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Irritable Bowel Syndrome (IBS)

BASICS

  • Chronic, functional GI disorder with abdominal pain and altered bowel habits.
  • No identifiable organic cause.
  • Subtypes:
  • IBS-D (diarrhea predominant)
  • IBS-C (constipation predominant)
  • IBS-M (mixed)
  • IBS-U (unclassified)

EPIDEMIOLOGY

  • Accounts for 25–50% of GI consults
  • Prevalence:
  • ~4% (Rome IV); ~10% (Rome III) globally
  • US: 10–15% of population
  • Female > male (3:1); IBS-C more common in women
  • More prevalent in low SES communities
  • Peak age: 20–39 years

ETIOLOGY & PATHOPHYSIOLOGY

  • Multifactorial: motility, inflammation, visceral hypersensitivity, microbiota, brain–gut axis
  • Post-infectious IBS (PI-IBS): ~10% after enteritis (6x risk)
  • Possible mechanisms:
  • Mast cell activation, lymphocyte infiltration
  • Altered gut microbiome
  • Low-grade mucosal & neuroinflammation
  • Psychological stress and early-life trauma

RISK FACTORS

  • Biologic: Female sex, genetics, GI infection, obesity
  • Psychosocial: Anxiety, depression, abuse history, somatization
  • Other: Family history of GI disorders, antibiotic use, abdominal surgery

ASSOCIATED CONDITIONS

  • Functional GI disorders: GERD, dyspepsia, pelvic floor dyssynergia
  • Chronic pain syndromes: Fibromyalgia, chronic pelvic pain, TMJ disorder
  • Psychiatric: Anxiety, depression, PTSD

DIAGNOSIS

Rome IV Criteria

  • Abdominal pain >1 day/week in last 3 months
  • Onset ≥6 months prior
  • Associated with ≥2 of:
  • Related to defecation
  • Change in stool frequency
  • Change in stool form

Subtypes (Bristol Stool Scale Based)

Subtype Criteria
IBS-D >25% diarrhea (types 6/7), <25% constipation
IBS-C >25% constipation (types 1/2), <25% diarrhea
IBS-M >25% of both
IBS-U Meets criteria but not subtype classified

RED FLAG SYMPTOMS

  • Age >50 with no prior screening
  • GI bleeding, anemia
  • Nocturnal pain/stools
  • Weight loss >10% over 3 months
  • FHx: CRC, IBD, celiac disease
  • Palpable mass, lymphadenopathy, fever

PHYSICAL EXAM

  • Usually normal; may have mild abdominal tenderness
  • Absent: peritoneal signs, hepatosplenomegaly, jaundice, ascites

DIFFERENTIAL DIAGNOSIS

Includes: - IBD, celiac disease, microscopic colitis - Infections (Giardia, E. histolytica, C. diff) - Endocrine: thyroid, Addison’s, diabetes - Malabsorption: lactose/fructose intolerance - Medication effects, SIBO, pancreatic insufficiency - Psychiatric: somatization, depression

DIAGNOSTIC STRATEGY

Minimal Labs (in absence of red flags)

  • CBC, TSH, electrolytes, CRP, fecal calprotectin or lactoferrin
  • Age-appropriate CRC screening

IBS-D Focused

  • Celiac panel (IgA + TTG)
  • Stool O&P, calprotectin, Giardia antigen, C. difficile

IBS-C Focused

  • Calcium, LFTs, lipase/amylase, abdominal imaging if needed

Advanced Evaluation

  • Breath testing (SIBO)
  • Colonoscopy: >60 years or persistent diarrhea
  • Balloon expulsion/anorectal manometry for severe constipation

TREATMENT

General Goals

  • Symptom relief
  • Improved quality of life

Lifestyle & Diet

  • Regular exercise (3–5x/week)
  • Low FODMAP diet
  • Avoid large meals, fatty food, caffeine
  • Gradual fiber introduction (3–4 g psyllium)
  • Consider lactose-free trial and gluten-free trial

PHARMACOLOGIC THERAPY

All IBS Types

  • Psyllium: 3–4 g/day (fiber for all subtypes)
  • Antispasmodics: hyoscyamine, dicyclomine (PRN)
  • TCAs (e.g., amitriptyline): best for IBS-D
  • Probiotics (multi-strain): limited evidence
  • Psychotropics: SSRIs, SNRIs for associated anxiety/depression

IBS-D

  • Loperamide, diphenoxylate-atropine
  • Bile acid sequestrants (cholestyramine, colesevelam)
  • Rifaximin: 2-week course
  • Alosetron: women, refractory severe cases
  • Eluxadoline: mixed opioid agonist-antagonist
  • Ondansetron: stool frequency/urgency control

IBS-C

  • PEG (polyethylene glycol)
  • Lubiprostone (8 mcg BID with meals)
  • Linaclotide (290 mcg QD)
  • Plecanatide (3 mg QD)
  • Tenapanor (50 mg BID)
  • Tegaserod: 5-HT4 agonist (women <65 yrs, emergency only)

Mixed IBS

  • Treat predominant symptom with IBS-D or IBS-C approach

ADDITIONAL & ALTERNATIVE THERAPIES

  • Behavioral therapy: CBT, relaxation, biofeedback
  • Peppermint oil: first-line for IBS-D (Europe)
  • Acupuncture, herbal supplements: insufficient evidence

FOLLOW-UP & MONITORING

  • Use IBS Severity Scoring System for treatment response
  • Monitor dietary compliance, side effects, and psychosocial health

PATIENT EDUCATION

  • IBS is chronic, non-progressive, non-malignant
  • Emphasize stress reduction and trigger avoidance
  • Symptom resolution may lead to functional symptom shift (e.g., to other systems)

ICD-10 CODES

Subtype ICD-10 Code
IBS with diarrhea K58.0
IBS without diarrhea K58.9
General IBS K58

CLINICAL PEARLS

  • Diagnosis = Rome IV criteria + absence of red flags
  • Low FODMAP + fiber = key lifestyle tools
  • Use TCAs and SSRIs based on predominant symptoms and psych comorbidity
  • Refractory symptoms = evaluate for SIBO, microscopic colitis, or malabsorption