Irritable Bowel Syndrome

Etiology and Risk Factors

Heightened visceral sensitivity may transform ordinary bowel distension into pain; altered motility and fluid handling contribute to diarrhea, constipation, or fluctuating stool patterns (Mills & Ciorba, 2019; Rej et al., 2023).

Epidemiology and Clinical Context

Population context: IBS is commonly recognized in younger and middle-aged adults and may follow enteric infection. New symptoms beginning later in life, unintentional weight loss, iron deficiency, bleeding, or a change from the prior bowel pattern require evaluation for an alternative explanation, not age-based exclusion or automatic labeling as IBS (Mills & Ciorba, 2019; Norris, 2020).

Pathophysiology

IBS is heterogeneous. Heightened visceral sensitivity may transform ordinary bowel distension into pain; altered motility and fluid handling contribute to diarrhea, constipation, or fluctuating stool patterns. Bidirectional signaling between the enteric nervous system, autonomic pathways, central pain processing, and stress-response systems shapes symptoms. A history of gastroenteritis may precede persistent sensitivity and altered bowel function. In some patients, microbiome changes, immune activation, and dietary fermentation contribute, but no single mechanism explains every case (Mills & Ciorba, 2019; Rej et al., 2023).

Clinical distinction: The symptoms are physiologically mediated and genuine. Psychosocial stress can amplify symptoms but is neither required nor sufficient to explain IBS. Likewise, low-grade immune or microbial changes in IBS are not equivalent to the mucosal ulceration and progressive tissue injury characteristic of inflammatory bowel disease.

Diagnostic Evaluation

• Clarify frequency, location, character and duration of abdominal pain; relationship to defecation; Bristol stool form; bloating; nocturnal symptoms; meal-related worsening; and effect on work, sleep, eating and social functioning.
• Screen for alarm findings: hematochezia or melena, iron-deficiency anemia, progressive weight loss, fever, nocturnal diarrhea, persistent vomiting, palpable mass, new symptoms at older age, or strong family history of colorectal cancer/IBD. These findings change the differential and may warrant further testing.
• Review medications, supplements, diet, laxatives, antimicrobial history, travel/exposures, thyroid or metabolic disease, and prior infections or surgery. Consider pelvic-floor symptoms, urinary or gynecologic pain conditions, and comorbid disorders of gut–brain interaction (Carbo & Smetana, 2012; Fortin, 2012).
• When the presentation fits IBS without alarm findings, limited targeted testing may suffice. In diarrhea-predominant symptoms, evaluate for celiac disease and evidence of inflammation as indicated (for example, CBC, CRP and/or fecal calprotectin). Routine colonoscopy is not required solely to establish IBS in younger low-risk patients; age-appropriate colorectal screening still applies.

Expected Findings

A positive clinical diagnosis: Rome V framework

The 2026 Rome V criteria describe recurrent, but not continuous, abdominal pain or discomfort on at least three days per month during the previous three months, with symptom onset at least six months before diagnosis. Symptoms must be associated with at least two of the following: defecation, a change in stool frequency, or a change in stool form. Rome V replaces the Rome IV threshold of abdominal pain at least one day per week and restores discomfort as a qualifying symptom (Rome Foundation, n.d.).

Subtype	Stool pattern	Clinical implication
IBS-C	Predominantly hard/lumpy stools with recurrent abdominal pain or discomfort.	Distinguish from chronic idiopathic constipation or outlet dyssynergia, which may require another approach.
IBS-D	Predominantly loose/watery stools with recurrent abdominal pain or discomfort.	Assess diet and medication effects; consider celiac disease, bile acid diarrhea, and microscopic colitis when appropriate.
IBS-M	Both hard and loose stools at different times.	Avoid treating every loose stool as separate infection or every constipated phase as an isolated problem.
IBS-U	Criteria for IBS met without sufficient stool-pattern predominance.	Focus on pain, function, patient goals, and reassess classification if bowel habits change.

IBS subtype is determined by stool consistency during abnormal bowel movements, not by counting total bowel movements alone. Loose stools or constipation alone do not establish IBS; the qualifying recurrent pain or discomfort and its bowel associations must be present (Rome Foundation, n.d.).

Differential Diagnosis

Key mimics and discriminators

Alternative	Discriminator	Clinical next step
Inflammatory bowel disease	Blood, objective inflammatory markers, weight loss, nocturnal stools, endoscopic inflammation.	Stool biomarkers and endoscopic investigation when indicated.
Celiac disease	Chronic diarrhea or bloating, iron deficiency, nutrient loss; may resemble IBS.	Serologic testing while eating gluten when indicated.
Microscopic colitis	Chronic watery, often nocturnal diarrhea, sometimes normal gross colonoscopy.	Colonic biopsies in the appropriate risk context.
Bile acid diarrhea	Persistent watery diarrhea, sometimes after ileal disease/resection or cholecystectomy.	Test for bile acid diarrhea when feasible; if testing is unavailable, a monitored trial of a bile acid sequestrant may be considered. Treatment response alone does not definitively establish the diagnosis (Rej et al., 2023; Smalley et al., 2019).
Lactose or FODMAP-related symptoms	Predictable meal-associated pain, gas and bloating.	Review diet and consider a structured, time-limited trial.
Chronic idiopathic constipation / pelvic floor dysfunction	Constipation predominant without recurrent pain; excessive straining or incomplete evacuation.	Targeted bowel and defecatory assessment.

Diagnostic Limitations and Reassessment

When symptoms are primarily loose stools without recurrent abdominal pain or discomfort, assess functional diarrhea and organic mimics rather than applying Rome V IBS criteria by habit (Lacy et al., 2021; Rome Foundation, n.d.).

Treatment and Management

• Establish a positive diagnosis and explain the gut–brain physiology in clear language. Identify the symptoms that matter most to the patient and develop measurable goals for pain, bowel function, sleep, and daily activity.
• Use individualized regular meals, adequate fluid intake, and soluble fiber when appropriate; soluble fiber may be preferable to indiscriminate increases in insoluble bran, which can worsen bloating in some patients. A time-limited low-FODMAP trial with guided reintroduction may be useful; avoid indefinite restrictive diets without nutrition oversight.
• For constipation-predominant symptoms, consider stool-directed therapy and appropriate secretagogues or other approved agents when needed. For diarrhea-predominant symptoms, use subtype-specific treatment while first addressing inflammatory or infectious red flags. For persistent global symptoms and pain, tricyclic neuromodulators can be useful after assessing contraindications, anticholinergic effects, and bowel-pattern consequences. The 2021 ACG guideline recommends against routinely using antispasmodics to treat global IBS symptoms because supportive evidence is limited; this does not preclude individualized short-term relief of specific spasms, but it should not substitute for evidence-based global treatment (Lacy et al., 2021).
• Gut-directed cognitive behavioral therapy, hypnotherapy, and related behavioral interventions can modify visceral pain amplification and disability. Their value does not imply symptoms are imaginary or exclusively psychological.
• Assess response and harms over time; reassess the original diagnosis if the pattern changes or alarm features develop. Match management to subtype and treatment target: the ACG guideline supports a limited low-FODMAP trial, constipation-directed secretagogues for IBS-C, rifaximin for IBS-D and gut-directed psychotherapy when appropriate. Prescribing still requires agent-specific contraindication and label review (Lacy et al., 2021).

A subtype-specific decision framework

In IBS-C, separate infrequent or hard stool from persistent abdominal pain. Soluble fiber or an osmotic laxative may improve stool passage but may not adequately treat pain; persistent global symptoms may justify an appropriate bowel secretagogue or other approved IBS-C treatment after reviewing contraindications. In IBS-D, loperamide can decrease stool frequency but does not reliably resolve the pain component; recurrent pain, bile-acid diarrhea or postinfectious features may require a different approach. IBS-M needs adjustment to the current predominant bowel pattern rather than simultaneously treating diarrhea and constipation with opposing drugs (Rej et al., 2023).

Complications and Red Flags

When to reopen the diagnosis

New rectal bleeding, iron deficiency, unintended weight loss, persistent nocturnal diarrhea, fever, or a new change in bowel pattern at older age should not be attributed automatically to a preexisting IBS diagnosis. In IBS-C with marked straining, sensation of incomplete evacuation and manual maneuvers, consider a defecatory disorder and pelvic-floor evaluation. When symptoms are primarily loose stools without recurrent abdominal pain or discomfort, assess functional diarrhea and organic mimics rather than applying Rome V IBS criteria by habit (Lacy et al., 2021; Rome Foundation, n.d.).

Clinical Reasoning Priorities

A patient develops recurrent lower abdominal discomfort and fluctuating stool form after a resolved gastrointestinal infection. Discomfort occurs on several days each month, is frequently related to bowel movements, and has persisted for six months; weight is stable, with no bleeding, anemia, nocturnal diarrhea or fever. This supports a possible postinfectious IBS phenotype when the Rome V associations and appropriate targeted evaluation are met. Conversely, bloody nocturnal diarrhea with elevated fecal calprotectin requires investigation for inflammatory or infectious colitis rather than a reflexive IBS label (Rome Foundation, n.d.).

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References

Carbo, A. R., & Smetana, G. W. (2012). Diarrhea. In M. C. Henderson, L. M. Tierney Jr., & G. W. Smetana (Eds.), The patient history: An evidence-based approach to differential diagnosis (2nd ed., Chapter 33). McGraw Hill.

Fortin, A. H., VI. (2012). Constipation. In M. C. Henderson, L. M. Tierney Jr., & G. W. Smetana (Eds.), The patient history: An evidence-based approach to differential diagnosis (2nd ed., Chapter 32). McGraw Hill.

Lacy, B. E., Pimentel, M., Brenner, D. M., Chey, W. D., Keefer, L. A., Long, M. D., & Moshiree, B. (2021). ACG clinical guideline: Management of irritable bowel syndrome. American Journal of Gastroenterology, 116(1), 17–44. https://doi.org/10.14309/ajg.0000000000001036

Mills, J. C., & Ciorba, M. A. (2019). Gastrointestinal disease. In G. D. Hammer & S. J. McPhee (Eds.), Pathophysiology of disease: An introduction to clinical medicine (8th ed., Chapter 13). McGraw-Hill Education.

Norris, T. L. (2020). Porth’s essentials of pathophysiology (5th ed.). Wolters Kluwer.

Rej, A., Chew, T. S., & Sanders, D. S. (2023). Gastroenterology. In I. D. Penman, S. H. Ralston, M. W. J. Strachan, & R. P. Hobson (Eds.), Davidson’s principles and practice of medicine (24th ed., pp. 781–858). Elsevier.

Rome Foundation. (n.d.). Rome V criteria. Retrieved October 19, 2026, from https://theromefoundation.org/rome-v-criteria/

Smalley, W., Falck-Ytter, C., Carrasco-Labra, A., Wani, S., Lytvyn, L., & Falck-Ytter, Y. (2019). AGA clinical practice guidelines on the laboratory evaluation of functional diarrhea and diarrhea-predominant irritable bowel syndrome in adults (IBS-D). Gastroenterology, 157(3), 851–854. https://doi.org/10.1053/j.gastro.2019.07.004