The Science of IBS
Given the lack of definitive organic markers for IBS, the absence of a unifying hypothesis regarding its underlying pathophysiology is not surprising. Nevertheless, important advances in research made during the past 50 years have brought us closer than ever to understanding the numerous putative etiologic factors involved in this multifaceted disorder, including environmental factors, genetic links, previous infection, food intolerance, and abnormal serotonergic signaling in the GI tract.
Although a patient's psychological state may influence the way in which he or she presents, copes with illness, and responds to treatment, no evidence supports the theory that psychological disturbances are the cause of IBS.[39,40] The biopsychosocial model proposed by Engel takes into account the interplay between biologic, psychological, and social factors. This model proposes that there is an underlying biologic predisposition for IBS that may be acted on by environmental factors and psychological stressors, which contribute to disease development, the patient's perception of illness, and impact on treatment outcomes.[42,43]
Studies evaluating the role of acute stress have shown that stress can result in release of stress-related hormones that affect colonic sensorimotor function (eg, corticotropin-releasing factor [CRF] and inflammatory mediators [eg, interleukin (IL)-1]), leading to inflammation and altering GI motility and sensation. For example, CRF-1 receptors located in the central nervous system (CNS) and gut can affect colonic motility, epithelial water transport, and gut permeability. Sagami and colleagues determined that the peripheral administration of a nonselective corticotropin-releasing hormone (CRH) receptor antagonist improved GI motility, visceral perception, and negative mood in response to gut stimulation in patients with IBS. These findings suggest that CRH may play an important role in the pathophysiology of IBS.
Studies with twins have shown that IBS is twice as prevalent in monozygotic twins as in dizygotic twins.[47,48,49] Limited research on familial aggregation has found that individuals who have a family member (other than a spouse) with a history of abdominal pain or bowel disorder have more than 2-fold increased odds of having IBS. It is likely that environmental influences may help explain this finding (eg, awareness of the symptom status of family members may make sufferers more open to discussing their symptoms and seeking help for the condition). Preliminary findings also suggest that IBS may be associated with select gene polymorphisms, including those in IL-10, G-protein GNb3, alpha adrenoceptor, and serotonin reuptake transporter (SERT).[47,51,52,53,54] Despite these potential links, however, conclusive evidence for a genetic basis for IBS has not been established.
The presence of postinfectious (PI)-IBS, referring to the development of IBS symptoms -- particularly abdominal pain and diarrhea -- shortly after an enteric infection, is based on research from prospective studies in which IBS symptoms developed in 7% to 32% of patients after they recovered from bacterial gastroenteritis.[52,55,56] Specific risk factors for the development of PI-IBS have been identified, including younger age, female sex, presence of severe infectious gastroenteritis for a prolonged period, use of antibiotics to treat this infection, and presence of concomitant psychological disorders (eg, anxiety).[39,52,55,57] Difficulty in downregulating intestinal inflammation in the colonic mucosa has been suggested as a potential underlying mechanism in this condition. Also suggested as a potential underlying mechanism is the presence of colonic changes shown in patients with PI-IBS compared with controls, including increased gut permeability, increased mucosal enterochromaffin cell production, and increased concentration of mast cells and T lymphocytes in the gut mucosa.[39,52,55,57] Despite considerable evidence linking IBS with an inflammatory etiology (perhaps triggered by enteric infection), in a controlled trial of patients with PI-IBS, anti-inflammatory treatment with prednisolone was not more effective than placebo in improving patient symptoms. The true role of prior infection as a key factor in PI-IBS remains to be established.
The use of probiotics (products containing live or attenuated bacteria that have a positive effect on the host) in alleviating symptoms in patients with PI-IBS is an area of recent focus.[60,61] The potential utility of probiotics in this setting stems from their antibacterial, antiviral, and immune-modulating properties; their ability to modify intestinal flora; and their potential to enhance intestinal mucus secretion or influence stool consistency or volume and gas handling. The number of studies evaluating the efficacy of probiotic preparations in patients with IBS is limited but growing.[60,61,62,63,64,65,66,67,68] Because trials vary in study design, dose, and strain (Lactobacillus and Bifidobacteria alone or in combination; mixture of Lactobacillus, Bifidobacteria, and Streptococcus), direct comparison of results is challenging. Overall, some degree of IBS symptom improvement has been demonstrated in symptoms such as abdominal pain,[65,66] bloating,[63,66] gas, and daily symptom scores.[62,65] O'Mahoney and colleagues have recently demonstrated that results with the Bifidobacterium infantis strain are particularly promising. In a separate analysis, these investigators showed that the baseline characteristics of urgency and hard stool increased the odds ratio of response to this strain, whereas straining and alcohol consumption reduced the likelihood of response.[69,70] The ultimate place in therapy of probiotics in IBS remains to be elucidated.
The presence of a higher than usual population of bacteria in the small intestine (leading to bacterial fermentation of poorly digestible starches and subsequent gas production) has been proposed as a potential etiologic factor in IBS. Pimentel and colleagues have shown that, when measured by the lactose hydrogen breath test (LHBT), small intestinal bacterial overgrowth (SIBO) has been detected in 78% to 84% of patients with IBS.[71,72] However, the accuracy of the LHBT in testing for the presence of SIBO has been questioned. Sensitivity of the LHBT for SIBO has been shown to be as low as 16.7%, and specificity approximately 70%. Additionally, this test may suboptimally assess treatment response. The glucose breath test has been shown to be a more reliable tool, with a 75% sensitivity for SIBO vs 39% with LHBT for the "double-peak" method of SIBO detection. In a recently conducted retrospective study involving review of patient charts for the presence of gastrointestinal-related symptoms (including IBS) in patients who were referred for glucose hydrogen breath tests for SIBO, of 113 patients who met Rome II criteria for IBS, 11% tested positive for SIBO. Thus, results demonstrated that IBS symptoms are often unrelated to the presence of SIBO. Despite the controversy regarding the contribution of SIBO to the underlying pathophysiology of IBS and its symptoms, short-term placebo-controlled clinical studies with select antibiotics, including neomycin and rifaximin, have demonstrated symptom improvement in IBS patients.[61,72,79] Antibiotics may therefore have potential utility in select subgroups of IBS patients in whom SIBO contributes to symptoms. However, the chronic nature of IBS symptoms often leads to the need for long-term treatment. Given the fact that long-term use of antibiotics is generally undesirable, the place of antibiotics in IBS therapy remains to be established.
Food intolerance has been proposed as a potential cause of GI symptoms in some patients with IBS; however, this link is not well established. Although some patients associate onset of IBS symptoms with ingestion of particular foods, identification of a true food intolerance is challenging, and elimination diets are typically time-consuming and difficult to implement. Recent research involving exclusion of foods to which patients had immunoglobulin (Ig) G antibodies, which are associated with a more delayed response after antigen exposure than IgE antibodies, resulted in significantly better symptom improvement than in patients in the nonexclusion group. Further research into the role of food intolerance in IBS is warranted.
Of the putative mechanisms underlying the pathophysiology of IBS, the strongest evidence points to the role of serotonin in the GI tract. The effect of serotonergic mechanisms in the manifestation of IBS symptoms has led to development of a new drug class for the treatment of IBS patients: the GI serotonergic agents.
Normal GI function relies on a properly functioning brain-gut axis, which involves the coordinated interplay of the GI musculature, the CNS, the autonomic nervous system, and the enteric nervous system (ENS). The ENS contains millions of neurons embedded in the wall of the digestive tract and functions, at least in part, independently of the CNS. The size, complexity, and independent function of the ENS has resulted in application of the terms "the second brain" and "the mini-brain." Impaired function or coordination of any of these systems, or the communication between these systems and the GI musculature, can lead to symptoms of dysmotility and altered sensory perception, which are characteristic of IBS and select other GI motility disorders.
The neurotransmitter serotonin (5-hydroxytryptamine [5-HT]) is a predominant signaling molecule in the ENS. Most (90% to 95%) of the body's serotonin is found in the gut, and smaller amounts are found in the brain (about 3%) and in platelets (about 2%). In the GI tract, serotonin facilitates communication between the ENS and its effector systems (muscles, secretory endothelium, endocrine cells, and vasculature of the GI tract), thus playing a key role in normal GI tract functioning. In addition, serotonin plays a role in the communication between the ENS and the CNS.
In the gut, serotonin is synthesized by and stored in the enterochromaffin cells, which are located within the mucosa of the intestinal wall. When material passes through the lumen and the mucosa is stimulated, enterochromaffin cells release serotonin, which then binds to its receptors (primarily 5-HT1P receptors) on intrinsic primary afferent neurons, initiating peristalsis and secretion. Serotonin also binds to 5-HT4 receptors on interneurons, which augments the transmission of signals to motor neurons, resulting in enhanced peristaltic activity. In transgenic mice lacking 5-HT4 receptors, colonic motility is abnormally slow, confirming the role of these receptors in facilitating normal colonic motility. By binding to 5-HT3 receptors on efferent sensory innervations coming from the vagus and the spinal nerves, serotonin mediates signaling between the ENS and the CNS and, thus, modulates pain perception.
To regulate the signaling process, excess serotonin must be removed; this is accomplished by the SERT molecule expressed by intestinal epithelial cells. Human studies have shown that defects in serotonin signaling contribute to the pathophysiology of IBS and, potentially, other GI motility disorders. In a recent study by Coates and colleagues, biopsy specimens from patients with IBS showed significantly lower mucosal serotonin concentrations than those from healthy controls, potentially the result of lower mRNA levels for tryptophan hydroxylase (the rate-limiting enzyme in serotonin synthesis), which were also significantly lower in patients with inflammatory bowel disease. There was no significant difference in the number of enterochromaffin cells or in the capacity of these cells to release serotonin under stimulated conditions. In another study, higher serotonin levels were observed in mucosal biopsy samples from patients with IBS with constipation (IBS-C) than in patients with IBS-D or in healthy volunteers.
Serotonin levels may also be affected by altering the amount or function of SERT. The study by Coates and colleagues showed a significant decrease in the level of SERT mRNA and SERT protein expressed in the intestinal epithelial cells of IBS patients compared with that of healthy volunteers. In another study, SERT expression and binding capacity in platelets were decreased in women with IBS-D compared with expression and binding capacity in healthy controls. Furthermore, Chen and colleagues showed that mice with a SERT gene deletion had altered colonic motility. It is interesting to note that the mice thrived in laboratory housing conditions, indicating that other transporters could compensate for the lack of SERT. Additional studies have focused on SERT polymorphisms. Yeo and colleagues showed an association between patients with IBS-D and the homozygous short polymorphism of the SERT gene promoter. This mutation results in lower levels of SERT gene transcription and reduced amounts of SERT protein available for reuptake of serotonin. In addition, Camilleri and colleagues showed a possible link between the long promoter polymorphism and patient response to therapy.
Thus, a substantially large body of work shows that normal gut physiology is predicated on the interplay between the GI musculature and the ENS, autonomic nervous system, and CNS. One of the central mediators of this complex interplay is the neurotransmitter serotonin. Impairment or imbalance in serotonergic signaling, which can affect GI motility, secretion, and visceral sensitivity, may be affected by defects or deficiencies in serotonin production, specific serotonin receptors, or proteins such as SERT. These changes can manifest in symptoms associated with IBS, including abdominal pain, altered bowel habits (constipation, diarrhea, or alternation between these 2 states), and bloating.
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