Irritable Bowel Syndrome – An Overview

Irritable bowel syndrome (IBS) is a “disorder of gut-brain interaction” characterized by a group of symptoms that commonly include abdominal pain and or abdominal bloating and changes in the consistency of bowel movements. These symptoms may occur over a long time, sometimes for years. IBS can negatively affect quality of life and may result in missed school or work or reduced productivity at work. Disorders such as anxiety, major depression, and chronic fatigue syndrome are common among people with IBS.

The causes of IBS may well be multi-factorial. Theories include combinations of “gut–brain axis” problems, alterations in gut motility, visceral hypersensitivity, infections including small intestinal bacterial overgrowth, neurotransmitters, genetic factors, and food sensitivity. Onset may be triggered by an intestinal infection (“post-infectious irritable bowel syndrome”) or a stressful life event.

Diagnosis is based on symptoms in the absence of worrisome features and once other potential conditions have been ruled out. Worrisome or “alarm” features include onset at greater than 50 years of age, weight loss, blood in the stool, or a family history of inflammatory bowel disease. Other conditions that may present similarly include celiac disease, microscopic colitis, inflammatory bowel disease, bile acid malabsorption, and colon cancer.

Treatment of IBS is carried out to improve symptoms and can be very effective. This may include dietary changes, medication, probiotics, and counseling. Dietary measures include increasing soluble fiber intake, or a diet low in fermentable oligosaccharides, disaccharides, monosaccharides, and polyols (FODMAPs). The “low FODMAP” diet is meant for short to medium term use and is not intended as a life-long therapy. The medication loperamide may be used to help with diarrhea while laxatives may be used to help with constipation. There is strong clinical-trial evidence for the use of antidepressants, often in lower doses than that used for depression or anxiety, even in patients without comorbid mood disorder. Tricyclic antidepressants such as amitriptyline or nortriptyline and medications from the selective serotonin reuptake inhibitor (SSRI) group may improve overall symptoms and reduce pain. Patient education and a good doctor–patient relationship are an important part of care.

About 10–15% of people in the developed world are believed to be affected by IBS. The prevalence varies according to country (from 1.1% to 45.0%) and criteria used to define IBS; however pooling the results of multiple studies gives an estimate of 11.2%. It is more common in South America and less common in Southeast Asia. In the Western world it is twice as common in women as men and typically occurs before age 45. However, women in East Asia are not more likely than their male counterparts to have IBS, indicating much lower rates among East Asian women. There is likewise evidence that men from South America, South Asia and Africa are just as likely to have IBS as women in those regions, if not more so. The condition appears to become less common with age. IBS does not affect life expectancy or lead to other serious diseases. The first description of the condition was in 1820, while the current term irritable bowel syndrome came into use in 1944.

IBS can be classified as diarrhea-predominant (IBS-D), constipation-predominant (IBS-C), with mixed/alternating stool pattern (IBS-M/IBS-A) or pain-predominant. In some individuals, IBS may have an acute onset and develop after an infectious illness characterized by two or more of: fever, vomiting, diarrhea, or positive stool culture. This post-infective syndrome has consequently been termed “post-infectious IBS” (IBS-PI).

Signs and symptoms
The primary symptoms of IBS are abdominal pain or discomfort in association with frequent diarrhea or constipation and a change in bowel habits. Symptoms usually are experienced as acute attacks that subside within one day, but recurrent attacks are likely. There may also be urgency for bowel movements, a feeling of incomplete evacuation (tenesmus) or bloating. In some cases, the symptoms are relieved by bowel movements. People with IBS, more commonly than others, have gastroesophageal reflux, symptoms relating to the genitourinary system, fibromyalgia, headache, backache, and psychiatric symptoms such as depression and anxiety. About a third of adults who have IBS also report sexual dysfunction, typically in the form of a reduction in libido.

While the causes of IBS are still unknown, it is believed that the entire gut–brain axis is affected. Recent findings suggest that an allergy triggered peripheral immune mechanism may underlie the symptoms associated with abdominal pain in patients with irritable bowel syndrome. IBS is more prevalent in obese patients.

Risk factors
The risk of developing IBS increases six-fold after acute gastrointestinal infection. Post-infection, further risk factors are young age, prolonged fever, anxiety, and depression. Psychological factors, such as depression or anxiety, have not been shown to cause or influence the onset of IBS, but may play a role in the persistence and perceived severity of symptoms. Nevertheless, they may worsen IBS symptoms and quality of life. Antibiotic use also appears to increase the risk of developing IBS. Research has found that genetic defects in innate immunity and epithelial homeostasis increase the risk of developing both post-infectious as well as other forms of IBS.

Publications suggesting the role of the brain–gut axis appeared in the 1990s and childhood physical and psychological abuse is often associated with the development of IBS. It is believed that psychological stress may trigger IBS in predisposed individuals.

Given the high levels of anxiety experienced by people with IBS and the overlap with conditions such as fibromyalgia and chronic fatigue syndrome, a potential explanation for IBS involves a disruption of the stress system. The stress response in the body involves the hypothalamic–pituitary–adrenal axis (HPA) and the sympathetic nervous system, both of which have been shown to operate abnormally in people with IBS. Psychiatric illness or anxiety precedes IBS symptoms in two-thirds of people with IBS, and psychological traits predispose previously healthy people to developing IBS after gastroenteritis.

Approximately 10 percent of IBS cases are triggered by an acute gastroenteritis infection. The CdtB toxin is produced by bacteria causing gastroenteritis and the host may develop an autoimmunity when host antibodies to CdtB cross-react with vinculin. Genetic defects relating to the innate immune system and epithelial barrier as well as high stress and anxiety levels appear to increase the risk of developing post-infectious IBS. Post-infectious IBS usually manifests itself as the diarrhea-predominant subtype. Evidence has demonstrated that the release of high levels of proinflammatory cytokines during acute enteric infection causes increased gut permeability leading to translocation of the commensal bacteria across the epithelial barrier; this in turn can result in significant damage to local tissues, which can develop into chronic gut abnormalities in sensitive individuals. However, increased gut permeability is strongly associated with IBS regardless of whether IBS was initiated by an infection or not. A link between small intestinal bacterial overgrowth and tropical sprue has been proposed to be involved as a cause of post-infectious IBS.

Small intestinal bacterial overgrowth (SIBO) occurs with greater frequency in people who have been diagnosed with IBS compared to healthy controls. SIBO is most common in diarrhea-predominate IBS but also occurs in constipation-predominant IBS more frequently than healthy controls. Symptoms of SIBO include bloating, abdominal pain, diarrhea or constipation among others. IBS may be the result of the immune system interacting abnormally with gut microbiota resulting in an abnormal cytokine signalling profile.

Certain bacteria are found in lower or higher abundance when compared with healthy individuals. Generally Bacteroidota, Bacillota, and Pseudomonadota are increased and Actinomycetota, Bifidobacteria, and Lactobacillus are decreased. Within the human gut, there are common phyla found. The most common is Bacillota. This includes Lactobacillus, which is found to have a decrease in people with IBS, and Streptococcus, which is shown to have an increase in abundance. Within this phylum, species in the class Clostridia are shown to have an increase, specifically Ruminococcus and Dorea. The family Lachnospiraceae presents an increase in IBS-D patients. The second most common phylum is Bacteroidota. In people with IBS, the Bacteroidota phylum has been shown to have an overall decrease, but an increase in the genus Bacteroides. IBS-D shows a decrease for the phylum Actinomycetota and an increase in Pseudomonadota, specifically in the family Enterobacteriaceae.

There is growing evidence that alterations of gut microbiota (dysbiosis) are associated with the intestinal manifestations of IBS, but also with the psychiatric morbidity that coexists in up to 80% of people with IBS. The role of the gut mycobiota, and especially of the abnormal proliferation of the yeast Candida albicans in some people with IBS, was under investigation as of 2005.

Vitamin D
Vitamin D deficiency is more common in individuals affected by irritable bowel syndrome. Vitamin D is involved in regulating triggers for IBS including the gut microbiome, inflammatory processes and immune responses, as well as psychosocial factors.


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  4. Images attributed to:

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