Irritable Bowel Syndrome

Irritable bowel syndrome (IBS) is a common functional bowel disorder, now framed as a disorder of gut-brain interaction, in which recurrent abdominal pain is associated with defecation or with changes in stool frequency or form, in the absence of a structural lesion that explains the symptoms. It is classified by predominant bowel habit into constipation-predominant, diarrhoea-predominant, mixed, and unsubtyped patterns.

Definition

Irritable bowel syndrome is a disorder of gut-brain interaction defined by recurrent abdominal pain, on average at least one day per week in the preceding three months, related to defecation and associated with a change in stool frequency or form, without a structural or biochemical abnormality that accounts for the symptoms (Rome IV criteria).

Scope

The entry covers the definition and Rome IV symptom criteria of IBS, its proposed pathophysiology, epidemiology, and the broad evidence base, treated as a reference topic. It does not provide a diagnostic checklist for individual patients or specific treatment regimens, which are set by current clinical guidelines.

Core questions

• How is IBS defined and subtyped under the Rome IV symptom-based criteria?
• Which mechanisms — altered motility, visceral hypersensitivity, gut-brain signalling, microbiota, immune activation — contribute to symptoms?
• How is IBS distinguished from organic disease without exhaustive investigation?
• What is the basis for the considerable overlap of IBS with other functional and psychological conditions?

Key concepts

• Disorder of gut-brain interaction
• Rome IV diagnostic criteria
• Bowel-habit subtypes (IBS-C, IBS-D, IBS-M, IBS-U)
• Visceral hypersensitivity
• Altered gastrointestinal motility
• Post-infectious IBS
• Gut microbiota and low-grade immune activation
• Brain-gut axis and psychological comorbidity

Mechanisms

IBS is understood as multifactorial. Disturbed gastrointestinal motility and visceral hypersensitivity — an amplified perception of normal gut stimuli — are core features, modulated through bidirectional brain-gut signalling. Additional contributors include altered intestinal permeability, low-grade mucosal immune activation, changes in the gut microbiota, bile-acid and carbohydrate handling, and prior enteric infection (post-infectious IBS). Genetic susceptibility has been implicated, including variants in the sucrase-isomaltase gene associated with IBS risk, illustrating how luminal carbohydrate handling may contribute in some patients. No single mechanism explains all cases, and psychological factors interact with peripheral changes.

Clinical relevance

IBS is a leading reason for primary-care and gastroenterology visits and a frequent consideration when patients present with chronic abdominal pain and altered bowel habit; recognising its symptom-based definition helps frame evaluation and counselling. This entry describes the disorder for reference and education and is not a substitute for individualised assessment or for the diagnostic and management recommendations of current guidelines.

Epidemiology

IBS affects a large share of the general population across many countries, with prevalence estimates varying by the criteria used and generally higher in women and in younger and middle-aged adults. It commonly overlaps with other functional gastrointestinal disorders and with anxiety and depression, and it imposes substantial effects on quality of life and health-care use.

Evidence & guidelines

The Rome IV criteria provide the reference definition and subtyping of IBS, and professional societies publish periodically updated management guidelines; specific recommendations evolve and should be drawn from the current versions rather than from this overview.

History

Symptom complexes resembling IBS were described under various names such as 'spastic' or 'irritable colon' through the twentieth century. The successive Rome consensus processes formalised symptom-based criteria, and Rome IV reframed IBS as a disorder of gut-brain interaction, shifting emphasis from a purely motility-based view toward an integrated model of gut and central nervous system contributions.

Debates

Is IBS best conceived as a peripheral gut disorder or a disorder of gut-brain interaction?

Evidence supports both peripheral mechanisms (motility, hypersensitivity, microbiota, immune activation) and central processing changes; the Rome IV framing as a disorder of gut-brain interaction integrates these, but the relative weight of peripheral versus central factors varies between patients.

Key figures

• Douglas A. Drossman
• Michael Camilleri
• Alexander C. Ford
• Nicholas J. Talley

Seminal works

• lacy-2016
• ford-2017
• enck-2016

Frequently asked questions

Is irritable bowel syndrome the same as inflammatory bowel disease?

No. IBS is a functional disorder of gut-brain interaction without structural damage, whereas inflammatory bowel disease (such as Crohn's disease or ulcerative colitis) involves chronic intestinal inflammation and tissue damage; they are distinct conditions, though symptoms can overlap.

How is IBS diagnosed?

It is a clinical diagnosis based on the Rome IV symptom criteria — recurrent abdominal pain related to defecation and changes in stool form or frequency — together with limited testing to exclude conditions suggested by alarm features; the specifics are defined by current guidelines.