Chronic diarrhea is defined as the abnormal passage of ≥3 loose or liquid stools per day for ≥4 week with or without a daily stool weight >200 g/day.
The definition of chronic diarrhea based on symptoms alone will lead to an overlap with functional bowel disorders eg irritable bowel syndrome.
Diarrhea that is continuous or nocturnal and lasting <3 months is more likely due to an organic disease.
The absence of abdominal pain during defecation and presence of weight loss are points against the diagnosis of functional bowel syndrome eg irritable bowel syndrome.

Differential Diagnosis

Osmotic Diarrhea

  • Water is retained due to substances that are poorly absorbed; stool osmotic gap is >125 mOsm/kg
    • Ingestion of exogenous Mg
    • Ingestion of osmotic laxatives
    • Ingestion of poorly absorbable carbohydrates

Secretory Diarrhea

  • Reduced water absorption, stool volume >1 L/day, frequently occurs at night, continues despite fasting, stool osmotic gap <50 mOsm/kg
    • Small bowel bacterial overgrowth
    • Endocrine diarrhea
    • Structural diseases [eg short bowel syndrome, inflammatory bowel disease (IBD), tumors, gastrocolic or enteroenteric fistula, mucosal diseases]
    • Peptide-secreting tumors [eg carcinoid syndrome, Zollinger-Ellison syndrome (ZES)]
Inflammatory Diarrhea
  • Increased white cell count with occult or frank pus or blood
    • Invasive infections
    • Pseudomembranous colitis, IBD, ischemia, radiation enteritis
    • Neoplasm
    • The above conditions may produce a secretory diarrhea without inflammatory markers in the stool and therefore should be considered in the evaluation of secretory diarrhea as well

Fatty Diarrhea 

  • Abdominal distension and bloating with malodorous, large, floating, pale fatty stool
    • Maldigestion (inadequate luminal breakdown of triglyceride): Pancreatic exocrine insufficiency, inadequate luminal bile acid [eg primary biliary cirrhosis (PBC)]
    • Malabsorption (inadequate mucosal transport of the products of digestion): Celiac disease, Orlistat drug, giardiasis
    • Small bowel bacterial overgrowth

Other Category

Functional Diarrhea

  •  A type of watery diarrhea that has hypermotility, small-volume stool (<350 mL/day), and improves at night and with fasting
  • Diagnostic criterion includes watery or loose stools, without bothersome bloating or predominant abdominal pain in >25% of bowel movements for the last 3 months with symptoms starting at least 6 months prior to diagnosis (Rome IV) 
    • Should not include patients fulfilling the criteria for diarrhea-predominant IBS

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