Anorectal morphology and function: analysis of the Shafik legacy

Flow simulations of rectal evacuation: towards a quantitative evaluation from video defaecography

Mechanistic understanding of anorectal (patho)physiology is missing to improve the medical care of patients suffering from defaecation disorders. Our objective is to show that complex fluid dynamics modelling of video defaecography may open new perspectives in the diagnosis of defaecation disorders. Based on standard X-ray video defaecographies, we developed a bi-dimensional patient-specific simulation of the expulsion of soft materials, the faeces, by the rectum. The model quantified velocity, pressure and stress fields during the defaecation of a neostool with soft stool-like rheology for patients showing normal and pathological defaecatory function. In normal defaecation, the proximal-distal pressure gradient resulted from both the anorectal junction which formed a converging channel and the anal canal. The flow of the neostool through these anatomical parts was dominated by its shear-thinning viscous properties, rather than its yield stress. Consequently, the evacuation flow rate was significantly affected by variations in pressure applied by the rectum, and much less by the geometry of the anorectal junction. Lastly, we simulated impaired defaecations in the absence of obvious obstructive phenomena. Comparison with normal defaecation allowed us to discuss critical elements which should lead to effective medical management.

Rheology of human faeces and pathophysiology of defaecation

BACKGROUND

Rectal evacuation involves multiple mechanisms that are not completely understood. The aim of this study was to quantify the rheologic property, i.e., yield stress, which governs the ease of deformation of a range of faeces of differing consistency and understand its influence on the pathophysiology of defaecation.

METHODS

Yield stresses of faeces of differing consistencies and Bristol scores were determined by the Vane test. We then explored the effects of this property on ease of defecation using a simple static model of the recto-anal junction based on the laws of flow for yield stress pastes and checked the conclusions by X-ray defaecography experience.

RESULTS

The yield stress of faeces increased exponentially with their solid content, from 20 to 8000 Pa. The static model of the recto-anal junction showed that evacuation of faeces of normal consistency and yield stress is possible with moderate dilatation of the anal canal, whilst the evacuation of faeces with higher yield stress requires greater dilatation of the anal canal. X-ray defaecography showed that such increases occurred in vivo.

CONCLUSIONS

The diameter of the recto-anal junction is increased to enable the passage of feces with high yield stress. The finite limits to such dilation likely contribute to fecal impaction. Hence, difficulties in defaecation may result either from unduly high yield stress or pathologies of reflex recto-anal dilatation or a combination of the two.

A PASSIVE STATE SIMULATION OF AN ANAL SPHINCTER USING SIMMECHANICS

Nowadays, different mechanical artificial sphincters can be found implanted in human beings, trying to overcome a deficiency in the performance of the natural one. However, they do not take into account the natural anal sphincter’s (AS) dimensions, and autonomous response; they also lack in basic contraction and relaxation properties. In this paper, by addressing the AS behavior, an AS model designed with Matlab/SimMechanics is shown. The model comprises bodies of concentrated mass interconnected by springs. The mass–spring system is arranged in concentric rings where every concentrated mass is interconnected by a spring. Each spring takes specific stiffness, which varies with length, in accordance to an experimental curve. The system described can be loaded or unloaded, describing then the muscle behavior.

Each element that forms the model of rings is subject to displacements caused by forces of traction and compression, when a radial force is applied from the center towards the inner ring. The springs of the inner ring experience forces of traction, whereas the springs that connect the body of the inner ring with the outer ring perpendicularly are submitted to compression forces.The data used in the proposed model corresponded to dimensions of the humanAS: width, height, rigidity, stress, tension, basically obtaining an initial deformation behavior according to the sphincter in the passive state. The model remained stable with some mechanical oscillations due to the elastic elements; by modifying one of the parameters, the behavior became unstable and unmanageable. It was verified that it is a sensitive model when modifying the initial conditions that the concrete data requires in case of reproducing the sphincter muscle with particular dimensions.

The male bulbospongiosus muscle and its relation to the external anal sphincter

PURPOSE

The bulbospongiosus muscle is part of the superficial muscular layer of the perineum and pelvic floor. Its morphology remains controversial in the literature. Therefore, we reinvestigated the fascial arrangement and fiber courses of the bulbospongiosus muscle and its topographical relation to the external anal sphincter.

MATERIALS AND METHODS

The perineum was dissected in 9 male cadavers (mean ± SD age 78.3 ± 10.7 years). Select samples were obtained for histology and immunohistochemistry. In 43 patients (mean age 60.7 ± 12 years) the topographical relation between the bulbospongiosus muscle and the external anal sphincter was determined by magnetic resonance imaging.

RESULTS

The perineum contains several fascial layers consisting of elastic and collagen fibers as well as bundles of smooth muscle cells. The bulbospongiosus muscle was subdivided into a ventral and dorsal portion, which developed in 4 variants. The ventral insertion formed a morphological unity with the ischiocavernous muscle while the dorsal origin had a variable relation to the external anal sphincter (5 variants). A muscle-like or connective tissue-like connection was frequently present between the muscles. However, in some cases the muscles were completely separated.

CONCLUSIONS

We suggest a concept of variations of bulbospongiosus muscle morphology that unifies the conflicting literature. Its ventral fiber group and the ischiocavernosus muscle form a functional and morphological unity. While the bulbospongiosus muscle and the external anal sphincter remain independent muscles, their frequent connection might have clinical implications for perineal surgery and anogenital disorders.

Immunohistochemical confirmation of the presence of smooth muscle in the normal neonatal anorectum and in neonates with anorectal malformations

AIM

To evaluate the presence of smooth muscle in relation to the anorectum in neonates that are normal and in neonates with anorectal malformations using immunohistochemical staining of antibodies specific to smooth muscle actin (SMA).

METHODS

Research ethics committee approval was obtained to study historical neonatal tissue sections. 7-15-μ thickness, sagittal sections of both normal anorectums (NA) and neonates with anorectal malformations (ARM) were available to study. The fragile tissue predates the development of charged glass microscope slides that are inherent to contemporary immunohistochemistry (IHC). Normal appendix sections on non-charged slides were used to establish and optimise a novel, steam generated heat, antigen retrieval protocol. This technique of "gentle" antigen retrieval allows preservation of tissue architecture whilst maintaining adequate staining of tissue on non-charged glass microscope slides. NA and ARM tissue was stained and compared to control NA and ARM sections.

RESULTS

SMA staining was observed in sagittal sections of both normal neonatal anorectums and neonates with anorectal malformations, confirming the presence of smooth muscle.

CONCLUSION

This preliminary work confirms the presence of smooth muscle in the anorectum of neonates that are normal and in neonates with anorectal malformations. It is likely that the smooth muscle complex demonstrated represents the internal anal sphincter in the normal anorectum. It is postulated that the smooth muscle complex seen in anorectal malformations may represent a similar internal anal sphincter complex which maybe of functional significance. Further IHC of tissue sections will allow reconstruction of the exact three-dimensional smooth muscle structure of the anorectum in neonates that are normal and neonates with anorectal malformations.

Geometry assessment of anal sphincter muscle based on monopolar multichannel surface EMG signals

Anatomical studies on the external anal sphincter (EAS) indicate that superficial muscle fibres are circular at low depth within the anal canal. A more complex geometry of the fibres is documented for increasing depth within the muscle and along the anal canal. Monopolar intra-anal EMG signals recorded using an array of electrodes placed in circular direction have no common mode components if the muscle fibres are circular, with constant depth within the muscle and parallel to the detection array. Thus, the presence of common mode signals may provide indications about the geometry of muscle fibres of EAS. Intra-anal EMG signals were recorded from EAS of 12 subjects using an anal probe carrying three circumferential arrays of 16 electrodes at three depths within the anal canal. Contribution of common mode components in single MUAPs was lower for MUs located superficially in the muscle (Pearson correlation coefficient: R=-0.75, p≪0.001) and at a lower depth within the anal canal (non-parametric one way Kruskal-Wallis ANOVA, Χ=17.3, p<0.001), in line with EAS anatomy. A large contribution of common mode components was found in the interference signal, suggesting that the signal receives contributions from far, large muscles (e.g. puborectalis, glutei).

MRI anatomy of the anal region

PURPOSE

The aim of this study was to identify the normal anatomy of the anal region on magnetic resonance images.

METHODS

T1-weighted turbo spin-echo images of anal sagittal sections, anal coronal sections, and oblique anal transverse planes were obtained with a body coil in 60 normal volunteers (30 women and 30 men, aged 19-25 years) at rest in the supine position.

RESULTS

T1-weighted images showed fat spaces and muscles simultaneously, allowing visualization of 7 image layers, including the mucosa, submucosa, anal smooth muscle, inner (intersphincteric) space, vertical levator, outer (intersphincteric) space, and external anal sphincter. The anal smooth muscle was derived from the rectal smooth muscle, and the inner space originated from the perirectal space. The outer space lay between the vertical levator and the external sphincters. The puborectalis did not have a longitudinal portion. The deep, superficial, and SC sphincters were 3 separate muscle bundles. The perianal spaces had a complex interconnection.

CONCLUSIONS

Multiplanar body-coil MRI studies can show anorectal fat spaces and musculature simultaneously, allowing fat spaces and musculature to serve as mutual referents. The results of imaging of the anal region with this method are different from previous imaging descriptions and may provide a more accurate and systemic description of the anal region structures than was previously available.