Perianal fistulas
DEFINITION AND ANATOMY
A fistula-in-ano is an abnormal epithelialised or granulation-lined tract connecting the anal canal/rectum to perianal skin. Most arise after cryptoglandular infection. The internal opening is usually near the dentate line. Relationship to the internal and external sphincters determines treatment and continence risk.
CLASSIFICATION
Parks classification: intersphincteric (between sphincters), transsphincteric (crosses external sphincter into ischioanal fossa), suprasphincteric (passes above puborectalis then descends) and extrasphincteric (from rectum above levator to skin, often due to trauma/Crohn disease rather than cryptoglandular infection). Fistulas are also described as simple or complex; high tracts, multiple branches, anterior fistula in women, recurrent disease, Crohn disease and pre-existing incontinence are complex features.
AETIOLOGY AND CLINICAL FEATURES
The commonest cause is a previous perianal abscess. Other causes include Crohn disease, tuberculosis, malignancy, radiation, trauma and hidradenitis. Symptoms include intermittent purulent/bloody discharge, recurrent swelling, pain, irritation and an external opening that may close and reopen. Ask about bowel symptoms, weight loss, prior abscesses, continence and inflammatory bowel disease.
INVESTIGATIONS
Examination identifies external openings, scars and induration; digital rectal examination and anoscopy may identify an internal opening. MRI pelvis is the preferred mapping investigation for complex/recurrent fistula and Crohn disease. Endoanal ultrasound is an alternative in experienced hands. Examination under anaesthesia allows probing and drainage but probing should be gentle to avoid creating a false passage. Colonoscopy is indicated when Crohn disease or colorectal pathology is suspected.
MANAGEMENT
The primary goals are eradication of sepsis and fistula while preserving sphincter function. Simple low fistula involving minimal sphincter may be treated by fistulotomy. High/complex fistulas generally require a loose draining seton followed by a sphincter-preserving strategy such as LIFT (ligation of intersphincteric fistula tract), advancement flap or selected endoscopic/laser techniques. Treat Crohn disease medically with gastroenterology input; surgery controls sepsis and selected tracts. Antibiotics are not definitive fistula treatment unless infection/cellulitis or systemic illness is present.
COMPLICATIONS
Recurrence, persistent sepsis, secondary extensions, scarring and faecal incontinence are key complications. Accurate preoperative mapping and sphincter-preserving planning are essential.