Myoarchitectural and functional alterations in rabbit external anal sphincter muscle following experimental surgical trauma

Abstract

Rajasekaran MR, Sinha S, Seo Y, Salehi M, Bhargava V, Mittal RK. Myoarchitectural and functional alterations in rabbit external anal sphincter muscle following experimental surgical trauma. Am J Physiol Gastrointest Liver Physiol 307: G445-G451, 2014. First published July 3, 2014; doi:10.1152/ajpgi.00450.2013.-Obstetrical trauma to external anal sphincter (EAS) is extremely common; however, its role in the development of anal incontinence is not clear. We examined the regenerative process and functional impact of experimental surgical trauma to EAS muscle in an animal model. Surgical myotomy, a craniocaudal incision extending along the entire length and thickness of the EAS, was performed in rabbits. Animals were allowed to recover, and anal pressures were recorded at weekly intervals for 12 wk using a custom-designed probe system to determine the length-tension property of EAS muscle. Animals were killed at predetermined time intervals, and the anal canal was harvested for histochemical studies (for determination of muscle/connective tissue/ collagen) and sarcomere length measurement. In addition, magnetic resonance diffusion tensor imaging (MR-DTI) and fiber tracking was performed to determine myoarchitectural changes in the EAS. Myotomy of the EAS muscle resulted in significant impairment of its length-tension property that showed only partial recovery during the 12-wk study period. Histology revealed marked increase in the fibrosis (connective tissue ϭ 69% following myotomy vs. 28% in controls) at 3 wk, which persisted at 12 wk. Immunostaining studies confirmed deposition of collagen in the fibrotic tissue. There was no change in the sarcomere length following myotomy. MR-DTI studies revealed disorganized muscle fiber orientation in the regenerating muscle. We conclude that, following experimental injury, the EAS muscle heals with an increase in the collagen content and loss of normal myoarchitecture, which we suspect is the cause of impaired EAS function. length-tension; fibrosis; magnetic resonance diffusion tensor imaging FECAL INCONTINENCE (FI) is extremely common and reported by 7-10% of women over 60 years of age (1, 3, 4). Childbirthrelated injury to the muscles of the anal sphincter complex is a possible cause of FI in women (11,26). The injury to these muscles occurs through several mechanisms. First is the stretch of the muscles caused by the fetus during its passage through the birth canal. Second is the trauma related to instrumentation, and third is surgical episiotomy. Women who sustain thirddegree obstetrical tear develop FI soon after delivery if muscles are not properly repaired (13). However, in the significant majority of women, injury to the anal sphincter and other pelvic floor muscle is occult. Symptoms develop in the fifth to sixth decade of life, the reasons for which are not clear.Of the three components of the anal sphincter complex [the internal anal sphincter, external anal sphincter (EAS), and puborectalis muscle (PRM)], it is generally believed that EAS muscle plays a significan...

Wnt-β Catenin Signaling Pathway: A Major Player in the Injury Induced Fibrosis and Dysfunction of the External Anal Sphincter

“…The results of this study confirm our previous findings that the EAS surgical myotomy results in, (1) loss of muscle function, i.e., decrease in anal canal pressure and alteration in EAS length-tension property, and (2) spontaneous EAS repair following EAS myotomy occurs through excessive collagen deposition and reduction in muscle mass 9 . The new findings of our studies are, (1) following EAS myotomy, there is upregulation of Wnt-β catenin and TGF-β signaling pathways, (2) we monitored the sphincter muscle healing after interventions with a Wnt antagonist (sFRP-2) for 15 weeks post-myotomy.…”

“…We found that following surgical myotomy of the EAS muscle in rabbit, there is marked impairment of its function (length-tension function) and the anal canal pressure 9 . Histological evaluation revealed that the spontaneous repair following surgical myotomy to the EAS muscle occurs not by replacement with normal muscle, instead with fibrous tissue 9 .…”

“…4B) as described previously 9 and animals were allowed to recover. Prior to myotomy and immediately afterwards, the anal canal pressure recordings in response to EAS muscle electrical stimulation were obtained to study the length-tension (L-T) property of the EAS muscle, as described earlier 32–34 .…”

“…Histological evaluation revealed that the spontaneous repair following surgical myotomy to the EAS muscle occurs not by replacement with normal muscle, instead with fibrous tissue 9 . The molecular mechanism(s) that dictate whether an injury heals by fibrosis or muscle is not known.…”

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

“…The central longitudinal muscles identified from the DTI data that span from the proximal to the distal sphincters may potentially be assigned to the longitudinal smooth muscle identified by anatomical dissection, while the circular muscle surrounding it (identified on anatomical dissections) is probably too thin to be detected at the resolutions afforded at 3T in a DTI acquisition. However, it should be noted that a similar circular smooth muscle surrounding the longitudinal smooth muscle has been visualized in the anal sphincter muscle complex in an animal model acquired at higher resolutions on an MR scanner operating at 7T …”