Advances in the evaluation of anorectal function

Abstract

Fecal incontinence and evacuation disorders are common, impair quality of life and incur substantial economic costs worldwide. As symptoms alone are poor predictors of underlying pathophysiology and aetiology, diagnostic tests of anorectal function could facilitate patient management in those cases that are refractory to conservative therapies. In the last decade several major technological advances have improved our understanding of anorectal structure, co-ordination and sensorimotor function.

This Consensus Statement provides the reader with an appraisal of the current indications, study performance characteristics, clinical utility, and strengths and limitations of the most widely available tests of anorectal structure (ultrasonography and MRI) and function (anorectal manometry, neurophysiological investigations, rectal distension techniques and tests of evacuation including defecography). Additionally this article provides our consensus on the clinical relevance of these tests.

INTRODUCTION

Evacuation of bowel contents is a highly regulated and coordinated function of the combined activities of the colon, rectum and anus¹. Dysfunction of this unit can lead to fecal incontinence and/or symptoms of an evacuation disorder, and can have a devastating effect on quality of life². In North America, between 7% and 18% of community dwelling adults fecal incontinence^{3, 4} and 12–19% of the population report evacuation disorders⁵ with an age and sex-adjusted incidence approximately 3-fold greater than Crohn’s disease⁶. Since the underlying aetiology and pathophysiology of fecal incontinence and evacuation disorders is multifactorial, reliance on symptoms alone to guide therapy is inadequate⁷.

The primary approach to a patient presenting for the first time with fecal incontinence or constipation with difficult defecation should be to exclude serious underlying pathology (such as colorectal malignancy and IBD)⁸. In patients with symptoms refractory to first-line therapies such as lifestyle modification and optimization of stool consistency, it is justifiable to proceed with evaluation of anorectal structure, motor and sensory function ^{9, 10}. The selection of appropriate investigations is often guided by the clinical history and examination. Such an evaluation should focus on determining the duration, type and severity of the patient’s symptoms as well as identification of risk factors for symptom onset¹¹. Epidemiological studies have identified a number of such risk factors including increasing age, elevated BMI and presence of diarrhea^{12, 13}. In women, obstetric injury is particularly relevant^14–16 owing to the risk of damage to the pelvic floor, anal sphincters and pudendal nerves during the second stage of labour^{17, 18}. In men, iatrogenic injury to the sphincter complex secondary to anal surgery is a factor in up to 59% presenting for assessment¹⁹, and co-existent benign perianal disease (such as hemorrhoids, fistula-in-ano and radiation proctitis) is also common²⁰. In all patients, particular attention should be paid towards symptoms of other anorectal complaints (for example, fecal incontinence in a patient presenting with constipation) as data increasingly suggest that both fecal incontinence and evacuation disorders commonly co-exist²¹. Also, anorectal evaluation begins with a carefully performed digital rectal examination that can reveal several abnormalities including dyssynergia, weak anal sphincters, sphincter defects, fecal impaction and others^15,23 .

No single test can fully characterize the causes of faecal incontinence and/or an evacuation disorder. Instead, several tests are used to assess anorectal structure, motor and sensory function (Table 1). Data on the usefulness of these tests is conflicting, and some studies suggest that clinical examination alone is an adequate method for stratification of treatment^{22, 23}; however, there are data to suggest that quantification of function can directly influenced clinical decision-making^{24, 25} and could provide biomarkers that predict response to treatment^26–28. However, despite several attempts at consensus^29–31, there is widespread discordance in practices between institutions³². Such a disparity probably reflects factors such as access to technology, resource availability and local expertise; nevertheless, it is generally accepted that the clinical utility of tests improves when anorectal function is assessed in a structured and systematic manner³³. Thus, the aim of this Consensus Statement is to provide the practicing clinician with a background and framework regarding the indication(s), application and clinical interpretation(s) of tests of anorectal function and notable key advances (Box 1). Recommendations are based on a review of the literature and discussion by members of the International Anorectal Physiology Working Group (IAPWG) under the auspices of the International Working Group for Disorders of Gastrointestinal Motility and Function, a collective of clinicians and academics with particular interest and experience in the field of anorectal function testing.

TABLE 1.

Clinical utility for commonly performed investigations of anorectal physiological function.

Function	Investigation	Clinical utility
Anus
Motor	Anorectal manometry (conventional)	****
	Anorectal manometry (high resolution)	****
	Anorectal manometry (3D)	***
	Electromyography	***
	Pudendal nerve terminal motor latencies	*
Structure	Endoanal ultrasonography	****
	Transperineal ultrasonography	***
	Endoanal or pelvic MRI	***
	MRI muscle fiber tracking	*
	Electrostimulation	*
Sensory	Light touch stimulation	*
	Anal evoked potentials	**
Rectum
Sensory	Balloon distension	****
	Rectal barostat	***
	Rectal evoked potentials	**
Motor	Distal colonic manometry	**
	Rectal barostat	***
	Rectal motor evoked potentials	*
Anorectal unit
Motor	Anorectal manometry(conventional, high-resolution or 3D high resolution)	****
	Balloon expulsion	****
Motor, sensory and structure	Barium defecography	****
	Magnetic resonance defecography	***
	Functional lumen imaging probe	*

^*Limited clinical utility or of research interest only.

^**Emerging technology with limited data of clinical utility.

^***Recognized clinical utility but less commonly performed.

^****Good clinical utility and commonly performed.

Box 1. Key advances in evaluating anorectal function.

• Investigations of anorectal structure, function and sensation are indicated for the assessment of patients with symptoms suggestive of an evacuation disorder and/or faecal incontinence that are unresponsive to conservative therapy

• No single investigation can fully assess anorectal function; for this reason, a range of techniques are generally used to characterize pathophysiology and aetiology of symptoms

• Anal endosonography and anorectal manometry (ARM) provide an assessment of sphincter structure and function in patients with symptoms of faecal incontinence; ARM with balloon expulsion and defecography identify functional and/or structural pathology in patients with evacuation disorders

• Investigations of anorectal sensation are a vital component of assessment in both faecal incontinence and evacuation disorders

• Owing to the overlap of normal and abnormal values, the results of such functional investigations should be interpreted carefully taking into context the clinical picture and the multifactorial etiology of anorectal disorders

METHODS

The IAPWG steering committee (E.V.C., S.M.S., H.H., M.F. and S.S.R.) was appointed by the International Working Group for Disorders of Gastrointestinal Motility and Function. Under the guidance of the steering committee, the authors performed focused literature reviews in the following areas: anorectal manometry, anorectal neurophysiology, endoanal ultrasonography, pelvic floor ultrasonography, rectal sensory testing, balloon expulsion and defecography. Consensus was achieved through careful evaluation and discussion of available literature, and expert agreement when recommendations lacked supporting evidence

This Consensus Statement will focus on the following tests that are widely available to the practicing clinician, and can be divided into: tests of anal motor function, anorectal manometry (ARM), high resolution (HR-ARM) and high-definition 3D anorectal manometry [3D-HR-ARM) and neurophysiology; tests of anal and pelvic floor structure, endoanal or pelvic floor ultrasonography; tests of rectal sensory and motor function, simple balloon distension and rectal barostat; tests of evacuation, balloon expulsion and both barium and magnetic resonance (MR) defecography; emerging technologies are also briefly addressed.

TESTS OF ANAL MOTOR FUNCTION

Anorectal manometry

ARM is the most widely used technique for the detection of abnormalities of sphincter function and/or rectoanal co-ordination^{15, 29, 31}. This investigation consists of a series of pressure measurements that assess: involuntary function of the anal canal during rest; voluntary function during squeeze; reflex rectoanal co-ordination during rectal distension; and rectoanal co-ordination during simulated defecation (‘push’)^{7, 29}.

Manometric equipment can record pressure data from single points in the anal canal (termed ‘conventional anal manometry’) or can record and display detailed information simultaneously from the whole anal canal and distal rectum (high-resolution manometry)^{34, 35}. Although conventional anal manometry using water perfused system remains in clinical practice, studies suggest a move towards the more detailed high-resolution solid state methodology, probably in part due to the ability of this technique to more accurately characterise sphincteric function³⁶.

Within the paradigm of high-resolution manometry, two technologies exist. HR-ARM records luminal pressures circumferentially from sensors mounted on a flexible catheter with data presented either topographically in color plots or as an average circumferential pressure at different longitudinal levels of the anorectum³⁴. On the other hand, 3D-ARM records point pressures longitudinally and radially from sensors mounted on a rigid probe with morphology represented in both 2D and 3D³⁴.

Study indications

ARM is typically indicated for the assessment of fecal incontinence and constipation, especially if characterized by symptoms of disordered evacuation. Relative indications for ARM include assessment of functional anorectal pain, pre-operative assessment of anorectal function and assessment of anorectal function in patients after obstetric injury and/or traumatic birth to inform treatment decisions concerning future mode of delivery^{7, 29}.

Study performance

Although not expected to be fully diagnostic, a digital rectal examination should be performed before intubation to provide an overview of anorectal and pelvic floor structure and function, to exclude fecal loading, stricture, bleeding and pain. It is also helpful to check patient understanding of instructions such as ‘squeeze’ and ‘push’. Studies are typically performed in the left lateral position and any lubricant to aid probe placement should be non-anesthetizing. The probe is then positioned ensuring that the sensors span the distal rectum to beyond the anal verge. Both conventional and high-resolution techniques can use either water-perfused or solid state technology for data collection and detailed description of hardware and software setup and catheter design is described elsewhere³⁷.

A standardized protocol for ARM would improve utility of the procedure, peer-acceptance, translation and dissemination of results. The protocol recommended by the IAPWG is shown in Figure 1. This protocol consists of the following standardized measurements: rest - basal anal pressures at rest over 60 s; squeeze - anal pressure during voluntary effort; long squeeze - anal pressure during sustained voluntary effort; cough - anorectal pressure changes during cough (that is, reflex increase in rectal and anal sphincter pressure during abrupt change in intra-abdominal pressure); push - anorectal pressure changes during simulated defecation; RAIR - reflex anal response to rectal distension; rectal sensation - assessment of rectal sensitivity to distension, typically performed as part of an ARM protocol (discussed in detail later).

Figure 1.

Schematic of standardized protocol for high-resolution anorectal manometry.

Normal reference values for many of these variables have been described in three studies using HR-ARM^38–40 and five using 3D-ARM (three adult populations^41–43, one pediatric population⁴⁴, one series of primigravid women (first pregnancy)⁴⁵. A number of similar reference values exist for conventional manometry, however, many studies use historical set-ups and protocols no longer in current use.

Clinical utility

A number of contemporary and historical studies have demonstrated differences in manometric findings between healthy volunteer (Figure 2a) and patient groups. Several clinically relevant features have been observed. Sphincter hypotonia (low anal resting pressure), though of low sensitivity, is associated with passive fecal incontinent^46–51 (Figure 2b), whereas sphincter hypertonia (high anal resting pressure) can be a feature of anal fissure^52–54 or constipation⁵⁵. Sphincter hypocontractility (impaired ability to voluntarily contract the anal sphincter) is associated with fecal incontinence, particularly fecal urgency^{47, 56} (Figure 2c) and poor propulsion (impaired rectal force during push), dyssynergia (paradoxical anal sphincter contraction during push) (Figure 2d) and pelvic floor akinesia (failure of movement of the pelvic floor)^57–59 have been noted. An absent rectoanal inhibitory reflex is classically seen in Hirschsprung disease⁶⁰; however, abnormal responses can also be observed in patients with fecal incontinence⁶¹ and constipation⁶². and after anorectal surgery⁶³ .

Figure 2.

Representative high-resolution anorectal manometry traces and resultant line traces to assess anorectal function. a | Sphincter hypotonia in a patient with fecal incontinence, visualized in the color contour plot as a band of pale green (~ 20–25mmHg) set between normal blue (~5mmHg) rectal (superiorly) and atmospheric (inferiorly) pressures. b | Dyssynergia (paradoxical anal sphincter contraction during push visualized in the color contour plot as a band of purple (~ 150–175mmHg) within the anal canal and band of yellow (~50mmHg) in a patient with evacuation disorder.

In addition to these findings, 3D-ARM has the ability to illustrate the normal asymmetry of pressures within the anal canal, with higher pressures in the posterior proximal and anterior distal regions of the sphincter. Deviation from this normal manometric anatomy can be detected on either 3D or 2D pressure plots at rest and/or squeeze, and can be suggestive of pathology, though studies only demonstrate slight concordance with anal sphincter defects detected by endoanal ultrasonography^{64, 65}. Thus, pressure defects detected by 3D-ARM should not be used as surrogate markers of anatomical anal sphincter defects without exercising caution.

Some evidence supports the hypothesis that pelvic floor abnormalities not previously identified by conventional anorectal manometry can be found with 3D-ARM. Pilot studies suggest that this technique can measure pelvic floor descent⁶⁶ and that results have a high positive predictive value (up to 100%) for the presence of an intra-anal intussusception diagnosed by defecography^{67, 68}. These findings might be a useful indicator of the existence of pelvic floor disorders and help direct further investigation (such as defecography), especially in patients with symptoms of evacuation disorders.

Strengths and limitations

ARM is the best-established technology that provides a direct assessment of anal sphincter pressure and rectoanal co-ordination during simulated defecation. It is widely available, easy to perform and well accepted by patients. However, interpretation of findings can be difficult due to the wide variability (and overlap) of manometric measurements in health and disease^{50, 69–72}. Furthermore, some studies suggest that ARM offers little additional utility over digital rectal examination for planning patient management²². Additionally, due to the striking variability in practices between institutions³², appropriate caution should be paid to using reference ranges unless equipment design and test protocol mirrors one’s own. Study set-up and patient position, presence or absence of the perception of the desire to defecate among others can each have a major effect on absolute values reported^73–78.

Anorectal neurophysiology

Although less commonly used in clinical practice³⁶ (due to the adoption of less invasive surrogate measures of sphincter function), anal electromyography and pudendal nerve terminal motor latencies (PNTML) remain an important component of anorectal neurophysiological function. The branches of the pudendal nerve, which course over the pelvic floor, are vulnerable to stretch injury (during the third trimester, second stage of labor and forceps-assisted vaginal delivery), which can lead to denervation of the external anal sphincter (EAS), muscle weakness and fecal incontinence^{79, 80}. Owing to the complex nature of symptom generation, such assessment is always performed in conjunction with other investigations (for example, ARM) to enable accurate understanding of the pathophysiological mechanisms.

Recording of pelvic floor electromyography (from both the EAS and levator ani muscles) enables mapping of the EAS to identify sphincter defects, determination of striated muscle function and assessment of denervation-reinnervation potentials (indicative of neural injury)⁸¹. PNTML measurement evaluates the neuromuscular integrity between the pudendal nerve and the anal sphincter^{7, 31, 81}.

Study indications

Neurophysiological assessment is typically indicated for the investigation of symptoms of fecal incontinence thought to be secondary to neurological injury. Relative indications for neurophysiology include symptoms of anorectal pain and characterization of complex pelvic floor disorders, especially prior to anorectal surgery⁷.

Study performance

Studies of electromyography activity are typically performed using a needle, skin or anal plug electrode. Disposable needle electrodes are inserted into the muscle under study and parameters are recorded to describe insertional activity, spontaneous activity, motor unit action potential morphology and recruitment during voluntary or reflex activity. For PNTML, stimulation of the pudendal nerve using a disposable bipolar electrode generates a compound muscle action potential response of the EAS⁸². The PNTML is the time between stimulus artifact and the onset of the compound muscle action potential response.

Clinical utility

In the striated anal musculature of patients with fecal incontinence, motor unit action potential activity, fiber density and jitter (the stability of consecutive muscle fiber discharges, reflecting the stability of terminal motor axons and neuromuscular transmission) through recording of electromyography activity, have all been shown to be altered in comparison with controls^{81, 83–85}. For PTNML, abnormal results (prolonged latencies) are used as a surrogate marker of pudendal neuropathy and indicate either demyelination or damage to a number of fast firing fibers⁸⁶.

Strengths and limitations

Electromyography recordings correlate well with sphincter pressures⁸⁷ (e.g. electromyographic recruitment is seen simultaneously with increased anal pressures during squeeze). Intraluminal electrodes are believed to be more accurate because they are closer to the external anal sphincter muscle and are less likely to pick up gluteal or other muscles⁸⁸. Currently, the most frequent application of electromyography is as a biofeedback signal for pelvic floor retraining of the external anal sphincter in patients with fecal incontinence or constipation^89–91, and it can be especially useful to detect paradoxical contraction or impaired relaxation of the sphincter in those with evacuation disorders⁹¹.

The clinical utility of PNTML remains controversial, as there are several test limitations: it is operator-dependent³¹; sensitivity and specificity are poor^{83, 84}; normal latencies can be recorded in a damaged nerve as long as some fast-conducting fibers remain⁹²; and prediction of clinical outcomes after intervention are conflicting⁹³. In addition, the upper limit of normal for latency is ill-defined due to substantial variability in healthy individuals ⁷.

TESTS OF ANAL STRUCTURE

Endoanal and pelvic floor ultrasonography

Endoanal ultrasonography⁹⁴ is a simple and well-tolerated technique that is widely used for detecting both internal anal sphincter (IAS) and EAS defects and has substantially increased our understanding of the pathogenesis of fecal incontinence. Total pelvic floor ultrasonography (integrating endoanal, transvaginal and transperineal ultrasonography) has been used to assess pelvic organ prolapse (that is, rectocele, enterocele, intussusception and cystocele)⁹⁵. Endoanal and/or pelvic MRI, although not widely available, are alternative imaging modalities only available in specialist centers⁹⁵

Study indications

Endoanal or pelvic floor ultrasonography can be used for a number of indications: to evaluate morphological integrity of the anal sphincters in patients with fecal incontinence, particularly when surgery is being considered and to provide information on the pelvic viscera and pelvic floor movement in patients with symptoms of an evacuation disorder as a complementary investigation to ARM and defecography⁹⁶. Moreover, these approaches can be used to assess obstetric anal sphincter injuries (OASIS) after childbirth either to guide early repair and/or to inform planning of subsequent deliveries⁹⁵. Other indications exist, particularly for the staging of rectoanal neoplasms (beyond the scope of this article)⁹⁷. Transperineal ultrasonography has additional indications such as assessment of urinary incontinence, voiding difficulties, pelvic organ prolapse symptoms (again considered outside the scope of this article)^{98, 99}.

Study performance

Endoanal ultrasonography is performed in the lateral or prone position using a rigid endoprobe (3–20 MHz), providing 360° axial views of the sphincter complex⁹⁴. Modern systems enable continuous capturing of images as the probe is withdrawn through the anal canal with post-hoc multiplanar 3D image reconstruction and calculation of anal sphincter volumes.

During the performance of total pelvic floor ultrasonography, the patient is placed in the dorsal lithotomy position with the hips flexed and abducted. Transperineal ultrasonography uses a curved array probe (5–9 MHz), whereas transvaginal scanning is performed using a linear array endoscopic probe (12 MHz) to obtain dynamic 2D posterior and anterior mid-sagittal views. In addition to the detection of anal sphincter defects⁹⁵, this technique enables evaluation of the dynamic interaction between the pelvic floor and viscera.

Anal anatomy is complex. On endoanal ultrasonography, two discrete rings of tissue are principally visualized, the inner hypoechoic IAS and the outer hyperechoic EAS, which is caudad to puborectalis⁹⁴. Several other structures including the subepithelial tissues and conjoined longitudinal muscle are also imaged¹⁰⁰. By contrast to 2D ultrasonography, 3D ultrasonography can also measure the length and volume of the EAS and might be better for distinguishing EAS defects from defects in surrounding structures (that is, transverse perineii and puboanalis muscle)⁹⁴. Transperineal ultrasonography, in addition to characterization of the sphincter complex, provides a dynamic evaluation of the interaction of the pelvic floor viscera and musculature, comprising assessment of the posterior, central and anterior compartments¹⁰¹.

Clinical utility

Endoanal ultrasonography is widely available and should be considered the cornerstone for anal imaging¹⁰². The IAS can be classified as normal or pathological, with the latter defined by several factors: sphincter discontinuity (defect)¹⁰³ (Figure 3b), atrophy (identified by diffuse thinning of the sphincter [thickness ≤1 mm]⁵⁰) (Figure 3c) and/or degeneration¹⁰⁴ or hypertrophy^{48, 105}. IAS defects and/or atrophy are appreciated to be associated with symptoms of (particularly passive) fecal incontinence^{106, 107}. IAS hypertrophy is primarily seen in relation to rectal intussusception and/or prolapse^{47, 48, 50}.

Figure 3.

Representative endoanal ultrasonography images. a | The mid anal canal in healthy volunteer, demonstrating an intact internal anal sphincter (IAS) (arrow) appearing hypoechoic, and an intact external anal sphincter (EAS) (arrowhead) appearing hyperechoic. b Mid anal canal in a patient with fecal incontinence, demonstrating an IAS defect between the 1 and 5 o’clock positions (between the arrows). c | Mid anal canal in a patient with fecal incontinence demonstrating an EAS defect, evident as an area of hypoechoic discontinuity between the 12 and 2 o’clock positions (extent of defect between dashed lines). d | Mid anal canal in a patient with fecal incontinence demonstrating IAS atrophy (global thinning of the smooth muscle ring, which is of mixed echogenicity difficult to distinguish from surrounding structures, arrow). The EAS is intact.

The EAS can be characterized as normal, or pathological by sphincter discontinuity (defect) (Figure 3d), or interruption of its fibrillar echotexture manifest as focal thinning, scarring, or atrophy^{108, 109}. All abnormalities can be described in cross-section according to a clock face (for example, defect between 1 and 3 o’clock), and also in the longitudinal plane. EAS disruption is a characteristic feature of OASIS⁹⁴, is associated with anal hypocontractility^{110, 111} and can be found in up to 68% of individuals presenting with symptoms of fecal incontinence (n = 200) ^{112, 113}.

Total pelvic ultrasonography has the additional ability to enable classification of organ prolapse, structural and functional causes contributing to evacuation disorders¹¹⁴ and the identification of pubovisceral avulsion (abnormal insertion of the levator ani on the inferior pubic ramus), which is an important pathophysiological mechanism for pelvic organ prolapse and fecal incontinence^{95, 102}. Though not widely available, studies have also used a 4D approach to evaluate dynamic pelvic floor movement in real time¹¹⁵.

Strengths and limitations

Imaging of the anal sphincters can identify defects that are often clinically unrecognized and might be amenable to surgical repair. However, whereas abnormalities of anal structure are associated with anal hypotonia and hypocontractility,¹¹⁶ it can be challenging to interpret the clinical relevance of this finding, because ~10% of women have postpartum (occult) sphincter defects without any symptoms¹¹⁷.

TESTS OF ANORECTAL SENSORY AND MOTOR FUNCTION

Simple balloon distension and rectal barostat

Awareness of rectal filling is critical to normal bowel function^{101, 118}. Abnormal visceral sensitivity and/or biomechanical function (most commonly described by evaluation of rectal compliance) are often found in fecal incontinence and evacuation disorders⁷, providing the rationale for measurement of anorectal sensory and motor function via balloon distention and rectal barostat in clinical practice.

Study indications

Rectal sensory testing should be considered integral to physiological assessment of anorectal function. This testing is most commonly performed with simple balloon distension. Assessment with a rectal barostat, which is less widely available, should be considered in select patients with alterations of rectal sensation on ‘standard’ balloon distension, and/or in whom there is a high index of suspicion of abnormal rectal compliance or capacity ⁷. These tests enable the detection of heightened (hypersensitivity) or impaired and/or blunted (hyposensitivity)) rectal sensation and/or abnormal rectal compliance or capacity (that is, ‘stiff’ or small (hypocompliant) or ‘lax’ or large (hypercompliant)).

Study performance

Rectal sensation is evaluated by assessing the perception of rectal distension^{7, 119, 120}. For simple balloon distension, this test is performed by distending an elastic balloon, secured to a catheter placed within the rectum, with air (manually using a handheld syringe or via a pump). Either ramp (continuous at 1–2 ml/s) or intermittent (phasic or stepwise) distension paradigms can be used⁷. During balloon inflation, individuals are instructed to report perceived sensations (first sensation; desire to defecate; urgency; and maximum toleration or pain). The distending volume (or, less frequently pressure) at each of these sensory thresholds is then recorded⁷.

Evaluation of both motor and sensory function is performed with a computerized barostat that enables distension at a specified and precise rate, thereby minimizing observer bias and measurement error^{7, 119}. An ‘oversized’, non-elastic bag is used that can be regarded as infinitely compliant (in that its own properties have no influence on internal pressure) (Figure 4a). This is preferable to measurements using balloons, which require correction to account for their intrinsic elasticity^{7, 119}. Patients are examined in a semi-prone or lateral position to reduce pelvic hydrostatic pressure. An initial ‘conditioning’ distention documents the minimal distending (that is, intra-abdominal) pressure and ensures that subsequent measurements are reproducible (Figure 4b)¹²¹. Similar to the ‘simple balloon’ test described earlier, sensory thresholds are recorded during ramp or phasic distensions. Intra-bag (intra-rectal) volumes and pressures are recorded concurrently enabling rectal compliance to be calculated from the derived pressure-volume curve. Rectal capacity can also be measured (defined as barostat bag volume at a prospectively defined, supra-physiological pressure; for example, 40mmHg¹²²). Perceived sensations can alternatively be described using visual analog scores (for example, urgency, pain) recorded on a 0–100 mm scale during distensions at set pressures (typically at 8, 16, 24, and 32 mmHg above operating pressure)¹²³.

Figure 4.

Schematic of rectal barostat setup. a | Typical rectal barostat setup. b | Barostat conditioning distension protocol.

Clinical utility

The thresholds for rectal sensation can be normal, reduced (hypersensitivity) or increased (hyposensitivity) in both fecal incontinence and chronic constipation¹²⁴. Demonstration of altered sensation can guide therapeutic measures aimed at normalizing sensory thresholds and relieving bowel symptoms.

Rectal hypersensitivity is a common finding in patients with symptoms of urgency and frequent defecation in diarrhea-predominant IBS, ulcerative colitis and radiation proctitis⁵⁰. In IBS, this finding can be associated with increased symptom severity^{69, 117, 125}. Rectal hypersensitivity is also a feature of fecal urgency¹²⁶, urge fecal incontinence¹²⁷ and low anterior resection syndrome¹²⁸. Such hypersensitivity can be related to to reduced compliance, capacity or an exaggerated response to rectal distension¹²⁹. Normalization of sensory thresholds in patients with hypersensitivity has been associated with positive clinical outcomes following the use of behavioral therapy, pharmacological agents and surgical interventions^{117, 130}; however, good outcomes are not necessarily associated with post-intervention changes in sensory parameters.

Rectal hyposensitivity (often found alongside an attenuated or absent call to stool^131–133) is observed in 18–66% of patients with chronic constipation¹³⁴, constipation-predominant IBS, fecal incontinence, and evacuatory dysfunction secondary to spinal cord injury¹³⁵. This finding can be ‘primary’ (due to direct impairment of afferent pathway function), ‘secondary’ (due to altered biomechanical properties, e.g. megarectum), or both^{135, 136}. When hyposensitivity is present, the assumed mechanism is that stool is involuntarily expelled before the individual is alerted to the need to respond¹³⁷. In such patients, sensory retraining has been shown to facilitate timely contraction of the external sphincter and improve continence¹³⁸.

Demonstration of rectal hyposensitivity can indicate a severe clinical phenotype and predict a poor response to treatments such as biofeedback or bowel retraining^{139, 140} and surgery with colectomy¹⁴¹. However, in those patients who do respond, normalization of impaired sensation is generally associated with an improvement in symptoms¹⁴², most notably during treatment with neuromodulation^{143, 144}.

Strengths and limitations

Several consensus statements and technical reviews acknowledge that evaluation of rectal sensory function has an accepted place in the clinical management of patients with anorectal disorders²⁷. Nevertheless, methods for simple, elastic balloon distension are poorly standardized^{29, 30, 145, 146}. Though the use of a barostat overcomes these limitations, this technology is not widely available. However, development of a ‘rapid’ barostat⁴⁰ could enable this approach to be performed in routine clinical practice (as a typical barostat protocol may take approximately 1 hour to complete)⁷. For either technique, measurements can be affected by age, the rate and pattern of distention, patient position, and biomechanical and structural properties of the rectum¹⁴⁷, hence it is essential for results to be interpreted in the context of appropriate normative values.

TESTS OF EVACUATION

Balloon expulsion test

Inability to expel solid stool from the rectum is a key feature of patients with constipation characterized by symptoms of an evacuation disorder. The balloon expulsion test (BET) is a direct method by which to assess this function.

Study indications and performance

Balloon expulsion is a simple, office-based test, which is indicated as a first-line screening investigation for assessment of the ability to evacuate. With a patient lying in the left lateral position with hips and knees flexed, a lubricated, preferably non-latex balloon attached to a plastic catheter is inserted into the rectum and inflated with 50 ml of warm water. The patient is then seated on a commode in privacy and asked to expel the balloon. The ability (or inability) to expel the balloon and the time taken for expulsion is recorded.^{7, 148, 149} Although reported cut-offs for normality vary, the generally accepted limit for expulsion is between 1–3 min. Expulsion times longer than this can indicate disordered evacuation ¹⁵⁰.

Clinical utility

Though often considered to be synonymous with dyssynergic defecation, it is pertinent to note that the sensitivity and specificity of this test is variable (ranging between 68–94% and 71–81%, respectively^{29, 150, 151}). BET in isolation is, therefore, not sufficient to clearly diagnose an evacuation disorder^{150, 151}. Furthermore, agreement with other tests of evacuation is suboptimal (a prospective study of 100 patients with functional constipation published in 2016 showed only fair agreement between BET and defecography in diagnosing evacuation disorders, and no agreement between BET and anorectal manometry¹⁵²) and the BET provides no information about anatomical phenomena that might impair evacuation (for example, rectocele or occluding intussusception).

Nevertheless, studies do indicate that a positive result can predict response to biofeedback therapy with demonstration of a clinical response in up to 85% of patients,¹⁵³ though this finding is not consistent across all studies^154–156.

Strengths and limitations

In spite of good reproducibility¹⁵⁷, test setup and study performance are poorly standardized¹⁵⁰. Demographic factors have an influence, with male participants having a shorter balloon expulsion time than women, and expulsion time increasing with age^{150, 152}.

As a simple to perform, office-based screening test, balloon expulsion can be considered useful for the initial assessment of patients with symptoms of evacuation disorder, however a firm diagnosis requires confirmation with other allied tests of evacuation.

Defecography

Barium (X-ray) defecography, or evacuation proctography, is an established clinical tool for the diagnosis of evacuation disorders¹⁵⁸. Barium defecography evaluates rectal wall morphology, pelvic floor motion and evacuation in real time¹⁵⁹. MRI defecography enables imaging of all pelvic compartments¹⁶⁰. In comparison with other tests of evacuatory function (for example, manometry, balloon expulsion and transperineal ultrasonography), defecography provides better overall evaluation of the defecatory process and structure and/or function of the anorectum^161–163.

Study indications

Primary indications for both investigations are: to identify structural or ‘functional’ obstructive features associated with impaired evacuation in patients with refractory symptoms of constipation consistent with an evacuation disorder; to identify impaired evacuation and/or pelvic organ prolapse in patients with fecal incontinence^{7, 164}; and to evaluate the effects of treatment, for example after surgical repair of anorectal or pelvic floor pathology^{21, 165}.

Study performance

Barium defecography involves fluoroscopic imaging of the anorectum during contraction of pelvic floor muscles and rectal evacuation (that is, simulated defecation) after barium paste, composed of barium sulfate, porridge oats and water, has been instilled into the rectum^{166, 167}. As rectal sensory function is critical to normal defecation^{159, 168}, introduction of a thick paste (approximating normal stool), to a volume individualized to a patients’ desire to defecate,¹ might be preferable to a fixed volume of liquid barium. The patient is then seated upright on a radiolucent commode (in privacy behind a screen) during fluoroscopic screening, and instructed to squeeze the anal sphincter then expel rectal contents until evacuation is felt to be complete, or the patient reports that they are unable to empty further. Rectal dimensions (length, diameter, capacity), anorectal angles, rectal wall morphological features, perineal descent, and evacuatory efficacy (rate, percentage of contrast expelled)¹⁶⁹ can be measured. However, the technique lacks standardization and numerous modifications to the original method¹⁶⁸ have been described, most notably opacification of bladder and/or vagina and/or small bowel (for example colpocystodefecography) which enables concurrent visualization of cul-de-sac hernias (for example, enterocele), and other pelvic organ prolapses.

MRI defecography can be performed either with the patient supine, within a closed-configuration magnet, or upright (sitting) within an open-configuration system¹⁶³. Limited availability of the latter, however, means that the majority of studies are performed with the patient in a non-physiological supine position. The rectum is filled with a stool substitute (for example, mashed potatoes or ultrasonography gel) mixed with gadolinium if necessary.

Clinical utility

Barium defecography can identify impaired rectal evacuation and diagnose anatomical and ‘functional’ features that can contribute to symptoms of an evacuation disorder, such as rectocele (Figure 5a), obstructing intussusception (Figure 5b), rectal prolapse and megarectum, as well as dyssynergic defecation, levator ani and descending perineum syndrome¹⁵⁹. With small bowel opacification, the effect of enterocele can be assessed. MR defecography additionally enables concurrent evaluation of bladder and vaginal vault descent, and enables imaging in different orthogonal planes¹⁶⁰.

Figure 5.

Representative barium defecography images. a | A significant rectocele; the left panel shows a lateral view of the rectum at rest, opacified by barium neostool with the anal canal closed (arrow). The right panel clearly demonstrates a large retaining rectocele at end evacuation (extent of anterior bulging highlighted by dashed line) b) Obstructing full thickness intussusception; the left panel shows a lateral view of the rectum at rest, with the anal canal closed (arrow). The right panel shows an image at mid evacuation with clear invagination of the mid rectum (between arrows) secondary to a full thickness rectal intussusception; this is causing occlusion of the distal rectal lumen with retention of neostool proximal to this.

Various grading systems exist for defining structural abnormalities and pelvic organ descent, both for barium¹⁶³ and MRI modalities,^170–172 and these can be used as a basis for guiding therapy, particularly in those abnormalities deemed amenable to surgical repair. However, anatomical findings such as a small rectocele or minor intussusception are frequently found in healthy individuals^{161, 173}, and failure to recognize such variants of normal can lead to overdiagnosis. A study using barium defecography has shown that an anterior rectocele is almost always present in asymptomatic female study participants^{168, 170}, and that only large (>4 cm) and/or retentive rectoceles (that might be smaller) should be regarded as clinically relevant. Intra-anal (as opposed to intra-rectal) intussusception¹⁶⁸ and enterocele¹⁶⁸ are also appreciated to be pathological.

Limited comparative studies exist comparing modalities. Barium defecography better detects intussusceptions than MR, either using supine¹⁷⁴ or upright imaging¹⁷⁵, and is more sensitive for identifying retentive rectoceles¹⁷⁶. Upright MR defecography is superior to dynamic supine MRI in imaging intussusceptions; diagnostic utility is otherwise equivalent^{177, 178}.

Strengths and limitations

Of the available tests used to investigate patients with symptoms of an evacuation disorder, diagnostic agreement is imperfect¹⁷⁹; however, only defecography evaluates evacuation and pelvic organ structure. For this reason, consensus documents recommend defecography either as a first-line^{180, 181} or second-line^{182, 183} test. MRI has several advantages over barium defecography, in that it lacks exposure to ionizing radiation, and provides excellent soft tissue resolution of all pelvic floor compartments and supporting structures, enabling assessment of coexisting cystocele and uterovaginal prolapse^{10, 184}.

There remain several limitations to defecography in general. Radiation exposure for barium studies make this test unsuitable for certain patient groups, particularly pregnant women. As with all tests of evacuatory function, defecography is not performed in response to the spontaneous desire to defecate and embarrassment in the patient can inhibit normal behavior, leading to overdiagnosis of impaired evacuation¹⁶³; nevertheless, diagnostic yield of functional disorders on defecography is approximately half that using other tests of evacuatory function¹⁸⁵. A paucity of normative data are available,¹⁸⁶ particularly for MRI defecography¹⁶⁸ and a large degree of overlap in results between patients and controls limits interpretability of results¹⁷⁴.

OTHER TECHNOLOGIES

A number of further technologies designed to describe anorectal function exist, some of which are established but not in routine clinical use (e.g. endoanal MRI), and others which are emerging (e.g. the functional lumen imaging probe). Primarily acknowledged as tools in the research setting, they provide novel insights into the mechanisms of anorectal (dys)function. Though the following list is not exhaustive, the following is a brief introduction to those modalities which the Consensus group believe to have some diagnostic capability.

Functional lumen imaging probe

The anal canal’s passive ability to withstand opening pressure (that is, distensibility) is thought to have a role in the continence mechanism¹⁸⁷. A novel device, the functional lumen imaging probe (Flip; Crospon, Ireland) measures the cross-sectional diameter of a saline-filled balloon within the anal canal as distension pressure increases^{168, 170}. Early studies show that patients with fecal incontinence have increased distensibility of the anal canal and this finding can be more sensitive to clinically relevant pathology than anorectal manometry.

Rectal and anal motor evoked potentials

The integrity of spinoanorectal pathways that govern anorectal neuronal function can be assessed using magnetic stimulation of the lumbar and sacral regions overlying the nerve plexi and recording the motor evoked potentials¹⁸⁸. More specifically, it can reveal either unilateral or bilateral prolonged motor evoked potentials at the lumbar region and/or sacral region and at the rectal and/or anal sites¹⁸⁹.

A study showed that translumbar and trans-sacral MEPs of the rectum and anus provide better delineation of peripheral neuromuscular injury in individuals with fecal incontinence and spinal cord injury than PNTML¹⁹⁰. This approach is relatively easy to perform but it is not widely available, which is the major limitation.

Endoanal MRI

Endoanal MRI^{114, 192, 193} is a technique that enables high-resolution imaging of the EAS (with the ability to differentiate between defects, scaring and atrophy) together with visualization of surrounding structures pertinent to pelvic organ prolapse and fecal incontinence (detecting, for example, pubovisceral avulsion). Additionally, MRI muscle fibre-tracking is a research technique that has enabled detailed functional anatomy of the continence mechanism, including morphology of the EAS and puborectalis complex¹⁹¹.

ANORECTAL DYSFUNCTION

Physiological classification

No single test can fully characterize the cause(s) of fecal incontinence and evacuation disorders, and instead, a range of investigations should be applied to assess anorectal structure, function and sensitivity (Table 1). We also recognize that there is no widely accepted consensus on the physiological nomenclature for the classification of anorectal disorders or the use of findings from anorectal investigations to broadly describe phenotypes.

A broad summary of the expert consensus view regarding findings of each investigation discussed earlier is presented in Table 2, with the clinical relevance classified as ‘major’, ‘minor’ and ‘of questionable significance’.

TABLE 2.

Clinical relevance of findings of commonly performed investigations of anorectal physiological function.

Function	Investigation	Finding	Clinical relevance
Anus
Motor	Anorectal manometry	Anal hypotonia	***
		Anal hypertonia	**
		Anal hypocontractility	***
	Electromyography	Reduced or abnormal myogenic activity	***
	Pudendal nerve terminal motor latencies	Prolonged latency	*
Structure	Endoanal ultrasonography	IAS defect	***
		IAS degeneration or atrophy	**
		IAS hypertrophy	**
		EAS atrophy	**
		EAS defect	***
Sensory	Anal mucosal electrosensitivity	Anal hyposensitivity	**
Rectum
Sensory	Balloon distension or rectal barostat	Rectal hypersensitivity	***
		Rectal hyposensitivity	***
Motor, sensory and structure	Rectal barostat	Rectal hypercompliance	**
		Rectal hypocompliance	**
		Increased rectal capacity	**
		Decreased rectal capacity	**
Anorectal unit
Motor	Balloon expulsion Anorectal manometry	Prolonged expulsion time	***
		Pelvic akinesia (can be described as type IV dyssynergia)	***
		Poor propulsion with dyssynergia(can be described as type II dyssynergia)	**
		Normal propulsion with dyssynergia(can be described as type I or III dyssynergia)	**
		Anorectal areflexia	***
Motor, sensory and structure	Defecography (barium or MRI)	Obstructing intussusceptione	***
		Retaining rectocelef	***
		Megarectum	*
		Rectal prolapse	***
		Enterocele or sigmoidocele	**
		Cystocele	**
		Vaginal vault prolapse	**
		Excessive perineal descent	**
		Impaired rectal emptyinga	***
		Impaired anorectal angle openinga	**

EAS, external anal sphincter; IAS, internal anal sphincter

^*Finding of questionable clinical significance

^**Finding of minor clinical significance

^***Finding of major clinical significance

^aFunctional (as opposed to structural) abnormality of evacuation

Fecal incontinence

In the context of fecal incontinence, major physiological findings with clear implications for clinical management and/or prognosis include anal hypotonia, anal hypocontractility and large sphincter defects on endoanal ultrasonography (Table 2). However, due to the multifactorial nature of symptom generation, such abnormal physiological findings are rarely found alone, and are often seen in overlapping clusters. Examples of well recognized physiological phenotypes include: anal hypotonia, IAS hypertrophy and rectoanal intussusception or full thickness rectal prolapse in patients with symptoms of passive fecal leakage;^{188–190, 192–194} and anal hypocontractility, rectal hypersensitivity and rectal hypocompliance in patients with fecal urgency or incontinence or diarrhoea-predominant IBS^{108, 195, 196}. However, although anecdotally recognized, prospective studies are required to better characterize these phenotypes, symptom clusters and disorders.

Evacuation disorders

For evacuation disorders, major findings include impaired emptying secondary to abnormal rectal structure (intussusception, prolapse or rectocele) or function (impaired propulsion or dyssynergia) and abnormalities of rectal sensitivity (in particular rectal hyposensitivity). Again, recognized phenotypes are often characterized by more than one physiological abnormality (for example, rectal hyposensitivity with impaired evacuation secondary to enlarged rectal capacity, hypercompliance or megarectum^{127, 130, 197–199}), and these also merit further study.

Effect of physiological evaluation on management

Epidemiological research has demonstrated that diarrhea is the most important cause of fecal incontinence (odds ratio 53, 95% CI 6.1–147, compared with continent controls)¹³⁷ and hard stool is the most common finding in constipation¹⁶⁵. However, symptoms of disordered defecation can be secondary to abnormalities outside the anorectum, such as colonic dysfunction²⁰⁰, or due to obsessive compulsive disorders and/or somatization²⁰¹. Hence, physiological assessments should be considered for patients whose symptoms have not responded to stool regulation.

The clinical utility of physiological measurement has previously been confirmed in large case series²¹; however, high-quality data from prospective trials using contemporary technology with outcome data is limited. Study design is challenging due to difficulties defining patient groups based on (non-specific) symptoms, the wide range of normal values, and the variety of potential physiological findings.

The problem of overlap between health and disease is particularly pertinent in this field: as demonstrated in a blinded study of ARM in healthy individuals and patients with constipation in which 90% of healthy individuals were classified as ‘abnormal’ using conventional descriptors of rectoanal co-ordination^{24, 202}. Test results should, therefore, be interpreted with an appropriate degree of caution (particularly when planning irreversible surgical correction of a physiological or structural ‘abnormality’) until further robustly performed stratified medicine studies become available.

Furthermore, it should not be assumed that a single measureable physiological change directly correlates with a single symptom, as these diseases are often multifactorial. As has been demonstrated elsewhere, treatment directed at one physiological abnormality might yield poor results when the true pathological driver is not fully appreciated (e.g. gastric acid suppression therapy for duodenal ulceration secondary to Helicobacter pylori infection)²⁰³.

However, particular findings on physiological testing do currently act as a basis for clinical management (for example, anal hypocontractility with an EAS defect after obstetric injury) but there remains a need for further development of an evidence-based classification system of physiological phenotypes. This development will require serial diagnostic and outcome studies to assess the clinical utility of the system for the direction of specific behavioral, medical, and surgical interventions.

Conclusions

If anorectal symptoms persist despite empiric stool regulation therapy without identification of a treatable cause, anorectal function testing can provide information that might explain the causes of fecal incontinence or evacuation disorders. This Consensus Statement has identified challenges concerning data acquisition, analysis and interpretation of results (Box 2); however, there is a high level of agreement that the evaluation of anorectal structure and function and a mechanistic understanding of anorectal pathophysiology can identify disease phenotypes and direct effective management.

Box 2. Open research questions.

• Optimal manometric measurements for diagnosis of sphincter dysfunction and rectoanal co-ordination need to be refined and better defined

• The clinical importance of measurements and tools used to assess rectal capacity, compliance and sensory function in patients with anorectal disorders require further validation in clinical practice

• Classification systems for physiological characterization of fecal incontinence and evacuation disorders that integrate the results of ARM, anorectal sensation, anal endosonography and defecography are required

• Serial diagnostic and outcome studies are needed to assess the clinical utility of anorectal investigations for stratifying patients to behavioral, medical or surgical therapies