Define fissure in ano

Fissure in ano is a longitudinal tear in the epithelial skin and anal mucosa, most commonly in the posterior (6 o'clock) position. Fissures may occur in the 12 o'clock position in approximately 10% of cases and are more common in women following childbirth.

What is the underlying pathophysiology of this condition?

It is thought that the initial insult may involve traumatic injury, e.g., from the passage of hard stool. However, in the majority of cases this heals without leading to development of a chronic anal fissure. It is likely that, in those patients developing anal fissure, there is an underlying abnormality of the internal anal sphincter leading to hypertonicity. This spasm exacerbates the relative ischaemia of the anodermal mucosa posteriorly and as a result can lead to the development of a fissure.