anal fistula

**Definition:** A chronic abnormal tract between the anal canal (internal opening, usually at the dentate line in an anal crypt) and the perianal skin (external opening).

**Pathophysiology (cryptoglandular theory):** - Anal glands drain into crypts at the dentate line. Obstruction and infection cause a perianal abscess (acute phase). - The abscess drains spontaneously or surgically, and the tract persists as a fistula (chronic phase). About 30–50% of abscesses lead to a fistula. - Other causes: Crohn disease (key association, often complex or recurrent), TB, HIV, trauma, radiation, malignancy, lymphogranuloma venereum, actinomycosis, and obstetric injury.

**Parks classification (relation to the sphincter complex):** 1. Intersphincteric (most common): tract stays between internal and external sphincters. 2. Transsphincteric: crosses the external sphincter into the ischioanal fossa. 3. Suprasphincteric: goes up over the puborectalis, then down to the skin. 4. Extrasphincteric: runs from the rectum through the levators to the skin, bypassing the sphincters (rare).

**Goodsall rule:** An external opening posterior to the transverse anal line has a curved tract to the posterior midline internal opening. An anterior external opening has a straight radial tract. Exception: an anterior opening more than 3 cm from the anal verge may track to the posterior midline.

**Clinical presentation:** - Persistent or intermittent purulent, bloody, or fecal perianal drainage - Perianal pain and swelling, pruritus, and recurrent abscesses - Exam: external opening (granulation tissue) with a palpable indurated cord

**Diagnosis:** - Clinical exam (inspection, digital rectal exam, anoscopy) - Imaging for complex cases: MRI pelvis (gold standard) or endoanal ultrasound - Exam under anesthesia - Evaluate for Crohn disease if recurrent or complex

**Management:** - Surgery is the mainstay. Fistulae rarely heal spontaneously. - Fistulotomy (lay-open) for simple, low fistulas involving little sphincter muscle. - Seton placement (a drainage or cutting seton) for high or complex fistulas, to preserve continence. - Advancement flap, LIFT (ligation of the intersphincteric fistula tract), fibrin glue, and plugs for complex fistulas. - Crohn disease: medical therapy (anti-TNF such as infliximab, with antibiotics) plus a seton. Avoid aggressive surgery.

**Complications:** - Fecal incontinence (from sphincter damage), recurrence, and chronic abscess - Rarely, malignant transformation (adenocarcinoma in long-standing fistulas)

**High-yield board pearls:** - Perianal abscess followed by a persistent draining tract points to an anal fistula. - Recurrent or complex perianal fistulas should prompt a Crohn disease workup. - Do not confuse with an anal fissure (painful tear, posterior midline, bright red blood on wiping) or hemorrhoids. - Goodsall rule predicts the internal opening location. - Priority in surgery: balance cure against preserving continence.