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Diarrhea — Complete Medical Review

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1. Definition

• ≥3 unformed/liquid stools per day, OR
• Stool weight >200 g/day in Western countries (up to 300 g on high-fiber diets in developing countries), OR
• Stool that "takes the form of the container"

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2. Pathophysiology — The Core Mechanism

Normal GI Fluid Handling

Three Fundamental Mechanisms

Infectious Mechanisms (4 specific types — Rosen's)

1. Ingestion of preformed toxins — toxin already made before eating (e.g., S. aureus, B. cereus); onset 1–6 hours
2. Adherence to intestinal cell walls — pathogen sticks to epithelium and disrupts brush border (e.g., ETEC, Giardia)
3. Mucosal cell invasion — organism penetrates mucosa (e.g., Shigella, Campylobacter, Salmonella)
4. Enterotoxin/cytotoxin production — toxin produced in vivo stimulates secretion (e.g., cholera toxin, C. difficile toxins A/B)

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3. Types / Classification of Diarrhea

A. By Duration

• Acute/Prolonged (≤13 days): Usually viral or bacterial
• Persistent/Chronic (>13 days): Protozoa, parasites, non-infectious

B. By Pathophysiological Mechanism

1. Osmotic Diarrhea

• Mechanism: Poorly absorbable solutes (non-electrolytes) remain in the lumen → osmotic gradient draws water in → intestinal distension → diarrhea
• Key feature: Stops with fasting
• Fecal osmotic gap: HIGH (>125 mOsm/kg) — unabsorbed solutes account for stool osmolality
  • Formula: Osmotic gap = 290 – 2×([Na⁺] + [K⁺]) in stool water
• Causes: Lactase deficiency (lactose malabsorption), magnesium-containing antacids/laxatives, lactulose, celiac disease, short bowel syndrome, mannitol/sorbitol ingestion

2. Secretory Diarrhea

• Mechanism: Active secretion of Cl⁻ (via CFTR) or inhibition of neutral NaCl absorption → net fluid secretion into lumen. Crypts of Lieberkühn are the primary secretory sites.
• Key feature: Persists with fasting; large volume (often >1 L/day)
• Fecal osmotic gap: LOW (<50 mOsm/kg) — stool osmolality accounted for almost entirely by Na⁺, K⁺, and accompanying anions
• Classic example — Cholera: Cholera toxin activates adenylyl cyclase → ↑ cAMP → opens CFTR Cl⁻ channels in crypts → massive Cl⁻ secretion followed by water → 10–12 L/day secretion (colon can only reabsorb max 6–8 L/day) → life-threatening losses (Guyton & Hall)
• Other causes: VIPoma (watery diarrhea, hypokalemia, achlorhydria — WDHA/Verner-Morrison syndrome), carcinoid syndrome (serotonin), gastrinoma, microscopic colitis, bile acid malabsorption, fatty acids in colon, congenital chloridorrhea

3. Inflammatory/Exudative Diarrhea

• Mechanism: Mucosal inflammation and ulceration → exudation of protein, blood, pus into the lumen → disrupted absorption + stimulated secretion; also increased motility
• Features: Mucus, blood, pus in stool; fever; fecal leukocytes positive; tenesmus
• Causes: IBD (UC — diffuse colonic inflammation and ulceration; Crohn's — transmural, skip lesions anywhere GI), invasive bacteria (Shigella, Campylobacter, Salmonella), C. difficile colitis, CMV, ischemic colitis, radiation colitis, colorectal cancer

4. Motility-Related Diarrhea

• Mechanism: Rapid intestinal transit reduces contact time for absorption; or paradoxically, slow motility → bacterial overgrowth → secondary malabsorption
• Causes: IBS-D (post-infectious), hyperthyroidism, diabetes (autonomic neuropathy with diarrhea predominant), post-vagotomy, psychogenic diarrhea (parasympathetic stimulation → ↑ motility + ↑ mucus secretion — Guyton), post-gastrectomy dumping syndrome

5. Malabsorptive / Fatty Diarrhea (Steatorrhea)

• Mechanism: Failure of digestion or absorption → unabsorbed fats/carbohydrates reach colon → osmotic effect + bacterial fermentation → gas + diarrhea; reduced water-binding
• Features: Bulky, greasy, foul-smelling, floating stools; Sudan stain positive; 72-hr fecal fat > 7 g/day
• Causes: Pancreatic exocrine insufficiency, celiac disease, Whipple's disease, SIBO, bile acid deficiency, lymphatic obstruction

C. By Volume

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4. Differential Diagnosis by Setting

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5. Diagnostic Criteria & Evaluation

Acute Diarrhea — When to Investigate

• Profuse diarrhea with signs of dehydration
• Overtly bloody stools
• Fever ≥38.5°C
• Duration >48 hours without improvement
• Recent antibiotic use
• New community outbreak
• Severe abdominal pain in patients >50 years
• Age ≥70 or immunocompromised patients

Diagnostic Workup

• Culture-independent multiplex PCR — first-line (more sensitive/specific/rapid)
• Stool culture for bacteria (Salmonella, Shigella, Campylobacter, E. coli O157:H7)
• Ova and parasites (O&P) × 3 specimens
• C. difficile toxin assay / GDH + PCR
• Fecal leukocytes / calprotectin / lactoferrin — marker of inflammatory diarrhea
• Giardia/Cryptosporidium antigen tests

• 24-hour stool weight: determines if true diarrhea exists (>200 g/day)
• Fecal osmotic gap = 290 – 2([Na⁺] + [K⁺]): high = osmotic, low = secretory
• Sudan stain / 72-hour fecal fat: steatorrhea screen
• Fecal pH: low (<5.5) suggests carbohydrate malabsorption
• Fecal elastase: pancreatic exocrine function
• Fecal calprotectin: IBD vs. functional (very useful noninvasive marker)
• α₁-antitrypsin clearance: protein-losing enteropathy
• Laxative screen (if factitious diarrhea suspected)

• CBC, CRP, ESR (inflammation)
• CMP (electrolytes, albumin — nutritional/absorption status)
• TSH (hyperthyroidism)
• Tissue transglutaminase IgA + total IgA (celiac screen)
• Vitamin B12, folate, iron — malabsorption markers
• Chromogranin A, 24-hr urine 5-HIAA (carcinoid)
• VIP level (VIPoma)

• Colonoscopy with biopsies: for chronic diarrhea with suspected IBD, microscopic colitis (normal gross appearance but biopsy shows collagenous/lymphocytic colitis), colorectal cancer evaluation, or blood/pus in stool
• Upper endoscopy with duodenal aspirate/biopsy: for malabsorption, giardiasis, celiac disease, Whipple's disease
• Sigmoidoscopy (without full prep): adequate for suspected proctitis or pseudomembranous colitis

• Abdominal CT: bowel wall thickening, mass lesions, mesenteric changes, obstruction
• CT enterography / MRI: Crohn's disease extent
• Abdominal X-ray: if ileus or toxic megacolon suspected

Management Algorithm for Acute Diarrhea

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6. Treatment & Management

A. Fluid & Electrolyte Replacement — CORNERSTONE

B. Diet

• Avoid: dairy (secondary lactase deficiency), caffeine (potentiates cramps), alcohol
• Allow: soups, saltines, bananas, mashed potatoes, rice (BRAT-type diet)
• In children: restart feeding immediately after tolerating oral intake
• Bowel rest provides only partial relief; early refeeding is preferred (Sleisenger)

C. Antimicrobial Therapy — Indications & Choices

D. Chronic Diarrhea Management

• Treat the underlying cause (IBD → 5-ASAs, steroids, biologics; celiac → gluten-free diet; microscopic colitis → bismuth, budesonide; pancreatic insufficiency → enzyme replacement; bile acid diarrhea → cholestyramine)
• Empirical anti-motility agents for symptomatic relief when cause cannot be eliminated
• IBS-D: dietary modification (low-FODMAP), antispasmodics, rifaximin (non-absorbed antibiotic), alosetron (women with severe IBS-D), loperamide

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7. Antidiarrheal Drugs — Detailed Pharmacology

Class 1: Opioid-Receptor Agonists (Anti-motility)

Loperamide (Imodium) — First-line antidiarrheal

• Class: Phenylpiperidine opioid derivative
• Mechanism: Agonist at peripheral μ-opioid receptors in the myenteric plexus → ↓ peristalsis, ↑ anal sphincter tone, ↓ intestinal transit time → more time for fluid and electrolyte absorption. Also inhibits Ca²⁺/calmodulin-dependent secretion.
• Key pharmacology: Very limited CNS penetration (P-glycoprotein pumps it out of the brain) → negligible abuse potential and no CNS opioid effects → available OTC without prescription
• Dosing: 4 mg initially, then 2 mg after each loose stool; max 16 mg/day
• Uses: Traveler's diarrhea, IBS-D, chronic diarrhea, chemotherapy-induced diarrhea, ileostomy output control
• Contraindications: Bloody diarrhea / dysentery, suspected invasive infection (especially STEC — risk of HUS/TTP, toxic megacolon), C. difficile (without antibiotic cover), pseudomembranous colitis, <2 years old, obstructive ileus
• Adverse effects: Constipation, abdominal distension, nausea; at very high doses (misuse) — QTc prolongation, cardiac arrhythmia

Diphenoxylate + Atropine (Lomotil) — Schedule V

• Class: Phenylpiperidine; metabolized to active metabolite difenoxin (Schedule IV)
• Mechanism: Same as loperamide — μ-opioid receptor agonism in the ENS; also small CNS effect
• Atropine is added in sub-therapeutic doses — too low to have antidiarrheal effect, but discourages misuse (anticholinergic side effects at doses required to get "high" from diphenoxylate). (Katzung)
• Poor solubility also limits IV abuse
• Dosing: 2 tablets (5 mg diphenoxylate / 0.025 mg atropine each) to start, then 1 after each diarrheal stool
• Contraindications: Same as loperamide; also jaundice; children <2 years (atropine toxicity risk)

Codeine / Tincture of Opium — used in refractory chronic diarrhea

• Full μ-opioid agonist; antidiarrheal + analgesic
• Reserved for severe chronic diarrhea unresponsive to other agents
• Higher abuse potential, CNS depression

Class 2: Bismuth Subsalicylate (Pepto-Bismol)

• Mechanism (multiple):
  • Anti-secretory: bismuth and salicylate both reduce intestinal secretion
  • Antimicrobial: bismuth is toxic to enteric pathogens (H. pylori, ETEC, other bacteria)
  • Anti-inflammatory: salicylate component inhibits prostaglandin synthesis
  • Adsorbent: binds toxins in the lumen
• Dose: 524 mg every 30–60 min as needed (max 8 doses/24 h)
• Uses: Traveler's diarrhea (prophylaxis and treatment), dyspepsia, ETEC diarrhea, vomiting with minimal diarrhea, H. pylori eradication regimens
• Efficacy: Safe and efficacious in bacterial diarrheas (Yamada's)
• Contraindications: Should NOT be used in immunocompromised patients or those with renal impairment (risk of bismuth encephalopathy — bismuth accumulates); avoid in aspirin-allergic patients; avoid in children with viral illness (Reye's syndrome risk from salicylate)
• Side effects: Black stools and black tongue (harmless — bismuth sulfide), tinnitus (salicylism at high doses), bismuth encephalopathy (with overuse)

Class 3: Antisecretory Agents

Racecadotril (Acetorphan)

• Mechanism: Enkephalinase (NEP/CD10) inhibitor → prevents breakdown of endogenous enkephalins → ↑ enkephalin activity at δ-opioid receptors → inhibits cAMP-stimulated Cl⁻ secretion without affecting motility
• Key advantage: Anti-secretory WITHOUT constipation (no motility effect) → preferred in children, secretory diarrhea
• Available in many countries (Europe, Asia, Latin America); not FDA-approved in the US

Octreotide (Somatostatin analog)

• Mechanism: Mimics somatostatin → ↓ secretion of GI hormones (VIP, serotonin, gastrin, glucagon) → ↓ intestinal secretion; also slows motility
• Uses: Carcinoid diarrhea (gold standard), VIPoma (WDHA syndrome), dumping syndrome, AIDS-related secretory diarrhea, chemotherapy-induced diarrhea, refractory secretory diarrhea
• Route: SC or IV; long-acting formulation (Sandostatin LAR) IM monthly
• Side effects: Steatorrhea (inhibits pancreatic enzyme and bile secretion), gallstone formation, bradycardia, hyperglycemia or hypoglycemia

Class 4: Adsorbents

Kaolin-Pectin (Kaopectate — old formulation)

• Mechanism: Adsorb toxins and bacteria to their surface; increase stool consistency
• Limited efficacy; replaced by bismuth in many markets
• No systemic absorption; safe in pregnancy

Attapulgite

• Hydrated magnesium aluminum silicate clay
• Binds water, toxins, and irritants in the gut
• OTC; symptomatic use only

Cholestyramine (bile acid sequestrant)

• Mechanism: Binds bile acids in the colon → prevents their pro-secretory/pro-motility effect on colonic mucosa
• Uses: Bile acid malabsorption diarrhea (type 1–3: ileal resection, primary bile acid diarrhea), post-cholecystectomy diarrhea, C. difficile toxin binding (adjunctive — limited evidence)
• Side effects: Bloating, constipation, malabsorption of fat-soluble vitamins (A, D, E, K), drug interactions (binds warfarin, thyroid hormone, digoxin — take other medications ≥1 hour before)

Class 5: Probiotics

• Mechanism: Competitive exclusion of pathogens, mucosal barrier reinforcement, immune modulation, production of antimicrobial substances
• Evidence-based uses:
  • Lactobacillus rhamnosus GG and Saccharomyces boulardii: reduce duration of acute infectious diarrhea by ~1 day in children
  • Reduce antibiotic-associated diarrhea (including C. difficile-associated)
• Generally safe; caution in severely immunocompromised (rare case of fungemia with S. boulardii)

Class 6: Antibiotics with Gut-Specific Action

Rifaximin

• Mechanism: Non-absorbable rifamycin antibiotic; inhibits bacterial RNA polymerase locally in the GI tract
• Uses: Traveler's diarrhea (non-invasive, non-bloody), IBS-D (reduces bacterial overgrowth; FDA-approved), hepatic encephalopathy prevention
• Advantage: Minimal systemic absorption → minimal systemic side effects, low resistance selection pressure for systemic pathogens
• Dosing for traveler's diarrhea: 200 mg TID × 3 days; for IBS-D: 550 mg TID × 14 days

Class 7: 5-HT3 Antagonists

Ondansetron

• Primarily antiemetic; also slows intestinal transit
• Used off-label for IBS-D and chemotherapy-induced diarrhea; reduces acute vomiting and diarrhea in gastroenteritis

Alosetron

• Selective 5-HT3 antagonist; slows colonic transit + reduces visceral hypersensitivity
• FDA-approved only for women with severe IBS-D unresponsive to other treatments
• Black box warning: Ischemic colitis and serious complications of constipation (requires REMS program)

Class 8: Enkephalin Analogs / μ-Opioid Agonists (Novel)

Eluxadoline (Viberzi)

• Mixed μ- and κ-opioid agonist + δ-opioid antagonist
• FDA-approved for IBS-D in adults
• Reduces abdominal pain and diarrhea; δ-antagonism prevents constipation rebound
• Contraindication: Patients without a gallbladder (pancreatitis risk); alcohol misuse

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Summary Table: Antidiarrheal Drug Classes

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