Irritable Bowel Syndrome (IBS)

A 29-year-old woman presents with a 6-month history of recurrent abdominal pain associated with bloating and altered bowel habits. She reports episodes of diarrhea alternating with constipation. Her symptoms are often worse during periods of stress and improve after defecation. She denies fever, weight loss, rectal bleeding, or nocturnal symptoms. Physical examination is unremarkable. Laboratory investigations including CBC, CRP, celiac serology, and stool studies are normal. Diagnosis?

Most likely diagnosis is Irritable Bowel Syndrome with mixed bowel habits (IBS-M).

The clinical presentation is highly characteristic of IBS-M. Strict fulfillment of Rome V criteria additionally requires recurrent, noncontinuous abdominal pain or discomfort occurring on average at least 3 days per month during the last 3 months, with symptom onset at least 6 months before diagnosis.

1. Definition

1.      Irritable bowel syndrome is a chronic disorder of gut-brain interaction (DGBI).

2.      It is characterized by:

1.      Recurrent abdominal pain or discomfort

2.      Altered bowel habits

3.      Symptoms not adequately explained by another medical condition after appropriate clinical evaluation

3.      IBS is a positive clinical diagnosis based on characteristic symptoms, appropriate limited investigation, and assessment for alarm features.

2. Etiology / Associations

1.      The pathogenesis is multifactorial and incompletely understood.

2.      Factors that may contribute include:

1.      Altered gastrointestinal motility

2.      Visceral hypersensitivity

3.      Altered gut-brain signaling

4.      Psychological and physiologic stress

5.      Alterations in the intestinal microbiota

6.      Post-infectious changes

7.      Mucosal immune activation

8.      Altered intestinal permeability in some patients

3.      Associated conditions include:

1.      Anxiety disorders

2.      Depression

3.      Fibromyalgia

4.      Chronic fatigue syndromes

5.      Other disorders of gut-brain interaction

3. Pathophysiology

1.      Visceral hypersensitivity contributes to exaggerated perception of intestinal stimuli.

2.      Altered gastrointestinal motility may contribute to constipation, diarrhea, or mixed bowel patterns.

3.      Altered central and enteric nervous system processing contributes to abnormal gut-brain signaling.

4.      Stress may influence gastrointestinal motility, sensation, and symptom severity.

5.      Alterations in the intestinal microbiota, mucosal immune activation, and increased intestinal permeability may contribute in subsets of patients.

6.      No single pathophysiologic abnormality explains IBS in all patients.

4. Clinical Features

4.1 Abdominal Symptoms

1.      Recurrent abdominal pain or discomfort

2.      Symptoms related to defecation

1.      Symptoms may improve or occasionally worsen with defecation

3.      Bloating

4.      Abdominal distension

5.      Excessive flatulence

4.2 Bowel Habit Changes

1.      Constipation

2.      Diarrhea

3.      Mixed constipation and diarrhea

4.      Passage of mucus

5.      Sensation of incomplete evacuation

6.      Urgency

4.3 Associated Features

1.      Fatigue

2.      Sleep disturbance

3.      Anxiety or depression

4.      Urinary symptoms

5.      Other functional gastrointestinal symptoms may coexist

4.4 Alarm Features / Red Flags

1.      Unintentional weight loss

2.      Gastrointestinal bleeding

3.      Iron deficiency anemia

4.      Persistent nocturnal diarrhea

5.      Fever

6.      Palpable abdominal or rectal mass

7.      Family history of colorectal cancer, inflammatory bowel disease, or celiac disease

8.      New onset of symptoms at age 50 years or older

9.      Need for age-appropriate colorectal cancer screening

10.  Progressive, severe, or otherwise atypical symptoms

5. Diagnosis

5.1 Rome V Criteria

Recurrent, but not continuous, abdominal pain or discomfort occurring on average at least 3 days per month during the last 3 months, associated with at least two of the following:

1.      Related to defecation

2.      Associated with a change in stool frequency

3.      Associated with a change in stool form or appearance

Criteria should be fulfilled during the last 3 months, with symptom onset at least 6 months before diagnosis.

A supportive criterion is that abdominal pain or discomfort should not occur exclusively in relation to menstruation.

5.2 IBS Subtypes

IBS subtypes are classified using the Bristol Stool Form Scale according to stool form on days with abnormal bowel movements.

1.      IBS-C: Constipation predominant

1.      More than 25% of abnormal bowel movements are Bristol Stool Form Scale types 1 or 2

2.      Fewer than 25% of abnormal bowel movements are types 6 or 7

2.      IBS-D: Diarrhea predominant

1.      More than 25% of abnormal bowel movements are types 6 or 7

2.      Fewer than 25% of abnormal bowel movements are types 1 or 2

3.      IBS-M: Mixed bowel habits

1.      More than 25% of abnormal bowel movements are types 1 or 2

2.      More than 25% of abnormal bowel movements are types 6 or 7

4.      IBS-U: Unclassified

1.      Diagnostic criteria for IBS are met, but the bowel pattern does not fulfill criteria for IBS-C, IBS-D, or IBS-M

Subtype assessment is preferably performed when the patient is not taking medications that substantially alter bowel habits.

5.3 Investigations

1.      Investigation should be limited and guided by the clinical presentation.

2.      Appropriate tests may include:

1.      CBC

2.      CRP

3.      Celiac serology, particularly in patients with diarrhea symptoms

4.      Fecal calprotectin in patients with diarrhea symptoms when inflammatory bowel disease needs to be excluded

3.      Routine stool testing for enteric pathogens is not recommended in all patients with suspected IBS.

4.      Stool infectious testing should be performed when clinically indicated, such as with relevant travel, exposure, epidemiologic risk, or suspected infection.

5.      Routine colonoscopy is not required in a young patient with typical IBS symptoms and no alarm features.

6.      Colonoscopy should be considered when:

1.      Alarm features are present

2.      Age-appropriate colorectal cancer screening is indicated

3.      Significant diagnostic uncertainty remains

4.      Microscopic colitis or another colonic disorder is suspected

6. Differential Diagnosis

1.      Inflammatory bowel disease

2.      Celiac disease

3.      Microscopic colitis

4.      Bile acid diarrhea

5.      Lactose or other carbohydrate intolerance

6.      Chronic gastrointestinal infection when epidemiologically appropriate

7.      Colorectal neoplasia

8.      Thyroid disease when clinically suspected

9.      Medication-induced bowel disturbance

10.  Small intestinal bacterial overgrowth in selected patients with appropriate risk factors

11.  Pelvic floor dysfunction in patients with refractory constipation or symptoms of difficult evacuation

12.  Gynecologic disorders such as endometriosis when symptoms are pelvic or cyclical

7. Management

Treatment should be individualized according to the predominant symptoms and IBS subtype.

7.1 General Measures

1.      Establish a clear positive diagnosis.

2.      Provide education regarding the chronic nature of the disorder.

3.      Identify individual symptom triggers.

4.      Encourage regular physical activity.

5.      Encourage adequate sleep.

6.      Address stress and relevant psychological comorbidities.

7.      Establish a therapeutic clinician-patient relationship and provide realistic treatment expectations.

7.2 Dietary Therapy

1.      Soluble fiber such as psyllium may improve global IBS symptoms.

2.      Insoluble fiber such as wheat bran may worsen symptoms in some patients.

3.      A limited trial of a low-FODMAP diet may improve global IBS symptoms.

4.      Low-FODMAP therapy generally involves:

1.      Short-term restriction, usually approximately 4 to 6 weeks

2.      Systematic food reintroduction

3.      Long-term dietary personalization

5.      Dietitian supervision is preferable when a restrictive diet is used.

6.      Dietary triggers should be individualized. Routine universal avoidance of caffeine, alcohol, high-fat foods, or lactose is unnecessary unless they provoke symptoms in the individual patient.

7.3 Pharmacologic Therapy

IBS-C

1.      Soluble fiber such as psyllium

2.      Polyethylene glycol for constipation

1.      Improves stool frequency and consistency

2.      Is less reliable for improving abdominal pain or global IBS symptoms

3.      Linaclotide

4.      Plecanatide

5.      Lubiprostone

6.      Tenapanor

IBS-D

1.      Loperamide

1.      Useful for reducing diarrhea and urgency

2.      Less effective for abdominal pain or global IBS symptoms

2.      Rifaximin

3.      Eluxadoline in appropriately selected patients

1.      Contraindicated in patients without a gallbladder

2.      Avoid in patients with a history of pancreatitis, sphincter of Oddi dysfunction, biliary obstruction, substantial alcohol use, or severe hepatic impairment

4.      Alosetron may be considered for severe refractory IBS-D in women where approved and appropriately regulated.

5.      Bile acid sequestrants are not routine treatment for global IBS-D symptoms but may be useful when bile acid diarrhea is suspected or demonstrated.

Abdominal Pain / Global Symptoms

1.      Low-dose tricyclic antidepressants

1.      May improve abdominal pain and global IBS symptoms

2.      May be particularly useful in IBS-D because they can slow gastrointestinal transit

3.      Should be used cautiously in IBS-C because anticholinergic effects may worsen constipation

2.      Enteric-coated peppermint oil may improve abdominal pain and global symptoms.

3.      Antispasmodics may improve episodic abdominal pain in some patients, although recommendations vary among clinical guidelines.

4.      SSRIs should not be routinely prescribed solely for treatment of global IBS symptoms but may be appropriate when there is an independent psychiatric indication.

5.      Chronic opioid therapy should be avoided for IBS-related abdominal pain.

7.4 Gut-Brain Behavioral Therapy

1.      Cognitive behavioral therapy

2.      Gut-directed hypnotherapy

3.      Other evidence-based gut-brain behavioral interventions

4.      Stress-management techniques as an adjunct to treatment

Gut-brain behavioral therapies may improve global IBS symptoms and should not be regarded solely as treatments for psychiatric disease.

8. Monitoring

1.      Monitor symptom burden and quality of life.

2.      Assess response to dietary and pharmacologic therapy.

3.      Reevaluate the diagnosis if new alarm features develop.

4.      Reassess patients whose clinical pattern changes substantially.

5.      Avoid repeated or extensive investigations in a stable patient with a well-established diagnosis unless a new clinical indication develops.

9. Complications / Prognosis

1.      Reduced quality of life

2.      Work and social impairment

3.      Increased healthcare utilization

4.      Anxiety and depression may coexist and increase symptom burden

5.      IBS does not cause destructive intestinal mucosal disease.

6.      IBS is not considered a premalignant disorder.

7.      Current evidence does not support IBS itself causing an increased long-term risk of colorectal cancer.

10. Key Clinical Insight

Chronic recurrent abdominal pain or discomfort related to defecation and associated with altered stool frequency or form, in a patient without alarm features and with an appropriate limited negative evaluation, strongly supports a positive diagnosis of IBS.

In this patient, alternating constipation and diarrhea are most consistent with IBS-M. Formal subtype classification is based on the Bristol Stool Form Scale and the proportion of abnormal bowel movements.

References

1.      Corsetti M, Shin A, Lacy BE, Cash BD, Simrén M, Schmulson MJ, Hou X, Lembo A. Bowel Disorders. Gastroenterology. 2026;170(6):1261-1282. doi:10.1053/j.gastro.2026.02.003.

2.      Lacy BE, Pimentel M, Brenner DM, Chey WD, Keefer LA, Long MD, Moshiree B. ACG Clinical Guideline: Management of Irritable Bowel Syndrome. Am J Gastroenterol. 2021;116(1):17-44. doi:10.14309/ajg.0000000000001036.

3.      Hung KW, Leiman DA, Kaza A, Watson R, Chang L, Maratt JK; American Gastroenterological Association Quality Committee. AGA Institute Quality Indicator Development for Irritable Bowel Syndrome. Gastroenterology. 2025;168(3):612-622.e4. doi:10.1053/j.gastro.2024.11.003.