Advances in the understanding of the pathophysiologic mechanism of diarrhea allow a more rational approach to diagnosis and treatment. Normally, the gastrointestinal tract processes
a large volume of fl uid (Figure 15-1 lists adult data). An infant can rapidly become fl uid depleted from diarrhea when such large gastrointestinal fl uid shifts take place each day. Under normal circumstances, about 90% of fl uid absorption takes place in the small bowel. However, the colon has a reserve capacity for fl uid absorption that must be overcome before diarrhea results. In adults, the colon can reabsorb as much as 2 L of ileal fl uid daily without diarrhea occurring.
Movement of water across the gastrointestinal tract mucosa is passive, following osmotic gradients created by electrolytes and other osmotically active solutes such as glucose and amino acids. Nutrients are absorbed by active transport, facilitated transport, or passive diff usion; some solutes fi rst require digestion to simpler compounds. Th e fl ux of electrolytes across the mucosa is bidirectional. Th e net result of absorption and secretion of these osmotically active solutes is net water retention or loss in the stool. In this sense, diarrhea can be considered the result of either malabsorption or net secretion of osmotically active substances.
Many nutrients, including glucose and most amino acids, are absorbed by active, carrier- mediated transport, which is coupled with sodium transport. Th e osmotic gradient created promotes the absorption of water. Movement of water, in turn, also carries small solutes such as sodium and chloride. Th is process is known as solvent drag and appears to be an important
route for sodium absorption during normal digestion. Th ese mechanisms of sodium move- ment associated with carrier-mediated nonelectrolyte transport are important to preserve
Mouth and esophagus
Duodenojejunal fluid load 9–10 Jejunal absorption 5–6 Ileal absorption 2–3 Colonic absorption 0.5–1.5 Fecal volume 0.1–0.2 Diet 2–3 Saliva 1 Gastric juices 2 Bile 1 Pancreatic juice 2 Succus entericus 1 Stomach Extracellular fluid Figure 15-1
Ingestion, secretion, and absorption of water in the gastrointestinal tract of an adult. Numbers refer to liters of water.
Diarrhea and Steatorrhea 175
normal fl uid and electrolyte balance during some episodes of diarrhea (see discussion on oral rehydration).
Active absorption of chloride in exchange for bicarbonate takes place in the ileum and colon. Potassium moves passively along electrochemical gradients in the small intestine, but both active absorption and secretion of potassium occur in the colon. Th e permeability of the intestinal mucosa to passive fl uid and electrolyte movement is high in the duodenum and proximal jejunum and decreases distally to the ileum and colon, which are poorly permeable. Th is feature allows the proximal intestinal contents to equilibrate rapidly with the isotonic extracellular fl uid and facilitates the rapid absorption of water and small solutes by diff usion (ie, solvent drag). Conversely, the ileum and colon are poorly permeable and are able to absorb water and sodium against high electrochemical gradients.
Th e pathophysiologic mechanisms for diarrhea fall into 4 basic groups1,2: osmotic diar- rhea, diarrhea resulting from secretion or altered absorption of electrolytes, exudative diarrhea, and diarrhea resulting from abnormal intestinal motility. Each mechanism has unique clinical characteristics and requires a diff erent therapeutic approach. Th erefore, for the physician con- sidering an individual patient who has diarrhea, this framework provides a rational approach for both diagnosis and treatment. Frequently, more than one mechanism of diarrhea will be involved in an episode of diarrhea, but this variation will be apparent in the evaluation. Osmotic Diarrhea
Th e ingestion of a poorly absorbable, osmotically active substance and its presence in the bowel lumen create an osmotic gradient that encourages movement of water into the lumen and subsequently into the stool. Electrolyte losses increase because electrolytes will follow water into the lumen through solvent drag and will tend not to be reabsorbed because of unfavorable electrochemical gradients.
Two main groups of poorly absorbed solutes exist, the ingestion of which result in osmotic diarrhea. Th e fi rst group includes normal dietary components that may be malab- sorbed either transiently or permanently. For example, disaccharides are usually hydrolyzed to monosaccharides before they are absorbed. If a mucosal disaccharidase (eg, lactase) is defi cient, then the disaccharide (in this case lactose) will be malabsorbed and will represent an osmotic load that will produce diarrhea. Similarly, monosaccharides may, at times, be poorly absorbed. Medium-chain triglycerides are also osmotically active and may lead occa- sionally to diarrhea when ingested in high concentration, such as when infants who have compromised mucosal function are given an elemental formula containing medium-chain triglycerides. Malabsorption of long-chain triglycerides (LCTs) does not lead to osmotic diarrhea because LCTs are large hydrophobic molecules and therefore have little osmotic activity. Malabsorption of LCTs, however, may lead to secretory diarrhea, as described later in this chapter. In addition, any osmotically active solute may produce diarrhea in healthy persons if given in quantities great enough to surpass the intestinal capacity for absorption. Th us some infants whose bowel function is normal will not tolerate the high osmolality of an elemental formula, especially if it is undiluted. Similarly, older children may develop functional gastrointestinal symptoms, including diarrhea, from ingesting large amounts of fructose in fruits and juices.3 Patients who have decreased mucosal surface area may have decreased functional capacity and resultant osmotic diarrhea, a problem seen in infants after small bowel resection. Protein malabsorption does not appear to be associated with