Mechanisms of Diarrhoea
Pathophysiological Classification
Understanding the mechanism of diarrhoea is essential for appropriate diagnosis and management.
Osmotic Diarrhoea
Occurs when a soluble compound cannot be absorbed by the small intestine, drawing fluid into the intestinal lumen.
Examples include: osmotic laxatives; magnesium-based antacids; foods containing mannitol, sorbitol, or xylitol.
Can also be due to generalized malabsorption (e.g., coeliac disease, pancreatic insufficiency).
Secretory Diarrhoea
Results from increased secretion of fluid and electrolytes into the intestine with decreased absorption.
Infections with organisms such as Vibrio cholerae, E. Coli, and C. difficile.
Other causes: bile salts in the colon (e.g., after ileal resection), certain drugs (laxatives, diuretics, theophylline, etc.), and gut allergies.
Inflammatory Diarrhoea
Damage to intestinal mucosal cells affects absorption of fluid and electrolytes and results in fluid and blood loss.
Infection (e.g., Shigella) and conditions such as ulcerative colitis and Crohn's disease.
Increased Intestinal Motility
Presents with increased frequency of stool passage without an increase in volume.
Can occur with endocrine conditions such as diabetes and hyperthyroidism.
Clinical Assessment
Red Flag Symptoms
Immediate attention required for: blood in stool, recent hospital treatment or antibiotics, weight loss, evidence of dehydration.
History Taking
Determine the onset, duration, frequency, and severity of symptoms.
Enquire about associated symptoms: fever, vomiting, abdominal pain, tenesmus.
Identify potential exposures: recent travel, contact with ill persons, food sources, antibiotic use.
Assess for risk factors: food handlers, healthcare workers, nursing home residents.
Physical Examination
Assess for signs of dehydration (see table below).
Perform abdominal examination: pain/tenderness, distension, masses, bowel sounds.
Consider rectal examination: assess for tenderness, stool consistency, blood, mucus.
Dehydration Assessment
| Mild Dehydration |
Moderate Dehydration |
Severe Dehydration |
| Lassitude |
Apathy/tiredness |
Profound apathy |
| Anorexia |
Dizziness |
Weakness |
| Nausea |
Nausea |
Confusion |
| Lightheadedness |
Headache |
Coma |
| Postural hypotension |
Muscle cramps |
Shock |
| Usually no signs |
Pinched face, dry tongue, sunken eyes |
Marked peripheral vasoconstriction |
|
Reduced skin elasticity |
SBP < 90 mmHg |
|
Postural hypotension, tachycardia |
Oliguria or anuria |
|
Oliguria |
|
Management
Emergency Admission Criteria
Arrange emergency admission if: vomiting and unable to retain oral fluids, features of severe dehydration or shock.
Assessment for Admission
Consider admission for:
- Older age (≥60 years at higher risk of complications)
- Poor home circumstances and limited support
- Fever, bloody diarrhoea, significant abdominal pain/tenderness
- Coexisting medical conditions or medications increasing risk
General Management Principles
Rehydration
Oral rehydration solution for mild-moderate dehydration. IV fluids for severe dehydration or vomiting.
Dietary Management
Continue breastfeeding in infants. BRAT diet (bananas, rice, applesauce, toast) may help. Avoid fatty foods.
Symptom Control
Loperamide can be used cautiously in adults without fever or bloody diarrhoea. Avoid in children.
Specific Treatments
Antibiotics are not routinely indicated for acute diarrhoea and may prolong some infections.
Specific antibiotic therapy may be needed for:
- Severe C. difficile infection
- Dysentery (bloody diarrhoea with fever)
- Systemic infection
- Immunocompromised patients
- Specific pathogens identified (e.g., Giardia, Campylobacter)
Treat underlying cause in chronic diarrhoea (e.g., gluten-free diet for coeliac disease, steroids for IBD).
Follow-up and Prevention
Advise on hand hygiene and food safety practices to prevent spread.
Exclude food handlers, healthcare workers, and children from daycare until symptom-free for 48 hours.
Follow up persistent or chronic diarrhoea for further investigation if not resolved.