Irritable Bowel Syndrome (IBS)

One of the most commonly diagnosed digestive illnesses worldwide, yet a persistent diagnostic and therapeutic dilemma: prevalence estimates rest on formal symptom-based criteria alone, in the absence of reliable biomarkers for what is a heterogeneous disease process. Relevant to a colorectal practice chiefly because IBS drives a disproportionate amount of surgical referral and misadventure — see "IBS and the relationship to surgery" below.

Definition and Rome IV subtyping

IBS is currently defined (Rome IV) as recurrent abdominal pain occurring at least weekly for the last 3 months, associated with ≥2 of: (1) relationship to defecation, (2) change in stooling frequency, and/or (3) change in stool form — with onset at least 6 months prior. Earlier definitions also required abdominal pain/discomfort improving with defecation, but the disease is fundamentally one of abdominal pain plus altered bowel habits.

Three subtypes are defined by stool form (Bristol scale), not by symptom severity:

IBS-D (diarrhea-predominant): >25% of bowel movements are Bristol type 6 or 7 (loose/watery).
IBS-C (constipation-predominant): >25% of bowel movements are Bristol type 1 or 2 (hard/lumpy).
IBS-M (mixed): >25% of bowel movements are both loose and hard/lumpy forms.

Beyond classification, Bristol stool form is a practical surrogate for colonic transit time and helps guide treatment selection (augmenting vs. slowing motility).

Diagnostic specificity/positive predictive value of the Rome criteria is as high as 100% in the absence of red-flag symptoms (abnormal exam, recent antibiotic use, documented weight loss, nocturnal symptoms, bloody stools, family history of colorectal cancer) — yet many patients still receive a positive diagnosis a mean of 6.6 years after symptom onset, reflecting persistent clinical underconfidence in symptom-based criteria alone.

Diagnostic workup by subtype

A limited workup guided by history/exam avoids unnecessary testing. Broad recommended testing across subtypes: age-appropriate CRC screening, CBC.

IBS-D (broadest differential): celiac serologies (TTG IgA + IgA level), CRP or fecal calprotectin, consideration of Giardia and bile acid diarrhea testing, stool studies for intestinal infection if post-infectious IBS is suspected, anti-CdtB/anti-vinculin antibodies in that setting.
Bile acid diarrhea: malabsorption in an anatomically normal small bowel (not just post-ileal- resection) is an increasingly recognized cause of diarrhea misattributed to IBS-D (up to 28.1% of such patients). Testing is difficult — fasting serum C4, 48-hour stool collection, or the gold standard SeHCAT nuclear medicine test (limited availability). Empiric bile acid sequestrant trial is the most common real-world diagnostic-and-therapeutic approach, though tolerance/palatability limits dose titration.
Microscopic colitis: an overlooked cause of watery diarrhea in older women, endoscopically normal with diagnosis requiring biopsy — see Microscopic Colitis. Colonoscopy yield for diagnosing microscopic colitis in IBS-D patients ≥45 years is only modestly higher than controls (2.3%), but supports a lower threshold for colonoscopy with random biopsies in older IBS-D patients, especially women.
IBS-M: same workup as IBS-D.
IBS-C: a narrower differential. Testing focuses on defining the physiology of constipation itself if symptoms are severe/refractory — see Constipation for the surgical/physiologic workup (anorectal manometry, abdominal plain film for fecal loading, referral for physiologic testing).

IBS and the relationship to surgery

Although surgery has no role in treating IBS itself, IBS patients are substantially more likely to undergo cholecystectomy, hysterectomy, and appendectomy — frequently representing preoperative misdiagnoses in a search for the cause of chronic pain. Extensive workups triggered by severe IBS's impact on quality of life (frequent PCP/ED visits, specialty referrals) culminate in unnecessary surgical procedures more often than desired.

Diverticular disease overlap: diverticulitis-associated inflammation can sensitize visceral afferent nerves in the affected colon segment, producing persistent hypersensitivity to normal stimuli even after the acute episode resolves. Low-grade inflammation can persist in diverticulosis and is associated with symptomatic uncomplicated diverticular disease (SUDD) — a controversial entity (recurrent abdominal symptoms with loose stools in the presence of localized diverticular disease, skewing toward an older population) that may not be entirely separate from IBS. See Diverticulitis, which does not yet carry this SUDD/IBS-overlap content. Patients with IBS-like functional complaints before elective resection for diverticular disease are less likely to have satisfactory symptomatic outcomes after surgery; persistent post-resection symptoms resembling IBS occur in up to 25% of patients despite resolved inflammation in the specimen. Conversely, surgery itself may precipitate new IBS symptoms in susceptible individuals via alteration of GI function — reported after a range of abdominal/pelvic operations (cholecystectomy, hysterectomy).

Treatment approach

A graduated approach (education/reassurance → dietary modification → pharmacotherapy → psychological treatment/multidisciplinary care), because assigning the diagnosis is itself unsatisfying without a confirmatory test — patient education on the disease's benign natural history and a strong therapeutic patient-provider relationship are independent predictors of improved health status.

Conservative and dietary measures

Exercise gives mild symptom improvement (particularly useful in IBS-M) plus general physical/mental health benefit. Probiotics give a mild benefit (low-quality evidence overall; Bifidobacterium infantis has the highest-quality supporting data). Peppermint oil gives a modest antispasmodic benefit, though a recent well-conducted RCT found no benefit for either small-intestinal- or colonic-release formulations against rigorous EMA/FDA endpoints. Antispasmodics are generally best used on-demand/prophylactically rather than as scheduled medication.

Fiber: a mainstay, associated with global symptom improvement though mechanism is poorly understood (bulking agent, substrate for short-chain-fatty-acid-producing colonic fermentation). Soluble fiber (psyllium, ispaghula, calcium polycarbophil) has more consistent benefit than insoluble fiber (corn, wheat, bran); psyllium is the most evidence-supported single agent. IBS-C patients who don't tolerate gas/bloating from fermentable soluble fiber may do better with non-fermentable insoluble agents (cellulose, methylcellulose).

Dietary/FODMAP treatments: growing in popularity because 60–70% of IBS patients report symptom worsening with meals, >70% believe food causes their symptoms, and up to 90% actively avoid trigger foods — despite true food allergy being no more prevalent in IBS than the general population. Lactase deficiency (~30% of Caucasians, ~70% of Asians) is a specific, common, testable contributor. Earlier literature on gluten avoidance (a double-blind RCT confirmed non-celiac gluten reintroduction worsens IBS symptoms) has since been superseded by the low-FODMAP framework: FODMAPs (fermentable oligo-/di-/monosaccharides and polyols) are poorly absorbed in the small bowel, increasing osmotic load and colonic bacterial fermentation; wheat is just one FODMAP source among many, and a dietary trial demonstrated symptom worsening on gluten-free-but-FODMAP-containing diets in patients with self-reported non-celiac wheat sensitivity. The full low-FODMAP protocol (elimination phase → systematic reintroduction) is complex, dietitian-dependent, and adherence-limited; the "FODMAP-gentle" bottom-up approach (restricting only the highest-yield foods) is a more practical alternative for many patients.

Pharmacotherapy by subtype

Matched to predominant symptom (abdominal pain, constipation, diarrhea, bloating) more than strict IBS subtype. Most FDA-/EMA-approved agents target a specific subtype/symptom; a strong placebo response (~10% benefit over active treatment in trials) is characteristic of functional GI disorder trials generally.

IBS-D: loperamide (peripheral mu-opioid agonist; improves urgency/consistency more reliably than global symptoms/pain; the authors favor it as prophylaxis before predictable bathroom-limited situations rather than monotherapy) → bile acid sequestrants when bile acid diarrhea is suspected (cholestyramine first-line for titratability/proof-of-principle despite poor palatability; colesevelam/colestipol better tolerated but more expensive, with only small-pilot- study evidence) → rifaximin (gut-specific, minimally absorbed antibiotic; ~10% improvement over placebo in adequate-relief endpoints per large trials, no increased C. difficile risk or resistance signal; approved for up to 3 courses in the US based on a retreatment trial showing sustained response) → eluxadoline (combined mu-/kappa-agonist, delta-antagonist; benefit over placebo in two phase 3 trials; risk of severe pancreatitis/Sphincter of Oddi dysfunction, especially without a gallbladder) → 5-HT₃ antagonists (alosetron — US-approved for women with refractory IBS-D, mitigated constipation/ischemic-colitis safety signal with lower starting doses and use restricted to severe/refractory cases; ondansetron and ramosetron used off-label/regionally; a meta-analysis found ramosetron and alosetron more effective than rifaximin/eluxadoline in IBS-D/IBS-M).
IBS-C: fiber (20–30 g/day) and PEG first-line, effective for constipation symptoms but PEG does not improve abdominal pain in the limited IBS-C literature — a key subtype-specific gap, since IBS-C management additionally requires pain control unlike functional constipation alone. Newer secretagogues (lubiprostone 8 μg BID, linaclotide 290 μg daily, plecanatide 3 mg daily) all improve global IBS symptoms and are generally well tolerated (diarrhea the most common adverse event; lubiprostone also associated with nausea); none have been compared head-to-head, and a meta-analysis suggests similar efficacy across the class, so agent selection is often driven by payer coverage/availability rather than a clear efficacy hierarchy.
Bloating: soluble fiber, low-FODMAP diet, or the same constipation-targeted secretagogues depending on the dominant co-symptom.

Neuromodulation for abdominal pain

IBS is conceptualized as lying on a spectrum with centrally mediated abdominal pain syndrome (CAPS) — frequent/continuous abdominal pain without bowel-habit change, distinguishing it from IBS. Chronic visceral hypersensitivity progresses from peripheral to central sensitization, particularly with comorbid emotional distress/trauma history; stress amplifies this maladaptive central pain response.

Neuromodulators are underused despite a well-developed rationale (downregulate central visceral pain processing, reduce catastrophizing/hypervigilance/anxiety, treat comorbid psychiatric illness, modulate GI transit, treat concomitant nausea/dyspepsia). Selection is bowel-habit-driven:

IBS-D: tricyclic antidepressants (TCAs) — most effective agents for pain — dosed well below psychiatric-treatment doses ("start low, go slow"); anticholinergic effects are partly therapeutic here (help diarrhea/urgency). Secondary amines (nortriptyline, desipramine) preferred over tertiary amines (amitriptyline, imipramine) for a better side-effect profile.
IBS-C / IBS-M: SNRIs (duloxetine, milnacipran preferred over venlafaxine, which needs higher doses for noradrenergic/pain effect and causes more nausea/withdrawal) and the α2δ ligands gabapentin/pregabalin (pregabalin has a positive small RCT). SSRIs are used adjunctively for comorbid anxiety/depression rather than as primary IBS pain treatment, being less effective for pain than TCAs.

Psychological treatment

Cognitive behavioral therapy, relaxation therapy, hypnotherapy, and multicomponent psychological therapy all have evidentiary support as monotherapy or combined with neuromodulators — access is limited mainly by availability of experienced psychological providers and the time-intensiveness of these modalities.

Diverticulitis — SUDD/post-diverticulitis-surgery symptom overlap described above is not yet reflected on that page; worth a cross-reference addition in a future pass.
Microscopic Colitis — a specific organic mimic of IBS-D requiring biopsy for diagnosis.
Constipation — IBS-C sits on a spectrum with functional constipation; shares first-line fiber/PEG treatment and the same anorectal-manometry-driven workup for refractory cases.
Functional Anorectal Pain & Coccygodynia — IBS-associated chronic pelvic pain is a distinct but related syndrome covered there, including the specific epidemiology of IBS/pelvic-pain overlap in women.

Open items / gaps

All content is textbook-level/secondary. Several individually load-bearing sources flagged in Research Priorities.md: the Ford et al. 2018 ACG monograph on IBS management (the field's central evidence synthesis) and the Black et al. 2020 secretagogue network meta-analysis.
No RCT has directly compared the three secretagogues (lubiprostone, linaclotide, plecanatide) head-to-head — current "similar efficacy" claims rest on indirect/network meta-analytic comparison only.
The SUDD/IBS overlap is explicitly described as controversial in the source itself, not a settled nosologic distinction — worth flagging if a diverticulitis-specific research question arises.

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