Neuromyogenic properties of the internal anal sphincter: Therapeutic rationale for anal fissures

Abstract

Lateral sphincterotomy diminishes internal anal sphincter hypertonia and thereby reduces anal canal pressure. This improves anal mucosal blood flow and promotes the healing of anal fissures. However, sphincterotomy can be associated with long term disturbances of sphincter function. The optimal treatment for an anal fissure is to induce a temporary reduction of anal canal resting pressure to allow healing of the fissure without permanently disrupting normal sphincter function. Broader understanding of the intrinsic mechanisms controlling smooth muscle contraction has allowed pharmacological manipulation of anal sphincter tone. We performed an initial Medline literature search to identify all articles concerning 'internal anal sphincter' and 'anal fissures'. This review is based on these articles and on additional publications obtained by manual cross referencing. Internal anal smooth muscle relaxation can be inhibited by stimulation of nonadrenergic non-cholinergic enteric neurones, parasympathetic muscarinic receptors, or sympathetic β adrenoceptors, and by inhibition of calcium entry into the cell. Sphincter contraction depends on an increase in cytoplasmic calcium and is enhanced by sympathetic α adrenergic stimulation. Currently, the most commonly used pharmacological agent in the treatment of anal fissures is topical glyceryl trinitrate, a nitric oxide donor. Alternative agents that exhibit a similar effect via membrane Ca2+ channels, muscarinic receptors, and α or β adrenoceptors are also likely to have a therapeutic potential in treating anal fissures.