Worldwide, diarrhea remains one of the leading causes of morbidity and mortality among the pediatric population, especially in children under 5 years of age. Although clean water and sanitation initiatives have reduced mortality, diarrhea remains the second leading cause of death in children aged 1 to 59 months.1
Diarrhea is the condition of passing loose or liquid stools, usually three or more per day (or more frequently than is normal for an individual).2 It requires both an increase in frequency and liquidity of stools. Normal daily stool volumes vary by age and size of the child. Neonates and infants have a normal daily stool weight of 5 g/kg/day. For preschool-age children, average stool weights range from 50 to 75 g/day. Older children and adults have stool weights up to 250 g/day. Diarrhea typically results in stool volumes >10 g/kg/day in children younger than 3 years and >200 g/day in children older than 3. Diarrheal episodes are typically classified into acute and chronic, based on duration. Acute diarrhea is defined as an episode of diarrhea that is acute in onset and lasts less than 14 days. Chronic diarrhea is defined as an episode that lasts equal to or longer than 14 days. This distinction has implications for etiology, management, and prognosis.
In the normally functioning gastrointestinal tract, nutrients are absorbed via active, carrier-mediated transport across the intact mucosal lining. Electrolytes such as sodium, potassium, chloride, and bicarbonate are transported via both active and passive mechanisms, with sodium transport creating the most significant gradient. This sodium gradient is responsible for promoting the passive transport of water across the mucosal lining. Under normal circumstances, there is a balance between the absorptive and secretory functions, which are the opposing unidirectional electrolyte fluxes. This results in a net water absorption, with 90% of the absorption occurring in the small intestine.
Altered pathophysiologic mechanisms resulting in diarrhea can be divided into four categories: osmotic, secretory, inflammatory, and motility disorders.
Osmotic diarrhea occurs when excessive amounts of solutes are retained in the lumen of the intestine, resulting in decreased water absorption. Osmotic diarrhea will typically cease with fasting or withholding of the poorly absorbed solute. Secretory diarrhea occurs when secretion of water into the lumen of the intestine exceeds absorption. In most cases, the diarrhea is not affected by alterations in enteral intake. Inflammation can cause diarrhea when there is mucosal damage resulting in decreased water absorption and increased fluid secretion. Inflammatory diarrhea may be associated with mucus, blood, and protein losses in the stool. Lastly, motility disorders cause either increased or decreased transit time in the intestine, leading to altered water absorption. A decreased transit time due to increased motility results in diarrhea.
The frequency, duration, volume, and character of the diarrhea (including smell, color, and presence of mucus or blood) should be determined during history-taking. It is important to understand the patient’s normal bowel habits to determine the degree of change from baseline. A careful diet history may help identify inciting factors contributing to osmotic diarrhea, as well as identify foods associated with food- or water-borne pathogens (Salmonella, Shigella, Escherichia coli, Giardia). Daycare attendance, sick contacts, exposures to pets, and recent travel history should also be explored. Giardia and Salmonella are the most common parasitic and bacterial pathogens, respectively, causing diarrhea in daycare attendees. One should inquire about exposure to animal vectors such as reptiles, which can be associated with Salmonella, as well as travel to areas with common endemic parasitic or bacterial infections. A history of recent antibiotic use should be obtained. Antibiotic-associated diarrhea typically resolves with cessation of medication, but Clostridium difficile can be associated with persistent diarrhea or blood in stool following recent antibiotic use.
It is also important to elicit any accompanying systemic symptoms. Fever is often associated with viral and bacterial infections. Rash or wheezing may accompany allergy-mediated diarrhea. Seizure activity associated with diarrhea in an otherwise healthy child can be seen with Shigella infection.
Growth history and nutritional status should be obtained, especially when considering the cause of chronic diarrhea. One of the most common causes of chronic diarrhea in toddler-age children is a disorder of small intestinal motility called nonspecific diarrhea of childhood (“toddler’s diarrhea”).
Post-infectious carbohydrate intolerance, a form of acquired lactose intolerance, can result in osmotic diarrhea in an otherwise well patient with recent acute gastroenteritis. Both nonspecific diarrhea of childhood and post-infectious carbohydrate intolerance may be related to ingestion of excessive amounts of sugary fluids (juice, soda). Constipation with encopresis should also be considered in children with chronic diarrhea, which actually represents leakage of watery stool around a blockage in a dilated colon.
Plotting a careful growth curve can assist in making a diagnosis in children with chronic diarrhea. Patients with cystic fibrosis often have poor growth beginning soon after birth. In contrast, children with celiac disease typically exhibit poor growth after 4 to 6 months of age, coincident with the introduction of gluten-containing solids to the diet.
