The etiology and pathogenesis of anal fissure (AF) is one of the most common causes of severe anal pain yet has never been fully understood. Factors which predispose people to developing AF include diarrhea, constipation, childbirth, medication as well as constant saddle vibration (amongst professional mountain-bikers) and using a jet of water from a bidet-toilet. For many years, it has been generally accepted that a sphincterotomy, whether surgical or pharmacologic, treats chronic AF because it produces a reduction in anal pressure, reverses sphincter spasms, and promotes fissure healing. However, recent studies cast doubt upon this explanation. A new theory explains that AF healing depends on biochemical processes in the anal passage. Eruption of tissues in the fissure region during defecation releases platelet products such as ADP, ATP, 5-HT, platelet activation factor, thrombin and substance P which cause the contraction of smooth muscles (of Internal Anal Sphicter and vessels) and result in difficulties in AF healing. The effect of a sphincterotomy is to reduce sphincter trauma during defecation and the consequent release of these potent smooth muscle contractors. An injection of botulinum toxin is thought to release the blockage in glyceryl trinitrate bioactivation in smooth muscle cells and suppress basal continuous sympathetic activity, resulting in AF healing.
