fisiologia da evacuacao

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BASIC SCIENCE
Physiology: evacuation,
pelvic floor and continence
mechanisms
Shashank V Gurjar
Oliver M Jones
Abstract
Faecal continence is defined by the ability to perceive, retain and evac-
uate bowel contents at socially convenient times. This is reliant upon
normal function of the main involved organs (i.e. rectum, pelvic floor
and anal sphincters) together with their associated sensorineural path-
ways. 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 integ-
rity 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 Very variable
rates, from 10% to over 40%, are reported in residential care and
nursing home residents.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 defae-
cation, 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
Shashank V Gurjar MSc FRCS is a Pelvic Floor Fellow at Oxford Radcliffe
NHS Trust, Oxford, UK. Conflicts of interest: none declared.
Oliver M Jones DM FRCS is a Consultant Colorectal Surgeon at Oxford
Radcliffe NHS Trust, Oxford, UK. Conflicts of interest: none declared.
SURGERY 29:8 358
with obstructive defaecation syndrome, is a not uncommon
presentation.
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.
Functional anatomy
Rectum
The rectum has a key role in the storage of faeces. Structurally,
its wall contains two layers of smooth muscle (longitudinal and
circular) in continuity with the rest of the intestinal tract.
Its nervous supply is from extrinsic autonomic nerves that act
at the submucosal plexuses (Meissner’s and Auerbach’s) con-
tained in its walls, which mediate rectal accommodation to
distension.
Pelvic floor
The primary functions of the pelvic floor are to support the
viscera of the pelvic cavity, help maintain continence and aid in
the process of defaecation. The floor consists of a broad, thin,
striated muscular sheet (levator ani) with a central ligamentous
structure, which allows the passage of viscera, namely rectum,
urethra and the vagina (in females). This striated muscle is
composed of four separate muscles, defined by their area of
attachment, which span the pubic bone, ischial spine and coccyx
posteriorly (puborectalis, pubococcygeus, ileococcygeus and
ischiococcygeus). It receives its innervation from branches of
the pudendal, inferior rectal, perineal and sacral (S3, S4)
nerves. Puborectalis is the most important component forming
a U-shaped sling around the rectum; it helps to pull the rectum
anteriorly and gives rise to the so-called anorectal angle.
Puborectalis can be palpated on digital rectal examination as
a taut muscle bulk, lying just above the external anal sphincter
at the top of the anorectal ring.
Anal canal and sphincters
The anal canal measures between 2 and 5 cm in length. It is
encircled by the anal sphincter, which itself comprises two key
muscular components: the caudal continuation of the circular
(involuntary) smooth muscle of the rectum forming the internal
anal sphincter, and the surrounding striated muscle (voluntary)
of the external anal sphincter.
The internal anal sphincter is approximately 3 cm long and
2 mm thick, increasing in thickness with age. Of note, it is shorter
in a woman; this fact must be borne in mind when considering
anterior perianal pathology in the post-episiotomy woman. It has
an intrinsic nerve supply arising from the myenteric plexus;
additional excitatory sympathetic innervation is via the hypo-
gastric and pelvic plexus, while inhibitory parasympathetic
supply is received from S1, S2 and S3 via the pelvic plexus.
The external anal sphincter receives its innervation from the
pudendal nerve (S2eS4), whilst autonomic supply is from
sympathetic sources derived from the hypogastric plexus and its
parasympathetic supply comes via the nervi erigentes. It is
composed of tonically active ‘slow-twitch’ and phasically active
‘fast-twitch’ fibres.
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http://dx.doi.org/10.1016/j.mpsur.2011.05.010
BASIC SCIENCE
Physiology of continence
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.
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.7
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.8
Rectal compliance is often altered in patients with faecal
urgency, constipation and incontinence. Extreme alterations in
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 intra-
mural reflex that is preserved after spinal cord injury or extrinsic
denervation of the rectum. It is not seen in Hirschspring’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.9
SURGERY 29:8 359
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 to
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. Addi-
tional excitatory input to the internal sphincter comes from
sympathetic neural input, whilst parasympathetic nerves to the
sphincter are inhibitory and mediated via nitric oxide.10 Isolated
damage to the internal anal sphincter typically causes predomi-
nantly passive faecal incontinence (leakage).
Whilst the external anal sphincter does make a small contri-
bution 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 incon-
tinence (the sensation of being unable to ‘hold on’).
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 preventing the passage of stool into the
lower anal canal.11 Other authors have demonstrated in radio-
logical 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.12 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.12
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 trig-
gers the rectoanal inhibitory reflex.
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 conse-
quence 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
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BASIC SCIENCE
position which, assisted by hip flexion, further straightens the
anorectal angle and allows more effective propulsion of faecal
residue.13 In theWesternworld, defaecation in the sitting position is
socially preferredwith reduced effect on the anorectal angle.Thenet
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 evac-
uation is complete, the pelvic floor rises and both sphincters and
puborectalis contract as part of a ‘closing reflex’.
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.
Sensory testing
Rectal balloon insufflation tests provide an indication of rectal
sensory function. Usually three variables are measured: first
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 func-
tional 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.
Figure 1 Endoanal ultrasound of a normal anal sphincter. The endoluminal
probe is delineated in this diagram by the dashed lines. The solid white
Electrophysiology
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.
Neurophysiological assessment of the anorectum includes elec-
tromyography of the sphincter mechanism, and nerve conduc-
tion studies to assess the pudendal and spinal nerves.
SURGERY 29:8 360
Electromyographyinvolves 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 (PTNML) is assessed
by stimulation of the nerve, as it enters the ischio-rectal fossa at
the ischial spines. Prolonged PTNML is associated with idio-
pathic faecal incontinence, rectal prolapse, solitary rectal ulcer
syndrome and sphincter defects.
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 tech-
nique 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
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
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Figure 2 Defaecating proctogram. The patient has been given oral contrast
to delineate the small bowel, which is dropping down into the recto-
vaginal septum to form an enterocoele (dashed line). The patient has also
received rectal contrast that delineates a rectocoele (dotted line) and
intussusception (indicated by arrows). The patient has a tampon soaked
in radiographic contrast medium placed in her vagina.
BASIC SCIENCE
detect a rectocoele (herniation of the rectal wall through a defi-
cient recto-vaginal septum), an enterocoele (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). Magnetic
resonance imaging (MRI) technology is used for defaecating
proctography in some centres.
Summary
The mechanisms controlling the processes of continence and
defaecation are highly complex and inter-dependent. Failure of
SURGERY 29:8 361
any part of the sensorineural pathway or dysfunction of the
component organs will lead to disordered continence.
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 Aug 14e20; 364: 621e32.
3 Longstreth GF, Thompson WG, Chey WD, Houghton LA, Mearin F,
Spiller RC. Functional bowel disorders. Gastroenterology 2006 Apr;
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 Feb; 90:
185e94. Table of Contents.
6 Bharucha AE. Pelvic floor: anatomy and function. Neurogastroenterol
Motil 2006 Jul; 18: 507e19.
7 Ehrenpreis ED, Chang D, Eichenwald E. Pharmacotherapy for fecal
incontinence: a review. Dis Colon Rectum 2007 May; 50: 641e9.
8 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. Gastroenterology 2003
Sep; 125: 786e94.
9 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 Sep; 35: 874e8.
10 Jones OM, Brading AF, Mortensen NJ. The physiology, pharmacology
and therapeutic manipulation of the internal anal sphincter. Can J
Gastroenterol 2002 Apr; 16: 249e57.
11 Parks AG. Royal Society of Medicine, Section of Proctology; Meeting
27 November 1974. President’s Address. Anorectal incontinence.
Proc R Soc Med 1975 Nov; 68: 681e90.
12 Bajwa A, Emmanuel A. The physiology of continence and evacuation.
Best Pract Res Clin Gastroenterol 2009; 23: 477e85.
13 Barleben A, Mills S. Anorectal anatomy and physiology. Surg Clin
North Am 2010 Feb; 90: 1e15. Table of Contents.
� 2011 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.mpsur.2011.05.010
	 Physiology: evacuation, pelvic floor and continence mechanisms
	 Introduction
	 Functional anatomy
	 Rectum
	 Pelvic floor
	 Anal canal and sphincters
	 Physiology of continence
	 Colonic motility and stool consistency
	 Rectal sensation and compliance
	 Local reflexes and anal sensation
	 Internal and external anal sphincters
	 Puborectalis and the pelvic floor
	 Physiology of evacuation
	 Physiological testing
	 Anorectal manometry
	 Sensory testing
	 Electrophysiology
	 Endoanal ultrasound
	 Defaecating proctography
	 Summary
	 References