Dehydration in Children

by Michael F. Cellucci, MD

Pediatrics - Merck Manuals Professional Edition

Dehydration is significant depletion of body water and, to varying degrees, electrolytes. Symptoms and signs include thirst, lethargy, dry mucosa, decreased urine output, and, as the degree of dehydration progresses, tachycardia, hypotension, and shock. Diagnosis is based on history and physical examination. Treatment is with oral or IV replacement of fluid and electrolytes.

Dehydration remains a major cause of morbidity and mortality in infants and young children worldwide. Dehydration is a symptom or sign of another disorder, most commonly diarrhea. Infants are particularly susceptible to the ill effects of dehydration because of their greater baseline fluid requirements (due to a higher metabolic rate), higher evaporative losses (due to a higher ratio of surface area to volume), and inability to communicate thirst or seek fluid.

Etiology

Dehydration results from

• Increased fluid loss
• Decreased fluid intake

The most common source of increased fluid loss is the GI tract—from vomiting, diarrhea, or both (eg, gastroenteritis—see Overview of Gastroenteritis). Other sources are renal (eg, diabetic ketoacidosis), cutaneous (eg, excessive sweating, burns), and 3rd-space losses (eg, into the intestinal lumen in bowel obstruction or ileus).

Decreased fluid intake is common during mild illnesses such as pharyngitis or during serious illnesses of any kind. Decreased fluid intake is particularly problematic when the child is vomiting or when fever, tachypnea, or both increase insensible losses. It may also be a sign of neglect.

Pathophysiology

All types of lost fluid contain electrolytes in varying concentrations, so fluid loss is always accompanied by some degree of electrolyte loss. The exact amount and type of electrolyte loss varies depending on the cause (eg, significant amounts of HCO 3 - may be lost with diarrhea but not with vomiting). However, fluid lost always contains a lower concentration of Na than the plasma. Thus, in the absence of any fluid replacement, serum Na rises (hypernatremia). Hypernatremia causes water to shift from the intracellular and interstitial space into the intravascular space, helping, at least temporarily, to maintain vascular volume. With hypotonic fluid replacement (eg, with plain water), serum Na may normalize but can also decrease (hyponatremia). Hyponatremia results in some fluid shifting out of the intravascular space into the interstitium at the expense of vascular volume.

Symptoms and Signs

Symptoms and signs vary according to degree of deficit (see Table: Clinical Correlates of Dehydration) and by the serum Na level. Because of the fluid shift out of the interstitium into the vascular space, children with hypernatremia appear more ill (eg, with very dry mucous membranes, a doughy appearance to the skin) for a given degree of water loss than do children with hyponatremia. However, children with hypernatremia have better hemodynamics (eg, less tachycardia and better urine output) than do children with hyponatremia, in whom fluid has shifted out of the vascular space. Dehydrated children with hyponatremia may appear only mildly dehydrated until closer to cardiovascular collapse and hypotension.

Clinical Correlates of Dehydration

Diagnosis

• Clinical evaluation