Irritable Bowel Syndrome

Slide 1 · Irritable Bowel Syndrome

Clinicopathologic mechanisms, clinical diagnosis, and differential reasoning

A symptom-defined syndrome

Clinical reasoning · pathogenesis · differential diagnosis

Dr Abu Suraih Sakhri

MBBS, MD (Pathology)

www.abusuraihsakhri.com

Slide 2 · Learning objectives

Define IBS as a symptom-based syndrome with no characteristic structural lesion.

Explain the contributions of motility, visceral sensation, gut–brain signaling and luminal factors.

Apply the Rome V clinical framework while recognizing its distinction from textbook-era wording.

Distinguish IBS from inflammatory, infectious and microscopic causes of bowel dysfunction.

Slide 3 · IBS: A syndrome, not a single lesion

Chronic or recurrent bowel symptoms without a defining macroscopic or microscopic lesion

A clinical umbrella for potentially heterogeneous mechanisms.

The symptoms are real even when endoscopy and biopsy are normal.

Clinical phenotype is more informative than a tissue diagnosis.

Slide 4 · Three symptom domains define the phenotype

Abdominal pain

Recurrent discomfort is central to case recognition.

Bloating

May accompany pain without tissue injury.

Altered bowel habit

Constipation, diarrhea, or both.

The combination—not any isolated symptom—is the clinical clue.

Slide 5 · Relapsing symptoms do not imply tissue injury

Symptoms recur

Intensity fluctuates

Bowel function changes

No obligatory visible lesion

A relapsing clinical course is compatible with normal structural assessment.

Slide 6 · A normal biopsy is expected in uncomplicated IBS

Macroscopic examination

No defining mass, ulcer, or inflammatory lesion

Microscopic examination

No characteristic structural change required for the diagnosis

Normal tissue does not invalidate an abnormal sensation or motility phenotype.

Slide 7 · Diagnosis begins with the symptom pattern

Characterize recurrent pain

Document stool changes

Review duration and context

Assess competing diagnoses

IBS does not have a pathognomonic histologic marker.

Slide 8 · IBS has recognizable bowel-habit subtypes

IBS-C

Constipation-predominant

IBS-D

Diarrhea-predominant

IBS-M

Mixed abnormal stool forms

Rome V also retains IBS-U for an unclassified bowel-habit pattern.

Slide 9 · Subtype and syndrome are different questions

Question 1: Is this IBS?

Recurrent pain/discomfort related to bowel function; Duration and associated features.

Question 2: Which subtype?

Determine predominant abnormal stool form, preferably off symptom-altering medication.

Subtyping follows—rather than replaces—syndrome recognition.

Slide 10 · IBS-C: Transit and stool retention

Reduced colonic contractions / transit

Longer colonic residence

Constipation pattern

Reduced colonic motility as a tendency, not a universal mechanism.

Slide 11 · IBS-D: Accelerated transit

Increased colonic contractions / transit

Reduced transit time

Diarrhea pattern

A tendency toward increased motility can coexist with sensory or bile-acid contributions.

Slide 12 · IBS-M: Abnormal stool forms coexist

Constipation episodes

Hard or lumpy stools occur in a substantial fraction of abnormal bowel movements.

Diarrheal episodes

Loose or watery stools also occur in a substantial fraction.

A single stool sample is insufficient to characterize the pattern.

Slide 13 · An integrative pathogenesis model

IBS symptoms

Gut–brain signaling

Altered motility

Sensory responsiveness

Dietary influences

Microbiome shifts

These are interacting contributors; Robbins does not assert one universal causal pathway.

Slide 14 · Enteric sensory responses may be heightened

Physiologic luminal stimulus

Increased sensory response

Disproportionate pain / discomfort

Symptom intensity need not correlate with visible mucosal damage.

Slide 15 · Visceral sensitivity explains the pathology paradox

Tissue-level assessment

No corresponding diagnostic lesion may be found.

Functional assessment

Responses to gastrointestinal stimuli can nevertheless be amplified.

This is a mechanistic explanation, not a stand-alone diagnostic test.

Slide 16 · The gut–brain axis is bidirectional

Central stress / emotion signals

Enteric neural function

Motility and sensory responses

Enteric signals can also influence central symptom perception; The relationship is not one-way.

Slide 17 · Enteric nervous system dysfunction is implicated

Altered enteric nervous system activity to IBS.

Disordered signaling can affect both motility and sensory processing.

The involvement is supported by physiologic and molecular associations.

No single neural defect is necessary or sufficient for all cases.

Slide 18 · Stress modifies symptoms without negating them

Stress-related input

Can modify gut–brain signaling, motility, and sensory responsiveness.

Clinical interpretation

Stress association is not proof that symptoms are imagined or voluntary.

A biopsychosocial mechanism is compatible with biologically mediated symptoms.

Slide 19 · Diet is a potential symptom modifier

Food-related luminal input

Gastrointestinal stimulation

Altered perception / bowel function

The source identifies diet as one factor; Individual food triggers are not universal.

Slide 20 · Microbiome perturbation is one proposed contributor

A shift in intestinal microbial communities may modify intestinal signaling.

The microbiome is discussed as part of an interacting pathogenesis model.

Robbins does not identify a single diagnostic IBS microbial signature.

Association alone does not establish a treatment target in every patient.

Slide 21 · Postinfectious IBS: A clinically identifiable pathway

Infectious gastroenteritis

Immune activation OR microbiome shift

Persistent bowel symptoms

Immune activation and dysbiosis as alternatives or joint hypotheses.

Slide 22 · Postinfectious IBS is not persistent infection by default

Preceding event

Documented or recalled infectious gastroenteritis before symptom onset.

Subsequent syndrome

Ongoing IBS-type symptoms may persist after the acute infection has resolved.

Evaluate active infection separately if clinical evidence indicates it.

Slide 23 · Excess bile acids can contribute to IBS-D

Increased synthesis OR impaired reabsorption

More bile acids reach colon

Diarrheal physiology

A recognized mechanism in a subset of patients—not synonymous with all IBS-D.

Slide 24 · Bile acids affect two intestinal functions

Motility

Bile acids can alter intestinal contractile and transit behavior.

Epithelial ion transport

Changes in ion handling can increase luminal water movement.

Both effects provide a biologic route to loose stools.

Slide 25 · Not all IBS-D is bile-acid mediated

IBS-D denotes the stool phenotype rather than one etiology.

Bile-acid excess or malabsorption accounts for some cases.

Motility, sensory and gut–brain factors may also be involved.

A therapeutic or diagnostic approach may need mechanism-specific reasoning.

Slide 26 · Serotonin signaling is a candidate pathway

Serotonin-related enteric signaling

Altered neural / motility responses

IBS symptom phenotype

Genetic links to serotonin reuptake transporters are associations, not diagnostic biomarkers.

Slide 27 · Other molecular associations remain exploratory

Cannabinoid receptors

Candidate regulatory signaling.

TNF-related mediators

Candidate inflammatory signaling.

Serotonin transporters

Candidate neurotransmitter handling.

Association studies do not establish a single-gene explanation for IBS.

Slide 28 · Genetic associations ≠ a clinical genetic test

What studies suggest

Several candidate gene families may influence susceptibility or phenotype.

What they do not prove

No reported candidate variant is a specific diagnostic test for IBS.

Separate etiologic hypotheses from clinically validated biomarkers.

Slide 29 · IBS is best understood as mechanistic heterogeneity

Similar clinical symptoms

Transit changes

Neural signaling

Visceral sensation

Luminal bile acids

Postinfection changes

Different mechanisms may converge on a shared symptom-defined syndrome.

Slide 30 · 5-HT₃ blockade links mechanism and treatment

Serotonergic signaling contributes

5-HT₃ receptor antagonism

Symptomatic benefit in some IBS-D

Treatment response supports pathway relevance but is not proof of etiology in each patient.

Slide 31 · Motility-directed agents target a symptom domain

Diarrhea-predominant IBS

Opioid-acting antidiarrheal agents can reduce stool frequency.

Important limitation

Bowel-habit improvement need not resolve abdominal pain or other mechanisms.

Robbins mentions opioid and anticholinergic effects; Specific prescribing needs separate guidance.

Slide 32 · IBS prevalence varies with the definition

Approximately 5–10% reported in high-income settings

Peak prevalence is reported at 20–40 years.

A female predominance is described.

Changes in symptom criteria affect prevalence estimates.

Slide 33 · Epidemiology: Avoid conflating case definitions

Reported prevalence

Dependent on population, survey method, and diagnostic threshold.

Biologic interpretation

A criteria-driven shift is not automatically a true change in disease occurrence.

State the criteria and population when interpreting an IBS frequency estimate.

Slide 34 · The textbook criterion requires interpretation

The extracted Robbins text describes abdominal pain or discomfort ≥3 days/month over 3 months.

It also mentions improvement with defecation and change in stool frequency or form.

This wording resembles earlier criteria and is not identical to Rome IV.

The current adult Rome V framework should be identified by its 2026 edition.

Slide 35 · Rome V (2026): Core adult IBS criteria

≥3 days per month

Recurrent, not continuous, abdominal pain or discomfort in the last 3 months

Associated with ≥2 bowel-related features.

Onset must precede diagnosis by ≥6 months.

Slide 36 · Rome V: The three associated features

Defecation

Symptoms are related to defecation.

Stool frequency

A change in frequency is present.

Stool form

A change in form/appearance is present.

At least two features accompany the recurrent pain or discomfort.

Slide 37 · Rome IV to Rome V: Two key revisions

Rome IV · 2016

Abdominal pain ≥1 day/week.

The word discomfort was not included.

Rome V · 2026

Pain OR discomfort ≥3 days/month.

Symptoms should be recurrent, not continuous.

Both versions require bowel-related features and chronic symptom onset.

Slide 38 · Duration matters in Rome V

Symptoms begin

At least 6 months elapse

Criteria fulfilled in last 3 months

A brief acute diarrheal illness alone is not established adult IBS.

Slide 39 · Defecation can improve or worsen symptoms

What the criterion says

Pain/discomfort is RELATED to defecation.

What not to assume

Improvement after defecation is not obligatory for every patient.

The modern wording is broader than “relieved by defecation.”

Slide 40 · Altered stool frequency is one association

Change from the patient’s usual bowel movement frequency

Frequency may increase in a diarrheal phenotype.

Frequency may decrease in a constipated phenotype.

Frequency alone does not classify every case or establish IBS.

Slide 41 · Stool form is distinct from stool frequency

Frequency

How often bowel movements occur.

Form

Whether stool is hard/lumpy or loose/watery.

Assess both rather than using “diarrhea” and “constipation” imprecisely.

Slide 42 · Rome V stool-form subtyping

Subtype

Hard stool (types 1–2)

Loose stool (types 6–7)

IBS-C

>25%

<25%

IBS-D

<25%

>25%

IBS-M

>25%

>25%

IBS-U

Not classifiable by the above patterns

Not classifiable by the above patterns

Applied to bowel movements on days with abnormal stool; Evaluate off bowel-habit medications.

Slide 43 · Bristol Stool Form Scale anchors terminology

Types 1–2

Hard / lumpy

Types 3–5

Intermediate formed stools

Types 6–7

Mushy / watery

IBS subtype rules emphasize hard and loose extremes, not an average over all seven types.

Slide 44 · Subtype assignment needs representative observations

Document abnormal bowel days

Classify form objectively

Calculate predominant pattern

Reassess if medication changes it

Rome V advises prospective stool diaries for clinical trial subtyping.

Slide 45 · IBS versus chronic constipation

IBS-C

Recurrent bowel-related pain/discomfort plus constipation pattern.

Chronic constipation

Constipation symptoms without sufficient IBS criteria.

Pain/discomfort is central to syndromic differentiation.

Slide 46 · IBS versus functional diarrhea

IBS-D

Diarrheal pattern with qualifying recurrent abdominal pain/discomfort.

Functional diarrhea

Predominantly loose/watery stool without the required IBS symptom constellation.

The two labels should not be used interchangeably.

Slide 47 · A positive diagnosis still requires judgment

Recognize typical IBS pattern

Look for competing explanations

Use indicated focused tests

Assign clinical diagnosis

ACG favors positive rather than exhaustive exclusionary diagnostic strategies.

Slide 48 · Alarm findings change the diagnostic pathway

Typical pattern

Stable recurrent symptoms with a convincing IBS relationship to stool function.

Concerning finding

Bleeding, anemia, weight loss or other evidence of organic disease demands evaluation.

Do not explain new objective abnormalities by IBS without independent assessment.

Slide 49 · Infection can mimic or precede IBS

Active enteric infection

An infectious cause must be considered for relevant acute or persistent diarrhea.

Postinfectious IBS

IBS-like symptoms begin after a gastrointestinal infection has resolved.

The temporal sequence alone does not demonstrate an active pathogen.

Slide 50 · IBS versus inflammatory bowel disease

Feature

IBS

Inflammatory bowel disease

Diagnostic basis

Characteristic symptom pattern

Evidence of chronic intestinal inflammation

Endoscopy

No characteristic lesion

May reveal ulcers/inflammatory changes

Histology

No defining inflammatory pattern

May show crypt injury, inflammation or granulomas

Concern

Functional impairment

Tissue injury and complications

Structural inflammation is not expected in uncomplicated IBS.

Slide 51 · Microscopic colitis is a crucial trap

Chronic watery diarrhea

Normal-appearing colonoscopy

Abnormal colonic biopsy

Normal endoscopy alone cannot distinguish IBS from microscopic colitis.

Slide 52 · Two microscopic-colitis morphologies

Collagenous colitis

A thickened subepithelial collagen band and inflammation.

Lymphocytic colitis

Increased intraepithelial lymphocytes without the collagenous band.

These are biopsy-defined entities, unlike IBS.

Slide 53 · Normal gross appearance ≠ normal histology

IBS

Endoscopy and routine histology may both be normal.

Microscopic colitis

Endoscopy can be normal despite diagnostic microscopic change.

The investigative question determines when sampling adds value.

Slide 54 · Targeted testing: Celiac disease in IBS-D

Compatible IBS-D symptoms

Celiac serologic testing

Interpret in clinical context

ACG 2021 advises screening for celiac disease in patients with IBS and diarrhea symptoms.

Slide 55 · Targeted testing: Inflammatory disease in IBS-D

Suspected IBS-D

Fecal calprotectin assessment

Evaluate possible IBD when indicated

ACG 2021 suggests fecal calprotectin to help exclude IBD in appropriate patients.

Slide 56 · Clinical reasoning: Combine pattern and testing

Pain/discomfort + stool change

Verify chronicity

Exclude indicated alternatives

Consider IBS subtype

Do not require normal colonoscopy in every patient as a universal gatekeeper.

Slide 57 · Case 1: Recurrent pain and loose stools

Clinical vignette

A 27-year-old reports 10 months of recurrent abdominal discomfort, frequently associated with defecation, and loose stools during episodes. There is no described inflammatory or bleeding feature.

What additional history confirms the symptom criterion, and which subtype is plausible?

Slide 58 · Case 1: Reason through the diagnosis

Establish recurrence on ≥3 days/month in the preceding 3 months.

Confirm at least two bowel-related features.

Characterize the proportion of loose and hard abnormal stools.

Consider IBS-D only if the subtype pattern fits and alternatives are assessed.

Slide 59 · Case 2: Watery diarrhea with normal endoscopy

Clinical vignette

A 58-year-old has persistent watery nonbloody diarrhea. The colon appears macroscopically normal. Biopsies show a thickened subepithelial collagen band.

Which diagnosis explains the microscopic finding, and why is IBS inadequate?

Slide 60 · Case 2: Tissue evidence overrides a symptom label

The collagen-band finding supports collagenous colitis.

This is a recognized cause of watery diarrhea with normal endoscopy.

IBS does not account for this diagnostic histopathology.

A normal endoscopic appearance must not be mistaken for normal mucosal biology.

Slide 61 · Case 3: Bowel symptoms after gastroenteritis

Clinical vignette

An adult recovers from infectious gastroenteritis but subsequently develops chronic episodic pain, bloating and altered stool form. No pathogen persistence is documented.

Which IBS pathway is suggested, and what competing explanation remains important?

Slide 62 · Case 3: A mechanism is not a confirmed cause

Postinfectious IBS is a reasonable clinical hypothesis.

Immune activation and microbiome shifts are proposed mechanisms.

Ongoing infection must be reconsidered when findings suggest it.

The history alone does not specify a microbial or immune biomarker.

Slide 63 · Management starts with the dominant problem

Pain / discomfort

Target gut–brain/sensory dimensions.

Diarrhea

Consider transit and selected mechanism-directed interventions.

Constipation

Consider measures directed at impaired transit or secretion.

Different symptom dimensions may require different approaches.

Slide 64 · Dietary intervention is a time-limited trial

Identify troublesome food-related symptoms

Consider structured low-FODMAP trial

Assess global symptom response

ACG 2021 recommends a limited trial; This is guidance beyond the Robbins paragraph.

Slide 65 · Gut-directed psychotherapy is biologically coherent

Brain–gut signaling contributes

Targeted psychological intervention

Possible improvement in global symptoms

ACG 2021 suggests gut-directed psychotherapy as an IBS treatment option.

Slide 66 · IBS-C: Approved secretory targets

Chloride-channel activators

Increase intestinal fluid movement; A guideline-endorsed treatment class.

Guanylate-cyclase activators

Another guideline-endorsed IBS-C treatment class.

ACG 2021 recommendations are treatment evidence, not textbook pathogenesis claims.

Slide 67 · IBS-D: Treatment need not imply infection

Rifaximin

ACG 2021 recommends treatment of global IBS-D symptoms in selected patients.

Interpretation

An antibiotic response cannot establish ongoing bacterial infection as the universal IBS-D cause.

Treatment selection requires patient-specific assessment.

Slide 68 · Mechanism-targeted care has limitations

Clinical subtypes are defined by stool pattern, not unique pathology.

Several mechanisms may coexist in one patient.

No single medication addresses all mechanistic contributors.

Response to intervention does not independently confirm a specific cause.

Slide 69 · Four errors to avoid

Error

Better interpretation

“Normal biopsy means no disease.”

IBS can involve sensory/motility dysfunction.

“All watery diarrhea is IBS-D.”

Consider infection, IBD and microscopic colitis.

“Stress proves symptoms are psychological.”

Gut–brain interaction is a physiologic model.

“A genetic association diagnoses IBS.”

Reported gene links are not validated diagnostic tests.

Slide 70 · Integrated model: From physiology to diagnosis

Multiple contributing mechanisms

Recurrent bowel-related symptoms

Rome V symptom classification

Targeted differential assessment

The clinicopathologic task is to interpret symptoms without inventing a lesion.

Slide 71 · Five clinically important conclusions

IBS is a heterogeneous symptom-defined syndrome with real functional impairment.

Endoscopic and microscopic assessments have no defining IBS lesion.

Motility, sensation, gut–brain signaling, bile acids and postinfection changes may contribute.

Rome V (2026) broadens the symptom threshold relative to Rome IV.

A positive diagnosis must remain sensitive to inflammatory and microscopic mimics.

Slide 72 · Principal sources

Robbins, Cotran & Kumar. Pathologic Basis of Disease. 11th ed. Chapter 17, pp. 727.

Rome Foundation. Rome V Diagnostic Criteria for Adult IBS and IBS Subtypes (2026).

Rome Foundation. Rome IV Diagnostic Criteria for Adult IBS (2016).

Lacy BE, et al. ACG Clinical Guideline: Management of Irritable Bowel Syndrome. Am J Gastroenterol. 2021;116:17–44.