Anorectal Physiology

Surg Clin N Am 82 (2002) 1115–1123

Anorectal physiology Alan G. Thorson, MDa,b,* a

Section of Colon and Rectal Surgery, Creighton University School of Medicine, 2500 California Plaza, Omaha, NE 65178, USA b University of Nebraska College of Medicine, 986585 Nebraska Medical Center, Omaha, NE, 68198-6585, USA

The normal function of the anorectum represents a complex interaction between neurologic, myogenic, sensory, anatomic, and hormonal components. Failure or weakness of any one part or combination of parts of this array may lead to symptoms recognized as many common and some not so common diseases and conditions seen in the clinical setting. When discussing anorectal physiology, the conversation usually refers to simple tests performed routinely in an anorectal physiology (ARP) laboratory. Such laboratories are designed to provide information about the function of the neurologic, sensory, and anatomic components of anorectal function. Resting and squeeze pressures, the rectal-anal inhibitory reflex (RAIR), dynamic proctography (DPG), contrast or balloon defecography, rectal compliance, sensory thresholds, estimates of neurologic function (particularly pudendal nerve terminal motor latency—PNTML), and assessment of anatomy via transanal ultrasound serve as the basic ARP laboratory tests. Various modifications of these tests and calculated values based on them have been described, but few have achieved routine clinical acceptance or application. The role that an ARP laboratory should play in the diagnosis of abnormalities of the anorectum has long been an area of controversy. Although such laboratories have been used for years in the research setting in attempts to quantify normal and abnormal function, their clinical usefulness has not been well accepted. Recent reviews have attempted to better define this role [1–4]. The primary clinical focus of an ARP laboratory is the evaluation of patients with either of the two extremes of bowel function: incontinence or constipation, and, in some instances, the simultaneous presentation of

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both. With this in mind, this article reviews the normal physiology of the anorectum, identifies the abnormalities associated with these clinical presentations, and considers how the ARP laboratory may provide useful assistance in patient management. Normal physiology Bowel continence Continence to stool represents a complex interaction of a rather large number of variables. These variables include stool consistency, reservoir capacity, rectal compliance, achievable sphincter pressures at rest and at squeeze, and sensation. Stool consistency The loss of control of stool, if gradual or incomplete, will nearly always first be noted with difficulties in controlling gas, then liquid stool, and finally solid stool. The importance of understanding this rests in the knowledge that a patient with incontinence to only gas or liquid stool may be corrected simply by effectively treating diarrhea. Reservoir capacity and compliance The compliance and the capacity of the rectal reservoir are closely integrated. Compliance refers to the ability of the rectum to distend. Although difficult to measure, conceptually compliance is an important component of the process of accommodation. In a normally functioning rectum, an increasing volume of stool is associated with passive distention of the rectum that allows intrarectal pressures to remain low. This accommodation can continue until a maximum tolerable volume is reached, at which point intrarectal pressures will begin to rise and ultimately overcome the pressures generated by the sphincters. The end result of this series of events is evacuation of the rectum. Disease states associated with decreased compliance include acute proctitis associated with inflammatory bowel disease (IBD) and radiation, postoperative status and the fibrosis associated with chronic radiation change, and chronic IBD. The latter chronic states can lead to permanently decreased reservoir capacity. IBD and other causes of diarrhea can overwhelm the continence mechanism by sheer volume of stool alone. With large volumes entering the rectum rapidly, the ability of the rectum to accommodate may reach its maximum tolerable volume in short order, which may lead to incontinence. The same can occur in cases of fecal impaction, where, due to the limits of accommodation, only a small additional amount of stool may be all that is necessary to reach maximum tolerable volume. Further aggravating the situation with impaction is the possibility of a chronically relaxed internal sphincter due to chronic rectal distention.

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Sphincter pressures The anal sphincter is composed of two major components that provide for normal continence. Immediately surrounding the canal is the internal anal sphincter, which accounts for approximately 80% to 85% of resting tone. The internal sphincter is involuntary smooth muscle. When performing anorectal manometry (ARM), the mean maximum resting pressure is largely a measure of internal sphincter function. Surrounding the internal sphincter is the external sphincter. It is composed of skeletal muscle. The external sphincter is responsible for enhancement of anal canal pressures during the process of accommodation and allows for the deferral of the call to stool. The exact structure and its relationship to the pelvic floor (levator ani) is still open to some debate; however, the puborectalis is generally felt to represent the superior component of the functional external sphincter unit. The function of the external sphincter is assessed in the ARP laboratory by measuring the maximum mean squeeze pressure with ARM. Generally, maximum squeeze pressures are twice the resting pressures. Maximum squeeze pressures can be maintained for less than one minute, however, before fatigue occurs. If accommodation can not occur within this time frame, evacuation of the rectum is likely. Sensation Sensory components of the mechanism of continence are critical in allowing an individual a satisfactory reaction time to react to a bolus of stool or gas entering the rectum. Sensation is associated with the sampling reflex. This reflex results in the transient relaxation of the internal sphincter, with contraction of the distal external sphincter, theoretically allowing for discrimination of the contents of the upper anal canal and rectum as either gas or liquid or solid. Two tests in the ARP laboratory can provide some assessment of this function of the continence mechanism. The RAIR measures the transient relaxation of the internal sphincter with distal contraction of the external sphincter. The absence of this reflex is seen in the presence of Hirschprung’s disease. A chronically relaxed internal sphincter (low resting pressures) and failure to further relax may be associated with fecal impaction and overflow incontinence. The minimum sensory threshold of distention can also be measured by sequentially inflating a balloon in the distal rectum to increasing volumes until the patient senses the distention. Failure to sense a small volume (2 cc–10 cc) is associated with anal leakage and can be a focus of re-education in situations of relatively minor incontinence [5]. Constipation To many patients’ minds, constipation represents the opposite end of the spectrum from bowel incontinence. This is particularly true when discussing

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bowel problems with patients who may not admit to incontinence while complaining bitterly of ‘‘diarrhea.’’ This is their polite way of stating that they have ‘‘lost control.’’ In fact, the two problems (constipation and incontinence) may coexist. It is not uncommon to find patients incontinent to solid stool. Such patients generally represent the most severe form of incontinence. To understand the role that the ARP laboratory can have in the evaluation of these patients, it is helpful to review the normal act of defecation. The development of a mass peristaltic wave by the left colon distended with stool results in the delivery of stool to the lower sigmoid colon and rectum. Once entering the rectum, the reflex relaxation of the internal sphincter and contraction of the distal external sphincter allows sampling and maintains continence. Assuming that accommodation does occur, the initial urge to defecate may pass as intraluminal pressures fall below the tonic resting pressure of the anal canal, so that voluntary contraction of the external sphincter is not necessary to maintain continence. If the urge occurs at a socially acceptable time to proceed with defecation, or if accommodation has reached its limits and there is little choice as to when to proceed with defecation, the individual will normally assume a squatting or sitting position. The process of defecation then normally will proceed with a straightening of the anorectal angle. Normally, intrarectal pressures then rise in response to a voluntary increase in intra-abdominal pressure due to a Valsalva maneuver. Normally there is a varying degree of pelvic descent with this maneuver. As rectal pressure increases, there is a combination of reflex and voluntary relaxation of the external sphincter and reflex relaxation of the internal sphincter. Once intrarectal pressures rise above the sphincter pressure, defecation can occur. If this process is accompanied by further mass peristalsis, the entire left colon may be evacuated. If not, evacuation may occur in a piecemeal fashion, with several defecations occurring in a 24-hour period. Following defecation, sphincter pressures, intra-abdominal pressure, and the anorectal angle return to their normal resting pressures and positions. Constipation manifested as outlet obstruction may occur when any portion of this process is interrupted. Such situations may occur when there is failure of the puborectalis to relax (paradoxical puborectalis, anismus, levator syndrome), abnormally high resting pressure (nutcracker anus), excessive accommodation (megarectum), external compression of the rectum resulting in blockage (enterocele), incomplete evacuation (large rectocele), or intussusception (overt or covert prolapse). In addition, excessive pelvic descent may be associated with outlet problems. The exact role of abnormal pelvic descent is frequently not clear. Whether the descent is a result of chronic straining due to outlet problems of another etiology or a primary cause of the complaint may be difficult to determine. Constipation can also be a manifestation of a dysfunctional colon rather than an outlet problem. Although the etiology of this problem lies outside of

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the pelvis, evaluation of this type of constipation is often a part of the evaluation in an ARP laboratory. A simple measure of colon transit using radio-opaque markers is probably the most commonly used test for this in the clinical setting [6].

Altered physiology When patients present with symptoms associated with pelvic floor dysfunction, the most important first step is a careful assessment of their true complaint. Because patients are frequently embarrassed by problems associated with the pelvic floor, they may try to protect themselves from that embarrassment by speaking in protective terms, including the complaint of ‘‘diarrhea’’ in lieu of incontinence, as mentioned previously. Once these initial tentative steps are overcome, it is important to quantify and qualify the complaints as much as possible. In the case of incontinence, frequency and type (gas, liquid, solid) are important to sort out. In the case of constipation, quality (hard or soft stool), type (excessive straining or failure to recognize a call to stool), and frequency may help direct the initial evaluation. Once a careful history has been obtained, a simple physical assessment may further define a role for the ARP laboratory. If a multiparous female patient complaining of incontinence is found to have an obvious anterior sphincter defect, further evaluation in the ARP laboratory may not be indicated prior to an initial repair. The presence of a fecal impaction in a patient with incontinence to smears of liquid stool as their primary complaint may simply need a recommendation for an appropriate bowel management program. An acute proctitis, an obstructing lesion, or an overt rectal prolapse may all allow for immediate treatment planning. If the initial physical assessment does not provide such obvious answers, however, an assessment in the ARP laboratory may prove helpful.

Basic ARP physiology laboratory tests For the purposes of an ARP laboratory devoted to a clinical practice, the basic tests which have been found to be most useful include anorectal manometry (ARM), anorectal electromyography (EMG), dynamic proctography (DPG), colon transit time (CTT) and transanal ultrasound (TAUS). Each will be considered in turn. Anorectal manometry Manometry is generally felt to be the least helpful of the physiologic tests in the clinical setting. The test is most commonly used in assessing the RAIR to rule out Hirschprung’s disease. It has been found to be complimentary in

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the assessment of sphincter injury and failed overlapping sphincteroplasty [7–9]; however, its usefulness is secondary to TAUS in this setting. For proper use, ARM must be assessed according to the appropriate subgroup, because normal variation occurs between males and females and between multiparous and nulliparous females. Males have longer sphincters at rest and squeeze and generate greater pressures at squeeze. Females and males have equal resting pressures but multiparous females have lower resting pressures than nulliparous females [10]. ARM may also offer some insight into appropriate management of recurring fissure following primary sphincterotomy. A patient with continued abnormally high resting pressures is a candidate for repeat sphincterotomy. In a patient with recurring fissure and normalized resting pressures following sphincterotomy, greater effort may be required to look for something other than an idiopathic etiology for the fissure. A functional stenosis with very high resting pressures can occasionally be the source of a complaint of outlet obstruction. Correction of the functional stenosis will relieve the constipation in this setting. Electromyography (EMG) Anorectal EMGs consist of two basic evaluations. A measure of pudendal nerve terminal motor latency (PNTML) assesses the conduction time in the pudendal nerve from Alcock’s canal to the end organ in the external sphincter. Normal conduction occurs in 2.0  0.2 milliseconds. Prolonged times suggest injury to the large fast-conducting fibers [11]. The significance of prolonged PNTML is debatable, however. There is general agreement that a single prolonged nerve has little impact on the expected functional results following anterior sphincteroplasty for a disrupted sphincter. There is less agreement on the effect of bilateral neuropathy, with some suggesting that bilateral prolongation is associated with poor function, whereas other studies show little correlation on a clinical or functional basis [4,12]. Functional assessment of pelvic floor activity can be obtained with an anal surface electrode. This electrode is contained on the surface of a small sponge, which, when inserted into the anal canal, will measure electrical activity in the pelvic floor, including the puborectalis. The results represent the ‘‘recruitment pattern.’’ With the electrode in place, the level of the pelvic floor activity is recorded with the patient first at rest, then while squeezing the anal sphincter, and finally while straining as if in an act of defecation. Ideally the pattern should show a baseline activity at rest, with significant augmentation at squeeze and then a fall below baseline at strain. With straining, patients with paradoxical puborectalis show further enhancement above the activity recorded at squeeze. This suggests further contraction rather than relaxation of the puborectalis while attempting to defecate. Single fiber EMG and electrophysiologic mapping have generally been supplanted by the much more accurate and less painful TAUS in the assess-

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ment of fecal incontinence. Both are used almost exclusively in the research setting for the purposes of incontinence evaluation and management. Dynamic proctography Dynamic proctography (DPG) is used to evaluate function of the pelvic floor at time of defecation. DPG has many variations in name as well as procedure. Video proctography, defecography, and defecating scintigraphy all accomplish the same essential assessment. DPG is performed by placing a thick mixture of barium contrast and methylcellulose in the rectum to simulate a stool. Additional liquid contrast is placed in the vagina. Generally the patient sits on a special commode while evacuating the mixture under videoflouroscopic monitoring. This test can demonstrate the existence and function of internal intussusception, paradoxical contraction of the puborectalis, enteroceles, rectoceles, pelvic floor hernias, and incomplete evacuation. The technique can be enhanced with the use of ‘‘triple contrast.’’ This technique combines the use of the standard DPG, with contrast in the small bowel hopefully timed to provide opacified loops of small bowel in the pelvis at the time the DPG is performed. Alternatively, water soluble contrast can be placed directly into the peritoneal cavity to outline the pelvis at the time of DPG [13]. Such peritoneography is less dependent on the timing of contrast reaching the appropriate small bowel loops than is a small bowel series. The third element of triple contrast is opacification of the bladder so that its interaction with the pelvic floor at time of straining can also be evaluated. This combination gives the most thorough evaluation of the pelvic floor in states of dysfunction. Colon transit time Evaluation of total and segmental colon transit time is an important component of the ARP laboratory evaluation. It is used to complement testing of the pelvic floor in patients with constipation. It provides an evaluation for colonic inertia. Because colonic inertia may coexist with outlet obstruction, the evaluation of both must occur prior to embarking on surgical therapy for constipation. The test consists of the ingestion of 24 radio-opaque markers each day for a three-day period. Abdominal radiographs are obtained on days four and seven and the numbers and distribution of markers recorded [6]. The transit time is then calculated for the right, left, and rectosigmoid regions, and for total transit. Dispersement throughout the colon on the final day is consistent with colonic inertia, whereas collection in the rectum suggests an outlet problem. Passage of all of the markers does not suggest an abnormal transit. An abnormal test showing inertia must be accompanied by a pelvic outlet evaluation, however, as it is not uncommon to find inertia in conjunction with pelvic floor dysfunction. A subtotal colectomy in a patient with concomitant pelvic outlet obstruction will fail on a functional basis.

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Transanal ultrasound Transanal ultrasound has become the primary ARP test in the assessment of fecal incontinence [3,8]. It provides the most accurate and complete evaluation for detecting anal sphincter defects. Generally, a probe with a 10 MHz, 360 degree rotating transducer is used. Using the TAUS, the internal sphincter is visualized as a hypoechoic band around the anal canal, and the external sphincter as a band of mixed echogenicity. Defects in either band can be readily identified and measured [7–9].

Summary Anorectal physiology, as assessed in an ARP laboratory, can provide helpful information in the management of patients with constipation and bowel incontinence. Beyond the uses described in this review, however, the ARP laboratory is most useful in the research setting. In this setting the laboratory can expand our understanding of function associated with other disease states, including anal fissure, fistula-in-ano, inflammatory bowel disease, and postoperative states. The lab can also provide improved understanding of the complex interactions of the enteric nervous and gut hormone systems with the smooth and skeletal muscle systems. A part of the failure of the ARP laboratory to enjoy more clinical usefulness lies in a lack of standardization of test protocols for many of the tests. Secondarily, there is a lack of normative data from large numbers of normal patients [1]. Finally, there is the difficulty in reproducing tests in situations where the patient has significant potential to compensate for deficits through the recruitment of adjacent muscle groups and other maneuvers. There is also some reluctance on the part of clinicians to make use of the ARP laboratory if the testing is not readily available in their community. Although the tests themselves are not difficult to learn to administer, lack of familiarity with the testing process can act as a barrier to acceptance. This is particularly true for clinicians that are used to making clinical decisions without the added benefit of physiologic testing. Despite these obstacles, the role of the ARP laboratory in the management of complex anorectal disease is likely to grow in the years ahead.

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[4] Thomas C, Lefaucheur JP, Galula G, et al. Respective value of pudendal nerve terminal motor latency and anal sphincter electromyography in neurogenic fecal incontinence. Neurophysiol Clin 2002;32:85–90. [5] Buser WD, Miner PB Jr. Delayed rectal sensation with fecal incontinence. Successful treatment using anorectal manometry. Gastroenterology 1986;91:1186–91. [6] Metcalf AM, Phillips SF, Zinsmeister AR, et al. Simplified assessment of segmental colonic transit. Gastroenterology 1987;92:40–7. [7] Falk PM, Blatchford GJ, Cali RC, et al. Transanal ultrasound and manometry in the evaluation of fecal incontinence. Dis Colon Rectum 1994;37:468–72. [8] Sentovich SM, Blatchford GJ, Rivela LJ, et al. Diagnosing anal sphincter injury with TAUS and manometry. Dis Colon Rectum 1997;40:1430–4. [9] Ternent CA, Shashidharan M, Blatchford GJ, et al. Transanal ultrasound and anorectal physiology findings affecting continence after sphincteroplasty. Dis Colon Rectum 1997; 40:462–7. [10] Cali RL, Blatchford GJ, Perry RE, et al. Normal variation in anorectal manometry. Dis Colon Rectum 1992;35:1161–4. [11] Kiff ES, Swash M. Slowed conduction in the pudendal nerves in idiopathic (neurogenic) fecal incontinence. Br J Surg 1984;71:614–6. [12] Suilleabhain CB, Horgan AF, McEnroe L, et al. Pudendal nerve terminal motor latency does not correlate with squeeze pressure. Dis Colon Rectum 2001;44:66–71. [13] Sentovich SM, Rivela LJ, Thorson AG, et al. Simultaneous dynamic proctography and peritoneography for pelvic floor disorders. Dis Colon Rectum 1995;38:912–5.