Endosonography of the anal sphincter: findings in healthy volunteers

Knowledge of the normal sonographic features of the anal canal is essential for the detection of anal carcinomas, anal sphincteric defects, or other anal abnormalities with endosonography. The anal sphincters consist of the circular smooth muscle fibers of the internal sphincter and the circular striated muscle fibers of the external sphincter together with the sling-shaped puborectalis muscle. Anal endosonography was performed in 14 healthy women with normal anophysiologic examinations. The procedure was performed during electromyographic registration in five. A radial 7-MHz probe and a multiplane 7-MHz probe were used, and transverse and longitudinal images were obtained. On transverse images, the internal anal sphincter was visualized as a circular hypoechoic band, which on longitudinal images was seen in continuity with the muscularis layer of the rectal wall. The external anal sphincter was seen as a thicker circular echogenic band just outside the internal sphincter. The puborectalis muscle sling, which is the medial part of the levator and muscle, was visualized in the upper anal canal and had the same echogenic appearance as the external sphincter. Our experience in volunteers provides information about the normal sonographic features of the anal canal as depicted on anal endosonography with high-frequency probes. The results suggest the procedure may be a useful diagnostic tool in detecting pathologic conditions in the anal canal.