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Anorectal Dysfunction and Chronic Pain
INTRODUCTION
The anus and the rectum are two different parts of the large intestinal tract, which have distinct physiology, function, and innervation. Their physiological role consists of controlling continence and defecation during bowel movement, a complex function that involves the activity of other pelvic organs and nearby muscles. The normal function of the anorectum is mainly unconscious, regulated by the autonomic nervous system, except for the final phase of defecation, which involves a conscious action via the central nervous system (CNS). Alteration of anorectal function can result in pain, which may be mechanical, chemical or neuropathic and can originate in the pelvic viscera or the pelvi-perineal muscles, whose innervations project to the same spinal cord segments. Chronic pain only develops when anorectal dysfunction occurs most of the time or during a prolonged period of time. In this context, neuroplastic changes in the nervous system ensue owing to stimulation of pain receptors (nociceptors) by innocuous inputs, which activate pain circuits, resulting in less inhibition and central amplification of pain-derived signals. The dysfunction may be mechanical, chemical or neuropathic. The pain may originate in the pelvic viscera or the pelvi-perineal muscles, whose innervations project to the spinal cord segments. Because pain is a subjective and complex experience with sensitive, cognitive, emotional and motivational elements, it should be considered real and be treated whether the dysfunction is based on mechanical or psychological dysfunction.
In this chapter, we will discuss chronic pain derived from anorectal dysfunction and the development of painful dysfunction from a normal anorectal state. An introduction to the normal anatomy and physiology of the anus and the rectum will help us describe its connection with the CNS and other pelvic organs to explain in detail the onset and development of chronic pain, its reciprocal effect on anorectal function and potential treatments.
BASIC ASPECTS
Anatomy of the Anorectum
The Anus
The anal canal is 4–5 cm in length and is the distal part of the colon, extending from the anorectal junction to the anal margin [5]. It contains two sphincters that overlap in the mid-anal canal and keep the lumen closed during continence.. The external anal sphincter is a voluntary, cylindrical striated muscle located distally in the anus and is about 4 mm thick as measured by endoluminal ultrasound. Whereas its posterior fibers connect to the tail bone through the anococcygeal ligament, some of the anterior fibers decussate into the superficial transverse perineal muscles and perineal body to attach the anal canal to the pubic bone. The proximal external sphincter is intimately related to the puborectalis muscle, which creates a posterior sling. The internal anal sphincter is an involuntary, smooth muscle structure located in the proximal 2/3 of the anus and the distal end of the inner circular muscle layer of the rectum [45].
The anus is normally closed to prevent leakage of fluids or gas. The internal anal sphincter, the external sphincter, and the haemorrhoidal cushion contribute to the permanent resting pressure in the anus (55%, 30%, and 15%, respectively) [21]. The puborectal muscle displays some resting tone; however, it contracts as a reflex in response to a brisk increase in intra-abdominal pressure thereby preventing incontinence. The external sphincter is a muscle under voluntary control that predominantly consists of slow-twitch fibers, capable of prolonged contraction. Its innervation comes from the inferior rectal branch of the pudendal nerve located in S2 and S3, and the perineal branch of the fourth sacral nerve. It has a dual somatic innervation by the levator ani nerve on its superior surface (S3-S4) and by the pudendal nerve.
In contrast to the colorectum, the anal canal is extremely sensitive to touch, pin prick, temperature and movements within the lumen [31]. Specific sensory receptors are therefore numerous, with the afferent nerve pathway for anal sensation coming from inferior hemorrhoidal branches of the pudendal nerve to S2–S4.
The Rectum
The rectum, which is located in the pelvis, begins at the level of the sacral promontory and extends distally to the anus, thus representing the end of the colon. The rectum has three submucosal folds called the valves of Houston, and acts as a reservoir for stool and as a pump to defecate. It has a double innervation through the pelvic plexuses on both sides of the rectum with the sympathetic innervation originating from lumbar regions L1–L3 and the parasympathetic nerves coming from sacral S2-S4 (nervi erigentes). The pelvic plexus then innervates not only the rectum but also the urinary and genital organs with both parasympathetic and sympathetic fibres.
Rectal sensation is transmitted by various receptors at different levels of the rectal wall. For example, rectal intraganglionic laminar endings (rIGLEs) are mechanoreceptors that respond to tension and rapid distension; other mucosal afferents that are both mechano and chemosensitive have been described [32].
Normal Anorectal Function
Continence and Defecation
Anal continence is achieved by a complex mechanism involving sphincters, pelvic muscles, visceral sensation and operates under the control of the autonomic and central nervous system. At rest, the anal pressure, which is higher than the rectal pressure, is responsible for continence. There is a causal relation between the post prandial increase of food residue and elevation of the abdominal pressure. Immediately before abdominal pressure is elevated and during contraction of the distal colon while propelling the stool to the rectum, the anus closure is enhanced through elevation of the pressure of the external sphincter and the puborectalis to maintain the continence. This normal anorectal sensation must be preserved to learn to postpone the urge to defecate when needed.
Once the rectal reservoir is stretched, the rectal pressure increases and the defecation process can begin. Upon relaxation of the internal sphincter (recto anal inhibitory reflex), the rectal contents progress into the upper anal canal, which allows the discrimination of solid from liquid and gaseous luminal contents. This process is finally followed by the conscious need to defecate, which is allowed by stimulation of the parasympathetic nerves, indicating that the rectal function involves a complex sensory-motor coordination.
Coordination of Pelvic Organs During Complex Functions: The Horizontal Links
Complex functions involving different organs and structures, such as defecation, micturition and sexual intercourse, are possible due to a higher level of cooperation between the different organs involved and the integration of signals by the CNS. The coordination needed for these functions emphasizes the importance of crosstalk between the pelvic organs. The pelvic organs are selectively activated according to the function they exert; for instance, the anus closes during sexual intercourse. During urination in humans, rising of the intraluminal pressure of the urinary bladder produces contractions of the anal sphincter, preventing defecation [4]; similarly, the urinary system is inhibited during defecation in humans, as functional stimulation of the anal sphincter inhibits detrusor muscle contraction which is involved in bladder emptying [13].
Because of this level of interactions and functional coordination between various systems, a pathological condition in one pelvic or visceral organ can affect the normal function of another organ. For example, some patients who suffer from irritable bowel syndrome have micturition problems, such as urinary incontinence [53]. Both the peripheral nervous system and CNS probably mediate the neural mechanisms underlying the interactions between the various pelvic and visceral organs. These complex visceral functions are controlled by viscerovisceral convergence within the CNS, both in the spinal cord itself, in which different viscera are stimulated by afferent dichotomized nerves [37] as well as in the brain.
DESCRIBING THE SUBJECT
Integration of Anorectal Sensations in The CNS: The Vertical Links
As previously described, the rectum and the anus have different innervation patterns but also differ in sensitivity. The afferent innervation of the rectum consists of C fibres and A-delta fibres which are only sensitive to distension [39]. A-delta fibres are spread in the rectal mucosa [47] and respond to changes in rectal distension [33], whereas the C fibres endings are mainly located within the muscular wall of the rectum and are activated by the intensity of rectal distension. The rectum and internal anal sphincter have an autonomic innervation through the inferior hypogastric plexus. Sensations from the rectum are poorly localized, often referred to somatic structures and result in greater autonomic response than with somatic sensation [39,47].
The external anal sphincter and levator ani muscle in the anal canal are under the control of the peripheral somatic nervous system (pudendal nerve) [38]. The presence of a high density of specialized receptors and afferent pathways allows for discrimination between different types of sensation, resulting in well-localized sensations at the level of the anal canal, in contrast to the rectum. These differences between rectal and anal sensation are partly based on the differences in their peripheral innervation and possibly to differences in their cortical representation [15].
The brain areas that control and process ano-rectal sensations, include those involved in spatial discrimination (primary and secondary somatosensory cortices SI and SII) and those that process affective and cognitive aspects of sensation (anterior cingular cortex, insula and prefrontal cortex) [16]. However, there are differences between the areas of somatosensory cortex responsible for processing the visceral and somatic components of sensation. The visceral sensation is processed in the inferior part of the SI, whereas somatic sensation is located in higher areas of the SI. The SII, which receives afferents from the SI [42], is activated by both rectal and anal stimulation; however, when the rectal stimulation is not painful the anterior cingulate cortex (ACC) is activated. The differences explained above may explain why a visceral stimulation induces autonomic reactions, whereas somatic stimulation triggers conscious reactions [39].
From Normal Function to Dysfunction and Pain
Normal Function and Some Alterations are Innocuous
The anus and the rectum are both active during continence and defecation, without any pain in the physiological situation. Most of the time the gastrointestinal sensory-motor functions are unconscious, although they can also be painful depending on certain factors such as acidity levels, intestinal contraction, and food composition. Only a few sensations, including postprandial fullness and the urge to defecate upon increased stool accumulation, become conscious, maybe because of their behavioral consequences. For instance, the feeling of incomplete defecation will trigger recurrent defecation efforts even when the rectum is actually empty.
Dysfunctions such as constipation, diarrhea and incontinence are occasionally painful, although different mechanisms can lead to functional alterations [14,35,30]. In a group of 67 patients with constipation, 24% had functional constipation, 27% showed pelvic floor dysfunction, and 49% were diagnosed with irritable bowel syndrome with constipation (IBS-C) [14]. Notably, there are often changes in diagnosis of this type of dysfunction; thus, after 12 months, a third of functional constipated patients developed IBS-C and a third of IBS-C reverted to functional constipation [54]. Anorectal dysfunctions can therefore vary over time within the same patient and may be associated with or without pain.
From Innocuous to Noxious Sensations and Development of Pain
In 1973, Ritchie J first described pain after mechanical distension of the rectum in patients with IBS, by showing that a balloon of 60 ml in the rectum caused pain in 55% of patients with IBS compared to 6% of the controls [44]. These results have been confirmed in many other studies [8,34,52].
The low threshold for pain is considered a hallmark of IBS in 20–90% of the cases [12,34]. Repetitive rectal stimulation in patients with IBS induces rectal hyperalgesia and viscerosomatic referral [36], suggesting that rectal hypersensitivity induced by repetitive distension, could be a reliable marker for IBS [40
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