Altered rectal sensory response induced by balloon distention in patients with functional abdominal pain syndrome

Abstract

Functional abdominal pain syndrome (FAPS) has chronic unexplained abdominal pain and is similar to the psychiatric diagnosis of somatoform pain disorder. A patient with irritable bowel syndrome (IBS) also has chronic unexplained abdominal pain, and rectal hypersensitivity is observed in a majority of the patients. However, no reports have evaluated the visceral sensory function of FAPS precisely. We aimed to test the hypothesis that FAPS would show altered visceral sensation compared to healthy controls or IBS. The present study determined the rectal perceptual threshold, intensity of sensation using visual analogue scale (VAS), and rectal compliance in response to rectal balloon distention by a barostat in FAPS, IBS, and healthy controls.
First, the ramp distention of 40 ml/min was induced and the thresholds of discomfort, pain, and maximum tolerance (mmHg) were measured. Next, three phasic distentions (60-sec duration separated by 30-sec intervals) of 10, 15 and 20 mmHg were randomly loaded. The subjects were asked to mark the VAS in reference to subjective intensity of sensation immediately after each distention. A pressure-volume relationship was determined by plotting corresponding pressures and volumes during ramp distention, and the compliance was calculated over the linear part of the curve by calculating from the slope of the curve using simple regression.
Rectal thresholds were significantly reduced in IBS but not in FAPS. The VAS ratings of intensity induced by phasic distention (around the discomfort threshold of the controls) were increased in IBS but significantly decreased in FAPS. Rectal compliance was reduced in IBS but not in FAPS.
An inconsistency of visceral sensitivity between lower and higher pressure distention might be a key feature for understanding the pathogenesis of FAPS.

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BioPsychoSocial Medicine
Open Access
Research
Altered rectal sensory response induced by balloon distention in
patients with functional abdominal pain syndrome
Tsukasa Nozu* and Miwako Kudaira
Address: Department of Comprehensive Medicine, Hokkaido University Hospital, Kita14, Nishi5, Kita-Ku Sapporo 0608648, Japan
Email: Tsukasa Nozu* - tnozu@sea.plala.or.jp; Miwako Kudaira - kmiwa@conpeito.gr.jp
* Corresponding author
Abstract
Background: Functional abdominal pain syndrome (FAPS) has chronic unexplained abdominal
pain and is similar to the psychiatric diagnosis of somatoform pain disorder. A patient with irritable
bowel syndrome (IBS) also has chronic unexplained abdominal pain, and rectal hypersensitivity is
observed in a majority of the patients. However, no reports have evaluated the visceral sensory
function of FAPS precisely. We aimed to test the hypothesis that FAPS would show altered visceral
sensation compared to healthy controls or IBS. The present study determined the rectal perceptual
threshold, intensity of sensation using visual analogue scale (VAS), and rectal compliance in
response to rectal balloon distention by a barostat in FAPS, IBS, and healthy controls.
Methods: First, the ramp distention of 40 ml/min was induced and the thresholds of discomfort,
pain, and maximum tolerance (mmHg) were measured. Next, three phasic distentions (60-sec
duration separated by 30-sec intervals) of 10, 15 and 20 mmHg were randomly loaded. The subjects
were asked to mark the VAS in reference to subjective intensity of sensation immediately after each
distention. A pressure-volume relationship was determined by plotting corresponding pressures
and volumes during ramp distention, and the compliance was calculated over the linear part of the
curve by calculating from the slope of the curve using simple regression.
Results: Rectal thresholds were significantly reduced in IBS but not in FAPS. The VAS ratings of
intensity induced by phasic distention (around the discomfort threshold of the controls) were
increased in IBS but significantly decreased in FAPS. Rectal compliance was reduced in IBS but not
in FAPS.
Conclusion: An inconsistency of visceral sensitivity between lower and higher pressure distention
might be a key feature for understanding the pathogenesis of FAPS.
Background
Functional gastrointestinal disorders (FGIDs) are charac-
terized as chronic or recurrent gastrointestinal symptoms
that are not explained by structural or biochemical abnor-
malities. They are diagnosed by Rome criteria [1]. Func-
tional abdominal pain syndrome (FAPS), one of the
FGIDs, is defined as "pain for at least six months that is
poorly related to gut function and associated with some
loss of daily activities". The prevalence is reported to be
much less than irritable bowel syndrome (IBS) or other
FGIDs [2], but this disease has a great impact on quality of
life and on the medical economy, because the patients
Published: 20 November 2009
BioPsychoSocial Medicine 2009, 3:13 doi:10.1186/1751-0759-3-13
Received: 30 August 2009
Accepted: 20 November 2009
This article is available from: http://www.bpsmedicine.com/content/3/1/13
© 2009 Nozu and Kudaira; licensee BioMed Central Ltd.
This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0),
which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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miss many work or school days and have high health care
resource use [2].
The pathogenesis of this disease is poorly understood, but
abnormality of pain modulation, particularly at the cen-
tral level, is thought to be the possible cause. However,
only a few studies addressing the visceral sensory function
of FAPS have been reported so far [3,4], and there is no
agreement on whether FAPS patients have altered visceral
sensory function. Recently, we demonstrated reduced rec-
tal pain threshold in response to balloon distention in
IBS, but no difference between FAPS and healthy controls,
suggesting the visceral sensory function of FAPS patients
may not be altered, in contrast to IBS [5].
The aim of the present study is to do a detailed assessment
of the sensory response induced by rectal distention in
FAPS patients. We evaluated rectal perceptual threshold,
perceived intensity by visual analogue scale (VAS), and
rectal compliance in response to rectal balloon distention
by a barostat in FAPS, IBS and healthy controls to clarify
the possible pathogenesis of FAPS.
Methods
Subjects
Controls. Thirteen healthy subjects (7 women and 6 men)
were recruited by advertisement to serve as controls. All
had normal bowel habits, and none had known gastroin-
testinal disease, was taking medication, or had a history of
acute or chronic illness.
Patients. Seven patients with IBS (5 women and 2 men)
and 6 patients with FAPS (4 women and 2 men) were
recruited from the Department of Comprehensive Medi-
cine, Hokkaido University Hospital. Selection criteria
included a positive diagnosis by the Rome II criteria [6].
All the IBS patients were diarrhea type and all the FAPS
patients had normal bowel habits. No patients had evi-
dence of organic disease by diagnostic studies including
abdominal ultrasonography and upper and lower gas-
trointestinal endoscopy. All the patients with IBS were
taking smooth muscle relaxant and calcium polycar-
bophil, and two were taking anxiolytics and/or tricyclic
antidepressants. On the other hand, half of the patients
with FAPS were given anxiolytics and/or tricyclic antide-
pressants, and two were taking selective serotonin
reuptake inhibitors.
To assess the symptom severity, the subjects were asked
the number of days work or school was missed because of
illness in the past three months and the length of hospital
stay because of illness. Verbal and written informed con-
sent was obtained from each subject. This study was
approved by the Hokkaido University Ethical Committee
on Human Studies.
Psychological status checklist
All subjects completed the hospital anxiety and depres-
sion scale (HADS) questionnaire, which assesses current
psychological status regarding anxiety and depression
[7,8]. HADS scores ≥ 11 were defined as clinically relevant
anxiety or depression, and a cutoff of ≥ 8 was defined as
borderline.
Visceral stimulation device
Distention of the rectum was effected by air inflation of a
balloon catheter. A computer-driven barostat device (Syn-
ectics Visceral Stimulator; Synectics, Stockholm, Sweden)
was used for the evaluation of rectal sensation and com-
pliance. It could be programmed to deliver distention
according to various protocols by air inflation of the bal-
loon in the rectum, to record pressures and volumes
simultaneously (sampling rate 1 per second), and to log
sensations by a push-button marker device onto a data
file. The balloon catheter consisted of a balloon (a thin-
walled polyethylene balloon) attached to a Silastic elas-
tomer tube (external diameter 18F, MAK-LA; Los Angeles,
CA, USA) at both proximal and distal ends. The distance
between the attachment sites was 9 cm, and distention to
a maximal volume of 500 ml resulted in a spherical bal-
loon shape. The open ends of the tube were connected to
the inflation channel and pressure sensor port of the baro-
stat device. Before placement in the rectum, the balloon
was checked for air leaks by maintaining an intra-balloon
pressure of 20 mmHg for 5 min in water.
Threshold
Rectal perceptual thresholds such as discomfort, pain or
maximum tolerance were determined as intra-balloon
pressure (mmHg).
Rectal compliance
The pressure-volume relationship was determined during
rectal balloon distention for each subject by plotting cor-
responding pressures and volumes. As the compliance
curve is S-shaped, the compliance was calculated over the
linear part of the curve by calculating from the slope of the
curve using simple regression.
Intensity
Subjective intensity of sensation in response to rectal bal-
loon distention was determined by VAS ranging from no
sensation (0) to severe (100) arrayed along a 100-mm bar.
The subjects marked the VAS in reference to subjective
intensity immediately after each distention.
Experimental protocol
All medications were discontinued 48 h before the proce-
dure. After a 15-h fast, bowel cleansing was performed by
warm water enema (250 ml). Subjects were placed in the
left lateral decubitus position on the bed, and the balloon,

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which was lubricated with olive oil, was inserted into the
rectum. Then, the subjects lay prone on the bed. The
experimental rectal distention protocol started after a 30-
min resting period. First, subjects were given ramp disten-
tion. The barostat device was programmed to inflate the
balloon at an inflation rate of 40 ml/min. During the
ramp distention, we determined three sensory thresholds;
discomfort, pain and maximum tolerance; and rectal
compliance. When the subject felt the distention was
intolerable and pressed the pushbutton (i.e. threshold of
maximum tolerance was obtained), the balloon was
instantaneously deflated and this first session was fin-
ished. Ten minutes later, to determine the intensity of sen-
sation using VAS, three phasic distentions of 10, 15 and
20 mmHg for 60 sec separated by 30-sec intervals at a rest-
ing pressure of 0 mmHg, were randomly loaded. These
distentions consisted of rapidly inflating the balloon at 14
ml/sec until the target pressure was reached, maintaining
it for 60 sec, and finally rapidly deflating it at 14 ml/sec.
The subjects were asked to mark the VAS in reference to
subjective intensity of sensation immediately after each
distention.
Statistical analysis
All data are expressed as means ± SE. To compare the clin-
ical characteristics and rectal compliance of each group,
an analysis of variance (ANOVA) or Kruskal-Wallis one-
way ANOVA followed by the least significant difference
test (LSD) or the Mann-Whitney rank sum test was per-
formed. Chi-square test was used to compare propor-
tional differences (male and female ratio) among groups.
For analysis of group differences in perceived intensity
(VAS ratings) or thresholds in response to rectal disten-
tion, a three-group (FAPS, IBS and control) × three-loaded
pressure (10, 15 and 20 mmHg) or threshold (discomfort,
pain and maximum tolerance) ANOVA design was fol-
lowed by LSD. For these analyses, the main effect of
grouping indicates an overall difference among groups
(FAPS, IBS and control) across three loaded pressures or
thresholds. The main effect of loaded pressure or thresh-
old indicates an overall difference among loaded pres-
sures or thresholds across the groups. A group × distention
or threshold interaction indicates that the three groups
differed in their response. Statistica (StatSoft Inc. Tulsa,
Okla., USA) was used for all statistical computations. An
α cutoff of P < 0.05 was used throughout the study.
Results
Table 1 summarizes the clinical characteristics of the sub-
jects. The mean age of the subjects (Kruskal-Wallis one-
way ANOVA: χ2 = 3.5, P > 0.05) and the ratio of males to
females (Chi-square test, χ2 = 22.6, P > 0.05) were not sig-
nificantly different among the groups. Stool frequency,the
mean number of bowel movements per day in the most
recent two weeks, was significantly greater in IBS as com-
pared with control or FAPS (Kruskal-Wallis one-way
ANOVA: χ2 = 12.8, P < 0.05, IBS vs. control or FAPS, P <
0.05). The body mass index was not different (ANOVA: F
= 2.5, P > 0.05). The number of days missed from work or
school because of illness in the most recent three months
was significantly greater in FAPS and IBS as compared to
the controls (Kruskal-Wallis one-way ANOVA: χ2 = 20.54,
P < 0.05, IBS or FAPS vs. control, P < 0.05). On the other
hand, the length of hospital stay because of illness was sig-
nificantly different among the groups by Kruskal-Wallis
one-way ANOVA (χ2 = 7.15, P < 0.05), but pairwise com-
parisons by Mann-Whitney rank sum test did not demon-
strate any significant difference between IBS or FAPS and
the controls (P > 0.05). The HADS scores were signifi-
cantly greater in both groups of patients as compared to
the controls (Kruskal-Wallis one-way ANOVA: χ2 = 22.6, P
< 0.05, for anxiety, χ2 = 16.4, P < 0.05, for depression, IBS
or FAPS vs. control, P < 0.05 for both scores). The mean
value of FAPS can be considered definite anxiety.
Figure 1 shows the perceptual thresholds in response to
rectal distention. The threshold increased in the order of
discomfort, pain and maximum tolerance across the
Table 1: Clinical characteristics
Clinical parameters FAPS IBS Control
No. of subjects 6 7 13
Mean age (years) 41.2 ± 4.7 26.4 ± 2.3 32.3 ± 3.4
Sex (M/F) 2/4 2/5 6/7
Stool frequency (/day) 0.92 ± 0.1 2.9 ± 0.6* 1.0 ± 0.1
BMI 21 ± 0.7 21 ± 1.0 20 ± 0.7
Days missed from work or school because of illness over past three months 33.8 ± 14.1* 20.4 ± 3.4* 0 ± 0
Length of hospital stay because of illness (days) 21.8 ± 14.2 7.7 ± 5.4 0 ± 0
HADS
Anxiety 12.5 ± 1.8* 7.4 ± 1.2* 2.2 ± 0.1
Depression 6.5 ± 1.8* 5.9 ± 1.5* 1.9 ± 0.2
FAPS, functional abdominal pain syndrome; IBS, irritable bowel syndrome; BMI, body-mass index; HADS, hospital anxiety and depression scale
*P vs. control <0.05; Kruskal-Wallis one-way analysis of variance followed by the Mann-Whitney rank sum test

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groups (ANOVA: F = 75.8; P < 0.05). On the other hand,
there was a significant main effect of group (ANOVA: F =
6.6; P < 0.05). IBS had significantly lower threshold as
compared to control or FAPS (P < 0.05), and it was not
different between FAPS and control (P > 0.05). Discom-
fort threshold was not different between control and IBS
(12.1 ± 1.0 mmHg for IBS vs. 16.9 ± 2.0 mmHg for con-
trol, P > 0.05), but pain threshold and maximum toler-
ance were reduced in IBS (pain threshold; 17.3 ± 1.4
mmHg for IBS vs. 32.2 ± 4.3 mmHg for control, P < 0.05,
maximum tolerance; 24.7 ± 3.7 mmHg for IBS vs. 42.9 ±
3.3 mmHg for control, P < 0.05). On the other hand,
thresholds were not different between FAPS and control
(discomfort threshold; 20.8 ± 2.0 mmHg, pain threshold;
34.8 ± 3.2 mmHg and maximum tolerance; 43.8 ± 2.2
mmHg for FAPS vs. control, P > 0.05). Significant interac-
tion (group × threshold, F = 3.51, P < 0.05) was also
observed.
The rectal compliance was also significantly reduced in
IBS (Fig. 2, ANOVA: F = 3.73; P < 0.05, 5.7 ± 1.1 ml/
mmHg for IBS vs. 9.8 ± 1.0 ml/mmHg for control, P <
0.05) but it was not different between control and FAPS
(9.7 ± 1.2 ml/mmHg for FAPS vs. control, P > 0.05).
Figure 3 shows VAS ratings of sensory intensity. There was
a significant main effect of loaded pressure (ANOVA: F =
70.25, P < 0.05), i.e. intensity increased as distended pres-
sure increased from 10 to 20 mmHg. Moreover, a signifi-
cant main effect of group was also observed (ANOVA: F =
7.53, P < 0.05). FAPS had significantly lower VAS ratings
as compared to control or IBS (P < 0.05), and IBS had sig-
nificantly higher VAS ratings as compared to control (P <
0.05). The VAS ratings at 10 and 15 mmHg were not sig-
nificantly different between FAPS and control (10 mmHg;
4.5 ± 4.1 for FAPS vs. 12.1 ± 2.6 for control, 15 mmHg;
12.7 ± 6.5 for FAPS vs. 30.9 ± 5.7 for control, P > 0.05),
but the value was significantly reduced in FAPS at 20
mmHg (27.9 ± 8.9 for FAPS vs. 54.0 ± 7.2 for control, P <
0.05). Although ANOVA demonstrated IBS had signifi-
cantly higher VAS ratings across the three loaded pres-
sures, each value tended to be higher, but not significantly
different from control (17.9 ± 4.3 at 10 mmHg, 47.8 ± 5.8
at 15 mmHg and 75.3 ± 7.1 at 20 mmHg for IBS vs. con-
trol, P > 0.05). Moreover, there was a significant interac-
tion (group × loaded pressure, ANOVA: F = 3.53, P <
0.05).
We calculated the sum of VAS ratings of intensity at three
distentions and set the cut off value at 60, high efficiency
(85%) with very good sensitivity (83%) and specificity
(85%), to discriminate FAPS patients from other subjects.
Discussion
No definitive neurophysiological study in FAPS has been
published to date, but neuropathic pain induced by cen-
tral sensitization is thought to be the most probable
pathogenesis [9]. Central sensitization is characterized by
a decrease in threshold, an increase in response duration
and magnitude to stimuli, and an expansion of the mech-
anosensitive receptive field of dorsal horn neurons [10].
In the present study, we assessed perceptual threshold and
intensity of sensation induced by rectal distention in
order to examine whether FAPS patients have altered vis-
ceral sensory function, including central sensitization.
Our study revealed that FAPS had a normal perceptual
threshold, but it was reduced in IBS. Previous reports by
other researchers demonstrated FAPS had normal rectal
sensory threshold [4] and rectal hypersensitivity [3,11].
However, these studies were investigated in children, and
no data of adult FAPS patients is available except for our
previous brief study demonstrating that they had normal
rectal thresholds of pain and maximum tolerance [5]. The
present study reconfirmed this result clearly, although it
should be noted that in the threshold study the discom-
fort threshold was not statistically different but tended to
be higher in FAPS as compared to the controls (Fig. 1).
The percent difference from the control value was + 23%
at the discomfort threshold and it was + 8 and + 2% at the
pain threshold and maximum tolerance, respectively. On
the basis of this finding, we tested the perceived intensity
at the distended pressure around the discomfort threshold
of the control subjects at 10, 15 and 20 mmHg in order to
detect possible differences between FAPS and the controls.
Our pilot study revealed that the threshold for first sensa-
tion in response to rectal ramp distention was 8.9, 12.2
Thresholds of discomfort, pain and maximum tolerance in response to ramp distentionFigure 1
Thresholds of discomfort, pain and maximum toler-
ance in response to ramp distention. Values are shown
as mean ± SE. * P vs. control < 0.05, analysis of variance fol-
lowed by the least significant difference test. FAPS, functional
abdominal pain syndrome; IBS, irritable bowel syndrome.

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and 14.9 mmHg for IBS, controls and FAPS, respectively,
Moreover, some IBS patients did not tolerate the disten-
tion over 20 mmHg. In this context, this range of disten-
tion was adequate to assess the perceived intensity of all
of the subjects tested in the present study.
The most important point of this study is that FAPS
patients demonstrated reduced intensity of sensation. On
the other hand, IBS patients had significantly higher per-
ceived intensity, which is consistent with previous reports
[12,13]. These results suggest that altered visceral sensory
function may be observed not only in IBS but also in
FAPS. However, the altered pattern was opposite, i.e.
hypersensitivity for IBS but hyposensitivity for FAPS.
Moreover, this hyposensitivity was only induced by non-
painful distentions. It is not clear whether this phenome-
non actually occurred associated with the pathogenesis or
resulted from response bias. A high level of anxiety may
influence sensory parameters [14,15], but both IBS and
FAPS patients with high anxiety scores on HADS showed
completely different sensory profiles, suggesting anxiety
did not modify the results in the present study. Moreover,
we could discriminate FAPS patients from other subjects
successfully using VAS ratings, suggesting this result may
not be induced by bias but that it is related to the patho-
genesis to some extent. FAPS overlaps the psychiatric diag-
nosis of somatoform pain disorder (pain disorder
associated with psychological factors) in the DSM-IV,
wherein symptoms are localized to the abdomen [16],
and any of several psychiatric diagnosis such as depres-
sion, anxiety, personality disorders or other somatoform
disorders frequently coexist [9]. In this context, this syn-
drome frequently needs early psychological treatment,
and accurate and prompt diagnosis is very important.
Because discrimination between IBS and FAPS is some-
times difficult, our procedure would seem to be useful for
clinical practice.
The mechanism of this response remains unclear. Rectal
compliance was reduced in IBS, which is consistent with
the previous reports [17,18]. However, it was not different
between FAPS and the controls, suggesting rectal tone
does not seem to contribute. The rectum is innervated by
both pelvic and lumbar splanchnic nerves. The pelvic
nerve afferents are activated at lower stimulation intensi-
ties and they mediate non-noxious physiological sensa-
tions such as the presence of stool or gas. On the other
hand, lumbar splanchnic afferents, with their higher stim-
ulus response threshold, would be better tuned to signal
the onset of higher-intensity mechanical events, such as
muscular contraction or passage of material [19]. These
lines of evidence suggest that hyposensitivity at lower
pressure of distention might result from altered pelvic
nerve function in FAPS. A number of investigators have
reported the presence of rectal hyposensitivity in patients
with constipation, which may result from altered pelvic
nerve activity [20,21]. This alteration induces abolished
perceptual response to physiological stimuli such as rectal
filling by stool, which is thought to lead to constipation.
Although all the patients with IBS tested in the present
study were of the diarrhea type who demonstrated rectal
hypersensitivity, IBS with constipation was also reported
to include hypersensitivity [22]. However, interestingly, at
the same time, hyposensitivity to physiological stimuli
was also detected in constipated IBS [22]. These lines of
evidence suggest that selectively altered pelvic nerve func-
tion is not so unique a phenomenon and it comes as no
surprise that this change occurs in FAPS.
We do not know the meaning of this response in the
pathogenesis; moreover, it is not clear whether this result
is contrary to possible pathogenesis, i.e. central sensitiza-
tion. Peripheral neuropathic conditions resulting from
various types of nerve injury could provide ongoing affer-
ent input to the spinal cord, keeping it in a constant state
of central sensitization [23]. Such nerve injury could
result from abdominal surgeries or injuries to pelvic
nerves during pregnancy or delivery. In fact, some FAPS
patients were reported to undergo several abdominal sur-
gical interventions in order to disclose the origin of
chronic abdominal pain [24]. However, once central sen-
sitization is established, symptoms can persist in the
absence of ongoing abnormal peripheral stimulation. In
the present study, only two patients had borne children
and none of the patients had a history of abdominal sur-
gery. However, other factors such as viral infection etc. are
also well known to be related to nerve injuries; moreover,
an important role of genetic factors in the predisposition
to develop peripheral neuropathic pain is suggested by
Rectal compliance in response to ramp distentionFigure 2
Rectal compliance in response to ramp distention.
Values are shown as mean ± SE. * P vs. control < 0.05, analy-
sis of variance followed by the least significant difference test.

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animal models, indicating that preexisting factors sepa-
rate from the degree of neural injury may influence these
processes [25]. In this context, our result, i.e. suggestive
dysfunction of pelvic nerve may support this pathophysi-
ological hypothesis. In any event, inconsistency of visceral
sensitivity between lower and higher pressure of disten-
tion might be a key feature to understanding the patho-
genesis of FAPS. Paying attention to the fact that FAPS
would qualify as a somatoform pain disorder [16], the
question we have to ask here is whether there are any
pathophysiological relations between FAPS and this dis-
order with pain localized in other areas. Patients with
chronic pain often report abnormal tactile sensitivity in
the affected area, suggesting pain disorder may have
altered sensory function [26]. But this abnormality was
reported to vary from hypoesthenia to allodynia [26].
These lines of evidence suggest that some patients with
pain disorder may have a common pathogenesis as FAPS;
moreover, this disorder may not be a homogenous but a
heterogeneous disease entity. Further study is needed.
Our study has several limitations. It must be noted that
there is no proof that this result would be the same for
more severe cases. Symptom severity may be related to the
sensory profiles assessed in the present study [27]. It is
also important to recognize that the report of pain is not
the sensation of pain and it is seriously influenced by the
decision processes of the individual [28]. Therefore sub-
jects are able to modify the results, even deliberately. As
stated in the Rome criteria, ruling out malingering is
essential for the diagnosis of FAPS [6] but sometimes it is
difficult. Thus, FAPS patients are often suspected to have
factitious pain by physicians, and some patients may have
a strong desire to prove that they have true pain. It was
also reported that patients with FAPS often deny or mini-
mize the role of psychological factors in their abdominal
pain [9]. In this context, they may show a pretend attitude
such as stoical or sensitive through this examination in
order to achieve this desire. These biases are inevitable for
this type of experiment, but it would also occur in IBS or
even in healthy subjects, suggesting that our results may
have some meaning.
Conclusion
FAPS patients had normal rectal perceptual thresholds,
such as discomfort, pain and maximum tolerance and, at
the same time, reduced VAS ratings of perceived intensity
at the lower pressure of rectal balloon distention. FAPS
patients may have hyposensitivity to non-noxious physio-
logical distention with normal sensitivity to painful dis-
tention, which may be a key feature to understanding the
pathogenesis.
Competing interests
The authors declare that they have no competing interests.
Authors' contributions
TN conceptualized and designed the study, collected and
analyzed the data, interpreted the results, and drafted the
manuscript. MK collected the data. All authors read and
approved the final manuscript.
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Intensity of sensation assessed by visual analogue scale (VAS) at three different phasic distentionsFigure 3
Intensity of sensation assessed by visual analogue
scale (VAS) at three different phasic distentions. Val-
ues are shown as mean ± SE. * P vs. control < 0.05, analysis
of variance followed by the least significant difference test.

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