Portal de Periódicos da CAPES

Abdominal Bloating

2005; Elsevier BV; Volume: 129; Issue: 3 Linguagem: Inglês

10.1053/j.gastro.2005.06.062

1528-0012

Fernando Azpiroz, Juan‐R. Malagelada,

Intestinal and Peritoneal Adhesions

Abdominal bloating is a common and significant clinical problem that remains to be scientifically addressed. Bloating is one of the most bothersome complaints in patients with various functional gut disorders. However, in the current standard classification, abdominal bloating is merely regarded as a secondary descriptor, which masks its real clinical effect. Four factors are involved in the pathophysiology of bloating: a subjective sensation of abdominal bloating, objective abdominal distention, volume of intra-abdominal contents, and muscular activity of the abdominal wall. The primer to elicit subjective bloating may be any of the other 3 factors, or the sensation may be related to distorted perception. All of these mechanisms may play an independent role or may be interrelated. Gas transit studies have evidenced that patients with bloating have impaired reflex control of gut handling of contents. Segmental pooling, either of gas or of solid/liquid components, may induce a bloating sensation, particularly in patients with altered gut perception. Furthermore, altered viscerosomatic reflexes may contribute to abdominal wall protrusion and objective distention, even without major intra-abdominal volume increment. Bloating probably is a heterogeneous condition produced by a combination of pathophysiological mechanisms that differ among individual patients and that in most cases are subtle and undetectable by conventional methods. Further advances in the pathophysiology and clinical forms of bloating are warranted to develop mechanistic strategies rather than the current empiric treatment strategies for comprehensive and effective management of this problem. Abdominal bloating is a common and significant clinical problem that remains to be scientifically addressed. Bloating is one of the most bothersome complaints in patients with various functional gut disorders. However, in the current standard classification, abdominal bloating is merely regarded as a secondary descriptor, which masks its real clinical effect. Four factors are involved in the pathophysiology of bloating: a subjective sensation of abdominal bloating, objective abdominal distention, volume of intra-abdominal contents, and muscular activity of the abdominal wall. The primer to elicit subjective bloating may be any of the other 3 factors, or the sensation may be related to distorted perception. All of these mechanisms may play an independent role or may be interrelated. Gas transit studies have evidenced that patients with bloating have impaired reflex control of gut handling of contents. Segmental pooling, either of gas or of solid/liquid components, may induce a bloating sensation, particularly in patients with altered gut perception. Furthermore, altered viscerosomatic reflexes may contribute to abdominal wall protrusion and objective distention, even without major intra-abdominal volume increment. Bloating probably is a heterogeneous condition produced by a combination of pathophysiological mechanisms that differ among individual patients and that in most cases are subtle and undetectable by conventional methods. Further advances in the pathophysiology and clinical forms of bloating are warranted to develop mechanistic strategies rather than the current empiric treatment strategies for comprehensive and effective management of this problem. Bloating, like some other descriptors for abdominal sensations, is an ambiguous term that alludes both to the subjective sensation and to the objective abdominal distention. The ambiguity of the English term is shared by other languages. Furthermore, bloating means different things to different patients (and to their doctors). Some use the term bloating to refer to the sensation of a swollen/distended abdomen, and others use it to refer to the sensation of a full belly, the feeling of abdominal pressure or wall tension, or the sensation of excess gas. Yet others use it for various apparently unrelated sensations, such as needing to burp; nausea; crampy, gurgling, or rumbling stomach; or needing to go to the bathroom. Many patients with bloating, approximately 24%, report no visible abdominal distention.1Chang L. Lee O.Y. Naliboff B. Schmulson M. Mayer E.A. Sensation of bloating and visible abdominal distention in patients with irritable bowel syndrome.Am J Gastroenterol. 2001; 96: 3341-3347Google Scholar As clinicians, when inquiring about bloating, do we really know what we are specifically asking the patient? In this review, we primarily focus on bloating, meaning the subjective sensation of abdominal distention, and, among other aspects, we will also address whether the subjective sensation of the patient corresponds with objective distention of the abdomen. In the current gold standard classification of functional gut disorders, abdominal bloating and distention are merely regarded as secondary descriptors.2Talley N. Stanghellini V. Heading R.C. Koch K.L. Malagelada J.-R. Tytgat G.N. Functional gastroduodenal disorders.in: Drossman D.A. Corazziari E. Talley N. Thompson W.G. Whitehead W.E. The functional gastrointestinal disorders. Rome II. 2nd ed. Degnon Associates, McLean, VA2000: 299-350Google Scholar, 3Thompson W.G. Longstreth G. Drossman D.A. Heaton K. Irvine E.J. Muller-Lissner S.C. Functional bowel disorders in functional abdominal pain.in: Drossman D.A. Corazziari E. Talley N.J. Thompson W.G. Whitehead W.E. The functional gastrointestinal disorders. 2nd ed. Degnon Associates, McLean, VA2000: 351-432Google Scholar Thus, patients with bloating as their predominant complaint are dispersed among different categories, such as functional dyspepsia and irritable bowel syndrome (IBS), and lumped together with nonbloating patients.2Talley N. Stanghellini V. Heading R.C. Koch K.L. Malagelada J.-R. Tytgat G.N. Functional gastroduodenal disorders.in: Drossman D.A. Corazziari E. Talley N. Thompson W.G. Whitehead W.E. The functional gastrointestinal disorders. Rome II. 2nd ed. Degnon Associates, McLean, VA2000: 299-350Google Scholar, 3Thompson W.G. Longstreth G. Drossman D.A. Heaton K. Irvine E.J. Muller-Lissner S.C. Functional bowel disorders in functional abdominal pain.in: Drossman D.A. Corazziari E. Talley N.J. Thompson W.G. Whitehead W.E. The functional gastrointestinal disorders. 2nd ed. Degnon Associates, McLean, VA2000: 351-432Google Scholar Only patients who do not meet the criteria that define these categories, ie, by exclusion, are categorized as having functional bloating.3Thompson W.G. Longstreth G. Drossman D.A. Heaton K. Irvine E.J. Muller-Lissner S.C. Functional bowel disorders in functional abdominal pain.in: Drossman D.A. Corazziari E. Talley N.J. Thompson W.G. Whitehead W.E. The functional gastrointestinal disorders. 2nd ed. Degnon Associates, McLean, VA2000: 351-432Google Scholar In the past few years, various clinical studies have raised the significance of abdominal bloating as an important, troublesome, and poorly understood clinical problem. It is also highly prevalent. The sensation of abdominal bloating may affect 10%–30% of the population in community-based studies.3Thompson W.G. Longstreth G. Drossman D.A. Heaton K. Irvine E.J. Muller-Lissner S.C. Functional bowel disorders in functional abdominal pain.in: Drossman D.A. Corazziari E. Talley N.J. Thompson W.G. Whitehead W.E. The functional gastrointestinal disorders. 2nd ed. Degnon Associates, McLean, VA2000: 351-432Google Scholar In a survey of US householders, 15.9% of the adult population reported abdominal bloating or distention within the month before the interview, and in more than 75% of them, the symptom was moderate or severe.4Sandler R.S. Stewart W.F. Liberman J.N. Ricci J.A. Zorich N.L. Abdominal pain, bloating, and diarrhea in the United States prevalence and impact.Dig Dis Sci. 2000; 45: 1166-1171Google Scholar Bloating is one of the most common and bothersome complaints in a large proportion of patients with various functional gut disorders, such as functional dyspepsia5Knill-Jones R.P. A formal approach to symptoms of dyspepsia.Clin Gastroenterol. 1985; 14: 517-529Google Scholar, 6Talley N.J. Phillips S.F. Melton L.J. et al.A patient questionnaire to identify bowel disease.Ann Intern Med. 1989; 111: 671-674Google Scholar and IBS,7Lembo T. Naliboff B. Munakata J. Fullerton S. Saba L. Tung S. Schmulson M. Mayer E.A. Symptoms and visceral perception in patients with pain-predominant irritable bowel syndrome.Am J Gastroenterol. 1999; 94: 1320-1326Google Scholar, 8Sach J. Bolus R. Fitzgerald L. Naliboff B. Chang L. Mayer E. Is there a difference between abdominal pain and discomfort in moderate to severe IBS patients?.Am J Gastroenterol. 2002; 12: 3131-3138Google Scholar, 9Wiklund I.K. Fullerton S. Hawkey C.J. Jones R.H. Longstreth G.F. Mayer E.A. Peacock R.A. Wilson I.K. Naesdal J. An irritable bowel syndrome-specific symptom questionnaire development and validation.Scand J Gastroenterol. 2003; 38: 947-954Google Scholar and it is frequently associated with constipation10Marcus S.N. Heaton K.W. Irritable bowel-type symptoms in spontaneous and induced constipation.Gut. 1987; 28: 156-159Google Scholar, 11Mertz H. Naliboff B. Mayer E.A. Symptoms and physiology in severe chronic constipation.Am J Gastroenterol. 1999; 94: 131-138Google Scholar and diarrhea.1Chang L. Lee O.Y. Naliboff B. Schmulson M. Mayer E.A. Sensation of bloating and visible abdominal distention in patients with irritable bowel syndrome.Am J Gastroenterol. 2001; 96: 3341-3347Google Scholar The importance of bloating is placed in perspective by considering the enormous economic burden imposed by this type of functional gut disorder.12Longstreth G.F. Wilson A. Knight K. Wong J. Chiou C.F. Barghout V. et al.Irritable bowel syndrome, health care use, and costs a US managed care perspective.Am J Gastroenterol. 2003; 98: 600-607Google Scholar Despite its clinical, social, and economic importance, bloating remains substantially ignored, without a proper clinical classification, a known pathophysiology, or an effective treatment.13Azpiroz F. Serra J. Treatment of excessive intestinal gas.Curr Treat Opin Gastroenterol. 2004; 7: 299-305Google Scholar It is not even clear to what extent individual patients' complaints of bloating correlate with objective evidence of abdominal distention, and the uncertainty regarding the subjective or the objective origin of the complaints further adds to the confusion. In summary, bloating is a common and highly significant clinical problem that remains to be scientifically addressed. Our aim was to clarify the concept, clinical importance, and pathophysiology of abdominal bloating and, thereby, to pave the way for the comprehensive management of this problem. Specifically, we performed a critical analysis of the following topics: the pathophysiological mechanisms involved in bloating, the various forms of presentation of bloating in clinical practice, and the current treatment options for this heterogeneous condition. To this aim, a literature review was performed based on a PUBMED search over January 1989 to September 2004 on the following terms: "abdominal bloating," "IBS and intestinal gas," and "IBS and flatulence." A total of 473 articles were identified. These articles, as well as chapters on intestinal gas in standard textbooks,14Suarez F.L. Levitt M.D. Intestinal gas.in: Feldman M. Friedman L.S. Sleisenger M.H. Gastrointestinal and liver diseases pathophysiology/diagnosis/management. Saunders, Philadelphia2002: 155-163Google Scholar, 15Calloway D.H. Gas in the alimentary canal.in: Code C.F. Handbook of physiology. Section 6: alimentary canal. Volume 5. Bile; digestion; ruminal physiology. American Physiological Society, Washington, DC1968: 2839-2859Google Scholar, 16Levitt M.D. Bond J.H. Levitt D.G. Gastrointestinal gas.in: Johnson L.R. Physiology of the gastrointestinal tract. Volume 2. Raven, New York1981: 497-502Google Scholar served as literature sources for articles published before 1989. However, only articles relevant to the areas of controversy have been quoted. The experimental evidence available on this subject is scarce; hence, this review is necessarily, to some extent, based on theoretical analysis and the authors' interpretation of existing data. References indicate the original source of information, but the referenced article may not necessarily support the concepts expressed. Four factors are included in the pathophysiology of bloating: subjective sensation, objective girth changes, volume of intra-abdominal contents, and muscular activity of the abdominal walls (Figure 1). The primer to elicit subjective bloating may be any of the other 3 factors, or the sensation may be related to distorted perception. These mechanisms, ie, abnormal perception, objective distention, intra-abdominal volume increment, and abdominal wall dystony, may play an independent role or may be interrelated. Indeed, objective abdominal distention, whether accompanied or not by a subjective sensation of bloating, may be due to changes in abdominal wall activity produced either by a real volume increment of abdominal contents or just by intra-abdominal content redistribution. Intra-abdominal content may increase at the expense of either intraluminal volume (ie, gas or liquid/solid gut content) or extraluminal volume (ie, tissue water increment due to edema or vascular congestion). Changes in abdominal wall activity, which are potentially related to viscerosomatic reflexes, may increase the tension of abdominal muscles, which may be subjectively interpreted as a bloating sensation but may also produce objective abdominal distention due to redistribution of intra-abdominal contents even in the absence of net changes in intra-abdominal volume. The experimental evidence, in terms of the amount of data, supporting each one of these possible mechanisms is frankly uneven and will be analyzed below. Most of the information available relates to the role of intestinal gas in bloating, because this has been the primary focus of experimental studies on this topic. Bloating, like many other abdominal symptoms, is probably a heterogeneous condition produced by a combination of pathophysiological mechanisms that differ among individual patients. The pathophysiology of bloating will be reviewed below by analyzing the relation of bloating to objective distention, abdominal wall dystony, abnormal perception, and intra-abdominal contents. The latter issue is by far the best documented, including aspects on the role of intestinal gas, impaired handling of gut contents, the responsible area of the gut, and the intra-abdominal component that gives rise to bloating. This is a deceptively simple and key question, yet it is not easy to answer. Most patients indicate that their abdomen is or becomes episodically distended, and their claim is often corroborated by a proxy. However, the examining doctor may be mystified or uncertain and, in any case, unable to either prove or disprove the assertion. There are indeed patients whose abdomens appear to be truly distended. However, to show even the seemingly obvious may be difficult, because there are no practical office or bedside devices to reliably measure abdominal volume. Tape measures of abdominal girth have been shown to reproducibly detect even small changes in abdominal circumference induced experimentally by intestinal gas infusion.17Serra J. Azpiroz F. Malagelada J.R. Intestinal gas dynamics and tolerance in humans.Gastroenterology. 1998; 115: 542-550Google Scholar, 18Caldarella M.P. Serra J. Azpiroz F. Malagelada J.R. Prokinetic effects of neostigmine in patients with intestinal gas retention.Gastroenterology. 2002; 122: 1748-1755Abstract Full Text Full Text PDF Scopus (130) Google Scholar, 19Serra J. Salvioli B. Azpiroz F. Malagelada J.R. Lipid-induced intestinal gas retention in the irritable bowel syndrome.Gastroenterology. 2002; 123: 700-706Abstract Full Text Full Text PDF Scopus (158) Google Scholar However, tape measurement requires an immobilized patient fitted with a belt-type measuring device that remains in place during the entire experiment, and each individual serves as his or her own control. Measurements performed in the context of clinical research studies have not yielded uniform results (Table 1). A multicenter study evaluating patients with functional gut disorders (IBS and functional dyspepsia) failed to detect differences in abdominal circumference between those who reported visible abdominal distention and those who did not.20Poynard T. Hernandez M. Xu P. Couturier D. Frexinos J. Bommelaer G. Benand-Agostini H. Chaput J.C. Rheims N. Visible abdominal distention and gas surface description of an automatic method of evaluation and application to patients with irritable bowel syndrome and dyspepsia.Eur J Gastroenterol Hepatol. 1992; 4: 831-836Google Scholar Using a tape measure, Maxton et al21Maxton D.G. Martin D.F. Whorwell P. Godfrey M. Abdominal distension in female patients with irritable bowel syndrome exploration of possible mechanisms.Gut. 1991; 32: 662-664Google Scholar showed that in women with IBS, girth significantly increased during the day. Furthermore, the anteroposterior diameter of the abdomen measured by computed tomography was also shown to increase during the day.21Maxton D.G. Martin D.F. Whorwell P. Godfrey M. Abdominal distension in female patients with irritable bowel syndrome exploration of possible mechanisms.Gut. 1991; 32: 662-664Google Scholar Another study reported that in patients with bloating, girth, but not weight, increased during episodes of visible abdominal distention. However, in this study, measurements were performed by the patients themselves, and the variability of the changes reported (5.3 ± 3.0 cm in girth and 1.0 ± 0.9 kg in weight; mean ± SD) was quite large.22Sullivan S.N. A prospective study of unexplained visible abdominal bloating.N Z Med J. 1994; 1: 428-430Google ScholarTable 1Do Patients With a Bloating Sensation Have Objective Abdominal Distention?StudyMethodResultPoynard20Poynard T. Hernandez M. Xu P. Couturier D. Frexinos J. Bommelaer G. Benand-Agostini H. Chaput J.C. Rheims N. Visible abdominal distention and gas surface description of an automatic method of evaluation and application to patients with irritable bowel syndrome and dyspepsia.Eur J Gastroenterol Hepatol. 1992; 4: 831-836Google ScholarTape measureNoMaxton21Maxton D.G. Martin D.F. Whorwell P. Godfrey M. Abdominal distension in female patients with irritable bowel syndrome exploration of possible mechanisms.Gut. 1991; 32: 662-664Google ScholarTape measureYesMaxton21Maxton D.G. Martin D.F. Whorwell P. Godfrey M. Abdominal distension in female patients with irritable bowel syndrome exploration of possible mechanisms.Gut. 1991; 32: 662-664Google ScholarComputed tomographic scanYesSullivan22Sullivan S.N. A prospective study of unexplained visible abdominal bloating.N Z Med J. 1994; 1: 428-430Google ScholarTape measureYesLea25Lea R. Whorwell P. Reilly B. Houghton L. Abdominal distension in irritable bowel syndrome (IBS) diurnal variation and its relationship to abdominal bloating.Gut. 2003; 52 (abstr): A32Google ScholarPlethysmographyYesLea26Lea R. Houghton L.A. Whorwell P.J. Reilly B. Relationship of abdominal bloating to physical distension in irritable bowel syndrome (IBS) effect of bowel habit.Neurogastroenterol Motil. 2003; 15 (abstr): 587Google ScholarPlethysmographyVariableaConstipation-predominant yes; diarrhea-predominate no.a Constipation-predominant yes; diarrhea-predominate no. Open table in a new tab More recently, automated methods have been developed to measure girth changes.23Marino B. Ogliari C. Basilisco G. Effect of rectal distension on abdominal girth.Neurogastroenterol Motil. 2004; 16: 497-502Google Scholar, 24Lewis M. Reilly B. Houghton L. Whorwell P. Ambulatory abdominal inductance plethysmography towards objective assessment of abdominal distension in irritable bowel syndrome.Gut. 2001; 48: 216-220Google Scholar Preliminary results with an ambulatory technique using inductance plethysmography indicate that clinical variations in girth are significantly greater in IBS patients complaining of bloating than in healthy subjects.25Lea R. Whorwell P. Reilly B. Houghton L. Abdominal distension in irritable bowel syndrome (IBS) diurnal variation and its relationship to abdominal bloating.Gut. 2003; 52 (abstr): A32Google Scholar However, the relationship of distention to subjective bloating is variable, showing a good correlation in constipation-predominant IBS patients, but not in diarrhea-predominant IBS.26Lea R. Houghton L.A. Whorwell P.J. Reilly B. Relationship of abdominal bloating to physical distension in irritable bowel syndrome (IBS) effect of bowel habit.Neurogastroenterol Motil. 2003; 15 (abstr): 587Google Scholar Hence, it seems that subjective claims of distention represent true perceptions of a real event in a significant proportion of patients, but this may not be the case in many others. For one thing, some patients complain of bloating but readily acknowledge no physical evidence of abdominal distention. Others point toward a "distended" abdomen that the examining physician appreciates as normal. Finally, other patients, usually with persistent bloating, show a prominent, fatty abdomen associated with IBS-like symptoms that they interpret as secondary to gut distention. A prospective study reported that patients with bloating were more likely to have experienced recent weight gain than healthy controls, despite similar age, sex, and body mass index between groups.22Sullivan S.N. A prospective study of unexplained visible abdominal bloating.N Z Med J. 1994; 1: 428-430Google Scholar Thus, fat accumulation in the abdomen may favor the development or awareness of bloating as a symptom. The shape of the abdomen is determined by the disposition of the walls of the abdominal cavity, specifically, the vertebral column, which determines the configuration of the posterior abdominal wall, the diaphragm, and the anterolateral musculature. The influence of the pelvic floor, with its limited mobility, is probably insignificant. Even without increments in intra-abdominal volume, a change in the relative position of the walls may produce visible, objective distention. Furthermore, signals arising from the abdominal wall—for instance, because of a muscular dystony—may induce a subjective sensation of abdominal bloating, even in the absence of true abdominal distention, and this could explain some cases of apparently imaginary bloating. Is a classic article, Alvarez27Alvarez W. Hysterical type of nongaseous abdominal bloating.Arch Intern Med. 1949; 84: 217-245Google Scholar described in great detail a series of patients in whom pronounced abdominal distention was, in his view, related to the muscular activity of the abdominal wall. This hypothesis is substantiated by the fact that in some patients, visible abdominal distention has a very rapid onset27Alvarez W. Hysterical type of nongaseous abdominal bloating.Arch Intern Med. 1949; 84: 217-245Google Scholar, 28Maxton D.G. Whorwell P.J. Abdominal distension in irritable bowel syndrome the patient's perception.Eur J Gastroenterol Hepatol. 1992; 4: 241-243Google Scholar and resolves instantaneously by gentle abdominal palpation while asking the patient to relax, by anesthesia,27Alvarez W. Hysterical type of nongaseous abdominal bloating.Arch Intern Med. 1949; 84: 217-245Google Scholar or by hypnotic induction (Whorwell, personal communication, November 1995). Furthermore, distention may affect only part of the abdomen, and rapid resolution is not associated with gas evacuation.27Alvarez W. Hysterical type of nongaseous abdominal bloating.Arch Intern Med. 1949; 84: 217-245Google Scholar However, experimental evidence of these abdominal wall hypotheses has been difficult to obtain. Computed tomography has failed to identify differences in lumbar lordosis and diaphragmatic position in patients with bloating.21Maxton D.G. Martin D.F. Whorwell P. Godfrey M. Abdominal distension in female patients with irritable bowel syndrome exploration of possible mechanisms.Gut. 1991; 32: 662-664Google Scholar It has also been reported that patients with bloating have weak abdominal muscles as compared with healthy controls.22Sullivan S.N. A prospective study of unexplained visible abdominal bloating.N Z Med J. 1994; 1: 428-430Google Scholar In contrast, McManis et al29McManis P.G. Newall D. Talley N.J. Abdominal wall muscle activity in irritable bowel syndrome with bloating.Am J Gastroenterol. 2001; 96: 1139-1142Google Scholar studied a group of patients with IBS and abdominal distention by means of surface electromyography and found that patients and healthy subjects alike increased electromyographic activity in the lower abdomen while standing compared with lying supine, but there were no differences between groups. Using a more elaborate technique and simultaneously recording the muscular activity at 8 different sites, we recently reproduced these results.30Tremolaterra F. Serra J. Azpiroz F. Villoria A. Bloating and abdominal wall dystony.Gastroenterology. 2004; 126 (abstr): A53Google Scholar However, we were also able to show a dystonic response of the abdominal wall to intra-abdominal volume increments in patients with bloating. Intestinal gas retention modeled by rectal gas infusion during anal blockade increased the muscular tone of the abdominal muscles in healthy subjects in the upright position. Similar volumes of gas retention produced significantly greater objective abdominal distention and subjective symptoms in patients with bloating than in healthy subjects. Exaggerated abdominal distention in patients was associated with failed contraction of the abdominal musculature and even paradoxical relaxation of the internal oblique.30Tremolaterra F. Serra J. Azpiroz F. Villoria A. Bloating and abdominal wall dystony.Gastroenterology. 2004; 126 (abstr): A53Google Scholar Animal studies have shown the relevance of viscerosomatic reflexes. For instance, chemical irritation of the colon in rats induces abdominal wall contractions that are inhibited by colonic distention.31Martinez V. Thakur S. Mogil J. Differential effects of chemical and mechanical colonic irritation on behavioral pain response to intraperitoneal acetic acid in mice.Pain. 1999; 81: 179-186Google Scholar Abnormal viscerosomatic reflex activity may also participate in the mechanism of abdominal distention and muscular wall dystony in patients with bloating. Abnormal perception related to cognitive interpretation, abdominal wall sensations, or visceral sensitivity is probably a key contributing factor to the sensation of abdominal bloating. Some patients with a normal or simply fatty abdomen, but with a distorted interpretation of reality, may believe, sometimes to the point of the obsession, that their abdomen is distended. Suarez et al32Suarez F.L. Dennis A. Savalano D. Levitt M. A comparison of symptoms after the consumption of milk or lactose-hydrolyzed milk by people with self-reported severe lactose intolerance.N Engl J Med. 1995; 333: 1-4Google Scholar elegantly showed that people who regarded themselves as severely lactose intolerant and complained of bloating after consumption of even small amounts of dairy products, when specifically tested in a double-blind fashion, recorded negligible symptoms when consuming 250 mL of milk, whether it was lactose hydrolyzed or not. Hence, despite their conviction, the experimental evidence proved that customary milk-related symptoms in these patients had an imaginary origin. However, in other cases, altered perception seems to be due to genuine hypersensitivity with impaired modulation of sensory signals.33Azpiroz F. Gastrointestinal perception pathophysiological implications.Neurogastroenterol Motil. 2002; 14: 1-11Google Scholar Hence, the bloating sensation could plausibly arise from a hypersensitive abdominal wall (for instance, in case of abdominal wall trauma, injuries, or scars) and may give the patient the sensation of increased abdominal wall tension, which is interpreted as bloating. This mechanism may contribute to postoperative bloating after laparotomy. Alternatively, the sensation may arise from abdominal viscera. Indeed, visceral hyperalgesia has been well characterized in patients with functional gut disorders, such as functional dyspepsia and IBS.33Azpiroz F. Gastrointestinal perception pathophysiological implications.Neurogastroenterol Motil. 2002; 14: 1-11Google Scholar In these patients, physiological stimuli in the gut, normally unperceived, may induce abdominal symptoms—specifically, bloating. Furthermore, probing stimuli in the laboratory, such as gut distention, tend to reproduce the customary symptoms, depending on the area on the gut stimulated, and in some patients they induce the sensation of bloating.33Azpiroz F. Gastrointestinal perception pathophysiological implications.Neurogastroenterol Motil. 2002; 14: 1-11Google Scholar The area of the gut affected by the sensory dysfunction depends on the clinical syndrome.34Bouin M. Lupien F. Riberdy M. Boibin M. Plourde V. Poitras P. Intolerance to visceral distension in functional dyspepsia or irritable bowel syndrome an organ specific defect or a pan intestinal dysregulation?.Neurogastroenterol Motil. 2004; 16: 311-314Google Scholar It has been shown that patients with IBS have increased sensitivity in the large and the small bowel.3Thompson W.G. Longstreth G. Drossman D.A. Heaton K. Irvine E.J. Muller-Lissner S.C. Functional bowel disorders in functional abdominal pain.in: Drossman D.A. Corazziari E. Talley N.J. Thompson W.G. Whitehead W.E. The functional gastrointestinal disorders. 2nd ed. Degnon Associates,