Diarrhea

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Diarrhea: Overview & Classification

Definition

Diarrhea is a symptom, not a disease, that can arise from many conditions. Clinically, it is defined as:

Three or more loose or liquid bowel movements per day, or
Stool weight > 200 g/day in Western populations (though this alone is insufficient - consistency matters too)

Fecal incontinence is often mistaken for diarrhea and must be distinguished, particularly in older adults. - Sleisenger & Fordtran's Gastrointestinal and Liver Disease

Pathophysiology

Diarrhea results from excess stool water due to abnormal net intestinal water and electrolyte transport.

Normally, the small intestine and colon absorb 99% of the ~9-10 L of fluid that passes the ligament of Treitz daily (from oral intake + endogenous secretions). A reduction of net water absorption by as little as 1% is sufficient to cause diarrhea.

Three fundamental mechanisms underlie all diarrhea:

Altered mucosal transport - impaired water/electrolyte absorption in the small intestine or colon
Rapid transit - reduced contact time between luminal contents and the absorptive epithelium
Altered stool composition - changes in insoluble fecal solids (e.g., steatorrhea reduces water-binding capacity)

Classification

Classification Diagram

Classification of diarrhea by mechanism - Frameworks for Internal Medicine

1. Pathophysiologic Classification

A. Osmotic Diarrhea

Caused by nonabsorbable solutes in the intestinal lumen that retain water osmotically.

Mechanism: Unabsorbed solutes create an osmotic gradient, drawing water into the lumen
Example: Lactase deficiency - undigested lactose remains in the lumen; colonic bacteria may further degrade it to more osmotically active particles
Key feature: Stops with fasting (the offending solute is removed)
Other causes: Lactulose, sorbitol, magnesium-containing laxatives, malabsorption syndromes

B. Secretory Diarrhea

Caused by excessive secretion of fluid by intestinal crypt cells, rather than failure of absorption.

Mechanism: Pathogenic bacteria (e.g., Vibrio cholerae, enterotoxigenic E. coli) produce toxins that activate adenylyl cyclase → ↑ cAMP → opens Cl⁻ channels in the apical membrane → massive Cl⁻ and water secretion into the lumen
Endogenous causes: Endocrine tumors (VIPoma, carcinoid, gastrinoma), excess bile acids reaching the colon, inflammatory mediators
Key feature: Persists with fasting; large-volume, watery stools
Stool osmotic gap: Normal (< 50 mOsm/kg) - electrolytes account for the osmolality

C. Inflammatory Diarrhea

Caused by mucosal disruption with exudation of serum into the lumen, plus destruction of the absorptive epithelium leading to malabsorption.

Mechanism: Invasion of or damage to the intestinal mucosa
Features: Abdominal pain, fever, tenesmus, bloody or mucoid stools; fecal leukocytes/calprotectin/lactoferrin elevated
Infectious: Invasive bacteria (Shigella, Salmonella, Campylobacter, C. difficile), parasites (Entamoeba histolytica)
Noninfectious: IBD (Crohn's disease, ulcerative colitis), ischemic colitis, radiation colitis
Life-threatening complication: Toxic megacolon (abdominal distention + systemic toxicity - fever, tachycardia, delirium)

D. Dysmotility Diarrhea

Caused by abnormal intestinal motility that impairs adequate contact time between luminal contents and the absorptive surface.

Rapid transit ("intestinal hurry"): Oral-cecal transit may be as short as 10 minutes in severe cases; also adds an osmotic component due to nutrient malabsorption
Causes: Diabetes mellitus (enteric neuropathy), post-vagotomy, hyperthyroidism, IBS, endocrine tumors
Slow transit: Can paradoxically cause secretory diarrhea via small intestinal bacterial overgrowth (SIBO) - best example is scleroderma

Most etiologies of diarrhea act through a combination of these mechanisms. Pure osmotic or secretory diarrhea is uncommon in clinical practice. - Sleisenger & Fordtran's

2. Clinical Classification

Criterion	Categories
Duration	Acute (< 4 weeks) vs. Chronic (≥ 4 weeks)
Volume	Large-volume (small bowel/proximal) vs. Small-volume (colorectal)
Stool character	Watery vs. Fatty (steatorrhea) vs. Inflammatory (bloody/mucoid)
Mechanism	Osmotic vs. Secretory

By Duration

Acute diarrhea: Most commonly infectious (viral > bacterial > parasitic); usually self-limited within 1-2 days
Chronic diarrhea: Defined as loose stools for ≥ 4 weeks; affects ~6.6% of the population annually; causes include IBD, IBS, malabsorption, endocrine disorders, medications

By Volume/Location

Large-volume (small bowel): Watery, voluminous; often periumbilical cramping; no blood
Small-volume (colorectal): Frequent small stools; urgency, tenesmus; may contain blood/mucus

By Stool Character (Watery / Fatty / Inflammatory)

Watery: Osmotic or secretory mechanism; most common subtype
Fatty (steatorrhea): Malabsorption - greasy, floating, foul-smelling stools; causes include pancreatic exocrine insufficiency, celiac disease, Whipple's disease
Inflammatory: Blood and/or mucus in stool; suggests mucosal damage

3. Osmotic Gap to Distinguish Osmotic from Secretory

Stool osmotic gap = Measured stool osmolality - 2 × (stool [Na⁺] + stool [K⁺])

Gap	Interpretation
> 125 mOsm/kg	Osmotic diarrhea (poorly absorbed solutes present)
< 50 mOsm/kg	Secretory diarrhea (electrolytes account for osmolality)

Metabolic Consequences

Diarrheal fluid is rich in:

HCO₃⁻ (from salivary, pancreatic, intestinal secretions) - loss causes hyperchloremic, normal anion gap metabolic acidosis
K⁺ (flow-rate-dependent colonic secretion) - loss causes hypokalemia
Water - loss causes volume depletion and circulatory collapse in severe cases (e.g., cholera)

Epidemiology

Acute gastroenteritis affects 7.5% of Americans in any given month
Causes >3.5 million outpatient visits, >180,000 hospitalizations, and ~3,000 deaths per year in the USA
Chronic diarrhea affects 6.6% of the population annually
In developing countries, acute infectious diarrhea remains a leading cause of child mortality

Sources: Sleisenger & Fordtran's Gastrointestinal and Liver Disease; Frameworks for Internal Medicine; Costanzo Physiology 7th Edition

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