Diarrhea 

Acute diarrhea is defined as the abrupt onset of abnormally high fluid content in the stool: more than the normal value of approximately 10 mL/kg/d in the infant and young child, and more than 200 g/d in the teenager and adult. This situation typically implies an increased frequency of bowel movements, which can range from 4-5 to more than 20 times per day. The augmented water content in the stools is due to an imbalance in the physiology of the small and large intestinal processes involved in the absorption of ions, organic substrates, and thus water. A common disorder in its acute form, diarrhea has many causes and may be mild to severe.

Childhood acute diarrhea is usually caused by infection; however, numerous disorders may cause this condition, including a malabsorption syndrome and various enteropathies. Acute-onset diarrhea is usually self-limited; however, an acute infection can have a protracted course. By far, the most common complication of acute diarrhea is dehydration.

Although the term "acute gastroenteritis" is commonly used synonymously with "acute diarrhea," the former term is a misnomer. The term gastroenteritis implies inflammation of both the stomach and the small intestine, whereas, in reality, gastric involvement is rarely if ever seen in acute diarrhea (including diarrhea with an infectious origin); enteritis is also not consistently present. Examples of infectious acute diarrhea syndromes that do not cause enteritis include Vibrio cholerae– induced diarrhea and Shigella -induced diarrhea. Thus, the term acute diarrhea is preferable to acute gastroenteritis.

Diarrheal episodes are classically distinguished into acute and chronic (or persistent) based on their duration. Acute diarrhea is thus defined as an episode that has an acute onset and lasts no longer than 14 days; chronic or persistent diarrhea is defined as an episode that lasts longer than 14 days. The distinction, supported by the World Health Organization (WHO), has implications not only for classification and epidemiological studies but also from a practical standpoint because protracted diarrhea often has a different set of causes, poses different problems of management, and has a different prognosis.

Diarrhea is the reversal of the normal net absorptive status of water and electrolyte absorption to secretion. Such a derangement can be the result of either an osmotic force that acts in the lumen to drive water into the gut or the result of an active secre­tory state induced in the enterocytes. In the former case, diarrhea is osmolar in nature, as is observed after the ingestion of nonabsorbable sugars such as lactulose or lactose in lactose malabsorbers. Instead, in the typical active secretory state, enhanced anion secretion (mostly by the crypt cell compartment) is best exemplified by enterotoxin-­induced diarrhea.

In osmotic diarrhea, stool output is proportional to the intake of the unabsorbable substrate and is usually not massive; diarrheal stools promptly regress with discontinuation of the offending nutrient, and the stool ion gap is high, exceeding 100 mOsm/kg. In fact, the fecal osmolality in this circumstance is accounted for not only by the electrolytes but also by the unabsorbed nutrient(s) and their degradation products. The ion gap is obtained by subtracting the concentration of the elec­trolytes from total osmolality (assumed to be 290 mOsm/kg), according to the formula: ion gap = 290 – [(Na + K) × 2].

In secretory diarrhea, the epithelial cells’ ion transport processes are turned into a state of active secretion. The most common cause of acute-onset secretory diarrhea is a bacterial infection of the gut. Several mechanisms may be at work. After colonization, enteric pathogens may adhere to or invade the epithelium; they may produce enterotoxins (exotoxins that elicit secretion by increasing an intracellular second messenger) or cytotoxins. They may also trigger release of cytokines attracting inflammatory cells, which, in turn, contribute to the acti­vated secretion by inducing the release of agents such as prostaglandins or platelet-activating factor. Features of secretory diarrhea include a high purg­ing rate, a lack of response to fasting, and a normal stool ion gap (ie, 100 mOsm/kg or less), indicating that nutrient absorption is intact.

Frequency

United States

In the United States, one estimate assumes a cumulative incidence of 1 hospitalization for diarrhea per 23-27 children by age 5 years, with more than 50,000 hospitalizations in 2000. By these estimates, rotavirus is associated with 4-5% of all childhood hospitalizations, and 1 in 67 to 1 in 85 children are hospitalized due to rotavirus by age 5 years. Furthermore, acute diarrhea is responsible for 20% of physician referrals in children younger than 2 years and for 10% in children younger than 3 years.

International

In developing countries, an average of 3 episodes per child per year in children younger than 5 years is reported; however, some areas report 6-8 episodes per year per child. In these settings, malnutrition is an impor­tant additional risk factor for diarrhea, and recurrent episodes of diarrhea lead to growth faltering. Childhood mortality associated with diarrhea has constantly but slowly declined during the past 2 decades, mostly because of the widespread use of oral rehydration solutions; however, it appears to have plateaued over the past few years.

Because the single most common cause of infectious diarrhea worldwide is rotavirus, and because a vaccine has been in use for over 3 years now, a reduction in the overall frequency of diarrheal episodes is hoped for in the near future.

Mortality/Morbidity

Mortality from acute diarrhea is overall globally declining but remains high. Most estimates have diarrhea as the second cause of childhood mortality, with 18% of the 10.6 million yearly deaths in children younger than age 5 years.

Despite a progressive reduction in global diarrheal disease mortality over the past 2 decades, diarrhea morbidity in published reports from 1990-2000 slightly increased worldwide compared with previ­ous reports.

Furthermore, in countries where the toll of diarrhea is highest, poverty also adds an enormous additional burden, and long-term conse­quences of the vicious cycle of enteric infections, diarrhea, and malnutrition are devastating.

Sex

Most cases of infectious diarrhea are not sex specific. Females have a higher incidence of Campylobacter species infections and hemolytic uremic syndrome (HUS).

Age

Viral diarrhea is most common in young children. Rotavirus and adenovirus are particularly prevalent in children younger than 2 years. Astrovirus and norovirus usually infect children younger than 5 years. Yersinia enterocolitis typically infects children younger than 1 year, and the Aeromonas organism is a significant cause of diarrhea in young children.

Very young children are particularly susceptible to secondary dehydration and secondary nutrient malabsorption. Age and nutritional status appear to be the most important host factors in determining the severity and the duration of diarrhea. In fact, the younger the child, the higher is the risk for severe, life-threatening dehydration as a result of the high body-water turnover and limited renal compensatory capacity of very young children. Whether younger age also means a risk of run­ning a prolonged course is an unsettled issue. In developing countries, persis­tent postenteritis diarrhea has a strong inverse correlation with age.