Anal Fistula Chapter 42 Lower Gastrointestinal Problems-10th Edition-lewis

Abstract

The act of defaecation, although a ubiquitous human experience, requires the coordinated actions of the anorectum and colon, pelvic floor musculature, and the enteric, peripheral and central nervous systems. Defaecation is best appreciated through the description of four phases, which are, temporally and physiologically, reasonably discrete. However, given the complexity of this process, it is unsurprising that disorders of defaecation are both common and problematic; almost everyone will experience constipation at some time in their life and many will develop faecal incontinence. A detailed understanding of the normal physiology of defaecation and continence is critical to inform management of disorders of defaecation. During the past decade, there have been major advances in the investigative tools used to assess colonic and anorectal function. This Review details the current understanding of defaecation and continence. This includes an overview of the relevant anatomy and physiology, a description of the four phases of defaecation, and factors influencing defaecation (demographics, stool frequency/consistency, psychobehavioural factors, posture, circadian rhythm, dietary intake and medications). A summary of the known pathophysiology of defaecation disorders including constipation, faecal incontinence and irritable bowel syndrome is also included, as well as considerations for further research in this field.

Key points

• Defaecation is a fundamental physiological process resulting in the evacuation of faeces; it is dependent on the coordination of neural, muscular, hormonal and cognitive systems.

• Several factors influence defaecation, including gastrointestinal transit, stool volume and/or consistency, and dietary intake.

• Defaecation can be described in terms of four reasonably discrete temporal phases: basal phase, pre-expulsive phase, expulsive phase and end phase.

• The latest imaging and technological advances (such as high-resolution colonic and anorectal manometry, cine-MRI and magnetic resonance defaecography and wireless capsules) have improved our knowledge of defaecatory mechanisms.

• Knowledge of the physiology of normal defaecation could inform management of common disorders of defaecation such as constipation and faecal incontinence; however, future research needs are highlighted in this article.

Introduction

Defaecation is a fundamental physiological process that results in the evacuation of faeces. Continence requires the voluntary control of defaecation. Both defaecation and continence are dependent on a morphologically intact gastrointestinal tract and, additionally, the coordination and integration of multiple physiological systems including: neural (principally the enteric nervous system, modulated by the peripheral somatic, autonomic and central nervous systems); muscular (smooth and striated); hormonal (endocrine and paracrine); and cognitive (behavioural and psychosocial)^1,2. Disorders of defaecation, such as constipation and faecal incontinence, are common, frequently coexist^3,4,5, and incur a considerable burden of morbidity and health-care expenditure^6,7,8,9,10. Constipation, for example, is the third most common presenting gastrointestinal symptom reported at outpatient clinics in the USA, with 2.5 million estimated visits in 2014 (ref.¹¹). The direct costs per patient for faecal incontinence and constipation are estimated to be between US$1,594 per year¹² and $7,522 per year⁷.

Since this topic was last reviewed¹³, there have been major technological advances in the investigative tools used to assess colonic and anorectal function (Box 1) including high-resolution colonic^14,15 and anorectal manometry^16,17, wireless capsule devices^18,19 and MRI techniques^20,21,22. In this Review, we provide an overview of the anatomy and physiology of defaecation and continence in human studies, an overview of the pathophysiology of defaecation disorders and summarize considerations for further research (Box 2).

Overview of relevant anatomy

Several structures in the abdomen, pelvis and perineum are integral to defaecation and continence (Fig. 1). We highlight key features in this section, with knowledge gaps and considerations for further research summarized in Box 2.

Fig. 1: Neuromuscular anatomy of the colon and anorectum.

a | Extrinsic sensorimotor innervation of the colon and anorectum relating to the physiology of defaecation. b | A coronal diagram of the anorectum, demonstrating features of structural importance in continence.

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Colon

The colon is a viscoelastic²³, tubular organ, beginning proximally at the ileocaecal junction and ending distally at the rectosigmoid junction. The human colon is approximately 130 cm in length in adulthood²⁴, with a luminal diameter of 60–80 mm in the caecum, progressively narrowing to 25 mm in the sigmoid colon²⁵. An elongated or redundant colon has been suggested to have a causative role in the pathogenesis of constipation^26,27. Delayed colonic transit has been demonstrated with colonic elongation in mice²⁸, but it is unclear whether this has any functional relevance in humans.

The colon receives intrinsic neural innervation from the enteric nervous system, extrinsic sympathetic innervation from the lumbar nerves, and extrinsic parasympathetic innervation from the vagus nerve (proximal colon) and pelvic splanchnic nerves, which collectively govern the sensorimotor function of the colon² (Fig. 1).

Rectum

The rectum can be considered a specialized distal extension of the colon. Located in the pelvis, its high compliance (distensibility in response to filling) is necessary for accommodation of content immediately prior to defaecation (the rectal 'reservoir')^29,30. Although a number of landmarks have been used to define the upper border of the rectum, a consensus group suggested in 2019 that the rectum commences at the mesocolic–mesorectal transition, or the 'sigmoid take off'³¹. This location corresponds to the convergence of the taenia coli into one continuous sheath of longitudinal muscle³¹.

Anal canal

The lumen of the anal canal is shaped like an hourglass³², with the middle third being the least distensible region³³. The length of the anal canal can be described in terms of the anatomical length³⁴ (dentate line to anal verge), surgical length³⁵ (anorectal junction to anal verge) or functional length³⁶ (a high-pressure zone that exceeds the resting intraluminal rectal pressure). The anal canal is generally longer in men than in women (median functional anal canal length: 2.9 cm in women, 3.6 cm in men)^16,36,37.

At rest, the anal canal is angulated at approximately 65–108°^38,39 to the superior–inferior axis of the rectum, forming the anorectal angle⁴⁰. The puborectalis muscle forms a U-shaped sling around the anorectal junction, further supporting the anorectal angle via resting tonic activity (postural reflex). The anal canal contains vascular columns⁴¹ (columns of Morgagni) and the haemorrhoidal plexus, which are supported by the conjoint longitudinal muscle, or Treitz's muscle⁴², which is a continuation of the longitudinal muscle of the rectum in the anal canal⁴³. Collectively, these anatomical features contribute up to 10–20% of anal canal resting pressure⁴⁴.

The internal anal sphincter (IAS) is the continuation of the circular muscle layer of the rectum, forming a ring of smooth muscle that encases the anal canal circumferentially in a spiral orientation⁴⁵. The IAS is not under voluntary control, receiving autonomic innervation that mediates IAS relaxation via the release of nitric oxide from non-adrenergic, non-cholinergic neurons^46,47 (see the section Anorectal sensorimotor activity). This process differs from the conjoint longitudinal muscle, which contracts in response to cholinergic stimulation⁴⁸. Owing to the spiral orientation of the IAS, contraction causes shortening and narrowing of the anal canal and relaxation causes lengthening⁴⁵. The resting tone of the IAS can be neural or myogenic in origin⁴⁹. The resting IAS tone is responsible for the majority (70–85%) of anal canal resting pressure^40,44,50,51. The IAS also exhibits phasic contractile activity (termed 'slow waves' and 'ultra-slow waves') that occur at dominant frequencies of 16–18 cycles per minute (cpm) and 1–3 cpm, respectively^52,53,54,55; accordingly, the IAS should be considered as a phasically active muscle that generates tone, rather than a tonic muscle as it is conventionally described⁵⁵.

In contrast to the IAS, the external anal sphincter (EAS) is composed of skeletal muscle under spinal and cortical control⁵⁶. The EAS makes a small contribution to anal canal resting pressure, but is largely responsible for generating maximal squeeze pressure and the acute voluntary control of continence⁵⁰. The EAS is further supported by the action of the transverse perineal and bulbospongiosus musculature, to create a so-called purse string closure at the perineal body⁵⁷.

Pelvic floor musculature and attachments

The pelvic floor includes the striated muscles of levator ani (pubococcygeus, iliococcygeus and puborectalis) and coccygeus^34,58,59, as well as the endopelvic fascia and ligamentous attachments, which support the pelvic viscera and provide attachments to the pelvic wall. The principal ligamentous support is afforded by the pubourethral ligament, uterosacral ligament and cardinal ligament^60,61. The urogenital hiatus and rectal hiatus allow passage of pelvic viscera to the perineum, including the urethra and vagina anteriorly and rectum posteriorly. The neuromuscular integrity of the IAS, EAS and pelvic floor are critical to continence⁵⁹, and are most vulnerable to structural traction injury during vaginal delivery or iatrogenic damage secondary to perianal or anorectal surgery^62,63,64,65. Connective tissue disorders such as Ehlers–Danlos syndrome hypermobility type can also lead to laxity of pelvic floor ligaments resulting in descending perineum syndrome⁶⁶.

Four phases of defaecation

The process of defaecation is best appreciated through the description of four phases (Fig. 2), which are temporally and physiologically reasonably discrete¹³: basal phase, pre-expulsive phase, expulsive phase and end phase. However, some gaps in knowledge remain (Box 2).

Fig. 2: Principal events before and during defaecation.

This flowchart depicts the four phases of defaecation (basal, pre-expulsive, expulsive and end), detailing the specific changes occurring during each phase to either maintain continence or facilitate defaecation. EAS, external anal sphincter; IAS, internal anal sphincter.

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Basal phase

Physiology

The basal phase describes the non-defaecatory state, during which the colon performs several homeostatic functions including⁶⁷: mixing of luminal content; propulsion of content distally for eventual expulsion; bacterial fermentation of carbohydrates; transmural exchange of fluid, electrolytes and short-chain fatty acids; formation of solid stool; and storage of contents prior to defaecation.

Coordinated motor patterns are essential to these functions^68,69. The control of colonic smooth muscle involves the integrated actions of neural, myogenic and hormonal mechanisms^1,69, although much of the action, interactions and integration of these systems remains poorly understood⁷⁰. Continence, simplistically, is maintained by an intraluminal rectoanal pressure gradient, with tonic contraction of the anal sphincter resulting in an anal canal intraluminal resting pressure that exceeds rectal pressure³⁶. The rectum is generally empty during the basal phase and only begins to fill during the pre-expulsive phase^59,71.

Colonic motility

The colon receives approximately 1,500 ml of liquid enteric content (chyme) per day via the ileocaecal junction^72,73. Mean colonic transit time is ~24 h, ranging between ~4 and 50 h; this represents 70–80% of whole-gut transit time^18,19. Digesta enters the caecum and moves aborally (outside episodes of mass movement or defaecation), with a net antegrade flow rate of approximately 1 cm/h (ref.⁶⁷), and is characterized by a series of 'to-and-fro' motions. As demonstrated by studies tracking an ingestible magnetic capsule, regional transit time in the colon is not evenly distributed^74,75 (Supplementary Fig. 1). When the location of the capsule is tracked in real time, it can spend many hours in one region and move through an adjacent region in seconds to minutes⁷⁵. The motor patterns responsible for these movements might include low-amplitude propagating contractions, high-amplitude propagating contractions (HAPCs), the cyclic motor pattern and colonic pressurizations⁶⁹.

Low-amplitude propagating contractions and HAPCs, as recorded by intraluminal manometry, have both been temporally associated with luminal transit in humans^76,77 and are likely to be the motor patterns associated with the 'fast antegrade' and 'long fast antegrade' movements described in studies using a magnet tracking system^18,75. Low-amplitude propagating contractions can also travel in a retrograde direction^14,69. These have been associated with retrograde luminal transit and could aid in retarding flow of content through the colon, allowing absorption and fermentation to occur⁷⁷.

Synchronous pressure increases across long colonic segments, or pancolonic pressurizations, increase during a meal and decrease immediately afterward^78,79. These phenomena are associated with transient anal sphincter relaxations⁷⁸, which enable anal 'sampling' of intraluminal content (discussed below), and also with the flatal urge and expulsion of flatus^78,79, which might partly explain the pooling of gas in the distal colon after a meal⁸⁰. An increase in synchronous colonic pressure waves in the human distal colon was first shown with the administration of the acetylcholinesterase inhibitor neostigmine⁸¹, and this finding was confirmed by Corsetti et al. from high-resolution manometry recordings in healthy volunteers⁷⁸. Similarly, earlier studies using colonic barostat recordings demonstrated increased colonic wall tone in response to meals and neostigmine administration in healthy adults^82,83.

Through conventional low-resolution colonic manometry studies, non-propagating activity was commonly reported to be the most prevalent motor pattern in humans^84,85. Such activity was suggested to facilitate mixing and transmural exchange of water, electrolytes and short-chain fatty acids. A number of low-resolution manometry studies also identified non-propagating rhythmic activity in the rectum, which was labelled as 'rectal motor complexes'⁸⁶, 'periodic rectal motor activity'⁸⁷ or 'intermittent rectal motor activity'⁸⁸. However, the same motor pattern can be found throughout the entire colon⁸⁴.

With the introduction of high-resolution manometry, the majority of what was previously considered 'non-propagating' has been shown to consist of rhythmic, propagating contractions, the majority of which occur over short distances at a frequency of 2–8 per minute¹⁴. This activity has subsequently been termed the 'cyclic motor pattern', and can be stimulated by a meal¹⁴ (discussed below) and HAPCs^87,89,90. The cyclic motor pattern is active during sleep^86,87 and general anaesthesia⁹¹, and can be activated by sacral nerve stimulation in humans⁹². As it occurs at the same frequency as colonic slow waves^93,94,95, the cyclic motor pattern is likely to be generated by the interstitial cells of Cajal⁹⁴. In addition, the cyclic motor pattern can be modulated by neural pathways in response to physiological, pharmacological and electrical stimulation^69,96,97.

Functionally, the cyclic motor pattern in the distal colon has been hypothesized to inhibit transit, thereby acting as an intrinsic colonic 'gatekeeper'⁸⁷ or 'rectosigmoid brake'^15,87,98. Several findings support this hypothesis. For example, the propulsive HAPCs, which are associated with defaecation, increase in number after a meal (see later sections) (Fig. 3). Despite this change, defaecation does not occur after every meal. Concurrently, a meal also results in a major increase in the cyclic motor pattern, which mostly propagates in a retrograde direction. Thus, the presence of the cyclic motor pattern might have a functional role in preventing rectal filling. When an urge to defaecate is perceived, defaecation can be voluntarily deferred, after which time the sensation will usually abate. Radiological evidence suggests that, under such circumstances, retrograde motility patterns can return the contents of the rectum into the sigmoid colon⁹⁹. This observation is in keeping with findings of an impaired or absent rectosigmoid brake function in patients with conditions characterized by diarrhoea^{100,101,102,103} and might explain why sacral nerve stimulation can reduce the severity of faecal incontinence by inducing distal colonic motility^{92,104,105,106}.

Fig. 3: High-resolution colonic manometry recordings in four healthy volunteers.

a | The colonic response to a meal. b | Expanded section of the trace outlined (dashed rectangle) in part a showing the cyclic motor pattern (blue arrows). c | The cyclic motor pattern can be seen directly below the high-amplitude propagating contraction (HAPC; magenta arrow); no urge to defaecate was reported by the volunteer. d | By contrast, the HAPC extends into the sigmoid colon (S), the cyclic motor pattern is inhibited and the volunteer reported an urge to defaecate. e | The volunteer reported the passage of a small volume of liquid stool (manometry performed in a prepared colon without solid faecal content). Note that defaecation was associated with a HAPC and inhibition of the cyclic motor pattern. HF, hepatic flexure; R, rectum; SF, splenic flexure.

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Pre-expulsive phase

Colonic motor activity

Low-resolution colonic manometry in the unprepared colon (with faecal content present) of healthy adults has demonstrated that both propagating and non-propagating activity begins to increase up to 1 h prior to defaecation¹⁰⁷. Importantly, these changes are not associated with any conscious awareness or urge. A series of antegrade propagating contractions sequentially originate at a more distal location¹⁰⁷. These coordinated motor patterns are likely to move intraluminal content towards the rectum in readiness for evacuation. A study published in 2020 using a magnet tracking system demonstrated distal transit of the capsule from the descending colon to the sigmoid colon 30–60 min prior to defaecation in healthy volunteers (unprepared colon)¹⁸.

The functional relevance of increased non-propagating activity prior to defaecation is unknown. As described in the previous section, this activity might assist in slowing antegrade movement. To date, high-resolution colonic manometry studies have only been performed in the prepared colon over short time periods (4–6 h) and, therefore, episodes of spontaneous defaecation are rarely captured¹⁰⁸ (Fig. 3).

Anorectal sensorimotor activity

The compliance of the rectal wall allows passive distension, but also adaptive reductions in rectal tone in response to distension, permitting storage of increasing volumes of content with minimal alteration in intraluminal pressure^29,109. Rectal distension is detected by mechanoreceptors or rectal intraganglionic laminar endings¹¹⁰, which transmit this information along S2–S4 parasympathetic neurons in the pelvic splanchnic nerves to the spinal cord. There might additionally be mucosal thermoreceptors and chemoreceptors with afferent signalling via spinal nerves; however, this has not been established in animal or human studies¹¹¹. Sensory receptors are also present in the extrarectal tissues and pelvic floor, as the defaecatory urge can still be perceived in patients following rectal excision with coloanal or ileoanal anastomoses⁷¹. Some investigators have suggested that rectal contractions are required to generate a conscious defaecatory urge^112,113. During balloon inflation in the rectum, the sensation of rectal distension is not consciously perceived until rectal contractions occur^114,115.

Distension of the rectum beyond a threshold initiates the rectoanal inhibitory reflex (RAIR)¹¹⁶, that causes reflex relaxation of the IAS and contraction of the EAS. The RAIR is an intramural reflex mediated by the myenteric plexus and is characteristically absent in Hirschsprung disease, in which the affected segment of rectum and/or colon lacks myenteric ganglia¹¹⁷. Intramural mediation is further evidenced by preservation of the RAIR in patients following spinal cord injury¹¹⁸, or following surgical mobilization and extrinsic denervation of the rectum¹¹⁹. In patients with an enlarged or hypercompliant rectum (for example, megarectum) allied to rectal hyposensitivity³⁰, the RAIR is still present; however, substantially higher rectal volumes are required to deform the rectal wall and elicit the reflex than those required in healthy volunteers^120,121.

Transient IAS relaxations occur approximately seven times per hour, and a proportion of these (~40%) might be consciously perceived¹²². The upper third of the anal canal is the region of greatest compliance³³ and, during these transient relaxations, intraluminal pressures within the proximal anal canal equalize with rectal pressures¹²³. This process enables luminal content to be 'sampled' by the mucosa of the anal canal^56,122,124, whereby specialized sensory receptors are present including Meissner's corpuscles (touch), Golgi–Mazzoni bodies and Pacinian corpuscles (pressure), Krause end-bulbs (thermal) and genital corpuscles (friction)^40,125,126. Sampling of content allows sensory discrimination between solid, liquid and/or gas^59,127,128.

The sensory information gathered from anal canal sampling is relayed to the lumbosacral defaecation centre in the spinal cord via parasympathetic neurons within the pelvic splanchnic nerves (S2–S4)¹²⁹. These afferent neurons include both myelinated Aδ fibres and unmyelinated C fibres¹³⁰. A spinal cord reflex arc can mediate contraction of the EAS⁵⁶, while sensory information is additionally relayed to the brainstem and cerebral cortex via the spinothalamic tracts¹³¹.

Central nervous system

The conscious perception of rectal distension involves multiple cortical areas including the prefrontal cortex, anterior cingulate gyrus, insula, thalamus and somatosensory cortex^132,133. The awareness of rectal filling is graded by the extent of distension, from a mild awareness initially to maximum tolerance¹¹⁵. Cortical input is critical to both voluntary inhibition or initiation of defaecation; notably, patients with spinal cord injury who lack cortical input require stimulation via manual digitation to initiate defaecation¹³⁴. Brainstem motor control of colonic, rectal and IAS smooth muscle is located in the pontine micturition centre (Barrington's nucleus)¹³⁵. Motor efferent neurons have cell bodies in Onuf's nucleus in the ventral sacral spinal cord. These neurons return to the anal canal via the inferior rectal branches of the pudendal nerves^136,137, whereby they elicit inhibition and relaxation of the anal sphincter¹³⁶.

Expulsive phase

Colonic motility

In the 15-min period preceding defaecation, antegrade propagating contractions in the colon increase in both frequency and amplitude¹⁰⁷. The site of origin of propagating contractions also migrates during this period, with each subsequent propagating event commencing at a more proximal location¹⁰⁷. Unlike the pre-defaecatory phase, these propagating contractions are associated with the urge to defaecate. Studies using a magnet tracking system have shown that the capsule can move from the ascending colon to the rectum during this period⁷⁵. Stool expulsion can be associated with HAPCs that span the entire length of the colon^107,138,139, emptying the colon from caecum to rectum^140,141. However, stool expulsion can also occur in the absence of HAPCs¹⁰⁷, which might require more voluntary effort via abdominal wall contraction.

Manometry studies^78,79 in healthy adults have also shown synchronous pressurizations across the distal pressure channels at the termination of HAPCs. This is hypothesized to function as a means to maintain colonic wall tone and facilitate transit as the lumen distal to a propagating contraction dilates to accommodate propulsion of content^142,143. These events are additionally associated with rectal balloon expulsion⁷⁹.

The cyclic motor pattern is inhibited during stool expulsion, presumably to allow intraluminal content to enter the rectum and anal canal. Although never specifically studied in humans or animals, the cyclic motor pattern seems to be inhibited during HAPCs that either terminate in the rectum or are associated with defaecation (Fig. 3d,e).

Rectoanal pressure gradient

In contrast to the basal and pre-expulsive phases, during which anal canal pressure exceeds rectal pressure, the rectoanal pressure differential is, in theory, reversed during the expulsive phase. This pressure gradient exceeds the frictional resistance of the anal canal¹⁴⁴ and provides the necessary yield stress to deform solid faeces to enable transit through the anal canal¹⁴⁵. This is facilitated by both voluntary and involuntary processes.

A reduction in anal pressure is elicited via a number of factors including voluntary relaxation of the EAS and a reduction in the acuity of the anorectal angle from 65–108° to 110–155°^39,146,147. The acuity of the anorectal angle can be further reduced by squatting, hip flexion and/or posterior pelvic tilt^148,149, or via the use of a defaecation postural modification device¹⁵⁰ (discussed below). Moreover, reflex relaxation of the IAS and pelvic floor musculature and dilation of the anal canal can reduce anal pressure. The extent of IAS relaxation is graded by the rectal stool volume, in that greater stool bulk will cause greater rectal distension and elicit more marked IAS relaxation and reduction in anal canal pressure¹⁵¹. For pelvic floor musculature and anal canal dilation, this contribution might be a combination of passive dilation to accommodate stool, as well as active dilation elicited by perineal descent and contraction of the conjoint longitudinal muscle^145,152. Finally, to reduce pressure, conjoint longitudinal muscle contraction flattens the anal endovascular cushions¹⁵³ and shortens and widens the anal canal^43,154, an action which lead Shafik to describe the conjoint longitudinal muscle as the 'evertor ani'¹⁵⁵. Simultaneously, an increase in rectal pressure is produced by performing a Valsalva manoeuvre (contraction of the diaphragm and abdominal wall musculature with a closed glottis to increase intra-abdominopelvic pressure) and there might additionally be low-amplitude, propulsive rectal contractions^59,156; however, the contribution of rectal wall contractions to increasing intraluminal pressure during the expulsive phase is unclear⁷¹.

Disturbance of the normal reversal of the rectoanal pressure gradient during attempted evacuation is described as 'dyssynergic defaecation', a functional disorder of defaecation characterized by failure of relaxation or paradoxical contraction of the anal canal and/or a failure to increase intrarectal pressure¹⁵⁷. Dyssynergic defaecation is generally described after anorectal manometric investigation of patients who report difficult evacuation. However, in several studies, a majority of healthy volunteers have also demonstrated a negative rectoanal pressure gradient (that is, anal pressure exceeding rectal pressure) during simulated defaecation^158,159,160. Thus, while a reversal of the rectoanal pressure gradient changes may explain stool expulsion conceptually, the investigative tools by which to optimally investigate this process have not yet been determined.

Anal canal dilation

It has been suggested that reflexive anal canal relaxation alone would be insufficient to permit evacuation of faeces; instead, Petros and Swash proposed an active anorectal opening mechanism during defaecation^60,152 (Fig. 4). Using defaecography (MRI and video myography), they demonstrated an increase in anorectal luminal diameter during defaecation. The ratio of rectal to anal luminal diameter decreased from approximately 4:1 at rest to 2:1 during defaecation¹⁴⁴. They proposed that this action was elicited by simultaneous muscle action in three directional vectors (anterior, posterior and inferior)¹⁵², and was achieved by: straightening of the anorectal angle via relaxation of the puborectalis and contraction of the levator plate (posterior vector) and conjoint longitudinal muscle of the anus (inferior vector)¹⁵²; and actively increasing the luminal diameter of the anal canal via contraction of the pubococcygeus to shift the perineal body (anterior vector) and contraction of the postanal plate to splint the posterior wall of the anal canal (posterior vector)¹⁵².

Fig. 4: Anorectal opening mechanism during defaecation.

A hypothesis for the anatomical basis of defaecation as detailed by Petros et al.¹⁵². Connective tissue attachments anchor the anterior wall of the rectum to the vagina and uterosacral ligaments. Several mechanical changes occur to facilitate defaecation: (1) a reduction in the acuity of the anorectal angle (red dashed lines) is enabled by relaxation of the puborectalis (marked by a red 'X') as well as contraction of the levator plate and conjoint longitudinal muscle of the anus (LMA); (2) the rectoanal pressure gradient is reduced by relaxation of the external anal sphincter; and (3) active dilation of the anal canal occurs via anterior displacement of the perineal body caused by pubococcygeus contraction and posterior movement of the post-anal plate. Adapted from ref.¹⁵², Springer Nature Limited.

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Evacuation

Dynamic assessment of defaecation has been performed using the balloon expulsion test¹⁶¹, 'push' manoeuvres during anorectal manometry¹⁶², fluoroscopic or MRI defaecography²¹, artificial stool¹⁶³ (Fig. 5), and, more recently, biomechtronics devices such as Fecobionics (a synthetic stool device that provides data on anorectal geometry and manometry during defaecation)^164,165. In MRI studies in healthy volunteers, rectosigmoid and total colonic volume was reduced by 44% and 19%, respectively, during defaecation¹⁶⁶. Colonic gas volume was also reduced following defaecation in healthy adults, but only in the distal colon¹⁶⁷. Preliminary findings in three healthy adults using the Fecobionics device have demonstrated that some expelled the device using a single, sustained pressure effort, whereas others used several abdominopelvic pressure efforts¹⁶⁵. Similar patterns of expulsion have been demonstrated in healthy adults using defaecography³⁹. When expelling a slurry of barium sulfate, oats and water, the three patterns observed were: a single, rapid expulsive motion (type 1); frequent, pulsatile expulsion of small volumes (type 2); and slow, sustained, steady expulsion (type 3)³⁹.

Fig. 5: Barium (X-ray) defaecography images.

Barium defaecography images provide representative examples, obtained by the senior author from reviewing studies performed in the GI Physiology Unit at Barts Health NHS Trust at the Royal London Hospital, UK (S.M.S. is the director of that clinical service). All images have been anonymized. Barium defaecography provides a real-time anatomical and dynamic study of defaecation, involving fluoroscopic imaging of the process, rate and completeness of rectal emptying, and providing morphological information regarding the anorectum. Parts a–e show defaecation of neostool contrast in a healthy woman (lateral view, the sacrum is indicated by the arrow in part a). There is clear opening of the anorectal angle (parts a–c, angled lines), with progressive dilation of the anal canal (parts b–d) that allows rapid expulsion of most of the contrast. Parts f and g show the development of a huge retaining rectocele (dashed white lines) in a woman complaining of difficulty in rectal evacuation, sense of prolapse and post-defaecation leakage (part f at rest, part g at mid-evacuation). Marked distortion of the anal canal position can be noted (arrows). Parts h and i show the development of a striking full-thickness rectoanal intussusception (arrows) in a man presenting with coexistent faecal incontinence and tenesmus.

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End phase

This phase represents termination of defaecation and the 'closing reflex'. Following evacuation, a series of changes occur to re-establish the basal rectoanal pressure gradient and restore continence. The closing reflex is theorized to be initiated by cessation of traction on the anal sphincter¹⁰⁹. This process elicits: contraction of the anal sphincter and pelvic floor; relaxation of the conjoint longitudinal muscle of the anal canal to enable distension of the anal endovascular cushions; contraction of puborectalis to restore the anorectal angle; and perineal ascent.

Colonic motility patterns in the immediate post-defaecation period have not been described in animal or human studies. In the human stomach and small intestine, prolonged quiescent periods occur after events such as phase III of the migrating motor complex¹⁶⁸. However, based on years of colonic manometry studies performed by the authors (C.H.K., P.J.L., P.G.D., S.M.S.; unpublished work), there is no clear period of quiescence following defaecation before motility returns to basal activity. This notion is supported by the consistency of the colonic meal response that occurs irrespective of its temporal proximity to defaecation, and occurs in the prepared colon, demonstrating that it is independent of colonic intraluminal volumes and distension.

Frequency of normal defaecation

Frequency of defaecation varies widely in healthy adults but is most commonly reported as between three bowel motions per day and three bowel motions per week in both sexes^{169,170,171,172,173,174,175,176,177}. The largest study assessing stool frequency in healthy adults included 4,775 participants (2,304 women, 2,471 men) in the USA, of whom 95% self-reported a stool frequency within this range¹⁷¹. These findings are consistent with those from smaller samples in China (1,952 participants)¹⁷⁵, the UK (1,897 participants¹⁷⁶ and 1,055 participants¹⁶⁹), the USA (1,128 participants¹⁷⁷ and 789 participants¹⁷⁸), Iran (1,045 participants)¹⁷⁴, Singapore (271 participants)¹⁷⁹, Italy (140 participants)¹⁷⁰ and Sweden (124 participants)¹⁷².

Compared with adults, the normal frequency of defaecation is substantially higher in infants, particularly during the first month of life. In 240 healthy infants studied in the UK, the mean stool frequency was four bowel motions per day at 2 weeks of age, decreasing to two bowel motions per day by 12 weeks of age¹⁸⁰. Mean stool frequency gradually decreases during the first few years of life towards the frequency observed in adults¹⁸¹.

Factors influencing defaecation

Multiple factors influence defaecation, including psychobehavioural factors, diet and sex. Knowledge gaps and considerations for further research for this section are summarized in Box 2.

Psychobehavioural factors and stool withholding

The relationships between cortical activity and gastrointestinal function have been described for over a century¹⁸². Stress and psychosocial factors might alter colonic motility^{183,184,185,186,187} and can contribute to the onset and severity of functional gastrointestinal diseases¹⁸⁸ (now also known as 'disorders of gut–brain interactions' (DGBIs)). Contributing factors can include acute and chronic stressors, psychiatric diseases, personality disorders and a history of abuse¹⁸⁹. These factors can lead to local alterations in autonomic function, including activation of the hypothalamus–pituitary–adrenal axis and descending modulatory pathways^187,190,191, which can influence gastrointestinal motility, visceral afferent signalling¹⁹⁰ (hypersensitivity or hyposensitivity), vascular tone and gastrointestinal secretions. Heart rate variability may be used to measure autonomic function; patients with constipation-predominant symptoms show decreased parasympathetic influence (through either withdrawal or sympathetic dominance) compared with patients with diarrhoea-predominant symptoms. The same tendency is seen in patients with severe abdominal pain compared with those with moderate pain, and patients with depression and anxiety compared with those without¹⁹². Symptoms can be compounded by hypervigilance, somatization and maladaptive illness behaviours¹⁸⁹. Thus, bidirectional brain–gut interactions have a role in the pathogenesis of functional gastrointestinal disorders or DGBIs and psychiatric disorders^{193,194,195,196}. In a prospective longitudinal study, Koloski et al.¹⁹³ demonstrated that anxiety was predictive of later developing a functional gastrointestinal disorder in 1,002 participants (based on self-reported survey responses at an interval of 12 years and assessed using the Rome II criteria). Conversely, participants with a functional gastrointestinal disorder in the initial survey were found to have higher incidence of anxiety and depression 12 years later¹⁹³.

Voluntary behaviours also influence defaecation. Toilet training, the process of establishing continence, usually begins between 21 and 36 months of age¹⁹⁷ and can take over 7 months to complete¹⁹⁸. The voluntary suppression and deferral of defaecation, or stool withholding, is seen most commonly in children and is implicated in the pathophysiology of constipation¹⁹⁹. Stool withholding is thought to be associated with painful or unpleasant defaecation and can result in faecal retention, constipation and overflow incontinence^3,200,201. In adults, learned behaviours via operant conditioning are also the basis for biofeedback therapy in disorders of defaecation²⁰². Using visual and/or auditory feedback, patients are able to rehearse the activation and coordination of abdominopelvic musculature during defaecation^157,203,204.

Posture

Posture has a major influence on the biomechanics of the anorectum²⁰⁵. There are cultural differences in the postures assumed during defaecation, with a squatting position more common in African, Asian and Middle Eastern cultures, and a seated position more common in Western cultures. Squatting increases hip flexion and posterior pelvic tilt, facilitating straightening of the anorectal angle¹⁴⁹. When assessed with fluoroscopy, the mean anorectal angle in six healthy volunteers during defaecation in squatting was 126°, compared with 100° in sitting¹⁴⁹. Squatting is associated with a reduction in the duration of defaecation, as well as an increased sense of complete evacuation when compared with sitting (mean duration of defaecation: 0.85 min with squatting, 2.16 min with sitting; P < 0.0001)²⁰⁶.

From a seated position, leaning forward to adopt the 'Thinker' position (like the Rodin sculpture Le Penseur) also increases hip flexion¹⁴⁸. This position, when compared with sitting upright, was shown on anorectal manometry to increase intrarectal pressure²⁰⁷ and, on cinedefaecography, to result in greater puborectalis relaxation, straightening of the anorectal angle, increased perineal plane distance and improved ease of evacuation of barium neostool¹⁴⁸. However, in another study assessing the anorectal angle with fluoroscopy, there was no statistically significant difference in the anorectal angle comparing a seated position and a forward-leaning seated position¹⁴⁹.

In a Western population, the use of a foot stool or 'defaecation postural modification device'¹⁵⁰ to partially replicate the biomechanics of squatting can result in a reduction in straining and the duration of defaecation and an increase in the perceived completeness of evacuation¹⁵⁰. However, these findings were self-reported and unquantified, and have been refuted by others²⁰⁷.

Colonic transit, stool volume and consistency

Faeces is composed predominantly of water (median water content 75%; mean range 63–86% across studies)²⁰⁸, in addition to a suspension of bacterial biomass, protein, carbohydrates and lipids^208,209. An analysis combining a distribution of the means from 116 studies in healthy adults found a median faecal wet mass of 128 g per day (mean range 51–796 g per day across studies)²⁰⁸. Stool volume increases considerably in diarrhoeal illnesses, which is predominantly due to an increase in water volume²¹⁰. Enormous increases in stool water volume, in excess of 10 l per day, can be seen in severe, secretory diarrhoeal illnesses such as cholera²¹¹.

Stool consistency is determined by the proportion of solid matter to fluid and is commonly described using the Bristol stool form scale²¹², which includes a range in consistency from stool type 1 ('separate hard lumps, like nuts') to type 7 ('watery, no solid pieces'). In healthy adults, normal stool consistency has considerable variation, from stool type 2 ('sausage-shaped but lumpy') to type 6 ('fluffy pieces with ragged edges, a mushy stool')^171,213. Extremes of stool consistency, from hard stools to watery stools, are associated with slow and rapid colonic transit, respectively²¹⁴. Lewis and Heaton initially designed the Bristol stool form scale in order to estimate transit time²¹². Using a combination of radiopaque marker studies and stool diaries, it was demonstrated that whole-gut transit time was most strongly correlated with stool consistency, followed by stool volume and stool frequency²¹². Jarunvongvanich et al.²¹⁵ demonstrated similar findings in a Thai population sample, with the Bristol stool form score independently associated with both colonic transit time (faster transit with softer stools) and stool frequency (more frequent stools of softer consistency). Using simultaneous radiopaque markers and wireless motility capsules, Saad et al.²¹⁶ found that Bristol stool form types 1 and 2 were predictive of delayed whole-gut transit in 46 patients with chronic constipation (sensitivity 85%, specificity 82%). However, in contrast to previous studies, there was no correlation between consistency, transit or stool frequency in the healthy control group (n = 64).

Stool consistency is considerably softer in infancy, with ~60–80% of healthy infants demonstrating soft or liquid stools^181,217,218. Stools tend to be firmer in formula-fed infants than in breast-fed infants. In a study including 911 infants (78.2% breast-fed), the prevalence of firm stools was 1.1% in breast-fed infants and 9.2% in formula-fed infants²¹⁷. Stool consistency seems to normalize from early childhood onwards, with no difference observed in children between the ages of 4 and 15 years²¹⁹. Similarly to adults, stool consistency is strongly correlated with whole-gut transit time in children²¹⁹.

Work published in 2020 on the rheology of human faeces demonstrated that stool consistency alters faecal yield stress, which describes the pressure required to deform the faeces to enable rectoanal transit¹⁴⁵. Bannister et al.¹⁵¹ used balloons and beads of differing size, volume and consistency to demonstrate that soft, large, deformable balloons were more easily evacuated than harder, smaller beads, requiring a shorter time and lower rectal pressure, and with more complete evacuation in healthy adults.

Colonic transit and stool consistency are interrelated with colonic microbiota composition, diversity and metabolism^220,221. Microbial composition is altered by diet and transit time, which can, in turn, alter host physiology²²². While causal associations between the gut microbiota and bowel dysfunction remain unclear²²³, longer colonic transit times can be associated with a shift in colonic metabolism from carbohydrate fermentation to protein catabolism²²⁰, reduced short-chain fatty acid concentrations (probably through increased colonic absorption rather than decreased production)^224,225, and increased abundance of methanogens²²¹ (as indicated by positive methane breath tests following carbohydrate challenge, which is more common in patients with slow-transit constipation^226,227. However, independent of transit time, one study has demonstrated that the colonic microbiota profile has a 94% accuracy for discriminating between healthy adults and patients with constipation (25 women in each group)²²⁸. Further studies have demonstrated that the Prevotella-predominant enterotype is associated with softer stool consistency than the Ruminococcus–Bacteroides-predominant enterotype²²¹. Despite these findings, a systematic review published in 2019 demonstrated no differences in colonic microbiota when comparing patients with diarrhoea-predominant and constipation-predominant irritable bowel syndrome (IBS)²²⁹. Therapeutic modulation of the colonic microbiome using prebiotics, probiotics, synbiotics and antibiotics for the treatment of gastrointestinal diseases is of great interest^{230,231,232,233,234,235,236,237,238}; however, the specific mechanisms relating colonic microbiota and colonic function are yet to be determined.

Circadian rhythm

Colonic motility exhibits diurnal variation and, in humans, can be inhibited by sleep and increased following morning waking^{84,85,239,240,241}. Propulsive HAPCs can be associated with morning waking and also with the morning call to defaecate¹⁰⁷.

Colonic motor response to a meal

Over 100 years ago, eating a meal was identified as a stimulus for 'mass movements' of colonic content²⁴². This colonic response was termed the 'gastrocolonic reflex'²⁴³. More recently, the colonic meal response was hypothesized to be a neurohormonal response to gastric distension in humans, causing the release of neuropeptides including cholecystokinin, serotonin, neurotensin and gastrin²⁴⁴. However, the colonic response to a meal can occur independently of gastric stimulation, which was demonstrated by the presence of the colonic meal response following the smell of food or verbal discussion of a meal in healthy adults²⁴⁵, and the preservation of the response in patients after gastrectomy²⁴⁶. Although still commonly used in current journals and textbooks, the term gastrocolonic reflex is, therefore, misleading.

The colonic meal response occurs rapidly. Snape et al. demonstrated an increase in contractility of the sigmoid colon and rectum in 16 healthy adults within minutes of starting to eat a 1,000-calorie meal²⁴⁷. The intensity of the colonic meal response is dependent on the nutritional content of the meal. For example, a meal of 300 calories has a less marked effect on colonic motor function than a meal of 1,000 calories²⁴⁷ and dietary fats cause a greater increase in colonic contraction than carbohydrates^248,249. The colonic meal response must be mediated in part by the central nervous system, as this response is absent in patients with spinal injury²⁵⁰ and can be inhibited by the muscarinic receptor antagonist clidinium bromide²⁵¹.

Low-resolution colonic manometry studies have shown that meals are temporally associated with an increase in non-propagating and low-amplitude propagating contractions throughout the colon^{84,85,241,252}. In addition, HAPCs (which can be associated with defaecation^84,85,253) are seen more frequently in the post-prandial period^84,85,241. With the introduction of high-resolution colonic manometry (10–30 mm spacing between recording sensors), the timing and characteristics of the colonic meal response have been described in greater detail. Within 60 s of starting a 700-calorie meal, a substantial increase in cyclic motor activity, predominantly propagating in a retrograde direction, occurs in the rectosigmoid region (2.8 events per hour pre-prandial compared with 17.4 events per hour post-prandial; P < 0.001)^14,15. Other high-resolution colonic manometry studies have shown that synchronous intraluminal pressure increases (termed pan-colonic pressurizations)⁷⁸ also increase in frequency during a meal⁷⁸.

Influence of dietary intake

Dietary intake alters the composition of luminal content, colonic microbiota²⁵⁴ and bowel function²⁵⁵. Dietary fibre, found in cereals, fruits, vegetables and legumes, comprises carbohydrates that are poorly absorbed in the upper gastrointestinal tract. Different fibre sources can be described in terms of their water solubility (water soluble or water insoluble) or their amenability to fermentation by the colonic microbiota (degradable or non-degradable), or categorized by volume of intake as high residue or low residue (high-fibre or low-fibre diets)²⁵⁶.

Degradable fibres are fermented in the colon and increase stool volume predominantly via an increase in bacterial biomass, which can account for over half of the total dry stool volume^209,257. Degradable fibres include fermentable oligosaccharides, disaccharides, monosaccharides and polyols (FODMAPs) and resistant starches, and are often synonymous with prebiotics, defined in a consensus statement published in 2017 as compounds within the diet that are selectively utilized by colonic microbiota to confer a health benefit²⁵⁸. A systematic review and meta-analysis demonstrated that an increased dietary intake of resistant starch (22–45 g per day) decreased stool pH, and increased stool volume and stool butyrate concentration, but had no effect on stool frequency²⁵⁹. Using MRI in 14 healthy volunteers, consumption of kiwifruit²⁶⁰ increased retention of water in the small bowel and ascending colon and increased the volume of colonic contents, whereas a high-FODMAP diet²⁶¹ was associated with an increase in small bowel luminal water content and colonic gas volume in 16 healthy volunteers.

Fibre sources less amenable to colonic fermentation, such as cereal fibres (for example, wheat fibre or psyllium husk), confer a greater increase in stool volume than fermentable fibres^262,263. In a study investigating the relationship between diet and colonic content using MRI, non-gaseous colonic content was substantially higher in participants on a high-residue diet (616 ml) than in those on a low-residue diet (479 ml; P = 0.038)¹⁶⁷. A systematic review and weighted regression analysis including 65 studies demonstrated that, for every 1 g increase in wheat fibre, stool volume increased by 3.7 ± 0.09 g per day²⁶⁴. Wheat fibre consumption was associated with an increase in stool water content, stool frequency and an apparent normalization of delayed whole-gut transit time in healthy adults^262,264. A vegetarian diet, which in most cases contains a high-fibre content, was associated with a statistically significant increase in stool frequency in healthy adults when compared with healthy adults on an omnivorous diet; however, the difference was minimal (11.8 ± 4.5 bowel motions per week on a vegetarian diet compared with 11.3 ± 4.7 bowel motions per week on an omnivorous diet; P < 0.05)¹⁷³.

Age, sex, parity and BMI

Defaecation is influenced by age, sex and BMI. Most published studies^{265,266,267,268,269} have shown that ageing is associated with a higher prevalence of constipation (17% prevalence in those aged ≥ 60 years²⁶⁷). However, a 2020 population-based survey of nearly 6,000 adults in the USA, Canada and the UK found a significantly higher prevalence of constipation in those aged 18–34 years (9.9%) than in those aged >65 years (6.5%)²⁷⁰. This finding is in contrast to earlier North American epidemiological studies that demonstrated a higher prevalence of constipation in those aged > 60 years (23.3%) than in the overall population (12.8%)^268,269. Both scintigraphic^271,272 and wireless ingestible electromagnetic capsule transit studies¹⁸ demonstrate longer colonic and whole-gut transit time with increasing age (0.26% increase in colonic transit time per year of life; P = 0.021)¹⁸. An age-related decrease in cholinergic function in the ascending colon has also been demonstrated²⁷³. However, the cause of altered bowel function in older people is difficult to pinpoint, as a multitude of other changes occur with ageing that could contribute, including diet, medications and decreased physical activity^273,274.

In children, the prevalence of constipation is equally distributed between the sexes but, in adults, constipation is reported more commonly by women^267,275. Prolonged colonic transit times have been demonstrated in radiopaque marker studies^{276,277,278,279}, scintigraphic studies^272,280 and wireless capsule studies^18,19 in healthy women. Women also report less frequent bowel motions¹⁷⁹ and have greater variability in stool consistency than men (Bristol stool form types 3–5 in men, types 2–6 in women)¹⁷¹, with softer stool consistency during the perimenstrual period²⁸¹ and firmer stools during the postpartum period²⁸¹. Variability in bowel habit between the sexes has been hypothesized to be due to cyclical fluctuations in sex hormones²⁸². In one study, exogenous progesterone administered to postmenopausal women accelerated colonic transit (colonic geometric centre at 48 h: 3.2 in women receiving placebo, 3.9 in women receiving progesterone) and resulted in softer stool consistency²⁸³. However, other studies have demonstrated slower transit during the luteal phase of the menstrual cycle, during which serum progesterone levels reach their peak^214,284,285, or no variation in stool consistency or colonic transit during the luteal phase and follicular phase²¹⁴.

In terms of anorectal function, men have a greater functional anal canal length^16,36,37 and higher rectal sensory thresholds to mechanical distension than women^286,287. Women are at risk of pelvic floor and anal sphincter injury during pregnancy and childbirth. Anal sphincter injury is reported in approximately one-third of primiparous women^288,289,290, yet less than one-third of women with sphincter injuries report faecal incontinence during the postpartum period²⁹¹. In some women, the onset of symptoms can be delayed, often occurring decades after pregnancy^292,293,294.

Parity can be associated with a reduction in anal canal squeeze pressure¹⁶. In women with faecal incontinence or constipation, each successive child is associated with a mean reduction in anal canal resting tone of 4.3 cmH₂O and prolongation of pudendal nerve terminal motor latencies²⁹⁵. However, there is only a weak correlation between anal sphincter resting and squeeze pressures and faecal incontinence symptom severity^296,297. In the absence of direct anal sphincter injury, a Swedish observational population-based study published in 2019 including over 3.7 million women demonstrated that both caesarean section and vaginal delivery were associated with a risk of developing faecal incontinence, suggesting that other pregnancy-related factors are also involved in the pathogenesis of faecal incontinence (185,219 women with caesarean section only, 1,400,935 with vaginal delivery only)²⁹³. Vaginal delivery is also a risk factor for developing descending perineum syndrome, which can be associated with evacuation disorders and chronic constipation⁶⁶.

The effects of BMI on colonic and anorectal function have been assessed using scintigraphy²⁹⁸, wireless electromagnetic ingestible capsules¹⁸ and anorectal investigations³⁷. In a study including 72 participants, there was a trend towards more rapid colonic transit on scintigraphy in patients with a BMI of >30 kg/m², but the difference was not statistically significant after adjusting for sex²⁹⁸. When assessed with wireless capsules, a higher BMI was significantly related to shorter whole-gut transit time (P = 0.012)¹⁸. In a Swedish study including 1,001 people, obesity was additionally associated with stool urgency and the sensation of incomplete rectal evacuation²⁹⁹. Obesity is also an independent risk factor for faecal incontinence³⁰⁰. Among 96 healthy women, an increase in BMI was correlated with a longer balloon expulsion time (reflecting impaired evacuatory efficiency) and a higher threshold volume during rectal sensory testing³⁷.

Other influences

A multitude of other factors influence the physiology of defaecation, some of which include comorbidities, many common medications and physical activity (Box 3). It is beyond the scope of this review to describe each in detail.

Many common medications alter bowel function (Box 3). Opioids, for example, are the most frequently prescribed drug class in the USA³⁰¹ and are associated with constipation in >40% of patients with chronic non-cancer pain^302,303. It is well acknowledged that opioids delay gut transit, but opioid use is also associated with rectal hyposensitivity and 'functional' evacuation disorders in patients with constipation³⁰⁴.

A direct relationship between exercise and bowel function is not clear¹⁷³. Decreased stool frequency and new onset of constipation was shown in six of ten healthy volunteers after a strict 35-day period of bed rest³⁰⁵. Conversely, 'runner's diarrhoea' can be induced by periods of high-intensity running training (1–2 h per day) in endurance athletes, resulting in higher stool frequency, softer stool consistency, and more rapid small bowel and distal colonic transit than periods of inactivity³⁰⁶.

Common disorders of defaecation

Parameters that constitute a disorder of defaecation are ill-defined. They include the obvious, where defaecation is difficult to initiate or complete (constipation) or control (incontinence), but also include related syndromes such as functional diarrhoea and IBS. Some sense of where diagnoses start and end is provided by the Rome IV criteria using specific combinations of symptoms to define syndromes^307,308. For constipation, for example, the Rome IV criteria allow categorization of disorders into four subtypes: functional constipation, IBS with constipation (IBS-C), opioid-induced constipation and functional defaecation disorders. However, there is considerable overlap between these groups and accumulating clinical and mechanistic evidence suggests that these subtypes actually exist on a spectrum rather than as distinct entities^309,310,311. It is further acknowledged that it is sometimes difficult to distinguish one from another, and that transition from one functional bowel disorder or from one predominant symptom to another is common. Specifically, considerable overlap between IBS-C and functional constipation exists with regard to both symptoms (for example, abdominal pain and presence of hard stools)³¹¹ and pathophysiology (for example, visceral sensory dysfunction, functional evacuation disorders and transit abnormalities)^{310,311,312,313,314}, and transition from functional constipation to IBS-C, and vice versa, is common^311,315. Likewise, as noted previously, constipation and faecal incontinence are not distinct conditions, with >40% of patients having concurrent symptoms of both disorders^{3,4,5,316,317}. Regrettably, pathophysiological findings do not neatly equate with syndromes. Rather, it is possible to categorize common abnormalities affecting each of the four phases of defaecation and then note where these have been documented for various clinical syndromes (Table 1).

Table 1 Common disorders of defaecation categorized by defaecation phase

Full size table

Conclusions

Our understanding of the physiology of defaecation and continence (and also the pathophysiology of conditions such as constipation and faecal incontinence) has progressed considerably, although fundamental uncertainties still remain, particularly regarding the actions, interactions and integration of the myogenic, neural and hormonal mechanisms involved in colonic and anorectal function. It is through resolution of these uncertainties that more effective assessment and individualized treatment of disorders of defaecation will hopefully be achieved. Specific gaps in our knowledge have been summarized in this Review (Box 2), which the authors propose will assist in guiding ongoing research.

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Acknowledgements

P.T.H. is the recipient of the Peter King Research Scholarship, Royal Australasian College of Surgeons.

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Affiliations

1. College of Medicine and Public Health, Flinders University, Adelaide, SA, Australia

   Paul T. Heitmann & Phil G. Dinning

2. Centre for Neuroscience, Flinders University, Adelaide, SA, Australia

   Paul T. Heitmann & Phil G. Dinning

3. Departments of Surgery and Gastroenterology, Flinders Medical Centre, Adelaide, SA, Australia

   Paul T. Heitmann & Phil G. Dinning

4. GI Physiology Unit, Barts Health NHS Trust, London, UK

   Paul F. Vollebregt, Charles H. Knowles, Peter J. Lunniss & S. Mark Scott

5. Blizard Institute, Centre for Neuroscience, Surgery and Trauma, Queen Mary University of London, London, UK

   Paul F. Vollebregt, Charles H. Knowles & S. Mark Scott

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The authors contributed equally to all aspects of the article.

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Heitmann, P.T., Vollebregt, P.F., Knowles, C.H. et al. Understanding the physiology of human defaecation and disorders of continence and evacuation. Nat Rev Gastroenterol Hepatol 18, 751–769 (2021). https://doi.org/10.1038/s41575-021-00487-5

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• Accepted: 21 June 2021

• Published: 09 August 2021

• Issue Date: November 2021

• DOI : https://doi.org/10.1038/s41575-021-00487-5

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