Disorder of gut-brain interaction with recurrent abdominal pain and altered bowel habits without structural disease.
Also known as: IBS, irritable bowel syndrome, IBS-D, IBS-C, IBS-M, functional bowel disorder
Overview
A disorder of gut-brain interaction characterized by recurrent abdominal pain associated with defecation or change in bowel habits, in the absence of identifiable structural, biochemical, or inflammatory abnormalities. Diagnosed by Rome IV criteria.
Epidemiology
Prevalence about 4% by Rome IV criteria (about 10% by older Rome III criteria); 2:1 female-to-male predominance; typical onset under age 50. Major contributor to outpatient GI visits and loss of work productivity.
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Question 1GastrointestinalMedium
A 30-year-old woman has 12 months of bloating, alternating constipation and diarrhea, lower abdominal cramping relieved by defecation, and mucus in the stool. She has no weight loss, rectal bleeding, or nocturnal symptoms. Colonoscopy and laboratory studies, including celiac serologies, are normal. Which of the following is the most likely diagnosis and the most appropriate first-line management?
ABile acid diarrhea; oral cholestyramine
BIrritable bowel syndrome; low-FODMAP diet
CBacterial overgrowth; oral rifaximin
DLactose intolerance; lactose-free diet
EMicroscopic colitis; oral budesonide
Reveal answer & full explanation
Correct answer: B — Irritable bowel syndrome; low-FODMAP diet
ABile acid diarrhea; oral cholestyramine
BIrritable bowel syndrome; low-FODMAP diet✓
CBacterial overgrowth; oral rifaximin
DLactose intolerance; lactose-free diet
EMicroscopic colitis; oral budesonide
Why Irritable bowel syndrome; low-FODMAP diet is correct
The Rome IV criteria are met: recurrent abdominal pain at least 1 day per week, related to defecation, with a change in stool frequency and form (here the mixed, alternating subtype, IBS-M)
Absence of alarm features (no weight loss, rectal bleeding, or nocturnal symptoms) plus a normal colonoscopy and normal laboratory studies supports a positive diagnosis of IBS rather than organic disease
Per current ACG (2021) and AGA (2022) guidance, first-line management is dietary and symptom-directed: a trial of a low-FODMAP diet, soluble fiber such as psyllium, and peppermint oil or an antispasmodic as needed for cramping
Why the others are wrong
Microscopic colitis; oral budesonide — microscopic colitis causes chronic nonbloody watery diarrhea and can hide behind a grossly normal colonoscopy, but it is diagnosed only by random colonic biopsies and does not produce alternating constipation, mucus, and cramping relieved by defecation (confused-with normal-appearing mucosa)
Lactose intolerance; lactose-free diet — lactase deficiency causes bloating, cramping, and diarrhea within hours of a dairy load, but symptoms track lactose ingestion rather than defecation and it does not account for the constipation phase or mucus in the stool (buzzword-matching on bloating)
Bacterial overgrowth; oral rifaximin — small intestinal bacterial overgrowth causes bloating and watery diarrhea, usually with a predisposing motility or anatomic disorder, and does not explain constipation alternating with pain relieved by defecation; rifaximin is a later-line option for IBS with diarrhea, not first-line management of this mixed-subtype patient (right-concept-wrong-first-step)
Bile acid diarrhea; oral cholestyramine — bile acid malabsorption is a recognized mimic of diarrhea-predominant IBS that, like microscopic colitis, leaves the colonoscopy normal, but it causes chronic watery diarrhea and does not account for the constipation phase of this mixed-subtype patient, and a bile acid sequestrant often causes constipation itself (confused-with an IBS-D mimic)
Question 2GastrointestinalEasy
A 22-year-old female has 8 months of lower abdominal cramping partially relieved by defecation, alternating constipation and loose stools, and bloating. Colonoscopy, celiac antibodies, stool cultures, and inflammatory markers are all normal. Stress worsens symptoms. Which of the following is the most likely diagnosis?
AMicroscopic colitis
BFunctional abdominal bloating
CBile acid malabsorption
DIrritable bowel syndrome
ESmall bowel bacterial overgrowth
Reveal answer & full explanation
Correct answer: D — Irritable bowel syndrome
AMicroscopic colitis
BFunctional abdominal bloating
CBile acid malabsorption
DIrritable bowel syndrome✓
ESmall bowel bacterial overgrowth
Why Irritable bowel syndrome is correct
Irritable bowel syndrome (IBS) Rome IV criteria: recurrent abdominal pain at least 1 day/week in the last 3 months, associated with 2 or more of: related to defecation, change in stool frequency, change in stool form; criteria must be fulfilled for the last 3 months with symptom onset at least 6 months before diagnosis.
This patient has all features: months of cramping relieved by defecation, alternating constipation and loose stools, bloating, stress worsening, and an entirely normal workup (colonoscopy, celiac antibodies, stool cultures, inflammatory markers).
Microscopic colitis — causes chronic watery, nonbloody diarrhea, typically in women over 50 and often drug-associated (nonsteroidal anti-inflammatory drugs, proton pump inhibitors); it does not produce alternating constipation or pain relieved by defecation (confused-with chronic diarrhea).
Bile acid malabsorption — follows cholecystectomy or ileal disease or resection and causes urgent, watery postprandial diarrhea rather than a mixed bowel pattern with bloating and stress-modulated cramping (right-concept-wrong-mechanism).
Small bowel bacterial overgrowth — arises with anatomic stasis or dysmotility (prior bowel surgery, strictures, scleroderma) and causes malabsorptive diarrhea with weight loss and nutrient deficiencies, none of which is present in this otherwise healthy 22-year-old (premature closure on bloating).
Functional abdominal bloating — bloating is prominent here, and Rome IV functional bloating/distension describes recurrent bloating without another explanation, but that diagnosis requires that IBS criteria not be met; pain related to defecation with alternating stool form makes this IBS with bloating as an associated feature (naming the disorder after the loudest symptom).
Additional high-yield points
Dietary: low fermentable oligosaccharides, disaccharides, monosaccharides, and polyols (FODMAP) diet — reduces symptoms in 70%.
Psychosocial: cognitive behavioral therapy (CBT), low-dose tricyclic antidepressants (TCAs) such as amitriptyline — reduces visceral sensitivity.
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Ovarian cancer — Bloating in women >50; pelvic exam, CA-125, ultrasound
Diagnostic workup
Diagnostic criteria
Rome IV: Recurrent abdominal pain on average ≥1 day/week in the last 3 months, associated with ≥2 of: (1) related to defecation, (2) change in stool frequency, (3) change in stool form. Symptom onset ≥6 months prior to diagnosis. Subtypes by predominant stool form using Bristol Stool Scale: IBS-C, IBS-D, IBS-M, IBS-U.
Labs
CBC (exclude anemia)
CRP (exclude inflammation)
Fecal calprotectin (or lactoferrin) with CRP when diarrhea is present (IBS-D or IBS-M) to exclude IBD (ACG 2021)
TTG IgA + total IgA (exclude celiac)
TSH (exclude thyroid disease)
Stool studies (Giardia antigen, C. diff, ova/parasites) if recent infection or travel
Imaging
Colonoscopy — NOT routinely required for typical IBS without alarm features; perform if age ≥45 (CRC screening), alarm features, family history of IBD/CRC, or refractory symptoms
Random colonic biopsies if chronic watery diarrhea (microscopic colitis)
Diagnostic algorithm
IBS Subtype
Bristol Stool Scale Pattern
Preferred Pharmacotherapy
IBS-C
>25% type 1-2, <25% type 6-7
Linaclotide, plecanatide, lubiprostone, PEG
IBS-D
>25% type 6-7, <25% type 1-2
Rifaximin, loperamide, eluxadoline, alosetron
IBS-M
>25% both ends of spectrum
Tailor by predominant symptom; TCAs for pain
IBS-U
Insufficient abnormality to classify
Symptom-targeted
Rome IV IBS subtypes by Bristol Stool Scale and first-line pharmacotherapy.
Treatment
First-line
Establish therapeutic relationship; educate that IBS is a real, chronic, manageable condition
Dietary modification: regular meals, adequate fiber (soluble — psyllium); trial of low-FODMAP diet under dietitian guidance
Exercise, sleep hygiene, stress management
Cognitive behavioral therapy and gut-directed hypnotherapy (strong evidence)
IBS-C (constipation predominant)
Soluble fiber (psyllium)
PEG 3350
Linaclotide, plecanatide, or lubiprostone (prosecretory)
Tenapanor (NHE3 inhibitor)
Avoid stimulant laxatives long-term
IBS-D (diarrhea predominant)
Loperamide PRN
Rifaximin 550 mg TID × 14 days (can repeat up to twice for recurrence)
Bile acid sequestrants — cholestyramine, colesevelam — if bile acid diarrhea suspected
Eluxadoline (mu/kappa-opioid agonist, delta antagonist) — avoid post-cholecystectomy and in heavy drinkers (sphincter of Oddi spasm)
Alosetron — 5-HT3 antagonist; restricted use in women with severe refractory IBS-D
IBS-M (mixed)
Tailor to predominant symptom at the time
TCAs particularly useful for pain-predominant phenotypes
Second-line / adjunct
Symptom-based pharmacotherapy (see by_subtype)
Tricyclic antidepressants (amitriptyline, nortriptyline, desipramine) — pain and IBS-D; low dose 10-50 mg at bedtime
SSRIs (citalopram, sertraline, paroxetine) — not recommended for IBS symptoms themselves (AGA 2022 suggests against); use only to treat comorbid anxiety or depression. Low-dose TCAs are the preferred neuromodulator
Diagnostic uncertainty leading to missed structural disease (if alarm features ignored)
PANCE pearls
Diagnose IBS POSITIVELY using Rome IV criteria — avoid diagnosis of exclusion approach with exhaustive testing.
Alarm features mandating colonoscopy: age ≥45 (CRC screening), GI bleeding, unexplained anemia, weight loss, nocturnal symptoms, family history of IBD or CRC, palpable mass.
Low-FODMAP diet has strong evidence but should be supervised by a dietitian — strict phase ≤6 weeks, followed by reintroduction.
Rifaximin is non-absorbable and effective for IBS-D bloating; up to 3 courses approved for recurrence.
Linaclotide and plecanatide are guanylate cyclase-C agonists — improve both constipation and IBS-related pain.
Avoid chronic opioid use — opioid-induced constipation and narcotic bowel syndrome amplify IBS symptoms.
CBT and gut-directed hypnotherapy have effect sizes rivaling pharmacotherapy for refractory IBS.
References
ACG 2021 — Lacy BE et al. ACG Clinical Guideline: Management of Irritable Bowel Syndrome. Am J Gastroenterol 2021;116:17-44
AGA 2022 IBS-C — Chang L et al. AGA Clinical Practice Guideline on the Pharmacological Management of Irritable Bowel Syndrome With Constipation. Gastroenterology 2022;163:118-136
Rome IV — Lacy BE et al. Bowel Disorders. Gastroenterology 2016;150:1393-1407
AGA 2022 IBS-D — Lembo A et al. AGA Clinical Practice Guideline on the Pharmacological Management of Irritable Bowel Syndrome With Diarrhea. Gastroenterology 2022;163:137-151
Rome Foundation Global Study — Sperber AD et al. Worldwide Prevalence and Burden of Functional Gastrointestinal Disorders, Results of Rome Foundation Global Study. Gastroenterology 2021;160:99-114
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