Pelvic Floor Disorders: Surgical Approach

5. Anorectal Physiology

Ivana Giannini, Maria Di Lena, Simona Giuratrabocchetta and Donato F. Altomare 

(1)

Department of Emergency and Organ Transplantation, Aldo Moro University, Bari, Italy

Donato F. Altomare

Email: donatofrancesco.altomare@uniba.it

Abstract

Anorectal physiology deals with the defecatory function, which consists of the release of intestinal gas and feces through the anus. The control of the appropriate time and place of defecation is a complex mechanism of anal continence. In this chapter, the physiological mechanisms involved in the control of defecation and continence will be reviewed.

5.1 Introduction

Anorectal physiology deals with the defecatory function, which consists of the release of intestinal gas and feces through the anus. The control of the appropriate time and place of defecation is a complex mechanism of anal continence. In this chapter, the physiological mechanisms involved in the control of defecation and continence will be reviewed.

Anorectal physiology was poorly understood until the results of several new investigations were published, such as those on anorectal manometry, neurophysiology of pelvic floor muscles and pudendal nerves (electromyography, pudendal nerve terminal motor latency), and imaging techniques such as dynamic defecography, pelvic floor ultrasound, and defeco-magnetic resonance imaging.

The correct assessment of anorectal physiology has important clinical implications because any alteration of continence and defecation may result in incontinence and/or constipation causing severe impairment to the patient’s quality of life.

5.2 Pathophysiology of Continence

5.2.1 Internal Anal Sphincter

The internal anal sphincter (IAS) represents a thickening of the circular muscle layer of the terminal rectum at the level of the anal canal; it extends about 2 cm below the dentate line and is located between the anal mucosa and the longitudinal muscle of the rectum, separated by the intersphincteric space from the external anal sphincter (EAS). This muscle accounts for about 70% of the resting pressure and represents one of the most important factors for anal continence [1].

The IAS is involved in the rectoanal inhibitory reflex (RAIR) defined as “the transient decrease in resting anal pressure by ≥ 25% of basal pressure in response to rapid inflation of a rectal balloon with subsequent return to baseline” [2].This intramural reflex mediates relaxation of the IAS with specific duration and latency, followed by a gradual recovery of basal anal pressure as a response to rectal distension by the stool.

The RAIR is mediated by the enteric nervous system and does not require control by the peripheral or central nervous system, as demonstrated by its presence in spinal patients; nitrogen dioxide is the most common inhibitory mediator produced by the myenteric neurons [3]. Traumatic lesions of the IAS usually lead to soiling, and altered RAIR may be responsible for megarectum, such as that found in Hirschsprung disease [4].

5.2.2 External Anal Sphincter

The EAS is a voluntary striated muscle in which three components (deep, superficial, and subcutaneous) can be identified. These findings were described by Santorini, Von Holl, Milligan, Morgan, and Gorsch, but they became more accepted after the Shafik’s 1975 paper in which three “U-shaped” loops (top, intermediate, and base) were identified, with separate and counterbalanced sphincter functions [56]; however, modern imaging techniques have cast doubts on this theory.

The EAS, with its continuous tonic activity, contributes to resting anal tone and provides a strong increase in anal pressure during voluntary squeeze and during rapid increase of intrarectal or intra-abdominal pressure [78].

5.2.3 Puborectalis Muscle

The puborectalis muscle, together with ileococcygeal and pubococcygeal, is a component of the levator ani muscle, which lies below and around pelvic organs. The tone of levator ani muscles contributes to continence, making the lumen of the pelvic organs a virtual space.

Many hypotheses have been proposed to explain the role of the puborectalis muscle in the control of continence. Parks and colleagues argued that continence is achieved by creating a “flap valve”, whereby an increase of intraabdominal pressure forces the anterior rectal wall down towards the upper third of anal canal, occluding it and preventing feces outflow [9]. Subsequent studies have shown that the “flap valve” is just a theoretical mechanism and it is mainly the simultaneous voluntary contraction of the EAS and puborectalis that prevents the escape of stool [1011].

One of the most distinctive features of the puborectalis muscle is the creation of an angle between the rectum and the anal canal, known as anorectal angle (ARA). It is open posteriorly due to the anatomic sling-shaped configuration of the puborectalis muscle that surrounds the anorectal junction and fits on pubic bone.

The ARA, which is easily measurable by defecography, is approximately 90–110° during the resting period, but it is a “dynamic angle” because it becomes more obtuse in pushing (during defecation) and more acute in squeezing (during retention of feces).

It has also been demonstrated that the erect position modifies the ARA without contraction of pelvic floor muscles, making the angle more acute (< 80°). On the other hand, the sitting position on the toilet causes opening of the ARA to more than the value recorded in voluntary pushing in Sim’s position (121° vs. 113°).

These findings indicate that the erect position helps the maintenance of continence, whereas the sitting position on the toilet, probably due to the gravity and relaxation of pelvic muscles, allows defecation [12].

5.2.4 Rectal Compliance

The distal part of the colon, and particularly the rectum, works as “a reservoir” and has a great importance in anal continence. The rectum has the capacity of storing stools by adapting the tone of the muscular wall in order to reduce the pressure inside. This property, known as rectal compliance, is the ratio between rectal pressure and rectal volume (Δvp) and its control is mediated by the sacral parasympathetic nerves.

The rectal wall also has mechanoreceptors inducing extrinsic and intrinsic reflexes, which play an important role in defecation.

5.2.5 Hemorrhoidal Cushions

Internal hemorrhoids are cushions of vascular tissue rich in arteriovenous anastomosis, elastic fibers, and collagen, and they seem to play a role in the maintenance of fine continence, contributing to 15–20% of the resting tone of the anal canal [113].

In conclusion all these anatomical structures are involved in the maintenance of continence, but some are activated mainly during fecal urgency (EAS and puborectalis muscle), and others in the resting state (IAS, rectal compliance, hemorrhoidal cushions) [14].

5.2.6 Bowel Movement and Stool Characteristics

The occurrence of strong and coordinated contractions (mass movements) allows the migration of stool from distal colonic segments into the rectum. These contractions occur more frequently in the morning after awakening and after a meal (gastrocolic reflex). In contrast, colonic motility is reduced at night to avoid incontinence [15].

The volume and consistency of the feces are other important factors affecting anal continence. Leakage of gas or liquid stools is more difficult to control, while hard stools are difficult to expel even in the presence of damaged or poorly functioning anal sphincters.

5.2.7 Defecation

Sensory perception and physiological coordination of pelvic floor muscles are important components of defecation, which depends on both involuntary and voluntary mechanisms.

The site for the integration and control of defecation is located in the lumbosacral spinal cord and is modulated by higher centers (brainstem and cerebral cortex). Alterations of this brain-gut axis can lead to important dysfunctions: in fact the voluntary control of defecation is lost in patients who have a spinal cord injury with interrupted corticospinal connections [16].

At rest, the pressure in the rectum is lower than in the anal canal, but once the rectum has received the fecal mass from the distal colon, its intraluminal pressure increases and the rectal walls are stretched. The pressure becomes higher than pressure of the anal canal, and rectal distension activates the rectoanal inhibitory reflex causing the relaxation of the IAS so that the feces, according to the new pressure gradient, come down to the upper anal canal where the sensory receptors in the anal mucosa can discriminate between flatus and liquid or solid stools. This mechanism is called “sampling” and determines both the urgency of defecation and the reflected contraction of the EAS, which prevents the loss of stools.

When the conscious perception of the stimulus to defecate is realized, if the passage of stools must be prevented, a voluntary contraction of the EAS and puborectalis muscle forces the stool back into the high rectum. Here the feces are temporarily stored so that the urgency to defecate temporarily disappears and anal sphincters recover their basal tone.

In contrast, if the time and the place are appropriate, the subject sits on the toilet and by the Valsalva maneuver increases the abdominal pressure contracting the abdominal muscles. Simultaneously the EAS and puborectalis muscles voluntarily relax and the anorectal angle opens and the stool can be expelled through the anus. The “closing reflex” (transient contractions of the EAS and puborectalis muscle) after defecation closes the anal canal, restoring its basal tone [3].

5.2.8 Role of The Nervous System

Hormones, paracrine substances, enteric nervous system, autonomic (sympathetic and parasympathetic) nervous system, and cerebral cortex together regulate colorectal motility and sensitivity.

The enteric nervous system is composed of the myenteric plexus (Auerbach’s plexus) and the submucosal plexus (Meissner’s plexus), and it consists of a network of nervous fibers, ganglion cells (sensory and effectors neurons), and interneurons richly interconnected by reflex arcs located in the wall of the gastrointestinal tract and directed to innervate smooth muscle cells. The effector neurons of the myenteric plexus may be excitatory or inhibitory according to the substances released in contact with smooth muscle cells. Excitatory neurons release acetylcholine, substance P, and other tachykinins, while inhibitory neurons release vasoactive intestinal peptide and nitric oxide, which cause relaxation of smooth muscle cells [17]. As a result of this organization, the enteric nervous system acts like a semi-autonomous system: it is able to coordinate most of the activities, even in the absence of an extrinsic control.

Extrinsic innervation is provided by the sympathetic and parasympathetic nerves, which have only a modulatory function on the contractile activity.

Sympathetic innervation originates from postganglionic fibers of the hypogastric plexus and it has an inhibitory effect on the motor function, making connections with neurons of the enteric nervous system, which in turn sends fibers to smooth muscle cells, inhibiting the contraction.

On the other hand, parasympathetic fibers, which originate from the sacral plexus (S2–S4) and run into the pudendal nerve, send preganglionic fibers to neurons of the intramural plexus, which in turn sends fibers to smooth muscle cells, stimulating the contractile function.

Also important are the intrinsic reflexes located in the colon and rectal wall, as well as throughout the gastrointestinal tract; the colocolonic reflex is finely organized so that the stimulation of an intestinal segment causes contraction of the segments upstream and distension of segments downstream. The gastrocolic reflex acts to produce an increase in colonic motility and mass movements in response to the presence of food in the stomach.

Therefore, it is evident that continence and defecation are the effects of the integration of many functions involving the colon, anorectum, pelvic floor muscles, and nervous system.

5.3 Conclusions

The ability to retain stools, distinguish them from flatus, and allow defecation, is a complex process controlled by several anatomic factors including the pelvic floor musculature and the anorectum, with its complex innervation including the somatic, autonomic and enteric nervous systems.

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