Approach to Diarrhoea

A structured clinical approach rather than a single disease.

• Diarrhoea is classified by duration as acute (under 2 weeks, overwhelmingly infectious in etiology), persistent (2 to 4 weeks), and chronic (over 4 weeks, where the differential shifts toward non-infectious causes and requires a more systematic workup).

• Further classified by mechanism as secretory (large volume, watery, persists with fasting, for example cholera or carcinoid), osmotic (improves with fasting, for example lactose intolerance or laxative use), inflammatory (blood or mucus, associated with fever and pain, for example IBD or invasive bacterial infection), and motility related (IBS, hyperthyroidism, diabetic autonomic neuropathy).

A structured history should establish duration, stool characteristics (watery versus bloody versus fatty and greasy, suggesting malabsorption), frequency and volume, associated symptoms (fever, abdominal pain, weight loss, nocturnal symptoms which point away from a purely functional cause), relevant exposures (travel, food, sick contacts, antibiotic use, water source), and relevant background (immunosuppression, recent hospitalization, medication review, family history of IBD or colorectal cancer).

Examination should assess hydration status (see Severe Dehydration entry), nutritional state, abdominal tenderness or a mass, and signs pointing toward a specific cause (thyroid examination, perianal inspection for fistulizing Crohn's disease, skin or joint findings suggesting IBD or celiac disease).

Red flags warranting prompt further investigation include bloody diarrhoea, weight loss, nocturnal symptoms, age over 50 with new onset symptoms, family history of colorectal cancer or IBD, and iron deficiency anemia.

Acute diarrhoea in an otherwise well patient generally requires no investigation and is managed symptomatically (see Acute Gastroenteritis entry).

• Investigation is warranted for severe illness, bloody diarrhoea, immunocompromise, recent hospitalization or antibiotic use, or symptoms persisting beyond 7 to 10 days: stool microscopy, culture and sensitivity, stool ova and parasites, and Clostridioides difficile toxin or PCR if relevant exposure.

• Chronic diarrhoea requires a broader workup: FBC, inflammatory markers, TSH, celiac serology (anti-tTG IgA with total IgA), fecal calprotectin (distinguishes inflammatory from functional causes with good sensitivity), and stool studies as above.

• Colonoscopy with biopsy is indicated for red flag features, chronic bloody diarrhoea, or an elevated fecal calprotectin, to assess for IBD or malignancy.

• Further specialized testing (hydrogen breath testing for lactose intolerance or small intestinal bacterial overgrowth, fecal fat quantification or fecal elastase for malabsorption, bile acid testing) is directed by the clinical pattern once initial screening is complete.

Differentials span an extensive list covered throughout this atlas, including Acute Gastroenteritis, Cholera, Ulcerative Colitis, Irritable Bowel Syndrome, celiac disease, chronic pancreatitis with exocrine insufficiency, hyperthyroidism, and colorectal malignancy.

Management is directed entirely at the underlying cause once identified, as detailed in the relevant condition specific entries throughout this atlas.

General principles applicable regardless of cause include maintaining hydration (oral rehydration solution for significant fluid loss, see Severe Dehydration entry), nutritional support particularly where malabsorption or chronicity has caused weight loss or micronutrient deficiency, and judicious use of antimotility agents (loperamide), which are appropriate for functional or mild non-inflammatory diarrhoea but should be avoided when an invasive bacterial or inflammatory cause is suspected, given the risk of worsening the underlying process.

Referral: gastroenterology for chronic diarrhoea with red flag features, positive fecal calprotectin, abnormal celiac serology, or diagnostic uncertainty after initial workup.