The anatomical basis of anal endosonography - A study in postmortem specimens

Purpose: Anal endosonography is an imaging modality new to the diagnostic workup of incontinence. Interpretations even of normal endosonomorphologic findings now vary considerably. The conjoined longitudinal muscle (LM), a widely ignored structure, has until recently not been fully recognized by anal endosonography. The aim of this study, therefore, was to accurately determine the normal anatomy of the anal canal and correlate it with the findings obtained by anal endosonography. Methods: Eight postmortem specimens of the anal canal were examined by endosonography. The findings were correlated with macroscopical dissection and gross sectional histology of the same specimens. Results. The external echogenic ring is composed of two anatomical structures: the LM and the external anal sphincter (EAS). However, during anal endosonography the LM cannot always be differentiated from the EAS. Histologically, the relation of the diameters of the LM and the EAS ranged from 0.45:1 to 1.25:1. The narrow hyperechogenic ring between the inner hypoechoic layer and the external hyperechoic ring is an artificial finding that cannot be related to a distinct anatomical structure and most likely represents a sonographic interface. Conclusions: This study exactly outlines the relation of diameters of the conjoined longitudinal muscle and external anal sphincter for the first time. Until now, the LM has been underestimated in its dimensions. The role of such a thick muscular structure should be included in the conception of anal continence in the future. Especially in view of the fact that anal endosonography is increasingly used in the diagnostic workup of incontinence and fistula in ano, it is essential to understand the anatomical basis of endosonography. This study accurately delineates the sonomorphology of the anal muscles. When viewed in light findings reported here, endosonographic findings in diseases of the anal canal are now based on a correct idea of the correlation between endosonomorphology and anal anatomy.