Diarrhoea: A Comprehensive Clinical Overview

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1. DEFINITION

• Normal stool output: 100–200 g/day, with up to 100–150 mL of fluid excreted
• Up to 9 L of fluid (exogenous intake + endogenous secretions) enters the proximal bowel daily; ~90% is absorbed in the small intestine and the remainder in the large intestine
• "Acute" diarrhoea: sudden onset lasting ≤14 days (WHO classification)
• "Persistent" diarrhoea: lasting >14 days but <4 weeks
• "Chronic" diarrhoea: lasting >4 weeks (Sleisenger & Fordtran)

Henry's Clinical Diagnosis & Management by Laboratory Methods notes: "One of the most important parameters defining diarrhea in an individual patient is a change in the usual bowel habit to more frequent, looser stools."

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2. CLASSIFICATION / TYPES

A. By Duration

B. By Mechanism (Pathophysiological Classification)

1. Secretory Diarrhoea

• Results from increased intestinal secretion of water and electrolytes into the gut lumen, OR inhibition of normal absorption
• Characterized by:
  • Large volume, watery stools
  • Persists with fasting
  • Stool pH >6
  • No reducing substances in stool
  • Fecal osmotic gap <50 mOsm/kg (electrolytes account for most osmolality)
• Examples: cholera toxin (V. cholerae), VIPoma, carcinoid syndrome, bile acid malabsorption

2. Osmotic Diarrhoea

• Caused by poorly absorbed solutes in the gut lumen drawing water osmotically
• Characterized by:
  • Stops or reduces with fasting
  • Stool pH <6
  • Reducing substances present in stool
  • Fecal osmotic gap >100 mOsm/kg (unabsorbed solutes account for osmolality)
• Formula: Osmotic gap = 290 - 2 × (fecal Na⁺ + fecal K⁺)
• Examples: lactase deficiency, sorbitol/lactulose ingestion, magnesium-containing antacids

3. Inflammatory / Exudative Diarrhoea

• Destruction of intestinal villous cells or dysfunction of cellular transporters
• Results in loss of fluids, electrolytes, mucus, proteins, and blood
• Dysentery: diarrhoea associated with blood and mucus, indicating compromised bowel wall
• Examples: Salmonella, Shigella, Campylobacter, IBD (ulcerative colitis, Crohn's disease)

4. Malabsorptive / Fatty Diarrhoea (Steatorrhoea)

• Defective absorption of fat and nutrients in the small intestine
• Stools are greasy, foul-smelling, float
• Examples: pancreatic exocrine insufficiency, coeliac disease, small intestinal bacterial overgrowth (SIBO), tropical sprue

5. Motility-Related Diarrhoea

• Rapid intestinal transit reduces contact time for absorption
• Examples: hyperthyroidism, irritable bowel syndrome (IBS-D), post-vagotomy diarrhoea, autonomic neuropathy in diabetes

6. Psychogenic Diarrhoea

• Excessive parasympathetic stimulation causing increased motility AND mucus secretion in the distal colon
• Occurs during periods of emotional stress (e.g., examination anxiety, combat stress)

C. By Stool Characteristics (Clinical Classification)

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3. ETIOLOGY

A. Infectious Causes

Viral (most common in developed countries)

Bacterial

Protozoal / Parasitic

Fungal (mainly immunocompromised)

• Candidiasis, Histoplasmosis, Cryptococcosis

B. Non-Infectious Causes

Special Settings (Epidemiological)

• Travellers: bacterial infections (ETEC), amebiasis, giardiasis, tropical sprue
• AIDS patients: Cryptosporidia, Microsporidium, CMV, MAC, Kaposi's sarcoma
• Diabetics: altered motility, coeliac disease, SIBO, metformin/acarbose
• Hospitalised patients: C. difficile, drug side effects, ischaemic colitis
• Children: Rotavirus, norovirus; higher mortality risk in developing countries

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4. PATHOPHYSIOLOGY

Normal Fluid Physiology

• ~9 L/day of fluid enters the proximal small bowel (diet + endogenous secretions from saliva, gastric juice, bile, pancreatic juice)
• ~90% (7–8 L) is absorbed in the small intestine
• ~1.5 L passes to the large intestine, which absorbs all but 100–150 mL
• Three main absorption mechanisms:
  1. NaCl absorption in small bowel via Na⁺/H⁺ and Cl⁻/HCO₃⁻ exchange (damaged in acute diarrhoea)
  2. Electrogenic Na⁺ absorption in colon (damaged in acute diarrhoea)
  3. Na⁺ co-transport with glucose/amino acids (intact in acute diarrhoea → basis of ORS therapy)

Mechanisms of Diarrhoea

1. Increased Secretion

• Microbial toxins (e.g., cholera toxin) stimulate cAMP/cGMP in enterocytes → open CFTR chloride channels → massive Cl⁻ and water secretion into the lumen
• Cholera can produce 10–12 L/day of fluid; colon can absorb a maximum of 6–8 L/day → net massive fluid loss

2. Mucosal Invasion and Inflammation

• Enteroinvasive organisms (Salmonella, Shigella, Campylobacter) cause mucosal inflammation
• Neutrophil infiltration → release of enzymes and cytokines → both increased secretion and decreased absorption
• Destruction of villous cells → loss of absorptive surface area
• Dysentery results when bowel wall integrity is compromised → blood and mucus in stool

3. Osmotic Mechanism

• Acute viral gastroenteritis disrupts microvilli of small bowel epithelium
• Decreased absorptive surface → unabsorbed solutes remain in lumen
• These solutes exert osmotic gradient → water and electrolyte influx into lumen
• Unabsorbed carbohydrates fermented by bacteria → hydrogen gas, short-chain fatty acids → further osmotic load, bloating, flatulence

4. Motility Disturbance

• Rapid intestinal transit (hyperthyroidism, psychogenic diarrhoea via parasympathetic over-stimulation) → reduced contact time for absorption
• In IBD: the ulcerated, inflamed colon has such increased motility that mass movements occur throughout the day rather than 10–30 minutes

5. Paediatric Vulnerability

• Relatively larger extracellular fluid compartments → proportionately greater losses
• Fluid turnover in infants/young children is 3× that of adults (higher metabolic rate, greater body surface area:mass ratio, higher body water content)
• Limited glycogen/fat reserves, limited renal water conservation → high risk of hypoglycaemia, electrolyte abnormalities, dehydration, and shock

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5. CLINICAL FEATURES

General Symptoms

• Frequent, loose or liquid stools (watery, mucousy, bloody, or foul-smelling)
• Nausea, vomiting
• Decreased appetite, weight loss
• Abdominal pain or cramps
• Fever, malaise, headache
• Signs of dehydration (see below)

Signs of Dehydration (Assessment)

Stool Characteristics as Diagnostic Clues

Specific Clinical Presentations

• Infectious diarrhoea: symptoms begin 12–72 hours after exposure; viral causes typically resolve within 1 week
• Cholera: painless, profuse watery diarrhoea, "rice-water" stools, severe dehydration within hours
• C. difficile: watery diarrhoea post-antibiotic, fever, leukocytosis; toxic megacolon risk (tachycardia, hypotension, delirium)
• IBD: chronic bloody diarrhoea, weight loss, abdominal pain, extra-intestinal manifestations
• VIPoma (WDHA syndrome): Watery Diarrhoea, Hypokalaemia, Achlorhydria; voluminous secretory diarrhoea
• Post-vagotomy: diarrhoea after gastric surgery

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6. DIAGNOSTIC CRITERIA AND EVALUATION

History - Key Elements to Determine

1. Duration (acute vs. chronic)
2. Stool frequency, volume, and character (watery/bloody/fatty)
3. Presence of blood, mucus, or pus
4. Fever, tenesmus, weight loss
5. Travel history, antibiotic use, food ingestion (raw/undercooked)
6. Immunosuppression, HIV status
7. Similar illness in household contacts
8. Medications (laxatives, antacids, antibiotics, chemotherapy)
9. Family history of GI illness

Physical Examination

• Vital signs (tachycardia, hypotension = severe dehydration/shock)
• Hydration status (mucous membranes, skin turgor, capillary refill)
• Abdominal examination (tenderness, distension, bowel sounds)
• Anorectal examination
• Extra-intestinal signs (rash, arthritis, uveitis in IBD; petechiae in HUS)

Laboratory Investigations

Initial Screening

Infectious Work-Up

Endocrine/Systemic Causes

Imaging and Endoscopy

• Plain abdominal X-ray: toxic megacolon, obstruction
• Colonoscopy + biopsy: IBD, microscopic colitis, neoplasia, C. difficile pseudomembranes
• Upper GI endoscopy + biopsy: coeliac disease, Giardia (duodenal aspirate), tropical sprue
• CT abdomen/pelvis: complications of IBD, abscess, ischaemic colitis

Diagnostic Algorithm (Simplified)

Diarrhoea
├── Acute (<4 weeks)
│   ├── Mild, no blood, no fever, no systemic illness → supportive care only
│   └── Severe, bloody, febrile, or immunocompromised → stool culture, C. diff, O&P
└── Chronic (>4 weeks)
    ├── Watery → is it secretory or osmotic? (fasting test + osmotic gap)
    │   ├── Secretory → hormonal screen, colonoscopy with biopsies (microscopic colitis)
    │   └── Osmotic → dietary history, lactase deficiency, laxative screen
    ├── Inflammatory (blood/mucus/fecal WBCs) → colonoscopy + biopsy (IBD, infection, neoplasm)
    └── Fatty (steatorrhoea) → 72h faecal fat, pancreatic function, coeliac screen, small bowel imaging

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7. MANAGEMENT

A. Rehydration (First Priority)

• WHO reduced-osmolality ORS (245 mOsm/L) is the standard: Na⁺ 75 mmol/L, K⁺ 20 mmol/L, glucose 75 mmol/L, Cl⁻ 65 mmol/L, citrate 10 mmol/L
• The Na⁺-glucose co-transport mechanism (remains intact even in acute diarrhoea) allows ORS absorption even when other mechanisms are impaired - this is the physiological rationale for ORS
• High-sugar beverages (fruit juices, soft drinks) should be avoided - they can worsen fluid losses due to insufficient salt content
• Normal feeding should continue during treatment (do not fast infants/children)
• Rice-based ORS or amylase-resistant starch ORS may reduce duration and volume of diarrhoea in cholera

• For severe dehydration, shock, or inability to take oral fluids
• Ringer's lactate or normal saline; correct hypovolaemia rapidly
• Monitor electrolytes closely (hypokalaemia common in cholera, secretory diarrhoea)

B. Dietary Management

• Continue breastfeeding in infants
• Resume normal diet as tolerated (do not restrict to "BRAT diet" exclusively per current evidence)
• Lactose-free diet may help in antibiotic-associated diarrhoea (transient lactase deficiency)
• Avoid dietary triggers if intolerance identified

C. Antimotility and Symptomatic Agents

• Bloody diarrhoea / dysentery
• Suspected C. difficile colitis
• STEC (O157:H7) infection - risk of haemolytic-uraemic syndrome (HUS)

D. Antibiotic Therapy

• Acute watery diarrhoea in the immunocompetent - usually NOT required; most is viral and self-limiting
• Empiric antibiotics: only for moderate-to-severe disease with systemic symptoms while awaiting cultures
• Antibiotics can increase the risk of HUS in STEC (O157:H7) infection - avoid in suspected STEC

E. Management of Specific Conditions

• Stop the offending antibiotic if possible
• Oral vancomycin 125 mg QDS or fidaxomicin 200 mg BD for 10 days
• Metronidazole IV + vancomycin for fulminant disease with hypotension/shock/ileus
• Fecal microbiota transplant (FMT): highly effective for recurrent C. difficile

• Aminosalicylates (mesalazine), corticosteroids (acute flares), immunomodulators (azathioprine), biologics (anti-TNF agents)
• Surgical resection for refractory ulcerative colitis (colectomy)

• Budesonide (first-line)
• Stop offending drugs (NSAIDs, PPIs, SSRIs)

• Octreotide (somatostatin analogue) for VIPoma, carcinoid
• Surgical resection of tumour if possible

• Dietary avoidance of lactose/trigger carbohydrates
• Lactase enzyme supplements

F. Adjunctive Therapies

• WHO recommends 10–20 mg/day for 10–14 days in children with acute diarrhoea
• Reduces severity, duration, and risk of future episodes
• Recent meta-analysis (PMID: 39641338, 2024): confirms benefit for acute and persistent watery diarrhoea in children

• Lactobacillus rhamnosus GG and Saccharomyces boulardii have evidence for reducing duration of acute diarrhoea
• Recent systematic review (PMID: 39703988, 2024): supports use in children under 10 years for acute and persistent diarrhoea

• Supplementation in malnourished children reduces diarrhoea severity

G. When to Hospitalise / Red Flag Indicators

• Signs of severe dehydration or shock
• Inability to maintain oral intake
• Bloody diarrhoea with high fever
• Suspected toxic megacolon (tachycardia, hypotension, delirium, abdominal distension)
• Immunocompromised patients
• Very young infants (<3 months) or elderly
• Suspected HUS (pallor, oliguria, petechiae after bloody diarrhoea)
• Failure to improve with outpatient management

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Summary Table

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What Was Covered

1. Definition

2. Types (Classification)

• By duration: Acute (<14 days), Persistent (14-28 days), Chronic (>4 weeks)
• By mechanism: Secretory, Osmotic, Inflammatory/Exudative, Malabsorptive/Fatty, Motility-related, Psychogenic
• By stool character: Watery, Bloody (dysentery), Fatty (steatorrhoea) - each pointing to different aetiologies

3. Etiology

4. Pathophysiology

5. Clinical Features

6. Diagnostic Criteria

7. Management

• Rehydration: ORS composition and physiological rationale (Na-glucose co-transport), IV fluids for severe cases
• Antimotility agents: Loperamide, diphenoxylate, bismuth, octreotide, cholestyramine
• Antibiotics: Pathogen-specific table with drug choices and important caveats (avoid in STEC)
• Specific conditions: C. difficile (vancomycin/fidaxomicin/FMT), IBD, microscopic colitis, VIPoma
• Adjuncts: Zinc supplementation (WHO-recommended for children), probiotics (recent 2024 meta-analysis support)
• Hospitalisation criteria: Red flag indicators