**2. Pathogenesis**

The triggering factor is usually thought to be trauma to the anoderm from the passage of hard stool or chronic irritation from diarrhea, but the exact etiology of anal fissure remains unclear. Two major hypotheses have been proposed regarding the development of chronic fissures: the presence of hypertonicity in the internal anal sphincter and relatively decreased tissue perfusion in the posterior midline [3–5] (**Figure 1**).

It has also been suggested that hypertonicity may result in pressure on the perpendicular vessels in the internal anal sphincter muscle and may compromise perfusion to the posterior midline even more [6]. Therefore, most of the medical and surgical treatments have been developed to decrease the internal sphincter's tonicity. Regardless of these hypotheses, hypertonicity may not be found in all patients with chronic anal fissures. Also, constipation and hard bowel movements have only been reported in 13% of these patients [7, 8].
