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.
First principles
A fissure starts as simple mechanical trauma below the dentate line
The passage of a hard, large stool (most often from constipation) or forceful, high-pressure defecation over-stretches and tears the anoderm, typically at the posterior midline where blood supply from the inferior rectal artery is relatively sparse and the anal canal is least distensible. Because this tissue lies below the dentate line, it is somatically innervated, so the tear itself is exquisitely painful, unlike bleeding from insensate haemorrhoidal tissue above the line.
Educational content pending clinical review. Not medical advice.