Ultrasound Imaging of the Anal Sphincter Muscles: Comparison Between Endoanal and Transcutaneous Approaches

Abstract

Introduction: Damage to the anal sphincter muscles is extremely common after vaginal birth and is a major cause of anal incontinence in women. Endoanal US imaging is currently the gold standard to diagnose occult injuries to the anal sphincter muscles. 3D-US imaging of the anal sphincters can also be performed using a transcutaneous (transperineal) approach. Our goal was to compare these two US techniques in assessing the internal anal sphincter (IAS), external anal sphincter (EAS) and puborectalis muscle (PRM). Methods: Studies were performed in 5 nulliparous women using the endoanal BK probe and Philips endo-vaginal probe. For the endoanal approach: the imaging was done using the standard 16 mm diameter anal US probe placed endo-luminally. Endo-vaginal images that provide views of the puborectalis muscle (PRM) were also obtained in each subject using the BK probe. For transcutaneous US imaging, the transducer was placed on the perineum; it was directed in the posterior direction to image the anal sphincters and in the cranial direction to image the PRM. Endoanal and transcutaneous US volumes were analyzed along the cranio-caudal length of the anal canal (every 2.5 mm) to visualize details of the IAS, EAS and PRM. Images were reviewed by DCK, RKM and an experienced US radiologist (DHP). Results:Figure 1 shows views of the IAS and EAS from the endoanal and transcutaneous approaches. Figure 2 shows the PRM and the pelvic floor hiatus from two approaches. Image analysis suggests the following pros and cons between the two US imaging methods. The pros of the transcutaneous approach are: 1) better global view of the IAS and EAS muscles. In addition to circular portions of the EAS, transverse perinea and perineal body, two important components of the EAS muscle complex are also visualized; 2) no deformation related to the US transducer probe in the anal/vaginal canal; 3) ability to record dynamic changes in the anorectal angle and pelvic floor hiatus with voluntary contraction. The limitation of the transcutaneous approach is that the images are not clear with regards to the outer margins of the EAS.Figure 1Figure 2Conclusion: Transcutaneous 3D-US imaging is a patient friendly (no probe insertion into the anus or vagina) technique to image the anal sphincters, puborectalis muscle and pelvic floor hiatus. The transcutaneous approach may provide better information on structures such as the perineal body and transverse perinea, which are commonly implicated in obstetrical injury and are the key components of the EAS.