Anal Sphincter Morphology & Function Assessed with Novel MR Imaging Techniques

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DESCRIPTION (provided by applicant): Internal anal sphincter (IAS), external anal sphincter (EAS) and puborectalis muscle (PRM) are 3 sphincter mechanisms that provide triple security mechanism for the anal continence. EAS is most commonly affected muscle in patient with fecal incontinence, the reason being that it gets injured fairly frequently during vaginal child birth. Up to 35% of women have defects of the EAS muscle after the first vaginal delivery, as detected by US imaging. Overlapping sphincteroplasty is the most commonly performed surgery in patients with fecal incontinence when US images of the anal sphincter muscle show major anatomical defects. However, the success rate of the overlapping sphincteroplasty is less than 50% at 5 years. There are probably several reasons for the failure of overlapping sphincteroplasty; we propose that one of the reasons is incorrect understanding of the morphology of the EAS muscle. Current thinking is that the EAS is a ring or a donut shaped muscle and circumferential squeeze generated by the EAS muscle is responsible for the genesis of EAS contraction related increase in the anal canal pressure. Our preliminary data show that that the EAS is not a circular muscle; instead the muscle has the unique "figure of 8" type of morphology and the constricting action of the EAS is due to the "tying of knot" like action of what is currently known as the EAS and transverse perinea muscles. In fact, transverse perinea are actually extension of the EAS muscles. We use state of the art magnetic resonance imaging techniques, i.e., diffusion tensor imaging to determine the true morphology of EAS muscle. Furthermore, we use velocity encoded dynamic MR imaging to determine the dynamic motion of EAS muscle fibers to prove the nature of constricting action of EAS muscle. We believe that correct understanding of the EAS morphology is of paramount importance in prevention of injury in the first place and repair of EAS muscle during overlapping sphincteroplasty. Lateral episiotomy, which incises transverse perinea muscle, is thought to spare the EAS muscle, as compared to midline episiotomy in which the incision extends to the EAS muscle. We propose that the transverse perinea are indeed the EAS muscle and lateral episiotomy is not a sphincter sparing procedure. Our studies also show that the PRM is not only responsible for the formation of anorectal angle; it actually causes closure of the proximal half of the anal canal. For these studies, we utilize state of the art, 3D-US imaging and high definition manometery to prove our hypothesis. We are confident that our studies will provide a new understanding of the anatomy and function of anal canal and with this new understanding a significant stride can be made in the prevention and treatment of fecal incontinence.