Causes and pathophysiology of diarrhea

Diarrhea is primarily a consequence of excess water in the stool, which may be caused by infection, medication, food, surgery, inflammation, accelerated intestinal transit, or malabsorption. These causes can cause diarrhea by 4 different mechanisms: increased osmotic pressure in the intestinal lumen, increased secretion, inflammation, and decreased absorption time. Paradoxical diarrhea is a consequence of coprostasis and is fluid leakage around the stool. Acute diarrhea (< 4 days) is usually caused by etiologically defined causes such as food poisoning or infection.

Complications may result from diarrhea of any etiology. There may be fluid loss with dehydration, electrolyte loss (Na, K, Mg, Cl), and even occasionally vascular collapse. Collapse may develop rapidly in patients with severe diarrhea (eg, cholera), the very young, the elderly, or the malnourished. Loss of HCO3 _may cause metabolic acidosis. Hypokalemia may occur with severe or chronic diarrhea or if the stool contains much mucus. Hypomagnesemia after prolonged diarrhea may cause tetany.

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Osmotic diarrhea

Osmotic diarrhea occurs when non-absorbable, water-soluble substances that retain water are retained in the intestine. Such solutions include polyethylene glycol, magnesium salts (hydroxide and sulfate), and sodium phosphate, which are used as laxatives. Osmotic diarrhea occurs in sugar intolerance (e.g., lactose intolerance caused by lactase deficiency). Large amounts of hexitols (e.g., sorbitol, mannitol, xylitol), used as sweeteners in hard candies and chewing gums, cause osmotic diarrhea because of poor absorption of these substances. Lactulose, which is used as a laxative, causes diarrhea by a similar mechanism. Excessive intake of certain fruits can also cause osmotic diarrhea.

Secretory diarrhea

Secretory diarrhea occurs when the intestine secretes more electrolytes and water than are absorbed. Secretogens include bacterial toxins (eg, cholera and Clostridium difficile colitis), enteropathogenic viruses, bile acids (eg, after ileal resection), nonabsorbable dietary fats, and many drugs (eg, quinidine, quinine, colchicine, selective serotonin inhibitors, cholinesterase inhibitors, anthraquinone laxatives, castor oil, prostaglandins). Various endocrine tumors produce secretogens, including vipoma (vasoactive intestinal peptide), gastrinoma (gastrin), labrocytosis (histamine), medullary thyroid carcinoma (calcitonin and prostaglandins), and carcinoid tumors (histamine, serotonin, and polypeptides). Microscopic colitis (collagenous or lymphocytic) rarely causes secretory diarrhea, especially in women over 60 years of age.

Inflammatory diarrhea

Inflammatory diarrhea occurs with some infections and diseases that cause inflammation of the mucous membrane or ulceration (e.g., Crohn's disease, ulcerative colitis, tuberculosis, lymphoma, cancer). The resulting exudation of plasma, serum proteins, blood, and mucus into the intestinal lumen increases the volume of intestinal contents and fluid. Involvement of the rectal mucosa in the inflammatory process can cause sudden and frequent stools, since the inflamed rectum is more sensitive to stretching.

Dietary factors that cause diarrhea

Food factor	Source
Caffeine	Coffee, tea, cola, over-the-counter medications used for headaches
Fructose (in quantities exceeding the intestinal absorption capacity)	Apple juice, pear juice, grapes, honey, dates, nuts, figs, soft drinks (especially with fruits)
Hexitol, sorbitol and mannitol	Apple juice, pear juice, sugar-free chewing gum, mints
Lactose	Milk, ice cream, frozen yogurt, yogurt, soft cheeses
Magnesium-containing antacids	Antacids
Sucrose	Table sugar

Diarrhea due to decreased absorption time

Diarrhea due to decreased absorption time occurs when there is insufficient contact of chyme with the active absorptive surface of the gastrointestinal tract, resulting in too much water retention in the stool. Factors that decrease contact time include minor or major bowel resections, gastrectomy, pyloroplasty, vagotomy, intestinal bypasses, drugs (e.g., magnesium-containing antacids, laxatives), or humoral agents (e.g., prostaglandins, serotonin) that cause accelerated passage by stimulating intestinal smooth muscle.

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Malabsorption-associated diarrhea

Malabsorption diarrhea may result from osmotic or secretory mechanisms. The mechanism may be osmotic if non-absorbable, water-soluble substances of low molecular weight enter the intestine. Lipids are not osmotic substances, but some (fatty acids, bile acids) act as secretagogues and cause secretory diarrhea. Generalized malabsorption (eg, celiac disease), fat malabsorption cause colonic secretion, and carbohydrate malabsorption causes osmotic diarrhea. Malabsorption diarrhea may also develop in cases of delayed passage of chyme and proliferation of bacteria contained in the small intestinal contents, as occurs in intestinal stenosis, scleroderma with gastrointestinal lesions, adhesive disease, and after surgical interventions.

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