Dr Chamara Basnayake Gastroenterologist and head of functional gastrointestinal disorders at St Vincent ’ s Hospital Melbourne , and senior lecturer at the University of Melbourne , Victoria .

First published online on 24 March 2023

BACKGROUND

IRRITABLE bowel syndrome ( IBS ) is

a chronic functional gastrointestinal condition characterised by abdominal discomfort associated with altered bowel habits . 1 IBS can be sub-categorised by the predominant stool profile , diarrhoea predominant ( IBS-D ) or constipation predominant ( IBS-C ) ( see figures 1 and 2 ).

As a functional gastrointestinal syndrome , IBS is characterised by the absence of abnormalities on anatomical or biochemical testing . There are numerous causes of the symptoms of IBS , including diet , abnormal toileting behaviours , abnormal gut immunology and gut microbiota factors . Psychological comorbidity is highly prevalent among patients with IBS and is implicated in causing a heightened sensory perception of gut viscera and altered gut motility . 2 This intimate causal link between psychological factors and gut symptoms in IBS has led to this condition now being referred to as a ‘ disorder of gutbrain interaction ’ ( see figure 3 ). 3

IBS is highly prevalent in Australia , with a predominance among younger adults and females . 4 IBS incurs significant healthcare costs , and quality

of life is often impaired . 5 This How to Treat covers the causes , epidemiology , diagnosis and treatment of IBS and aims to ensure GPs can confidently manage patients independently .

GPs can make the diagnosis when characteristic symptoms are present in the absence of symptomatic red flags . The diagnosis can be made early , before receiving the results of basic non-invasive testing ; where possible , avoid the strategy of diagnosing IBS after extensive testing .

GPs can be reassured and optimistic about managing IBS as many effective treatments are available .

AETIOLOGY

IBS is a syndrome with multiple causative factors ( see figure 4 ). Several causative factors may be present in a single patient . The causes of the symptomatology can be divided into those that are currently treatable and those for which there is as yet no treatment .

Treatable causes of IBS symptoms

DIET In recent years there has been greater understanding of the role of

diet in the causation of the symptoms of IBS .

The best understood dietary factor is foods that are high in poorly absorbed fermentable carbohydrates . 6 These short-chain carbohydrates , including fructose , fructans , oligosaccharides , disaccharides monosaccharides and polyols , are collectively referred to as fermentable oligosaccharides , disaccharides , monosaccharides and polyols ( FODMAPs ). These poorly absorbed carbohydrates result in the formation of excess liquid and gas , leading to intestinal distention and the development of pain , bloating and diarrhoea .

These symptoms can be addressed by a low-FODMAP diet ( see table 1 and figure 5 ), which is structured to reduce a patient ’ s total FODMAP consumption ( see figure 6 ).

Another important dietary contributor to the symptoms of IBS is fibre . 7 Inadequate consumption of fibre was traditionally thought to explain poor stool form ; however , the exact pathophysiological contribution of fibre is still not completely understood . 7 Gluten is perceived by many patients to contribute to

symptoms of IBS . The exact mechanism by which gluten causes IBS symptoms is not completely understood ; however , the most recent studies support the idea that gluten avoidance helps via a reduction in fructan content ( a type of FODMAP ) through the exclusion of wheat . 8 There are likely to be other foodrelated pathophysiological mechanisms that cause symptoms in IBS . For example , a 2014 study demonstrated electron microscopic changes of the gut mucosa in patients with IBS who consume ‘ trigger foods ’. 9

PSYCHOLOGICAL AND EMOTIONAL STATE A patient ’ s psychological and emotional state can strongly influence gastrointestinal function . Patients will commonly describe anxiety or “ stress ” as worsening their symptoms .

Heightened perception of gut symptoms Psychological factors , such as anxiety , can lead to a heightened perception of symptoms within the gastrointestinal tract , referred to as ‘ visceral hypersensitivity ’. Clinical