How Big a Problem Is Diarrhea?

Diarrhea is the most common cause of morbidity and mortality in the world, although in developed nations it has much less severe consequences. Acute diarrhea in North America is typically viral in etiology, is usually self-limited, and requires no diagnostic studies or specific therapy. When bacterial infection is suspected, stool testing should be done. Chronic or recurrent diarrhea has a much broader differential diagnosis, but most cases are not caused by serious underlying pathology. To diagnose and treat acute and chronic diarrhea, it is important to have a good understanding of its pathophysiology.

What Is Diarrhea?

Parents concerned about diarrhea usually describe stool that has a liquid texture. Sometimes they might mean excessively frequent stools, or are concerned with large-volume stools. The accepted medical definition of diarrhea is excessive daily stool volume, more than the normal upper limit of around 10 ml/kg/day. It is certainly possible to have diarrhea by this definition with stools that are at least partially formed or to not have diarrhea even with liquid bowel movements.

What Causes Diarrhea?

Physiologic mechanisms of diarrhea include excessive secretion of water and electrolytes (secretory diarrhea), and malabsorption (malabsorptive or osmotic diarrhea).

Etiology may be infectious, inflammatory, or neither.

What Is Secretory Diarrhea?

Secretory diarrhea may be stimulated by infection or inflammation. Cholera, the classic example of secretory diarrhea, is caused by a bacterial toxin that stimulates intestinal secretion of water and electrolytes. Intestinal inflammation, as in ulcerative colitis and Crohn’s disease, is a much more common cause of secretory diarrhea. In this case, cytokines, prostaglandins, and enteric hormones are responsible for increased secretion. Regardless of the cause, secretory diarrhea yields watery stools with a relatively high electrolyte content. Secretory diarrhea continues even when fasting.

How Does Osmotic Diarrhea Develop?

Osmotic diarrhea commonly results from intestinal injury, dietary indiscretions, or specific defects in digestion leading to malabsorption. Celiac disease is a relatively common example of intestinal injury causing malabsorption. Cystic fibrosis causes diarrhea because of maldigestion of fats and proteins. Osmotic diarrhea can also be caused by the ingestion of nondigestible or nonabsorbable substances. Laxatives such as magnesium salts, polyethylene glycol, and lactulose all fit the latter category. When a single, specific digestive enzyme is lacking, as in lactose intolerance, osmotic diarrhea results only when the substrate of that enzyme (lactose) is eaten. Overindulgence of certain foods for which we have limited absorptive capacity can also cause diarrhea. A common example is the watery diarrhea seen with excessive intake of fructose in fruit juices or soft drinks. Stools in osmotic diarrhea have a relatively low electrolyte content because the osmolarity of the stool is largely accounted for by the malabsorbed material. Osmotic diarrhea ceases when enteral intake is withheld.

What Is “Toddler’s Diarrhea?”

Toddler’s diarrhea occurs when young children (“toddlers”) drink large quantities of sweet, clear liquids. The watery, osmotic diarrhea results from the large fluid and carbohydrate load that exceeds the intestinal absorption capacity. These children have excellent growth and no evidence of generalized malabsorption. The loose stools are particularly evident when the toddlers are still in diapers, a messy and unpleasant situation.

How Can I Distinguish the Types of Diarrhea?

History usually provides most of the information needed to decide if true diarrhea is present, to classify the diarrhea by type, and to consider the diagnostic approach (Table 28-1). It is seldom possible to determine the exact weight of stool in grams per day.

Table 28-1 Types of Diarrhea

How Do I Determine the Cause of Acute Diarrhea?

Table 28-2 lists common causes of acute diarrhea. Stool cultures are seldom indicated unless fever or bloody diarrhea suggests bacterial etiology. Rotavirus is the most common infectious agent. This is seen mostly during the winter months and also is accompanied by vomiting. Stool odor is particularly offensive. Testing for rotavirus is seldom necessary, except for epidemiologic reasons.

Table 28-2 Conditions Causing Acute Diarrhea