classification describes the track's relation to the sphincters: intersphincteric (commonest), transsphincteric, suprasphincteric, and extrasphincteric. Practically, fistulas are also grouped as 'low/simple' (crossing little sphincter — safe to lay open) or 'high/complex' (crossing much sphincter — where laying open would threaten continence).

Goodsall's Rule

Goodsall's rule helps predict the internal opening from the external one. If the external opening lies anterior to an imaginary transverse line across the anus, the track usually runs in a straight, radial line to the nearest point in the anal canal; if it lies posterior, the track usually curves to open in the posterior midline.

Clinical Features, Investigation & Management

Patients have persistent or recurrent perianal discharge (pus or blood), recurrent abscesses and discomfort, often with a palpable track or a visible external opening. Assessment uses examination, gentle probing and proctoscopy, with MRI or endoanal ultrasound for complex or recurrent fistulas. Treatment depends on the anatomy:

• Fistulotomy (laying the track open) — for low/simple fistulas; the laid-open track heals by secondary intention.
• Seton (a thread through the track) — for high/complex fistulas, to drain and gradually manage the track while protecting the sphincter (cutting, loose or draining setons).
• Sphincter-preserving techniques — advancement flap, the lift procedure, and fibrin glue/plug — for complex fistulas.

The guiding principle throughout is to eradicate the fistula while preserving continence.

WHY Continence Is the Key Concern

The reason fistula surgery is more nuanced than simply cutting the track open is the anal sphincter mechanism. A track that crosses a large proportion of the sphincter (a 'high' fistula) cannot be laid open without dividing enough muscle to cause incontinence. This is the whole rationale for the seton and the sphincter-preserving procedures: they allow the fistula to be treated while keeping the sphincter intact. Judging how much sphincter a track crosses — clinically and on MRI — is therefore the crucial decision in planning surgery.

How a Seton Works

The seton deserves explanation as it is frequently asked about. A thread is passed along the fistula track and tied. A loose (draining) seton keeps the track open so it drains and settles, preventing recurrent abscesses while preserving the sphincter. A cutting seton is gradually tightened so that it slowly divides the sphincter muscle while fibrosis forms behind it, so the muscle ends do not spring apart — allowing the track to be cut through over time with less risk of incontinence than a single division. This staged approach is the classic solution to the high fistula.

A Note on Crohn's Fistulas

Crohn's disease is an important cause of complex, recurrent and multiple perianal fistulas, and it changes management. Crohn's fistulas are often high, branching and associated with proctitis, and aggressive fistulotomy risks both poor healing and incontinence. Management therefore combines medical therapy (including anti-TNF agents such as infliximab) with conservative surgery (long-term draining setons) rather than attempts at cure by laying open. Suspecting Crohn's in any patient with atypical or recurrent perianal fistulas is a key clinical point.

• Usually follows an anorectal abscess (cryptoglandular); atypical/recurrent → Crohn's, TB, malignancy.
• Goodsall's rule: anterior external opening → straight radial track; posterior → curved track to posterior midline.
• Low/simple → fistulotomy (lay open); high/complex → seton or sphincter-preserving techniques (preserve continence).

Goodsall rule predicts the internal opening from the external one.

• Fistula-in-ano = epithelialised track from the anal canal (internal opening at dentate line) to perianal skin; usually post-abscess (cryptoglandular).
• Parks classification: intersphincteric (commonest), transsphincteric, suprasphincteric, extrasphincteric; low/simple vs high/complex.
• Goodsall's rule predicts the internal opening (anterior = straight/radial; posterior = curved to posterior midline).
• Persistent/recurrent perianal discharge and abscesses; MRI/endoanal ultrasound for complex/recurrent.
• Fistulotomy for low fistulas; seton or sphincter-preserving techniques for high fistulas (preserve continence); suspect Crohn's/TB if atypical.