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This is a clinician-written, evidence-based summary aligned to the 2026 MLA Content Map. It is intended for medical students and junior doctors preparing for the UKMLA. Always cross-reference with NICE guidance, local protocols, and clinical judgement.
The Bottom Line
- Tear in the anoderm, most commonly posterior midline (6 o'clock in lithotomy position) — if lateral or multiple, think Crohn's, HIV, TB, or malignancy
- Presents with severe sharp pain during and after defaecation + minor bright red rectal bleeding on wiping
- Acute (<6 weeks): usually heals with conservative treatment — stool softeners, adequate fibre and fluids, topical anaesthesia
- Chronic (>6 weeks): topical GTN 0.4% ointment or diltiazem 2% cream (chemical sphincterotomy) — first-line
- Refractory chronic fissure: botulinum toxin injection or lateral internal sphincterotomy (definitive but risk of incontinence)
Overview
An anal fissure is a longitudinal tear in the squamous epithelium of the anal canal, extending from the anal verge to the dentate line. Most (~90%) occur in the posterior midline, where the blood supply to the anoderm is poorest. The tear exposes the internal anal sphincter, which goes into spasm, reducing local blood flow and impairing healing — creating a cycle of pain, spasm, and ischaemia. Causes include passage of hard stool, straining, diarrhoea, childbirth, and inflammatory conditions (Crohn's disease). Fissures not in the posterior midline should raise suspicion of underlying disease.
Epidemiology
Anal fissures are very common, affecting all age groups with peak incidence in young adults (20–40 years). They are equally common in men and women. Most acute fissures heal spontaneously within 6 weeks with conservative measures. Approximately 40% of acute fissures become chronic (>6 weeks) if not appropriately managed. Chronic fissures are characterised by a visible white fibrous base (exposed internal sphincter), sentinel skin tag distally, and hypertrophied anal papilla proximally — the "triad of chronicity."
Clinical Features
Symptoms
Severe sharp or tearing pain during defaecation, lasting minutes to hours afterwards
Minor bright red blood on wiping or on surface of stool (NOT mixed with stool)
Fear of defaecation (leading to constipation, which worsens the fissure)
Perianal itching or irritation
Signs
Visible tear in posterior midline on gentle parting of buttocks (inspection — do NOT perform DRE in acute fissure due to pain)
Sentinel skin tag (chronic fissure — at distal end of tear)
Sphincter spasm on attempted examination
Lateral or multiple fissures (suggests Crohn's, HIV, TB, syphilis, or anal carcinoma)
Investigations
First-line
Clinical diagnosisVisual inspection of the anal verge is usually sufficient. Gentle parting of buttocks reveals the fissure. DRE often impossible due to pain and spasm
Second-line
Examination under anaesthesia (EUA)If unable to examine adequately in clinic, or if atypical features/suspicion of other pathology
Proctoscopy/sigmoidoscopyOnce healed or under anaesthesia — to exclude other pathology if indicated
Specialist
Anorectal manometryRarely needed — may assess sphincter tone before considering sphincterotomy, particularly if concerns about incontinence
BiopsyIf atypical appearance — exclude Crohn's disease, malignancy, or infection
1
Acute fissure (<6 weeks)
- Dietary fibre and adequate fluid intake
- Stool softeners: bulk-forming laxative (ispaghula husk) ± osmotic laxative (macrogol)
- Topical anaesthetic: lidocaine 5% ointment before defaecation for pain relief
- Warm sitz baths (10–15 min after bowel movements — relaxes sphincter)
- Most heal within 6 weeks with these measures
2
Chronic fissure (>6 weeks)
- Continue dietary and stool management
- First-line topical: GTN 0.4% ointment applied to anal margin BD for 6–8 weeks (relaxes internal sphincter, improves blood flow). Side effect: headache (~50%)
- Alternative if GTN not tolerated: diltiazem 2% cream BD (fewer headaches)
- Healing rate with topical therapy: ~50–70%
3
Refractory chronic fissure
- Botulinum toxin A injection into internal sphincter: second-line if topical therapy fails (~60–80% healing rate)
- Lateral internal sphincterotomy: definitive surgical treatment — division of lower portion of internal sphincter. Healing rate >95%
- Risk of sphincterotomy: minor faecal incontinence in ~5–10% (flatus/soiling) — discuss with patient
- Sphincterotomy is preferred to manual anal dilatation (Lord procedure), which has higher incontinence rates
Complications
- Chronicity: ~40% of untreated acute fissures become chronic
- Perianal abscess/fistula: From secondary infection of chronic fissure (uncommon)
- Constipation cycle: Fear of defaecation → stool withholding → harder stools → worsening fissure
- Post-sphincterotomy incontinence: Minor flatus/soiling incontinence in ~5–10%
UKMLA Exam Tips
- 1Posterior midline (6 o'clock) = typical fissure. LATERAL or MULTIPLE fissures = think Crohn's, HIV, TB, or cancer
- 2Do NOT perform DRE in acute anal fissure — it's too painful and the diagnosis is made by inspection
- 3GTN 0.4% ointment = first-line for chronic fissure. Main side effect: headache. Alternative: diltiazem 2% cream
- 4Triad of chronic fissure: white fibrous base, sentinel skin tag, hypertrophied anal papilla
- 5Lateral internal sphincterotomy = definitive treatment for refractory chronic fissure (>95% cure rate)
practicetest your knowledge on anal fissureApply what you've learnt with UKMLA-style questions from the iatroX Q-Bank — gastroenterology and beyond.
open q-bank