Anal Fissure
A linear tear in the sensate anal mucosa that triggers reflex internal sphincter spasm, and that spasm both causes the pain and starves the wound of blood, locking the fissure into a self-perpetuating cycle that pharmacological sphincter relaxation is designed to break.
In a nutshell
A hard stool tears the sensate anoderm, and the resulting pain triggers internal anal sphincter spasm. That spasm both re-injures the tissue at the next defecation and compresses the blood supply to the fissure base, locking it into a pain-spasm-ischaemia cycle that will not simply heal with time.
Classic presentation
Severe, tearing pain on defecation with a persistent throbbing ache afterwards, and a small streak of bright red blood, following passage of a hard stool, with a linear posterior midline tear on examination.
Key points
- The fissure lies below the dentate line in somatically innervated anoderm, which is why it is intensely painful, unlike haemorrhoidal bleeding above the line.
- The pain-spasm-ischaemia cycle is the core mechanism: pain raises internal sphincter tone, spasm re-tears the wound and compresses its blood supply, and ischaemia stops healing.
- Topical GTN and diltiazem work by relaxing the internal sphincter pharmacologically, restoring perfusion to allow healing: this is why they are first-line rather than analgesia alone.
- Lateral internal sphincterotomy achieves the same goal mechanically when topical treatment fails, by permanently lowering resting sphincter pressure.
- Lateral, multiple, or non-healing fissures are atypical and should prompt investigation for Crohn's disease rather than assuming a simple idiopathic fissure.
First-line investigation
Clinical inspection of the perianal skin; digital examination is usually deferred acutely due to pain.
First-line management
Stool softening with dietary fibre and laxatives, plus topical glyceryl trinitrate to relax the internal sphincter and permit healing.
Exam traps
- Do not perform a routine digital rectal examination in a patient with a suspected acute fissure in clinic: pain and sphincter spasm make it unnecessary and poorly tolerated; diagnosis is visual.
- A fissure that is lateral, multiple or recurrent is atypical for the idiopathic mechanical cause and should raise suspicion of Crohn's disease.
- GTN headache is a recognised and common side effect due to systemic vasodilation, not a sign of treatment failure; diltiazem is the alternative if not tolerated.
- The posterior midline location of most fissures follows from relatively poor blood supply there, not from anatomical trauma pattern alone: this is examinable mechanistic reasoning, not rote memorisation.
Educational content pending clinical review. Not medical advice.