The Musculo-Elastic Theory of anorectal function and dysfunction

The mechanics of defecation and fecal continence are poorly understood. Valvular theories for continence 2 rely on raised intra-abdominal pressure to force the anterior wall of the rectum downwards to close off the anorectal junction. Such theories are not consistent with EMG and radiological data which suggest a striated muscle sphincteric mechanism.3 It has been demonstrated that puborectalis and external anal sphincter muscles contract during effort, indicating a role for both in fecal continence.4 The internal anal sphincter is also said to be important for feces continence.5 The mechanism of defecation is even more poorly understood. According to one description,3 feces enter the anal canal, stimulate stretch receptors and produce the urge to defecate. The internal and external anal sphincters relax, decreasing the pressure within the anus. The rectum contracts and with the assistance of raised intraabdominal pressure (Valsalva) expels the feces. Shafik 6 suggested an active muscular mechanism for anorectal opening and closure. In his proposal, during straining, the puborectalis, acting as the upper part of a triple loop system, contracts to close off the anal canal. No role is assigned by Shafik for levator plate contraction during anorectal closure. According to Shafik, during defecation, the puborectalis muscle relaxes, the levator plate contracts, and the “suspensory sling” lifts upwards to open out the anorectal canal prior to evacuation by rectal detrusor contraction.