5 Recent Advances Concerning the Normal and Abnormal Anatomy of the Anus and Rectum
Total Page：16
File Type：pdf, Size：1020Kb
65 5 Recent Advances Concerning the Normal and Abnormal Anatomy of the Anus and Rectum Michael R.Q. Davies and Heinz Rode Contents 5.9.1 Position of the Anus . 74 5.1 Introduction . 66 5.9.2 The Anal (Canal Opening) Target Site . 74 5.2 The Pelvic Floor . 66 5.9.3 The Perineum . 75 5.2.1 Levator Ani . 66 5.9.4 The Common Wall Phenomenon . 75 5.2.2 The Nerve Supply . 66 5.9.5 Striated Muscle Tissue, “Potential” 5.3 The Anal Canal . 67 Sphincteric Fibers in ARM . 75 5.3.1 The Anal Canal in ARM . 67 5.9.5.1 The Levator Component 5.3.1.1 Agenesis of the Anal Canal . 67 – The Pubovisceralis . 75 5.3.1.2 Dysgenesis of the Anal Canal . 68 5.9.5.2 The Striated Muscle Complex (Peña) . 75 5.4 The Anal Canal – Relevant Anatomy . 68 5.10 The Rectum . 76 5.4.1 Potential Sphincteric Structures . 68 5.10.1 Rectal Ectasia . 76 5.4.1.1 Endoanal Anatomy . 69 5.10.1.1 Pouch Anatomy . 76 5.4.1.2 Mucosal Nerve Supply . 69 5.10.1.2 Ectasia . 76 5.5 The Internal Anal Sphincter . 69 5.11 Fetal Defecation . 76 5.5.1 Internal anal sphincter thickness . 69 5.12 The Pelvic Connective Tissue . 77 5.5.2 Nerve Supply . 70 5.12.1 The Outer Stratum, the Parietal 5.5.3 The Internal Anal Sphincter in ARM . 70 Layer (Leaf) . 77 5.6 The Longitudinal Muscle 5.12.2 The Intermediate Stratum . 77 of the Anal Canal . 71 5.12.3 The Inner Stratum: The Visceral 5.6.1 Dimensions of the Longitudinal Muscle . 71 Layer (Leaf) . 77 5.7 External Anal Sphincter . 71 5.12.4 The Aventitia Surrounding the Rectum . 78 5.7.1 Localization and Nature . 71 5.12.5 The Pelvic Plexus . 78 5.8 The Perineum and Perineal Body . 72 5.12.6 The Neurovascular Bundle of Walsh . 78 5.8.1 Gender Differences – External 5.12.7 The Rectogenital Septum Anal Sphincter . 72 (Denonvilliers’ Fascia) . 78 5.8.1.1 The Female Sphincter . 72 5.12.8 The Rectovaginal Fascia . 78 5.8.2 Nerve Supply of the External 5.12.9 The Perianal Connective Tissue . 78 Anal Sphincter . 72 5.13 Intestinal Neuronal Dysplasia . 79 5.8.2.1 Overlapping Innervation (Monkey) . 73 5.13.1 General . 79 5.8.2.2 Resting Tone . 73 5.13.2 Specific . 79 5.8.3 Spinal Control of the Pelvic 5.13.3 Normal Anatomy . 79 Floor Muscles . 73 5.13.4 Abnormal Anatomy . 79 5.8.4 The Voluntary Anorectal 5.14 Caudal Regression . 79 Inhibition Reflex . 73 5.15 Spinal Dysraphism . 80 5.8.5 Neurophysiological Dysfunction . 73 5.15.1 Defects in the Development of the Axial 5.8.6 Endosonography of the Anal Sphincters . 73 Vertebral Structures . 80 5.8.7 Individual Versus Common Sphincter Control 5.15.1.1 The Sacrum . 80 of Pelvic Organ Continence . 74 5.15.1.2 The Vertebral Canal . 80 5.9 The Pelvic Floor and External Anal 5.15.1.3 The Spinal Cord, Cauda Equina and their Sphincter in ARM . 74 Coverings and Attachments . 81 References . 81 66 Michael R.Q. Davies and Heinz Rode 5.1 Introduction the formation of the external anal sphincter, from which it can be distinguished, forming the most cra- Knowledge about a problem brings with it under- nial significant component of this structure. The -pu standing, allowing assessment of the situation and its bovisceralis is part of the levator, separated from it ultimate solution. This flow of activity is interrupted by its function, closing the urogenital and anorectal where knowledge remains conjectural and unclear. hiatuses by contraction. This state, which is characterized by uncertainty, clouds what is written and spoken about by experts on the subject of congenital anorectal malformations 5.2.2 The Nerve Supply (ARM). Major significant recent advances have been made, but ignorance due to a lack of clarification on Gross dissection studies carried out in the female the subject still pervades much of what we think is suggest that the levator muscle is not innervated the truth concerning these malformations. Because by the pudendal nerve, but by a levator ani nerve of this sorting out of “fact from fiction,” finding the (S3–S4), which travels on the superior surface of the true pathway to follow through the literature on the pelvic floor and is made up of nerves that are direct subject is extremely difficult and subject to observer branches that split from nerve roots proximal to the bias. sacral plexus [4]. It should be noted that this supply is anatomically distinct from that of the external anal sphincter, which is supplied by nerve fibers traveling 5.2 The Pelvic Floor with the pudendal nerve [73]. This dual motor supply indicates that the pelvic floor muscles do not neces- The sheet-like muscle of the pelvic floor has been arbi- sarily have to behave as a unitary mass. This has been trarily divided into seemingly different muscles based demonstrated in the cat, where bilateral section of the on their site of origin and perhaps insertion. This is pudendal nerves entirely abolishes both the tonic ac- of no practical importance to the pediatric surgeon. tivity and phasic responsiveness of the external anal Furthermore, conventional morphologic descrip- sphincter without affecting the activity of the levator tions, those of the anatomist, are of limited value to ani [27]. In spite of proposals that the external anal the clinical practitioner [12]. They do, however, form and urethral sphincters and bulbocavernosus muscle the basis, the core, around which the pelvic structures originate from the puborectalis and can be demon- can be discussed. In practice what can be visualized strated to act as one muscle (they contract or relax en in the living provides more guidance and insight to masse), a voluntary selective individual muscle func- the nature of the problem addressed [91]. It is for this tion exists in man, such that each named component reason that imaged appearances (sonar, magnetic res- can act independently of the other [108–110]. Yet to onance – MR imaging, and computed tomography – be clarified in the human is the suggestion that there CT scan) of the pelvic structures discussed are used is a clear segregation of the segmental motor neuron in preference to those seen by the anatomist [120]. pools innervating these muscular sections of the pel- vic floor. Quantitative analysis of these motorneu- rons in the rat model with ARM of various forms has 5.2.1 Levator Ani shown that there are significantly fewer motor nerve cells in these cases [135]. This is the major muscle of the pelvic diaphragm and Evolutionary considerations regarding the pelvic is attached anteriorly to the pubic bone. The ventro- floor muscles indicate that they develop with specific medial segment, is termed the pubovisceralis muscle attachments and function; they do not derive from as it holds the urethra, vagina, and anorectum within preexisting muscles [26]. For instance, the external its sling-like fibers. It is drawn caudally by the viscera anal sphincter has strong connectivity with its sur- passing through it to which it is attached. A further rounding skin and has muscle spindles, but it is de- subdivision of this muscle, a segment composed of void of the phasic monosynaptic component of the fibers passing, but intimately in contact with, the stretch reflex. Similarly, in quadrupedal mammals it anorectum in the shape of a U-loop from pubis to pu- has fast-twitch muscle characteristics, but in the hu- bis is named the puborectalis. It forms the anorectal man it is a slow-twitch muscle. Onuf’s nucleus, which junction, defining the rectum from the surgical anal innervates the pelvic floor muscles and is suggested canal, the anorectal ring. This muscle participates in to occupy an intermediate position between the vis- 5 Recent Advances Concerning the Normal and Abnormal Anatomy of the Anus and Rectum 67 ceral and somatic nuclei, receives an important group anal canal being that part of it held within the exter- of suprasegmental afferents, probably direct cortico- nal anal sphincter. This implies that the anal canal is spinal fibers. a modified part of the rectum [107]. New tissue in- Sensory perception originating from the levator troduced from surface structures is only encountered muscles plays a fundamental role in signaling the within its wall at and distal to the pectinate (valve) arrival of rectal content at the pelvic floor. Its role in line. anal continence mechanisms is undisputed. Sphincter- In ARM the anal canal may be either agenetic, saving operations in humans have shown clearly that where the terminal hindgut is not surrounded by a the rectum is essential for neither the appreciation of well-formed external anal sphincter, or dysgenetic, impending evacuation, nor for the sphincter inhibi- where the terminal hindgut is incompletely formed tory reflexes (rectoanal reflex) [62]. Because of this, but surrounded by an external sphincter [30,113]. sensory nerve endings in the puborectalis and anal region have been evaluated in humans with ARM and equated with normal material [65,82]. The reported 5.3.1 The Anal Canal in ARM findings must be interpreted in light of the fact that these sensory receptors appear to present themselves The following analysis is based on the assumption that at different ages (i.e., there are no sensory nerve end- the following anatomical considerations are correct: ings in the anal canal of the newly born, but they 1.