Anal endosonography for the study of anal canal anatomy: normal images and sonographic variants.

OBJECTIVE: To present normal images and sonographic variants of the anal canal to be used as reference for the study of sphincter and anal canal abnormalities. MATERIAL AND METHODS: Sixty subjects without known anal canal disease were studied by means of anal endosonography. Subject were divided according to age in two groups (up to 50 years and more than 50 years). All of them underwent an outpatient study with B&K medical ultrasound 2,003 scanner and 1,850 multifrequency transducer. RESULTS: Four layers can be sonographically identified in the anal canal: an inner hyperechoic layer which is the submucosa, a second hypoechoic layer which is the internal sphincter, a third one which is a longitudinal muscle and the outer hyperechoic layer which is the external sphincter and the only to be found in the low anal canal. In people older than 50 years, both sphincters were significantly thicker (0.3-0.5 mm). At the high anal canal 40% of women presented an anterior gap in the external anal sphincter. CONCLUSIONS: Anal endosonography allows an easy division in high-, mid-, and low anal canal. In some women there is a gap at the mid-high anal canal that must be taken into account in order to avoid diagnostic errors. An internal sphincter thickness greater than 3.5 mm should be considered abnormal at any sex or age.