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Diarrhoea

Medicine > Infection

Differential Diagnosis Schema 🧠

Infectious Causes

  • Viral gastroenteritis: Common cause, often with associated vomiting and mild fever.
  • Bacterial gastroenteritis: Sudden onset, may have blood in stool (e.g., Salmonella, Shigella, Campylobacter).
  • Clostridium difficile infection: Recent antibiotic use, hospital-acquired, watery diarrhoea.
  • Parasitic infections: Chronic diarrhoea, travel history (e.g., Giardia lamblia).

Inflammatory Causes

  • Inflammatory bowel disease (IBD): Chronic diarrhoea, blood in stool, weight loss (e.g., Crohn’s disease, Ulcerative colitis).
  • Ischaemic colitis: Sudden onset in elderly, abdominal pain, history of cardiovascular disease.
  • Radiation colitis: History of pelvic radiation, chronic diarrhoea, blood in stool.
  • Diverticulitis: LLQ pain, fever, change in bowel habits.

Malabsorptive and Osmotic Causes

  • Coeliac disease: Chronic diarrhoea, steatorrhoea, iron deficiency anaemia.
  • Lactose intolerance: Bloating, cramping, diarrhoea after dairy consumption.
  • Chronic pancreatitis: Fatty stools, weight loss, history of alcohol use.
  • Short bowel syndrome: History of bowel resection, chronic diarrhoea.

Functional and Idiopathic Causes

  • Irritable bowel syndrome (IBS): Alternating diarrhoea and constipation, abdominal pain relieved by defecation.
  • Microscopic colitis: Chronic watery diarrhoea, often in middle-aged women, normal colonoscopy.
  • Bile acid malabsorption: Chronic diarrhoea, previous cholecystectomy or ileal resection.
  • Hyperthyroidism: Weight loss, palpitations, heat intolerance.

Key Points in History 🥼

Onset and Duration

  • Acute (less than 2 weeks): Suggests infectious or toxin-mediated causes.
  • Chronic (more than 4 weeks): Points to inflammatory, malabsorptive, or functional disorders.

Stool Characteristics

  • Watery: Often viral, functional (IBS), or osmotic (lactose intolerance).
  • Bloody: Suggests inflammatory causes (IBD, ischaemic colitis, infection with E. coli O157:H7).
  • Fatty (steatorrhea): Points to malabsorption (coeliac disease, chronic pancreatitis).

Associated Symptoms

  • Fever: Suggests infection or inflammatory causes (IBD, diverticulitis).
  • Weight loss: Consider chronic infections, IBD, or malignancy.
  • Abdominal pain: Location may suggest cause (e.g., LLQ for diverticulitis).

Background

  • Travel history: Risk of parasitic infections (Giardia, Entamoeba histolytica).
  • Recent antibiotic use: Suggests Clostridium difficile infection.
  • Dietary changes: Consider lactose intolerance or food poisoning.
  • Family history of IBD or coeliac disease: Increased risk of these conditions.

Possible Investigations 🌡️

Blood Tests

  • Full blood count: Anaemia (IBD, chronic disease), raised white cells (infection).
  • C-reactive protein (CRP): Raised in inflammation (IBD, infection).
  • Thyroid function tests: Hyperthyroidism as a possible cause.
  • Coeliac serology: Anti-TTG and anti-EMA antibodies for coeliac disease.

Stool Tests

  • Stool culture: Identifies bacterial pathogens (Salmonella, Shigella).
  • Clostridium difficile toxin: Essential if recent antibiotic use or hospitalisation.
  • Faecal calprotectin: Marker for intestinal inflammation, useful in IBD.
  • Ova, cysts, and parasites: Consider if chronic diarrhoea or travel history.

Imaging and Endoscopy

  • Abdominal X-ray: May show signs of bowel obstruction or perforation.
  • CT abdomen/pelvis: Useful in suspected diverticulitis or malignancy.
  • Colonoscopy: Essential for diagnosing IBD, microscopic colitis, malignancy.
  • Upper GI endoscopy: Consider if coeliac disease or upper GI pathology suspected.

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Infection