Irritable Bowel Syndrome — Symptoms, Causes & Treatment | MyMedicPlus

Irritable bowel syndrome (IBS) is a common functional gastrointestinal disorder characterized by recurrent abdominal pain associated with a change in bowel habit (diarrhea, constipation, or both) in the absence of structural or biochemical abnormality. Diagnosed using the Rome IV criteria (abdominal pain at least 1 day per week for the past 3 months, associated with altered stool frequency and/or form), IBS affects 10–15% of adults globally. Despite no organic pathology, IBS significantly impairs quality of life and is the most common reason for gastroenterology referral. The Rome IV diagnostic criteria require abdominal pain onset at least 6 months before diagnosis, symptoms present for at least 3 months, with abdominal pain occurring on average at least 1 day per week in the last 3 months, related to 2 of: change in stool frequency, change in stool form, or association of pain with defecation. IBS is classified into subtypes based on predominant stool consistency using the Bristol Stool Form Scale: IBS-C (constipation predominant, BSS 1–2), IBS-D (diarrhea predominant, BSS 6–7), IBS-M (mixed, both patterns), and IBS-U (unclassified). Quality of life impairment in severe IBS is comparable to moderate heart failure and significantly impacts work productivity, social function, and mental health.

IBS is a disorder of gut-brain interaction with multiple contributing mechanisms: visceral hypersensitivity (altered pain processing in the gut), dysmotility (abnormal intestinal movement patterns), intestinal microbiome dysbiosis, increased intestinal permeability ('leaky gut'), altered mucosal immune activation, and dysregulated central nervous system processing of gut signals. Post-infectious IBS develops in 10–25% of individuals after acute gastroenteritis (Campylobacter, Salmonella, Giardia). Risk factors include female sex, young age, anxiety and depression (bidirectional relationship), prior adverse life events and psychological trauma, antibiotic use, celiac disease, and small intestinal bacterial overgrowth (SIBO). Serotonin (5-HT) plays a critical role in gut-brain signalling — 90–95% of the body's serotonin is produced by enterochromaffin cells in the gut mucosa; altered serotonin transporter (SERT) function causes abnormal gastrointestinal motility and visceral hypersensitivity. Bile acid malabsorption (affecting 10–25% of IBS-D patients) causes watery diarrhoea from excess bile acids reaching the colon — diagnosed by SeHCAT nuclear scan or serum 7-alpha-hydroxy-4-cholesten-3-one (C4) measurement; treated with bile acid sequestrants (cholestyramine, colestipol). Lactose intolerance, fructose malabsorption, and FODMAP intolerance overlap significantly with IBS symptoms and should be assessed before diagnosing pure IBS.

Core symptoms: recurrent abdominal pain or discomfort (typically lower abdominal or diffuse), relieved or altered by defecation, associated with change in bowel frequency (IBS-D: diarrhea predominant; IBS-C: constipation predominant; IBS-M: mixed; IBS-U: unclassified). Additional common symptoms: bloating and abdominal distension (worse through the day, relieved by passage of gas), urgency (need to rush to the toilet), sensation of incomplete evacuation, mucus in stools (without blood), and tenesmus. Alarm features excluding IBS and requiring investigation: rectal bleeding, weight loss, nocturnal symptoms waking patient, family history of bowel cancer, age above 50 with new symptoms. Bloating and abdominal distension are the most bothersome symptoms for many IBS patients — often worse as the day progresses and after meals. The distinction between functional bloating (subjective sensation) and visible abdominal distension (objectively measurable increase in abdominal girth by greater than 3 cm) is important as mechanisms differ. Pelvic floor dyssynergia — inappropriate contraction of the puborectalis and external anal sphincter during defecation — contributes to constipation and straining in IBS-C and is amenable to biofeedback retraining therapy.

IBS is primarily a clinical diagnosis based on Rome IV criteria and exclusion of alarm features. Investigations to exclude organic pathology: full blood count (anaemia), CRP (inflammation — elevated in IBD, not IBS), coeliac serology (anti-TTG antibodies — 5% of IBS have coeliac disease), thyroid function (hypothyroidism causes constipation, hyperthyroidism causes diarrhea), fecal calprotectin (elevated in IBD, normal in IBS), and stool cultures for post-infectious IBS. Colonoscopy is indicated for alarm features, age above 50, or suspected inflammatory bowel disease. Abdominal ultrasound excludes gallbladder disease. Faecal immunochemical test (FIT) for haemoglobin detects occult blood with high sensitivity and specificity — a positive FIT in any age group with bowel symptoms is a red flag requiring urgent colonoscopy regardless of IBS-like features. Breath testing for hydrogen and methane (after lactulose or glucose challenge) detects small intestinal bacterial overgrowth (SIBO) and lactose/fructose malabsorption — SIBO may explain some IBS-D cases and responds to rifaximin treatment.

The low-FODMAP diet (restricting fermentable oligosaccharides, disaccharides, monosaccharides, and polyols) significantly reduces symptoms in 50–75% of IBS patients — most effective when guided by a registered dietitian trained in the protocol. Antispasmodics (mebeverine, hyoscine butylbromide, peppermint oil capsules) relieve acute abdominal cramps. For IBS-C: osmotic laxatives (polyethylene glycol, lactulose), prucalopride (5-HT4 agonist), or linaclotide. For IBS-D: loperamide (for urgency/frequency), cholestyramine (for bile acid malabsorption), rifaximin (non-absorbable antibiotic — addresses SIBO). Low-dose amitriptyline (10–30 mg at night) or SSRIs provide global symptom relief through central pain modulation. Cognitive behavioral therapy (CBT) and gut-directed hypnotherapy are effective for overall IBS. Linaclotide (Constella) — a guanylate cyclase-C agonist — reduces visceral hypersensitivity through increased intestinal cyclic GMP and accelerates colonic transit; NICE-approved for moderate-to-severe IBS-C. Plecanatide (a second GC-C agonist) has similar efficacy. Amitriptyline 5–10 mg at night (NICE-recommended first-line centrally acting treatment for IBS) — the NNT for IBS symptom reduction is approximately 4–5 in primary care studies (ATLANTIS trial, Lancet 2023). Ondansetron (5-HT3 antagonist) reduces urgency and diarrhoea in IBS-D (Creed trial, Gut 2012) — off-label in IBS but increasingly used.

IBS does not progress to serious organic disease (cancer, IBD) and does not cause bowel damage. However, it profoundly impacts quality of life — patients with severe IBS rate quality of life impairment comparable to congestive heart failure. Social isolation, dietary restriction, anxiety about symptoms in public places, reduced work productivity, relationship difficulties, and depression are significant consequences. Healthcare utilization is high — multiple specialist referrals, investigations, and sick days are common. Unnecessary investigations and procedures (appendectomy, hysterectomy) occur more frequently in IBS patients due to diagnostic uncertainty. Nutritional deficiencies can occur from overly restrictive elimination diets.

There is no definitive prevention for IBS. Managing triggers significantly reduces symptom frequency and severity: follow a low-FODMAP diet under dietitian guidance, eat regular meals (3 meals per day, no skipping), chew food thoroughly, avoid carbonated drinks and gas-producing foods, limit caffeine and alcohol, exercise regularly (aerobic exercise reduces IBS symptoms through gut motility and mood effects), and manage psychological stress (mindfulness, yoga, CBT, psychological therapy). Identify and treat comorbid anxiety and depression — these perpetuate IBS severity. Keep a food and symptom diary to identify personal triggers.

See your GP if you have recurrent abdominal pain, bloating, or a change in bowel habit persisting for more than 4 weeks — particularly if symptoms significantly affect your quality of life. IBS is a clinical diagnosis made on the basis of the Rome IV criteria; however, it is a diagnosis of exclusion — 'red flag' features that should prompt urgent investigation include: blood in the stool (visible or on faecal immunochemical testing); unexplained significant weight loss (above 5% in 3 months); onset of symptoms after age 50; a family history of bowel cancer or inflammatory bowel disease; nocturnal symptoms that wake you from sleep; and progressive worsening of symptoms. These features require urgent referral to a gastroenterologist and colonoscopy rather than an IBS diagnosis. Emergency assessment is needed for: severe, acute-onset abdominal pain (different from usual IBS pain), peritonism (rigid abdomen), fever with abdominal pain, or rectal bleeding — these may indicate bowel obstruction, perforation, or inflammatory bowel disease complication.