A fistula-in-ano is an abnormal hollow tract lined by granulation tissue connecting the internal opening at the dentate line of the anal canal to one or more external openings on the perianal skin. The vast majority arise from infection and spontaneous or surgical drainage of a cryptoglandular abscess originating in the anal glands that empty into the anal crypts at the dentate line. When an anorectal abscess fails to heal completely, a persistent epithelialised tract forms establishing the fistula. Fistulas are anatomically classified by their relationship to the external anal sphincter using Parks' Classification: intersphincteric (most common, tracking between the internal and external sphincters), transsphincteric (crossing through the external sphincter), suprasphincteric (looping above the puborectalis) and extrasphincteric (passing entirely outside the sphincter complex). Secondary causes include Crohn's disease, tuberculosis, malignancy and radiation injury. The critical surgical principle governing all treatment decisions is preservation of sphincter function and prevention of faecal incontinence.
Symptoms
Patients typically present with persistent perianal discharge purulent, blood-stained, or serous which soils undergarments and causes significant perianal irritation, pruritus and discomfort. Recurrent episodes of perianal abscess formation with spontaneous or surgical drainage are the classical preceding history, often repeated over months or years before the fistula is formally diagnosed and treated. A palpable external opening on the perianal skin discharging on gentle pressure is the characteristic clinical finding. Goodsall's Rule predicts the internal opening location: external openings anterior to a transverse anal line track directly to the dentate line, while posterior openings curve to the posterior midline internally.
Diagnosis
The diagnosis is clinical, confirmed by probing and imaging. Examination Under Anaesthesia (EUA) remains the gold standard, allowing thorough assessment of the internal and external openings, the fistula tract course relative to the sphincters and any secondary extensions providing both diagnosis and the basis for immediate surgical treatment. MRI pelvis with endoanal protocol is the investigation of choice for complex, recurrent or Crohn's-related fistulas, providing precise mapping of the tract anatomy, sphincter integrity, and secondary tracks that may not be apparent clinically. Endoanal Ultrasound offers a complementary real-time assessment of sphincter involvement.
Treatment
Treatment is surgical and tailored to the fistula type and sphincter involvement. Fistulotomy laying open the entire fistula tract is the simplest and most effective treatment for low intersphincteric and low transsphincteric fistulas involving minimal sphincter muscle, achieving cure rates exceeding 90%. For complex or high fistulas where fistulotomy would risk incontinence by dividing a significant portion of the sphincter, sphincter-preserving techniques are employed: the Seton suture, a thread passed through the tract is used either as a cutting seton (slowly tightened over weeks to gradually divide the muscle) or a draining seton (maintaining drainage while the tract matures before definitive repair). Advancement flap repair, LIFT procedure (Ligation of the Intersphincteric Fistula Tract) and video-assisted anal fistula treatment (VAAFT) offer sphincter-preserving options with good success rates in selected patients. Crohn's-related fistulas require concurrent medical optimisation with biologics (infliximab) alongside surgical management.