# Aetiology

Aetiology

The cause of  an anal fissure, and particularly the reason why the posterior midline is so frequently a ff ected, is not completely understood. The location in the posterior midline may relate to the shearing forces acting at that site at defecation, combined with a less elastic anoderm endowed with an increased density of  longitudinal muscle extensions in that region of  the anal circumference. Anterior anal fissure is more common in women and may arise following vaginal delivery . A ff ected tonia, which, in turn, enhances the traumatic e ff ect of  the hard stool and perpetuates relative tissue ischaemia with a decrease in blood supply to the anal mucosa. After the initial tear, a vicious cycle of  non-healing and repeated trauma leads to the dev elopment of  chronic deep fissures. Local pain increases sphincter hypertonia, which worsens hard stool and local tissue ischaemia.