Pediatric Dehydration 

Pediatric dehydration is frequently the result of gastroenteritis, characterized by vomiting and diarrhea. However, other causes of dehydration may include poor oral intake due to diseases such as stomatitis, insensible losses due to fever, or osmotic diuresis from uncontrolled diabetes mellitus.

The terms dehydration and volume depletion are commonly used interchangeably to denote intravascular fluid depletion. However, it is useful for clinicians to understand that volume depletion is distinct from dehydration.^[1] Volume depletion denotes contraction of the total intravascular plasma pool, whereas dehydration denotes loss of plasma free water disproportionate to loss of sodium, the main intravascular solute. The distinction is important because volume depletion can exist with or without dehydration, and dehydration can exist with or without volume depletion.

In children with dehydration, the most common underlying problem actually is volume depletion, not dehydration. Intravascular sodium levels are within the reference range, indicating that excess free water is not being lost from plasma. Rather, the entire plasma pool is contracted with solutes (mostly sodium) and solvents (mostly water) lost in proportionate quantities. This is volume depletion without dehydration. The most common cause is excessive extrinsic loss of fluids in conditions such as vomiting and diarrhea.

Children, especially those younger than 4 years, tend to be more susceptible to volume depletion as a result of vomiting, diarrhea, or increases in insensible water losses. Significant fluid losses may occur rapidly. The turnover of fluids and solute in infants and young children can be as much as 3 times that of adults. This is because of the following:

• Higher metabolic rates
• Increased body surface area to mass index
• Higher body water contents

Water constitutes approximately 70% of body weight in infants, 65% in children, and 60% in adults.

Sodium considerations

Volume depletion can be concurrent with hyponatremia. This is characterized by plasma volume contraction with free water excess. An example is a child with diarrhea who has been given tap water to replete diarrheal losses. Free water is replenished, but sodium and other solutes are not.

In hyponatremic volume depletion, the patient may appear more ill clinically than fluid losses indicate. The degree of volume depletion may be clinically overestimated. Serum sodium levels less than 120 mEq/L may result in seizures. If intravascular free water excess is not corrected during volume replenishment, the shift of free water to the intracellular fluid compartment may cause cerebral edema.

With true dehydration, plasma volume contracts with a disproportionately larger loss of free water. An example is the child with diarrhea whose fluid losses have been replenished with hypertonic soup, boiled milk, water and baking soda, or improperly diluted infant formula. Volume has been restored, but free water has not.

In hypernatremic volume depletion, the patient may appear less ill clinically than fluid losses indicate. The degree of volume depletion may be underestimated. Usually, at least a 10% volume deficit exists with hypernatremic volume depletion.

As in hyponatremia, hypernatremic volume depletion may result in serious central nervous system (CNS) effects as a result of structural changes in central neurons. However, cerebral shrinkage occurs instead of cerebral edema. This may result in intracerebral hemorrhage, seizures, coma, and death. Overly rapid correction of hypernatremia, however, may result in cerebral edema. For this reason, volume restoration must be performed gradually over 24 hours or more. Gradual restoration prevents a rapid shift of fluid across the blood-brain barrier and into the intracellular fluid compartment.

Potassium considerations

Potassium shifts between intracellular and extracellular fluid compartments occur more slowly than free water shifts. Serum potassium level may not reflect intracellular potassium levels. Although a potassium deficit is present in all patients with volume depletion, it is not usually clinically significant. However, failure to correct for a potassium deficit during volume repletion may result in clinically significant hypokalemia. Potassium should not be added to replacement fluids until adequate urine output is obtained.

Acid and base problems

Derangements of acid-base balance may occur with volume depletion. Some degree of metabolic acidosis is common, especially in infants. Mechanisms include bicarbonate loss in stool and ketone production. Hypovolemia causes decreased tissue perfusion and increased lactic acid production. Decreased renal perfusion causes decreased glomerular filtration rate, which, in turn, leads to decreased hydrogen (H⁺) ion excretion. These factors combine to produce a metabolic acidosis.

In most patients, acidosis is mild and easily corrected with volume restoration; increased renal perfusion permits excretion of excess H⁺ ions in the urine. Administration of glucose-containing fluids further decreases ketone production.

Vomiting may be caused by central nervous system, gastrointestinal, endocrine, or renal processes. Psychogenic vomiting is not seen in infants and is rare in children. CNS causes of vomiting include infections and space-occupying lesions.

GI causes of vomiting include the following:

• Gastroenteritis
• Obstruction
• Hepatitis
• Liver failure
• Appendicitis
• Peritonitis
• Intussusception
• Volvulus
• Pyloric stenosis
• Drug toxicity (ingestion, overdose, drug effects)

Endocrine causes of vomiting include the following:

• Diabetic ketoacidosis (DKA)
• Congenital adrenal hypoplasia
• Addisonian crisis

Renal causes of vomiting include the following:

• Infection
• Pyelonephritis
• Renal failure
• Renal tubular acidosis

Diarrhea may be caused by GI or endocrine disorders or by anxiety. GI disorders that may cause vomiting include the following:

• Gastroenteritis
• Malabsorption
• Intussusception
• Irritable bowel
• Inflammatory bowel disease
• Short gut syndrome

Endocrine causes of diarrhea include the following:

• Thyrotoxicosis
• Congenital adrenal hypoplasia
• Addisonian crisis
• Diabetic enteropathy

Volume depletion not caused by vomiting or diarrhea may be divided into renal or extrarenal causes. Renal causes include use of diuretics, renal tubular acidosis, and renal failure (eg, trauma, obstruction, salt-wasting nephritis). The effects of diabetes insipidus, hypothyroidism, and adrenal insufficiency also fall into this category.

Extrarenal causes of volume depletion include the following:

• Third-space extravasation of intravascular fluid (eg, pancreatitis, peritonitis, sepsis, heart failure)
• Insensible losses from fever, sweating, burns, or pulmonary processes
• Poor oral intake
• Hemorrhage

Dehydration from volume depletion is a common complication of illness observed in pediatric patients presenting to the emergency department (ED). At times, it is the presenting complaint. Early recognition and early intervention are important to prevent progression to shock and cardiovascular collapse.

In most cases, volume depletion in children is from fluid losses from vomiting or diarrhea. On physical examination, combinations of findings can be used to determine the degree of dehydration. Laboratory studies are of limited utility in cases of mild dehydration, but they may be considered under certain conditions and are recommended in patients with more severe dehydration.

Mild or moderate volume depletion can be treated with oral rehydration. Intravenous fluid therapy is used when oral therapy fails or volume depletion is severe.

For patient education information, see the Children's Health Center, as well as Dehydration in Children.

Pediatric dehydration, particularly that due to gastroenteritis, is a common emergency department complaint. Approximately 200,000 hospitalizations and 300 deaths are attributed to gastroenteritis each year in the United States.

Worldwide, according to the Centers for Disease Control and Prevention (CDC), for children younger than 5 years, the annual incidence of diarrheal illness is approximately 1.5 billion, while deaths are estimated between 1.5 and 2.5 million. Though these numbers are staggering, they actually represent an improvement from the early 1980s, when the death rate was approximately 5 million per year.^[2]

Infants and younger children are more susceptible to volume depletion than older children. In general, however, pediatric patients with volume depletion have an excellent prognosis if they are appropriately treated.

Morbidity varies with the degree of volume depletion and the underlying cause. The severely volume-depleted infant or child is at risk for death from cardiovascular collapse. Hyponatremia resulting from replacement of free water alone may cause seizures. Improper management of volume repletion may cause iatrogenic morbidity or mortality.