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11. Investigation of Functional Bowel Disorders 221
Figure 11.11. Maximal squeeze pressure.
Figure 11.12. Low pressures in an incontinent patient.
222 P.J. Arumugam et al.
3.1. Rectoanal Inhibitory Reflex
Rectal distension causes a reflex inhibition of internal anal sphincter (IAS)
and contraction of the external anal sphincter (EAS). The mechanism of
this reflex is as follows: as stool fills the rectum it eventually reaches the
anorectal junction, as there is relaxation of the proximal anal canal. A
spinal-cord reflex is initiated resulting in the relaxation of the IAS and the
sensation of a call to stool. If there is a morphological damage to the EAS
or it is denervated as in pudendal neuropathy, then incontinence occurs.The
reflex, which is present in all healthy people, was first described by Gowers
and is believed to be mediated by myentric plexus and stretch receptors in
the pelvic floor [31]. The test is easily conducted by placing a manometry
catheter in the high-pressure zone of the anal canal and a balloon within
the rectum. The volume in the balloon is increased and then there is a rapid
and profound decrease in resting pressure. It is initiated by a volume of
10 mL or less. It is absent in Hirschprung’s disease, systemic sclerosis, and
Chagas disease [32]. It has been shown to recover after a coloanal anasto-
mosis [33]. This reflex allows the contents to be sampled by the transitional-
zone mucosa to discriminate between solid, liquid, and air [34].
Saigusa et al. analysed 100 patients who underwent double stapled-pouch
anal anastomosis and found presence of rectoanal inhibitory reflex in only
53% following surgery. There was no difference in the continence scores in
patients with or without the reflex [35]. However, they noted a decreased
incidence of nocturnal soiling if the reflex was present.
3.2. Vector Volume Manometry
Vector volume manometry allows a 3-dimensional reconstruction of the
anal canal and has the ability to assess radial asymmetry. Sphincter defects
are associated with indices of 0.6 or less and normal is a vector symmetry
index (VSI) of 1.
Vector volume manometry is performed by a radially arranged 8-channel
catheter, which records the pressure as the catheter is withdrawn and the
software constructs the image. It is very useful in demonstrating a traumatic
cause for faecal incontinence such as an isolated defect compared with
global weakness of the external sphincter, which may denote a nerve-
conduction defect [36]. However, the correlation with anal endosonogra-
phy and EMG is not good with 11% and 13% of sphincter defects being
detected, respectively [37].
3.3. Rectal Sensation
Rectal sensation is assessed using a water-filled balloon, which is gradually
inflated. Rectal filling and sensation can be variable in individuals with a
wide normal range with patients with obstructed defaecation having a very
high rectal threshold volume [38]. Normal first perception starts between
10–30 mL while the urge to defaecate starts from 60–100 mL and the
maximal tolerated volume ranges from 220–270 mL [39,40].
3.4. Rectal Compliance
From the change in volume and the associated pressure change, compliance
can be calculated:
This gives an indication of the distensibility of the rectum.Mean rectal com-
pliance is between 4–14 mL/cm H
2
0 and is measured by a rectal balloon
filled with water [40]. Rectal compliance is reduced in Behcet’s disease,
Crohn’s disease and after radiotherapy. However, rectal compliance is
found to be normal in incontinent patients.
3.5. Electromyography
Electromyography of the anal sphincter or levator ani can be used to show
that a muscle has been denervated and that this is part of the clinical
problem. Electromyography was used to map external-sphincter defects
before surgery by either concentric-needle studies or single fiber studies but
has been replaced by endo–anal ultrasound. Puborectalis EMG has been
shown to demonstrate paradoxical contraction in obstructed defaecation
but has been shown to be present in normal individuals [41].
Internal-anal-sphincter hypertonia has been shown to be associated with
nonhealing fissures and can be demonstrated by EMG [42].
3.6. Pudendal Nerve Terminal Motor Latency
Pudendal nerve terminal motor latency is a simple nerve-conduction study
used in the assessment of the pelvic floor. It is performed using a specially
designed finger cot with a stimulating electrode at the fingertip and a
recording electrode at the base of the finger. The latter electrode sits in the
anal canal while the other is applied to the pudendal nerve as its passes
around the iliac spine.The stimulus is applied at the iliac spine and the anal-
canal electrode records the response. The nerve latency is measured from
the stimulus to the motor response in the external sphincter and normal
values range from 1.5–2.5 ms. Sato et al. have shown that there is physio-
logic variation in PNTML related to where it is recorded in the anal canal
[43]. It is delayed with increasing age and is prolonged in 24% of patients
with faecal incontinence. It is also prolonged in patients with rectal pro-
lapse, solitary rectal ulcer syndrome, and severe constipation [44].
A prolonged PNTML is associated with a bad prognosis after an ante-
rior anal sphincter repair for external sphincter defects [45].
Compliance V P=d d
11. Investigation of Functional Bowel Disorders 223
Thomas et al. compared PNTML and EMG in 80 patients with faecal
incontinence secondary to obstetric surgical trauma and found both
PNTML and EMG were useful in providing additional information about
the neurogenic basis of incontinence [46].
4. Magnetic Resonance Imaging of the Pelvis
Magnetic resonance imaging allows a multiplanar assessment which is non-
invasive, quick, avoids ionising radiation, requires no preparation or con-
trast medium, and it is relatively non-operator–dependent. However, it is
expensive, time consuming, and does not provide good mucosal images as
seen on evacuatory proctography.
It is increasingly being used in obstructed defaecation (pelvic surface
coil) and faecal incontinence (endo–anal MRI coil).
Healy et al., in a controlled study comparing obstructed defaecation pa-
tients with asymptomatic controls, showed MRI is useful in demonstrating
more pelvic pathology compared to evacuatory proctography such as pelvic
visceral descent, levator muscle, and plate descent [47]. They recommended
MRI in symptomatic patients who were reported as normal following
proctography.
Endo–anal coil MRI was compared with endo–anal ultrasound in assess-
ing the sphincters in incontinent patients by Fletcher et al., who demon-
strated the superiority of MRI in evaluating the thickness of the
puborectalis and external sphincter [48]. MRI is also able to demonstrate
degeneration of external anal sphincter with age. Both procedures were
equally effective in identifying defects of the sphincters.
By comparing dynamic MRI with videoproctography, Matsuoka et al.
have showed MRI is unable to detect rectocoeles and intussusception
effectively [49]. Further, there were differences in the measurement
of the anorectal angle and perineal descent compared with proctography.
However, Lamb et al. believe dynamic MRI proctography to be a
useful alternative to conventional proctography in a small study of 40
patients [50].
The evidence thus far supports the role of MRI in evaluation of faecal
incontinence, either on its own or as an adjunct to endo–anal ultrasound;
however, its role in assessing obstructed defaecation patients is still
evolving.
5. Endo–anal Ultrasound
Endo–anal ultrasound (EAUS) is a minimally invasive easily accessible
technique that provides detailed anatomy of the anal canal. It complements
anal manometry and evacuatory proctography and is mandatory in the
224 P.J. Arumugam et al.
investigation of faecal incontinence. It also has a role in the management
of obstructed defaecation and other functional bowel disorders.
Indications for EAUS assessment in the functional bowel disease patient
include faecal incontinence, constipation and obstructed defaecation, post-
surgical assessment, anal pain, and solitary rectal ulcer syndrome.
5.1. Technique and Instrumentation
Ideally, a 360-degree cross-sectional image is required, and this is obtained
by using a mechanically rotating transducer. The most commonly used
scanner is manufactured by Bruel and Kjaer (Figure 11.13), and uses an
11. Investigation of Functional Bowel Disorders 225
Figure 11.13. An endoanal ultrasound scanner by Bruel and Kjaer.
1850 endoprobe with a 10 MHz transducer. The transducer is protected
by a plastic cone with an outside diameter of 17 mm (Figure 11.14) and
filled with degassed water to prevent artifacts caused by air bubbles.
The assembly is covered by a large condom with gel both inside and
outside to provide acoustic coupling. Patients are examined in the prone
position to avoid perineal asymmetry and the images are orientated such
that anterior is uppermost and viewed from the feet of the patient. The
anal canal is examined as a dynamic process and images are convention-
ally obtained at superficial, mid, and deep levels (Figure 11.15–
Figure 11.17).
The various layers of the anal canal as depicted on EAUS examination
are as illustrated in Table 11.2.
Variation in the anatomy of the anal canal is apparent between males and
females, particularly in the external anal sphincter (Figure 11.18), and
changes in the appearances of the sphincters with age are important in
interpretation of the endosonic anatomy of the anal canal.
The thickness of the internal sphincter shows a positive correlation with
age in the high- and middle-anal canal in the female asymptomatic
nulliparous women [52]. However, the external sphincter shows a negative
correlation with age at high-, mid-, and low-anal levels. There is no
significant correlation between thickness of longitudinal muscle or
puborectalis.
226 P.J. Arumugam et al.
Figure 11.14. The transducer protected by a plastic cone.
Figure 11.15. Endo–anal scan showing superficial level.
Figure 11.16. Endo–anal scan showing mid level.
In patients less than 55 years of age, the internal sphincter is between
2.4–2.7-mm thick, while above 55 years it is from 2.8–3.4-mm thick (Figure
11.19, Figure 11.20).
The conjoint longitudinal muscle coat surrounding the internal sphincter
is continuation of the outer longitudinal muscle coat of the rectum. It forms
a prominent hyperechoic layer between the internal and external sphinc-
ters. This layer is not sonographically uniform and thin short hypoechoic
areas within it may be due to segments of smooth muscle devoid of con-
nective tissue.
The external sphincter is complex and a trilaminar arrangement is
accepted with deep, superficial, and subcutaneous parts. As the probe is
228 P.J. Arumugam et al.
Figure 11.17. Endo–anal scan showing deep level.
Table 11.2. Endosonographic layers of the anal canal.
1. Mucosa/cone interface, hyperechoeic layer
2. (Mucosa)–(hypoechoeic layer) not always seen
3. Subepithelial layer, hyperechoic layer
4. Internal anal sphincter, hypoechoiec layer
5. Longitudinal muscle, hyperechoic layer
6. External sphincter, mixed echogenicity layer
Figure 11.18. Diagrammatic representation of the variation in the anatomy of the
anal canal in males and females.
Figure 11.19. Variation in the thickness in the internal sphincter with age.
withdrawn, the hyperechoic U-shaped sling of puborectalis comes in view.
The external sphincter is identified as converging fibres anteriorly as the
probe comes down and they converge to form an annular ring [53]. In
females, the external sphincter may be very thin anteriorly. In males, the
external sphincter is more symmetrical and annular at all levels with an
outer hypoechoic ring. In females, the echogenicity is identical and the
layers are difficult to delineate and there is a natural deficiency anteriorly
in the deep part of the anal canal.
Recently, 3-dimensional ultrasonography has been introduced [54]. Con-
ventional axial imaging may not give the accurate length of a sphincter tear
but 3-dimensional imaging allows visualisation of the sphincter complex in
multiple planes (Figure 11.21).
5.2. Faecal Incontinence
Endo–anal ultrasound has been shown in various studies to be a useful tool
in mapping sphincter defects.It distinguishes patients with sphincter defects
from idiopathic faecal incontinence and allows the surgeon to manage this
complex condition appropriately. Endo–anal ultrasound also has a role in
230 P.J. Arumugam et al.
Figure 11.20. Variation in the thickness in the internal sphincter with age.
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