gastrointestinal disorders
Diarrhea
Etiology of Diarrhea
Normally, the small intestine and colon absorb 99% of fluid resulting from oral intake and gastrointestinal (GI) tract secretions—a total fluid load of about 9 of 10 L daily. Thus, even small reductions (ie, 1%) in intestinal water absorption or increases in secretion can increase water content enough to cause diarrhea. There are a number of causes of diarrhea ( see Table: Some Causes of Diarrhea* ). Several basic mechanisms cause most clinically significant diarrheas. The three most common are increased osmotic load, increased secretions/decreased absorption, and decreased contact time/surface area. In many disorders, more than one mechanism is active. For example, diarrhea in (hydroxide and sulfate), and sodium phosphate, which are used as laxatives. Osmotic diarrhea occurs with sugar intolerance (eg, Carbohydrate Intolerance caused by Diarrhea occurs when the bowels secrete more electrolytes and water than they absorb. Causes of increased secretions include infections, unabsorbed fats, certain drugs, and various intrinsic and extrinsic secretagogues. Infections (eg, Overview of Gastroenteritis ) are the most common causes of secretory diarrhea. Infections combined with food poisoning are the most common causes of acute diarrhea ( Unabsorbed dietary fat and bile acids (as in Overview of Malabsorption and after ileal resection) can stimulate colonic secretion and cause diarrhea. Drugs may stimulate intestinal secretions directly (eg,quinidine,quinine,colchicine, anthraquinone cathartics,castor oil, prostaglandins) or indirectly by impairing fat absorption (eg,orlistat). Various endocrine tumors produce secretagogues, including Vipoma (vasoactive intestinal peptide), Impaired absorption of bile salts, which can occur with several disorders, can cause diarrhea by stimulating water and electrolyte secretion. The stools have a green or orange color. Rapid intestinal transit and diminished surface area impair fluid absorption and cause diarrhea. Common causes include small-bowel or large-bowel resection or bypass, gastric resection, and Overview of Inflammatory Bowel Disease . Other causes include microscopic colitis (collagenous or lymphocytic colitis) and Stimulation of intestinal smooth muscle by drugs (eg, magnesium-containing antacids, laxatives, cholinesterase inhibitors, selective serotonin reuptake inhibitors) or humoral agents (eg, prostaglandins, serotonin) also can speed transit. 