Correlation between gross anatomical topography, sectional sheet plastination, microscopic anatomy and endoanal sonography of the anal sphincter complex in human males

Abstract

This study elucidates the structure of the anal sphincter complex (ASC) and correlates the individual layers, namely the external anal sphincter (EAS), conjoint longitudinal muscle (CLM) and internal anal sphincter (IAS), with their ultrasonographic images. Eighteen male cadavers, with an average age of 72 years (range 62-82 years), were used in this study. Multiple methods were used including gross dissection, coronal and axial sheet plastination, different histological staining techniques and endoanal sonography. The EAS was a continuous layer but with different relations, an upper part (corresponding to the deep and superficial parts in the traditional description) and a lower (subcutaneous) part that was located distal to the IAS, and was the only muscle encircling the anal orifice below the IAS. The CLM was a fibro-fatty-muscular layer occupying the intersphincteric space and was continuous superiorly with the longitudinal muscle layer of the rectum. In its middle and lower parts it consisted of collagen and elastic fibres with fatty tissue filling the spaces between the fibrous septa. The IAS was a markedly thickened extension of the terminal circular smooth muscle layer of the rectum and it terminated proximal to the lower part of the EAS. On endoanal sonography, the EAS appeared as an irregular hyperechoic band; CLM was poorly represented by a thin irregular hyperechoic line and IAS was represented by a hypoechoic band. Data on the measurements of the thickness of the ASC layers are presented and vary between dissection and sonographic imaging. The layers of the ASC were precisely identified in situ, in sections, in isolated dissected specimens and the same structures were correlated with their sonographic appearance. The results of the measurements of ASC components in this study on male cadavers were variable, suggesting that these should be used with caution in diagnostic and management settings.

Fig. 1

A transilluminated 2.5 mm thick coronal section of a male pelvis processed for epoxy resin E12 sheet plastination showing the entire length of the anal canal and its muscular layers, namely the external anal sphincter (EAS), conjoint longitudinal muscle (CLM) and internal anal sphincter (IAS). Note the continuity of the EAS with the levator ani muscle superiorly, consisting of two parts, the upper part [EAS (UP)] and lower part [EAS (LP)]. The upper part of the CLM is continuous with the longitudinal muscular layer of the rectum and inferiorly its fibres traverse the lower part of the EAS to blend with or form fibro-fatty honeycomb-like compartments around the perianal region (asterisks). The IAS ends above the lower part of the EAS.

Fig. 4

An excised anal canal specimen showing sequential removal of its muscular layers (a,c,e,g) and their corresponding endoanal sonographic images (b,d,f,h). Full thickness of the anal canal (a,b), after the removal of the external anal sphincter (c,d), after the removal of the conjoint longitudinal layer (e,f) and after the removal of the internal anal sphincter (g,h). Note that the ring-shaped artefacts on the sonographic images are due to the effect of the air/tissue interface in this ex-vivo preparation.

Fig. 2

The anal canal and its muscular layers, external anal sphincter (EAS), conjoint longitudinal muscle (CLM) and internal anal sphincter (IAS). (a) Gross anatomical dissection, (b) longitudinal slice of the lateral wall of the anal canal, (c) longitudinal histological section, haematoxylin and eosin stain, (d) longitudinal histological section, elastic van Gieson stain and (e) longitudinal histological section, Mason's trichrome stain.

Fig. 3

Axial sections of the anal canal and its muscular layers, namely the external anal sphincter (EAS), poorly identified conjoint longitudinal layer (*) and internal anal sphincter (IAS). (a) Epoxy resin E12 sheet plastination, (b) histological section, haematoxylin and eosin stain, (c) histological section, Mason's trichrome stain and (d) endoanal sonographic image of the layers of the anal canal.