Anorectal Disorders — Anal Fissure, Fistula-in-Ano (Goodsall, Parks), Haemorrhoids and Perianal Abscess

Almost every anorectal question turns on the dentate line. Above it, the lining is columnar and pain-insensitive; below it, the anoderm is squamous and supplied by somatic sensory branches of the pudendal nerve, so it is exquisitely painful. That is why an internal haemorrhoid bleeds painlessly while a fissure or a thrombosed external haemorrhoid hurts.

• Anal glands lie mainly in the intersphincteric space and drain into the crypts of Morgagni at the dentate line — the starting point of the cryptoglandular abscess and fistula.
• Internal haemorrhoids arise above the dentate line from the superior haemorrhoidal cushions and drain through the superior rectal vein into the portal system.
• External haemorrhoids lie below the dentate line around the anal verge, covered by skin and anoderm.
• The internal anal sphincter (smooth muscle) is the target of fissure treatment; the external sphincter must be preserved in fistula surgery to protect continence.

An anal fissure is a longitudinal tear in the anoderm distal to the dentate line. Most are idiopathic and solitary, and are linked to hypertonia of the internal anal sphincter, which reduces perfusion, causes ischaemia and perpetuates a cycle of pain and spasm. Fissures are acute if symptoms last less than 6 weeks and chronic beyond that.

• Site: primary fissures occur mostly in the posterior midline, where blood flow is less than half that of the rest of the anal canal; a smaller number are anterior midline, often linked to external sphincter injury.
• Off-midline or multiple fissures are secondary — think Crohn disease, tuberculosis, HIV, syphilis/STIs, malignancy or trauma.
• Symptoms: severe pain during and for hours after defecation, with streaks of bright red blood rather than heavy bleeding.
• Chronic fissure triad: the fissure, a sentinel skin tag distally and a hypertrophied anal papilla proximally; exposed horizontal internal-sphincter fibres in the base.

Most anorectal abscesses are cryptoglandular: an anal gland duct blocks, the gland becomes infected and pus first collects in the intersphincteric space. From there it spreads downward to the perianal skin, across the external sphincter into the ischiorectal (ischioanal) fossa, upward to the supralevator space, or posteriorly to form a horseshoe abscess. Usual organisms are *E. coli* and *Bacteroides*.

• Risk factors: smoking, obesity, diabetes and other immunocompromised states, Crohn disease; also TB, actinomycosis, STIs and foreign bodies.
• Examination: tender fluctuant perianal swelling; an intersphincteric abscess may show little outside but is a boggy, tender bulge on rectal examination, typically at 6 o'clock. Supralevator abscesses cause deep pelvic pain, fever and urinary symptoms.
• Imaging is not routine — reserve CT, MRI or endoanal ultrasound for occult, deep, recurrent or Crohn-related abscesses.
• Treatment: prompt incision and drainage, ideally within 24 hours. Routine antibiotics after adequate drainage are not recommended; give them for cellulitis, systemic sepsis or immunocompromise.

A fistula-in-ano is an epithelialised tract between the anal canal and the perianal skin. Cryptoglandular infection is the commonest cause; the rest follow Crohn disease (more often than ulcerative colitis), trauma including obstetric tears, malignancy, TB and radiation. Parks classified them by their relation to the sphincters:

MRI is the gold-standard imaging for fistula-in-ano, identifying the internal opening, secondary tracts, abscesses and horseshoe extensions with very high accuracy — essential for complex fistulae. Simple fistulae rarely need imaging; CT is inaccurate for tract anatomy and is used mainly to find drainable collections.

Goodsall's rule predicts the site of the internal opening from the position of the external opening, with the patient in lithotomy and the anus divided by a transverse line. StatPearls gives it up to 75% accuracy.

On digital rectal examination the internal opening often feels like a firm, indurated point along the dentate line, and intersphincteric or low transsphincteric tracts may be palpable as a cord.

A fistula that has not healed by 12 weeks counts as treatment failure; discharge restarting within a year of healing counts as recurrence. Recurrence is more likely in smokers, obese patients, those with multiple tracts, high transsphincteric fistulae, horseshoe abscesses or an internal opening that was never found.

Haemorrhoids are normal vascular cushions in the anal submucosa — loose connective tissue, smooth muscle (Treitz's muscle) and arteriovenous connections, which is why the bleeding is bright red. They aid continence. Disease arises with straining, constipation and raised intra-abdominal pressure. The three primary cushions lie left lateral, right anterior and right posterior.

• Rubber band ligation is the most commonly performed office procedure — safe and cheap; StatPearls calls it the gold standard for symptomatic grade 1–3 disease failing conservative care. Contraindicated with acutely thrombosed haemorrhoids and anticoagulation. Delayed bleeding typically occurs 8–14 days later as the tissue sloughs.
• Surgery: excisional haemorrhoidectomy (closed Ferguson technique), stapled haemorrhoidectomy, Doppler-guided devascularisation. Recurrence after surgery is below 5%, versus 10–50% over 5 years with conservative care.
• Incarcerated/strangulated prolapsed internal haemorrhoids → urgent excisional haemorrhoidectomy.
• Acutely thrombosed external haemorrhoid → can be excised under local anaesthesia, with a pressure dressing and sitz baths.
• Rectal varices of portal hypertension are a different entity from haemorrhoids.

Related reading: inguinal canal for groin anatomy, acute appendicitis and intestinal obstruction for other high-yield general-surgery emergencies.