Purse-string morphology of external anal sphincter revealed by novel imaging techniques

Abstract

Mittal RK, Bhargava V, Sheean G, Ledgerwood M, Sinha S. Purse-string morphology of external anal sphincter revealed by novel imaging techniques. Am J Physiol Gastrointest Liver Physiol 306: G505-G514, 2014. First published January 23, 2014 doi:10.1152/ajpgi.00338.2013.-The external anal sphincter (EAS) may be injured in 25-35% of women during the first and subsequent vaginal childbirths and is likely the most common cause of anal incontinence. Since its first description almost 300 years ago, the EAS was believed to be a circular or a "donut-shaped" structure. Using three-dimensional transperineal ultrasound imaging, MRI, diffusion tensor imaging, and muscle fiber tracking, we delineated various components of the EAS and their muscle fiber directions. These novel imaging techniques suggest "purse-string" morphology, with "EAS muscles" crossing contralaterally in the perineal body to the contralateral transverse perineal (TP) and bulbospongiosus (BS) muscles, thus attaching the EAS to the pubic rami. Spin-tag MRI demonstrated purse-string action of the EAS muscle. Electromyography of TP/BS and EAS muscles revealed their simultaneous contraction and relaxation. Lidocaine injection into the TP/BS muscle significantly reduced anal canal pressure. These studies support purse-string morphology of the EAS to constrict/close the anal canal opening. Our findings have implications for the effect of episiotomy on anal closure function and the currently used surgical technique (overlapping sphincteroplasty) for EAS reconstructive surgery to treat anal incontinence. external anal sphincter muscle architecture; anal incontinence; childbirth-related injury; magnetic resonance diffusion tensor imaging AN UNDERSTANDING OF THE MORPHOLOGY of the external anal sphincter (EAS), one of the superficial muscles of the pelvic floor, is important, since damage to this muscle may occur in 25-35% of women during vaginal childbirth (34). Injury to the EAS muscle is an important cause of anal incontinence, which has a devastating effect on an individual's quality of life (8). Morphology of the EAS has intrigued many investigators. From the original description by Santorini at the turn of 18th century (27) to the most current texts (2, 19), the EAS is described as a three-component (subcutaneous, superficial, and deep) circular muscle structure (2, 19). Others have argued that the deep part of the EAS is actually the puborectalis muscle (PRM) (7,14,23,25). MRI studies of Hussain et al. (14) and our recent three-dimensional (3-D) ultrasound (US) images, combined with high-definition manometry maps (25), support the idea that the deep part of the EAS is indeed the PRM, because it has a "sling" shape. On the basis of 3-D US images and anal canal pressures obtained by high-definition manometry (HDM), our study shows that the increase in pressure with voluntary contraction of the proximal and distal halves of the anal canal is related to contraction of the PRM and EAS, respectively (25).Another aspect of EAS morphology that has never been questi...

Exploration of male urethral sphincter complex using diffusion tensor imaging (DTI)‐based fiber‐tracking

“…Additional validation for the current study comes from the good agreement of the DTI values of the external anal sphincter extracted from the current study to values reported earlier for the same structure in Ref. 15.…”

“…The DTI values of the EAS values from Ref. are: (λ 1 : 1.68 ± 0.02; λ 2 : 1.22 ± 0.02: λ 3 : 0.95 ± 0.02; FA: 0.295 ± 0.04) with the eigenvalues in units of 10 −3 mm 2 /sec. The current values are in good agreement with those reported in the literature …”

“…The anal sphincter muscle, which is also very well visualized in the images, was analyzed to provide a validation of the DTI values extracted from the current study by comparison to normal values that have been reported in an earlier study for the external anal sphincter. 15…”

Relationship Among Anal Sphincter Injury, Patulous Anal Canal, and Anal Pressures in Patients With Anorectal Disorders

“…36 It is conceivable but unknown if perianal bulking agents restore the anal seal. Finally, a modified external anal sphincteroplasty, designed to restore the purse string morphology of this muscle, 37 may be more likely to close a patulous anal canal.…”

High‐frequency ultrasound imaging of the anal sphincter muscles in normal subjects and patients with fecal incontinence

“…However, our work proves that the PB is the site of crossing of EAS muscle fascicles from right and left sides, which then continue as transverse perineal and bulbospongiosus muscles to be attached to the pubic rami of the opposite sides. The EAS is not composed of circular muscle fibers; rather, it is shaped like a “purse string” with crossing of the two sides of EAS in the PB . Therefore, we believe that PB is an extremely important part of the EAS muscle; it is also the most frequent site of damage in women due to childbirth‐related injuries and surgical episiotomy.…”