- Email: [email protected]
Clinical, radiological and physiological assessment of anorectal function
BASIC SCIENCE
Clinical, radiological and physiological assessment of anorectal function
Several techniques are available that allow assessment of the anatomy and physiological function of the key parts of the pathway. The most important initial step in assessment involves a detailed history (including obstetric history and previous proctological surgery) and physical examination.
Heman Joshi
Clinical assessment
Oliver Jones
General physical and abdominal examination is important. Assessment of fitness may be a major determinant of how far to pursue investigations and treatment, particularly surgical interventions. Patients with obesity and diabetes are more prone to pelvic floor problems but also the complications of intervention. Optimization of underlying medical problems should be encouraged. In extreme obesity correction of this may be more important than any efforts directed to the pelvic floor, even if considering surgery for obesity. In the UK it is traditional to examine patients in the left lateral position to assess the perineum and anorectum. The limitation of this assessment is that inspection of the vaginal introitus is compromised and the consequences of gravity are less obvious, even allowing for straining and bearing down. For this reason, examination with the aid of the gynaecological couch has much to advocate it and is routine in many parts of Europe. The traditional all-fours approach in the USA gives good access for anal procedures in the clinic but is less helpful at assessing prolapse. Careful inspection of the perineum will identify obvious scarring and deformity as a result of previous surgery or parturition. Morphology of the perineum has been described by Nivatongs and it may be useful to record this.7,8 The anus may be gaping from obstetric injury or pelvic neuropathy or tight with evidence of spasm or a bulky internal sphincter. The presence of soiling, excoriation, tags, and piles is relevant. It is essential to include a dynamic component to the examination, asking the patient to bear and push down. Here we observe the nature of movement and if it appears coordinated and appropriate. Understandably many patients are reluctant to unleash the full effort in doing this for fear of incontinence and it can be worth repeating at the end of the examination as the individual becomes relaxed. The amount of perineal descent should be noted as well as the presence of vaginal prolapse. Palpation around the anus will give the impression of sphincter bulk, spasm and signs of obstetric or surgical trauma. Digital rectal examination is undertaken gently and it is valuable to ask the patient to bear down again, squeeze and relax. An impression of paradoxical contraction of the puborectalis muscle may also be detected. Digital examination allows assessment of: the sphincter bulk; any evidence of previous damage to and the quality of the perineal body and posterior vaginal wall; and any rectocele. With the finger above the sphincter mechanism it is again useful to ask the patient to bear down and in this position one may become aware of a poorly supported anterior rectal wall and the presence of internal rectal prolapse as the descending intussusceptions strikes the examining finger. A bulky retroverted fibroid uterus may also be felt to intrude significantly into the rectum at this level. Inspection and digital rectal examination are followed by inspection using rigid sigmoidoscope, excluding mucosal disease
Abstract Faecal continence is defined by the ability to perceive, retain and evacuate bowel contents at socially convenient times. It may be associated with urgency, occur as a passive event, or be mixed. This is reliant upon normal function of the main involved organs (i.e. rectum, pelvic floor and anal sphincters) together with their associated sensorineural pathways. Incontinence may occur as a result of dysfunction in any one of these systems or due to factors such as systemic disease, emotion, bowel motility and stool consistency. The act of defaecation is a conscious process that involves interplay between motor and sensory elements, initiated by higher cortical function. Incontinence and evacuatory dysfunction are investigated using specialized tests that assess sphincter function and structure (anorectal manometry, endoanal ultrasound), anorectal and pelvic floor function (defaecating proctography, nerve conduction studies) and luminal integrity and colonic function (transit studies and endoscopy).
Keywords Anal sphincter; anorectal investigations; continence; defaecation; pelvic floor; rectum
Introduction Faecal incontinence is defined as the involuntary loss of faecal material. It is common, with a prevalence in the community estimated at 1.4% and rising to 7% in the elderly.1,2 Constipation and related functional bowel disorders have been formally defined by the Rome III collaboration.3 They affect up to 25% of the population and comprise a symptom complex associated with infrequent defaecation, hard stools, difficulty in emptying, straining and a feeling of incomplete evacuation.4 Maintenance of continence is a complex process that involves coordination between multiple neuronal reflexes, sensory and motor pathways, and the key pelvic organs, namely the rectum, pelvic floor and anal sphincters.5,6 An abnormality of any part of the pathway can be associated with dysfunctional defaecation, manifesting as constipation at one end of the scale, or as faecal incontinence at the other. However, a mixed picture, associated with obstructive defaecation syndrome, is a not uncommon presentation.
Heman Joshi BSc MRCS(Eng) is a Research Fellow in Pelvic Floor Surgery at Oxford University Hospitals Trust, Oxford, UK. Conflicts of interest: none declared. Oliver Jones DM FRCS is a Consultant Surgeon at Oxford University Hospitals Trust, Oxford, UK. Conflicts of interest: none declared.
SURGERY 32:8
404
Crown Copyright Ó 2014 Published by Elsevier Ltd. All rights reserved.
BASIC SCIENCE
The upper internal anal sphincter relaxes enabling anorectal sampling to occur. When the anal canal is deemed to have solid contents, and a decision to defaecate is made, relaxation of the pelvic floor occurs in conjunction with a rise in intra-abdominal pressure. The rise in intra-abdominal pressure occurs as a consequence of the Valsalva manoeuvre, in association with tensing of the anterior abdominal wall musculature and closure of the glottis. Optimal evacuation is achieved when the subject is in the squatting position which, assisted by hip flexion, further straightens the anorectal angle and allows more effective propulsion of faecal residue.12 In the Western world, defaecation in the sitting position is socially preferred with reduced effect on the anorectal angle. The net result is relaxation of puborectalis, straightening of the anorectal angle and slight pelvic floor descent. The external anal sphincter relaxes and rectal contents are evacuated. Further propulsive contractions of the rectum occur until it is fully empty. Once evacuation is complete, the pelvic floor rises and both sphincters and puborectalis contract as part of a ‘closing reflex’.
and occasionally allowing assessment of a high take-off rectal intussusceptions; however the return in pelvic assessment is limited. A short proctoscope is valuable in assessment of piles and mucosal prolapse particularly with the patient straining. A good impression of full-thickness internal prolapse can also be obtained as the proctoscope is delivered through the anus.8
Physiology of continence Internal and external anal sphincters The internal anal sphincter demonstrates sinusoidal ‘slow-wave’ activity with a frequency of 20e40 cycles/minute and is the main contributor to resting anal pressure, measured at between 50 and 120 mmHg in a healthy individual. Much of this resting pressure is attributable to myogenic tone, an intrinsic property of sphincteric smooth muscle independent of neural input. Additional excitatory input to the internal sphincter comes from sympathetic neural input, whilst parasympathetic nerves to the sphincter are inhibitory and mediated via nitric oxide.9 Isolated damage to the internal anal sphincter typically causes predominantly passive faecal incontinence (leakage). Whilst the external anal sphincter does make a small contribution to resting tone, its primary role is that of voluntary contraction of the anal sphincter which can be recruited consciously or as a reflex during times of additional risk of incontinence such as when lifting a heavy object or coughing (see below). A normal external sphincter may generate an additional pressure of between 50 and 200 mmHg. Damage to the external sphincter is strongly associated with urge faecal incontinence (the sensation of being unable to ‘hold on’).
Electrophysiology Neurophysiological assessment of the anorectum includes electromyography of the sphincter mechanism, and nerve conduction studies to assess the pudendal and spinal nerves. Electromyography involves the placement of needle electrodes into the puborectalis or external sphincter to assess the state of the muscle and its innervating nerve as a function of its electrical activity during the resting and contractile phase. The procedure can be painful and unpleasant. Increasing adoption of endoanal ultrasound has meant that electromyography is rarely used. Pudendal nerve terminal motor latency (PNTML) is assessed by stimulation of the nerve, as it enters the ischio-rectal fossa at the ischial spines. Prolonged PNTML is associated with idiopathic faecal incontinence, rectal prolapse, solitary rectal ulcer syndrome and sphincter defects. The maintenance of anal continence is a very complex process dependent on several processes including colonic motility, stool consistency, sensation, rectal compliance, local reflexes, pelvic floor and sphincter function.
Puborectalis and the pelvic floor The puborectalis plays a key role in continence. It is thought to achieve this function due to its sling-like anatomy and its effect on the anorectal angle when it is contracted. Sir Alan Parks proposed a ‘flap-valve’ mechanism of continence e namely that in a situation of increased intra-abdominal pressure (e.g. sneezing, coughing or straining), the puborectalis keeps the valve shut by driving the anterior rectal wall against the upper anal canal, and thereby prevents the passage of stool into the lower anal canal.10 Other authors have demonstrated in radiological studies that during the Valsalva manoeuvre, the relation between the anterior rectal wall and the anorectal angle is not clearly defined; some studies have shown little difference in the anorectal angle between incontinent patients and a normal control group.11 It is likely that the puborectalis functions as a further deeper sphincter mechanism and complements the activity of the external anal sphincter. Indeed, it has been shown that the puborectalis sling can maintain continence even in the presence of internal and external sphincter dysfunction.11
Colonic motility and stool consistency Even patients with normal sphincters will suffer urgency or actual incontinence if they develop severe diarrhoea (as can occur with ulcerative colitis). It is little surprise, therefore, that in many patients with diarrhoea and incontinence, significant improvements in control will be seen with the use of constipating medications.13 Rectal sensation and compliance The rectum plays a key role in continence, by functioning as a site for temporary storage of faeces. A normal rectum can accommodate significant volumes of faecal residue with minimal alteration in measured rectal pressures. This is termed rectal compliance. Recent animal studies have identified unique receptors, termed rectal intra-ganglionic laminar endings (rIGLEs), which act as slowly adapting mechanoreceptors responsive to tension and rapid distension.14 Rectal compliance is often altered in patients with faecal urgency, constipation and incontinence. Extreme alterations in
Physiology of evacuation An awareness of the need to defaecate occurs in the superior frontal gyrus and anterior cingulate gyrus of the cerebral cortex, as a result of a critical level of rectal filling. The process is initiated by faecal matter, which has moved into the lower rectum, causing distension. This leads to stimulation of pressure receptors on the pelvic floor (puborectalis), which in turn triggers the rectoanal inhibitory reflex.
SURGERY 32:8
405
Crown Copyright Ó 2014 Published by Elsevier Ltd. All rights reserved.
BASIC SCIENCE
detectable sensation, urge to defaecate and maximum tolerable volume. These parameters may be abnormal in patients with autonomic neuropathy from diabetes or in patients with functional and somatic alterations of the rectal reservoir. For example, patients with megarectum may have delayed sensation (hyposensitivity) whilst those with inflammatory bowel disease typically have low volumes for each of these three parameters (hypersensitivity). Clinically, patients with hypersensitivity often have urgency and stool frequency. Rectal sensation can also be evaluated by use of thermal and electric probes. There has also been interest in assessment of anal sensation by these means but the clinical utility of this is uncertain at present. Rectal compliance (reflecting distensibility and accommodation) can also be measured using a barostat but this is not used widely in clinical practice.
capacity and compliance can lead to faecal incontinence through a decreased capacity for accommodation or as a consequence of overflow states. Compliance is reduced as part of the normal ageing process, or in inflammatory states (e.g. ulcerative colitis). Normal sensation (the ability to sense that the rectum contains stool) is also fundamental to continence. Local reflexes and anal sensation The best studied of the local reflexes central to faecal continence is the rectoanal inhibitory reflex (RAIR). This is an intrinsic intramural reflex that is preserved after spinal cord injury or extrinsic denervation of the rectum. It is not seen in Hirschsprung’s disease when myenteric ganglia are absent in the low rectum and for a variable distance proximally. It manifests as a periodic relaxation of the upper internal anal sphincter in response to distension of the rectum, with extent of relaxation dependent upon degree of rectal distension. This permits faecal matter or flatus to come into contact with specialized receptors in the upper anal canal, where ‘sampling’ takes place. This process is instrumental in the discrimination of gas from stool and passage of flatus without faecal leakage. If evacuation is not desired, the high resting pressures of the lower part of the internal anal sphincter, coupled with contraction of the external anal sphincter and puborectalis sling, result in return of faecal content to the upper rectum, delaying the process of defecation. An attenuated rectoanal inhibitory reflex may lead to difficulty with evacuation, whilst an exaggerated reflex can result in faecal leakage. Several studies have looked at the impact of rectal excisional surgery on the rectoanal inhibitory reflex and noted that while it is nearly abolished in the early postoperative period, it appears to return, in some form, in due course in the majority of patients.15
Radiology Endoanal ultrasound Endoanal ultrasound is performed using an internal rotating micro-transducer. It evaluates sphincter integrity and structure. The external sphincter appears as a circumferential hyperechoic structure, while the internal sphincter appears as a hypoechoic (black) inner circle (Figure 1). Sphincter defects and scarring may be seen as ‘incomplete rings’. Defaecating proctography Defaecating proctography utilizes video fluoroscopy and is a dynamic examination, providing structural information as well as assessing function during defaecation. Its use is indicated as part of the work-up of a patient presenting with a picture of outlet obstruction and for many patients with incontinence. The technique requires barium paste to be introduced into the rectum. An additional amount is usually inserted into the vagina in women and oral contrast is given to opacify the small bowel. The patient
Physiological testing Anorectal manometry Anorectal manometry provides an assessment of anal sphincter pressures and local reflexes. There are various types of probes and recording systems including water-based catheter balloon systems placed into the distal rectum and withdrawn through the anal canal in a step-wise manner, or the newer solid-state devices that contain micro-transducers to measure anal canal pressure at various points along the length of the catheter. The technique can determine resting and voluntary squeeze pressures of the anal canal (reflecting internal and external sphincter tone respectively) as well as the length of the anal canal itself. Other parameters that might be recorded include testing for the cough or Valsalva reflex (a brief increase in anal tone to counter a sudden rise in intra-abdominal pressure) and the presence of a functional rectoanal inhibitory reflex (a fall in anal resting pressure in response to rectal distension by means of inflation of an intrarectal balloon). This latter reflex is typically absent in Hirschsprung’s disease (congenital lack of intramural rectal ganglia). The duration of external sphincter contraction (endurance squeeze), assessed by asking the patient to perform a sustained anal contraction is another useful test; a low result is often seen in incontinent patients.
Figure 1 Endoanal ultrasound of a normal anal sphincter. The endoluminal probe is delineated in this diagram by the dashed lines. The solid white arrows mark the margins of the external anal sphincter. The unfilled arrows delineate the internal anal sphincter that appears as a black circle because of its high water content.
Sensory testing Rectal balloon insufflation tests provide an indication of rectal sensory function. Usually three variables are measured: first
SURGERY 32:8
406
Crown Copyright Ó 2014 Published by Elsevier Ltd. All rights reserved.
BASIC SCIENCE
Managing such patients requires an informed approach with relevant investigations to identify the cause of their problem and thereby facilitate appropriate management. A REFERENCES 1 Bartolo DC, Paterson HM. Anal incontinence. Best Pract Res Clin Gastroenterol 2009; 23: 505e15. 2 Madoff RD, Parker SC, Varma MG, Lowry AC. Faecal incontinence in adults. Lancet 2004; 364: 621e32. 3 Longstreth GF, Thompson WG, Chey WD, Houghton LA, Mearin F, Spiller RC. Functional bowel disorders. Gastroenterol 2006; 130: 1480e91. 4 Chatoor D, Emmnauel A. Constipation and evacuation disorders. Best Pract Res Clin Gastroenterol 2009; 23: 517e30. 5 Mellgren A. Fecal incontinence. Surg Clin North Am 2010; 90: 185e94. 6 Bharucha AE. Pelvic floor: anatomy and function. Neurogastroenterol Motil 2006; 18: 507e19. 7 Nivatongs S, Fang DT, Kennedy HL. The shape of the buttocks. A useful guide to selection of anaesthesia and patient position in anorectal surgery. Dis Colon Rectum 1983; 26: 85e6. 8 Cunningham C. Establishing and developing a pelvic floor service: the multidisciplinary team and the approach to patient assessment. In: Lindsey I, Nugnet K, Dixon T, eds. Pelvic floor disorders for the colorectal surgeon 2011. 9 Jones OM, Brading AF, Mortensen NJ. The physiology, pharmacology and therapeutic manipulation of the internal anal sphincter. Can J Gastroenterol 2002; 16: 249e57. 10 Parks AG. Royal Society of Medicine, Section of Proctology; Meeting 27 November 1974. President’s Address. Anorectal incontinence. Proc R Soc Med 1975; 68: 681e90. 11 Bajwa A, Emmanuel A. The physiology of continence and evacuation. Best Pract Res Clin Gastroenterol 2009; 23: 477e85. 12 Barleben A, Mills S. Anorectal anatomy and physiology. Surg Clin North Am 2010; 90: 1e15. 13 Ehrenpreis ED, Chang D, Eichenwald E. Pharmacotherapy for fecal incontinence: a review. Dis Colon Rectum 2007; 50: 641e9. 14 Lynn PA, Olsson C, Zagorodnyuk V, Costa M, Brookes SJ. Rectal intraganglionic laminar endings are transduction sites of extrinsic mechanoreceptors in the guinea pig rectum. Gastroenterol 2003; 125: 786e94. 15 O’Riordain MG, Molloy RG, Gillen P, Horgan A, Kirwan WO. Rectoanal inhibitory reflex following low stapled anterior resection of the rectum. Dis Colon Rectum 1992; 35: 874e8.
Figure 2 Defaecating proctogram. The patient has been given oral contrast 7 to delineate the small bowel, which is dropping down into the rectovaginal septum to form an enterocoele (dashed line). The patient has also received rectal contrast that delineates a rectocele (dotted line) and intussusception (indicated by arrows). The patient has a tampon soaked in radiographic contrast medium placed in her vagina.
is then asked to sit on a commode and attempt evacuation during X-ray screening (Figure 2). Several abnormalities can be assessed including measurement of the anorectal angle and position of pelvic floor at rest, during squeeze and defaecation (perineal descent). Proctography may detect a rectocele (herniation of the rectal wall through a deficient recto-vaginal septum), an enterocele (descent of the small bowel into the recto-vaginal septum from above) or a rectal prolapse. This rectal prolapse may extend beyond the anal verge (external prolapse or procidentia) or remain within the rectum or anal canal (internal prolapse or intussusception). MRI technology is used for defaecating proctography in some centers.
Summary The mechanisms controlling the processes of continence and defaecation are highly complex and inter-dependent. Failure of any part of the sensorineural pathway or dysfunction of the component organs will lead to disordered continence.
SURGERY 32:8
407
Crown Copyright Ó 2014 Published by Elsevier Ltd. All rights reserved.
Clinical, radiological and physiological assessment of anorectal function
Several techniques are available that allow assessment of the anatomy and physiological function of the key parts of the pathway. The most important initial step in assessment involves a detailed history (including obstetric history and previous proctological surgery) and physical examination.
Heman Joshi
Clinical assessment
Oliver Jones
General physical and abdominal examination is important. Assessment of fitness may be a major determinant of how far to pursue investigations and treatment, particularly surgical interventions. Patients with obesity and diabetes are more prone to pelvic floor problems but also the complications of intervention. Optimization of underlying medical problems should be encouraged. In extreme obesity correction of this may be more important than any efforts directed to the pelvic floor, even if considering surgery for obesity. In the UK it is traditional to examine patients in the left lateral position to assess the perineum and anorectum. The limitation of this assessment is that inspection of the vaginal introitus is compromised and the consequences of gravity are less obvious, even allowing for straining and bearing down. For this reason, examination with the aid of the gynaecological couch has much to advocate it and is routine in many parts of Europe. The traditional all-fours approach in the USA gives good access for anal procedures in the clinic but is less helpful at assessing prolapse. Careful inspection of the perineum will identify obvious scarring and deformity as a result of previous surgery or parturition. Morphology of the perineum has been described by Nivatongs and it may be useful to record this.7,8 The anus may be gaping from obstetric injury or pelvic neuropathy or tight with evidence of spasm or a bulky internal sphincter. The presence of soiling, excoriation, tags, and piles is relevant. It is essential to include a dynamic component to the examination, asking the patient to bear and push down. Here we observe the nature of movement and if it appears coordinated and appropriate. Understandably many patients are reluctant to unleash the full effort in doing this for fear of incontinence and it can be worth repeating at the end of the examination as the individual becomes relaxed. The amount of perineal descent should be noted as well as the presence of vaginal prolapse. Palpation around the anus will give the impression of sphincter bulk, spasm and signs of obstetric or surgical trauma. Digital rectal examination is undertaken gently and it is valuable to ask the patient to bear down again, squeeze and relax. An impression of paradoxical contraction of the puborectalis muscle may also be detected. Digital examination allows assessment of: the sphincter bulk; any evidence of previous damage to and the quality of the perineal body and posterior vaginal wall; and any rectocele. With the finger above the sphincter mechanism it is again useful to ask the patient to bear down and in this position one may become aware of a poorly supported anterior rectal wall and the presence of internal rectal prolapse as the descending intussusceptions strikes the examining finger. A bulky retroverted fibroid uterus may also be felt to intrude significantly into the rectum at this level. Inspection and digital rectal examination are followed by inspection using rigid sigmoidoscope, excluding mucosal disease
Abstract Faecal continence is defined by the ability to perceive, retain and evacuate bowel contents at socially convenient times. It may be associated with urgency, occur as a passive event, or be mixed. This is reliant upon normal function of the main involved organs (i.e. rectum, pelvic floor and anal sphincters) together with their associated sensorineural pathways. Incontinence may occur as a result of dysfunction in any one of these systems or due to factors such as systemic disease, emotion, bowel motility and stool consistency. The act of defaecation is a conscious process that involves interplay between motor and sensory elements, initiated by higher cortical function. Incontinence and evacuatory dysfunction are investigated using specialized tests that assess sphincter function and structure (anorectal manometry, endoanal ultrasound), anorectal and pelvic floor function (defaecating proctography, nerve conduction studies) and luminal integrity and colonic function (transit studies and endoscopy).
Keywords Anal sphincter; anorectal investigations; continence; defaecation; pelvic floor; rectum
Introduction Faecal incontinence is defined as the involuntary loss of faecal material. It is common, with a prevalence in the community estimated at 1.4% and rising to 7% in the elderly.1,2 Constipation and related functional bowel disorders have been formally defined by the Rome III collaboration.3 They affect up to 25% of the population and comprise a symptom complex associated with infrequent defaecation, hard stools, difficulty in emptying, straining and a feeling of incomplete evacuation.4 Maintenance of continence is a complex process that involves coordination between multiple neuronal reflexes, sensory and motor pathways, and the key pelvic organs, namely the rectum, pelvic floor and anal sphincters.5,6 An abnormality of any part of the pathway can be associated with dysfunctional defaecation, manifesting as constipation at one end of the scale, or as faecal incontinence at the other. However, a mixed picture, associated with obstructive defaecation syndrome, is a not uncommon presentation.
Heman Joshi BSc MRCS(Eng) is a Research Fellow in Pelvic Floor Surgery at Oxford University Hospitals Trust, Oxford, UK. Conflicts of interest: none declared. Oliver Jones DM FRCS is a Consultant Surgeon at Oxford University Hospitals Trust, Oxford, UK. Conflicts of interest: none declared.
SURGERY 32:8
404
Crown Copyright Ó 2014 Published by Elsevier Ltd. All rights reserved.
BASIC SCIENCE
The upper internal anal sphincter relaxes enabling anorectal sampling to occur. When the anal canal is deemed to have solid contents, and a decision to defaecate is made, relaxation of the pelvic floor occurs in conjunction with a rise in intra-abdominal pressure. The rise in intra-abdominal pressure occurs as a consequence of the Valsalva manoeuvre, in association with tensing of the anterior abdominal wall musculature and closure of the glottis. Optimal evacuation is achieved when the subject is in the squatting position which, assisted by hip flexion, further straightens the anorectal angle and allows more effective propulsion of faecal residue.12 In the Western world, defaecation in the sitting position is socially preferred with reduced effect on the anorectal angle. The net result is relaxation of puborectalis, straightening of the anorectal angle and slight pelvic floor descent. The external anal sphincter relaxes and rectal contents are evacuated. Further propulsive contractions of the rectum occur until it is fully empty. Once evacuation is complete, the pelvic floor rises and both sphincters and puborectalis contract as part of a ‘closing reflex’.
and occasionally allowing assessment of a high take-off rectal intussusceptions; however the return in pelvic assessment is limited. A short proctoscope is valuable in assessment of piles and mucosal prolapse particularly with the patient straining. A good impression of full-thickness internal prolapse can also be obtained as the proctoscope is delivered through the anus.8
Physiology of continence Internal and external anal sphincters The internal anal sphincter demonstrates sinusoidal ‘slow-wave’ activity with a frequency of 20e40 cycles/minute and is the main contributor to resting anal pressure, measured at between 50 and 120 mmHg in a healthy individual. Much of this resting pressure is attributable to myogenic tone, an intrinsic property of sphincteric smooth muscle independent of neural input. Additional excitatory input to the internal sphincter comes from sympathetic neural input, whilst parasympathetic nerves to the sphincter are inhibitory and mediated via nitric oxide.9 Isolated damage to the internal anal sphincter typically causes predominantly passive faecal incontinence (leakage). Whilst the external anal sphincter does make a small contribution to resting tone, its primary role is that of voluntary contraction of the anal sphincter which can be recruited consciously or as a reflex during times of additional risk of incontinence such as when lifting a heavy object or coughing (see below). A normal external sphincter may generate an additional pressure of between 50 and 200 mmHg. Damage to the external sphincter is strongly associated with urge faecal incontinence (the sensation of being unable to ‘hold on’).
Electrophysiology Neurophysiological assessment of the anorectum includes electromyography of the sphincter mechanism, and nerve conduction studies to assess the pudendal and spinal nerves. Electromyography involves the placement of needle electrodes into the puborectalis or external sphincter to assess the state of the muscle and its innervating nerve as a function of its electrical activity during the resting and contractile phase. The procedure can be painful and unpleasant. Increasing adoption of endoanal ultrasound has meant that electromyography is rarely used. Pudendal nerve terminal motor latency (PNTML) is assessed by stimulation of the nerve, as it enters the ischio-rectal fossa at the ischial spines. Prolonged PNTML is associated with idiopathic faecal incontinence, rectal prolapse, solitary rectal ulcer syndrome and sphincter defects. The maintenance of anal continence is a very complex process dependent on several processes including colonic motility, stool consistency, sensation, rectal compliance, local reflexes, pelvic floor and sphincter function.
Puborectalis and the pelvic floor The puborectalis plays a key role in continence. It is thought to achieve this function due to its sling-like anatomy and its effect on the anorectal angle when it is contracted. Sir Alan Parks proposed a ‘flap-valve’ mechanism of continence e namely that in a situation of increased intra-abdominal pressure (e.g. sneezing, coughing or straining), the puborectalis keeps the valve shut by driving the anterior rectal wall against the upper anal canal, and thereby prevents the passage of stool into the lower anal canal.10 Other authors have demonstrated in radiological studies that during the Valsalva manoeuvre, the relation between the anterior rectal wall and the anorectal angle is not clearly defined; some studies have shown little difference in the anorectal angle between incontinent patients and a normal control group.11 It is likely that the puborectalis functions as a further deeper sphincter mechanism and complements the activity of the external anal sphincter. Indeed, it has been shown that the puborectalis sling can maintain continence even in the presence of internal and external sphincter dysfunction.11
Colonic motility and stool consistency Even patients with normal sphincters will suffer urgency or actual incontinence if they develop severe diarrhoea (as can occur with ulcerative colitis). It is little surprise, therefore, that in many patients with diarrhoea and incontinence, significant improvements in control will be seen with the use of constipating medications.13 Rectal sensation and compliance The rectum plays a key role in continence, by functioning as a site for temporary storage of faeces. A normal rectum can accommodate significant volumes of faecal residue with minimal alteration in measured rectal pressures. This is termed rectal compliance. Recent animal studies have identified unique receptors, termed rectal intra-ganglionic laminar endings (rIGLEs), which act as slowly adapting mechanoreceptors responsive to tension and rapid distension.14 Rectal compliance is often altered in patients with faecal urgency, constipation and incontinence. Extreme alterations in
Physiology of evacuation An awareness of the need to defaecate occurs in the superior frontal gyrus and anterior cingulate gyrus of the cerebral cortex, as a result of a critical level of rectal filling. The process is initiated by faecal matter, which has moved into the lower rectum, causing distension. This leads to stimulation of pressure receptors on the pelvic floor (puborectalis), which in turn triggers the rectoanal inhibitory reflex.
SURGERY 32:8
405
Crown Copyright Ó 2014 Published by Elsevier Ltd. All rights reserved.
BASIC SCIENCE
detectable sensation, urge to defaecate and maximum tolerable volume. These parameters may be abnormal in patients with autonomic neuropathy from diabetes or in patients with functional and somatic alterations of the rectal reservoir. For example, patients with megarectum may have delayed sensation (hyposensitivity) whilst those with inflammatory bowel disease typically have low volumes for each of these three parameters (hypersensitivity). Clinically, patients with hypersensitivity often have urgency and stool frequency. Rectal sensation can also be evaluated by use of thermal and electric probes. There has also been interest in assessment of anal sensation by these means but the clinical utility of this is uncertain at present. Rectal compliance (reflecting distensibility and accommodation) can also be measured using a barostat but this is not used widely in clinical practice.
capacity and compliance can lead to faecal incontinence through a decreased capacity for accommodation or as a consequence of overflow states. Compliance is reduced as part of the normal ageing process, or in inflammatory states (e.g. ulcerative colitis). Normal sensation (the ability to sense that the rectum contains stool) is also fundamental to continence. Local reflexes and anal sensation The best studied of the local reflexes central to faecal continence is the rectoanal inhibitory reflex (RAIR). This is an intrinsic intramural reflex that is preserved after spinal cord injury or extrinsic denervation of the rectum. It is not seen in Hirschsprung’s disease when myenteric ganglia are absent in the low rectum and for a variable distance proximally. It manifests as a periodic relaxation of the upper internal anal sphincter in response to distension of the rectum, with extent of relaxation dependent upon degree of rectal distension. This permits faecal matter or flatus to come into contact with specialized receptors in the upper anal canal, where ‘sampling’ takes place. This process is instrumental in the discrimination of gas from stool and passage of flatus without faecal leakage. If evacuation is not desired, the high resting pressures of the lower part of the internal anal sphincter, coupled with contraction of the external anal sphincter and puborectalis sling, result in return of faecal content to the upper rectum, delaying the process of defecation. An attenuated rectoanal inhibitory reflex may lead to difficulty with evacuation, whilst an exaggerated reflex can result in faecal leakage. Several studies have looked at the impact of rectal excisional surgery on the rectoanal inhibitory reflex and noted that while it is nearly abolished in the early postoperative period, it appears to return, in some form, in due course in the majority of patients.15
Radiology Endoanal ultrasound Endoanal ultrasound is performed using an internal rotating micro-transducer. It evaluates sphincter integrity and structure. The external sphincter appears as a circumferential hyperechoic structure, while the internal sphincter appears as a hypoechoic (black) inner circle (Figure 1). Sphincter defects and scarring may be seen as ‘incomplete rings’. Defaecating proctography Defaecating proctography utilizes video fluoroscopy and is a dynamic examination, providing structural information as well as assessing function during defaecation. Its use is indicated as part of the work-up of a patient presenting with a picture of outlet obstruction and for many patients with incontinence. The technique requires barium paste to be introduced into the rectum. An additional amount is usually inserted into the vagina in women and oral contrast is given to opacify the small bowel. The patient
Physiological testing Anorectal manometry Anorectal manometry provides an assessment of anal sphincter pressures and local reflexes. There are various types of probes and recording systems including water-based catheter balloon systems placed into the distal rectum and withdrawn through the anal canal in a step-wise manner, or the newer solid-state devices that contain micro-transducers to measure anal canal pressure at various points along the length of the catheter. The technique can determine resting and voluntary squeeze pressures of the anal canal (reflecting internal and external sphincter tone respectively) as well as the length of the anal canal itself. Other parameters that might be recorded include testing for the cough or Valsalva reflex (a brief increase in anal tone to counter a sudden rise in intra-abdominal pressure) and the presence of a functional rectoanal inhibitory reflex (a fall in anal resting pressure in response to rectal distension by means of inflation of an intrarectal balloon). This latter reflex is typically absent in Hirschsprung’s disease (congenital lack of intramural rectal ganglia). The duration of external sphincter contraction (endurance squeeze), assessed by asking the patient to perform a sustained anal contraction is another useful test; a low result is often seen in incontinent patients.
Figure 1 Endoanal ultrasound of a normal anal sphincter. The endoluminal probe is delineated in this diagram by the dashed lines. The solid white arrows mark the margins of the external anal sphincter. The unfilled arrows delineate the internal anal sphincter that appears as a black circle because of its high water content.
Sensory testing Rectal balloon insufflation tests provide an indication of rectal sensory function. Usually three variables are measured: first
SURGERY 32:8
406
Crown Copyright Ó 2014 Published by Elsevier Ltd. All rights reserved.
BASIC SCIENCE
Managing such patients requires an informed approach with relevant investigations to identify the cause of their problem and thereby facilitate appropriate management. A REFERENCES 1 Bartolo DC, Paterson HM. Anal incontinence. Best Pract Res Clin Gastroenterol 2009; 23: 505e15. 2 Madoff RD, Parker SC, Varma MG, Lowry AC. Faecal incontinence in adults. Lancet 2004; 364: 621e32. 3 Longstreth GF, Thompson WG, Chey WD, Houghton LA, Mearin F, Spiller RC. Functional bowel disorders. Gastroenterol 2006; 130: 1480e91. 4 Chatoor D, Emmnauel A. Constipation and evacuation disorders. Best Pract Res Clin Gastroenterol 2009; 23: 517e30. 5 Mellgren A. Fecal incontinence. Surg Clin North Am 2010; 90: 185e94. 6 Bharucha AE. Pelvic floor: anatomy and function. Neurogastroenterol Motil 2006; 18: 507e19. 7 Nivatongs S, Fang DT, Kennedy HL. The shape of the buttocks. A useful guide to selection of anaesthesia and patient position in anorectal surgery. Dis Colon Rectum 1983; 26: 85e6. 8 Cunningham C. Establishing and developing a pelvic floor service: the multidisciplinary team and the approach to patient assessment. In: Lindsey I, Nugnet K, Dixon T, eds. Pelvic floor disorders for the colorectal surgeon 2011. 9 Jones OM, Brading AF, Mortensen NJ. The physiology, pharmacology and therapeutic manipulation of the internal anal sphincter. Can J Gastroenterol 2002; 16: 249e57. 10 Parks AG. Royal Society of Medicine, Section of Proctology; Meeting 27 November 1974. President’s Address. Anorectal incontinence. Proc R Soc Med 1975; 68: 681e90. 11 Bajwa A, Emmanuel A. The physiology of continence and evacuation. Best Pract Res Clin Gastroenterol 2009; 23: 477e85. 12 Barleben A, Mills S. Anorectal anatomy and physiology. Surg Clin North Am 2010; 90: 1e15. 13 Ehrenpreis ED, Chang D, Eichenwald E. Pharmacotherapy for fecal incontinence: a review. Dis Colon Rectum 2007; 50: 641e9. 14 Lynn PA, Olsson C, Zagorodnyuk V, Costa M, Brookes SJ. Rectal intraganglionic laminar endings are transduction sites of extrinsic mechanoreceptors in the guinea pig rectum. Gastroenterol 2003; 125: 786e94. 15 O’Riordain MG, Molloy RG, Gillen P, Horgan A, Kirwan WO. Rectoanal inhibitory reflex following low stapled anterior resection of the rectum. Dis Colon Rectum 1992; 35: 874e8.
Figure 2 Defaecating proctogram. The patient has been given oral contrast 7 to delineate the small bowel, which is dropping down into the rectovaginal septum to form an enterocoele (dashed line). The patient has also received rectal contrast that delineates a rectocele (dotted line) and intussusception (indicated by arrows). The patient has a tampon soaked in radiographic contrast medium placed in her vagina.
is then asked to sit on a commode and attempt evacuation during X-ray screening (Figure 2). Several abnormalities can be assessed including measurement of the anorectal angle and position of pelvic floor at rest, during squeeze and defaecation (perineal descent). Proctography may detect a rectocele (herniation of the rectal wall through a deficient recto-vaginal septum), an enterocele (descent of the small bowel into the recto-vaginal septum from above) or a rectal prolapse. This rectal prolapse may extend beyond the anal verge (external prolapse or procidentia) or remain within the rectum or anal canal (internal prolapse or intussusception). MRI technology is used for defaecating proctography in some centers.
Summary The mechanisms controlling the processes of continence and defaecation are highly complex and inter-dependent. Failure of any part of the sensorineural pathway or dysfunction of the component organs will lead to disordered continence.
SURGERY 32:8
407
Crown Copyright Ó 2014 Published by Elsevier Ltd. All rights reserved.