Dehydration

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Updated: Nov 27, 2016
  • Author: Lennox H Huang, MD, FAAP; Chief Editor: Timothy E Corden, MD  more...
Overview

Background

Dehydration describes a state of negative fluid balance that may be caused by numerous disease entities. Diarrheal illnesses are the most common etiologies. Worldwide, dehydration secondary to diarrheal illness is the leading cause of infant and child mortality.

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

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Pathophysiology

The negative fluid balance that causes dehydration results from decreased intake, increased output (renal, gastrointestinal [GI], or insensible losses), or fluid shift (ascites, effusions, and capillary leak states such as burns and sepsis). The decrease in total body water causes reductions in both the intracellular and extracellular fluid volumes. Clinical manifestations of dehydration are most closely related to intravascular volume depletion. As dehydration progresses, hypovolemic shock ultimately ensues, resulting in end organ failure and death.

Young children are more susceptible to dehydration due to larger body water content, renal immaturity, and inability to meet their own needs independently. Older children show signs of dehydration sooner than infants due to lower levels of extracellular fluid (ECF).

Dehydration can be categorized according to osmolarity and severity. Serum sodium is a good surrogate marker of osmolarity assuming the patient has a normal serum glucose. (Osmolarity = [2 × sodium] + [glucose/18] + [blood urea nitrogen/2.8]) Dehydration may be isonatremic (130-150 mEq/L), hyponatremic (< 130 mEq/L), or hypernatremic (>150 mEq/L). Isonatremic dehydration is the most common (80%). Hypernatremic and hyponatremic dehydration each comprise 5-10% of cases. Variations in serum sodium reflect the composition of the fluids lost and have different pathophysiologic effects, as follows:

  • Isonatremic (isotonic) dehydration occurs when the lost fluid is similar in sodium concentration to the blood. Sodium and water losses are of the same relative magnitude in both the intravascular and extravascular fluid compartments.
  • Hyponatremic (hypotonic) dehydration occurs when the lost fluid contains more sodium than the blood (loss of hypertonic fluid). Relatively more sodium than water is lost. Because the serum sodium is low, intravascular water shifts to the extravascular space, exaggerating intravascular volume depletion for a given amount of total body water loss. [1, 2]
  • Hypernatremic (hypertonic) dehydration occurs when the lost fluid contains less sodium than the blood (loss of hypotonic fluid). Relatively less sodium than water is lost. Because the serum sodium is high, extravascular water shifts to the intravascular space, minimizing intravascular volume depletion for a given amount of total body water loss. [2, 3, 4]

Neurologic complications can occur in hyponatremic and hypernatremic states. Severe hyponatremia may lead to intractable seizures, whereas rapid correction of chronic hyponatremia (>2 mEq/L/h) has been associated with central pontine myelinolysis. During hypernatremic dehydration, water is osmotically pulled from cells into the extracellular space. To compensate, cells can generate osmotically active particles (idiogenic osmoles) that pull water back into the cell and maintain cellular fluid volume. During rapid rehydration of hypernatremia, the increased osmotic activity of these cells can result in a large influx of water, causing cellular swelling and rupture; cerebral edema is the most devastating consequence. Slow rehydration over 48 hours generally minimizes this risk (not to exceed 0.5 mEq/L per hour; 10-12 mEq/L in 24 hours).

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Etiology

Determination of the cause of dehydration is essential. Poor fluid intake, excessive fluid output, increased insensible fluid losses, or a combination of the above may cause intravascular volume depletion. Successful treatment requires identification of the underlying disease state.

Common causes of dehydration include the following:

  • Gastroenteritis: This is the most common cause of dehydration. If both vomiting and diarrhea are present, dehydration may rapidly progress. [5, 6]  Rotaviruses are the most common global causes for dehydration and severe diarrheal disease in infants and young children. [7]
  • Stomatitis: Pain may severely limit oral intake.
  • Diabetic ketoacidosis (DKA): Dehydration is caused by osmotic diuresis. Weight loss is caused by both excessive fluid losses and tissue catabolism. Rapid rehydration, especially rapid initial volume resuscitation, may be associated with a poor neurologic outcome. DKA requires very specific and controlled treatment (see  Diabetic Ketoacidosis).
  • Febrile illness: Fever causes increased insensible fluid losses and may affect appetite.
  • Pharyngitis: This may decrease oral intake.

Life-threatening causes of dehydration include the following:

  • Gastroenteritis
  • Diabetic ketoacidosis (DKA)
  • Burns: Fluid losses may be extreme. Very aggressive fluid management is required (see  Burns, Thermal).
  • Congenital adrenal hyperplasia: This may have associated  hypoglycemia, hypotension,  hyperkalemia, and hyponatremia.
  • GI obstruction: This is often associated with poor intake and emesis. Bowel ischemia can result in extensive capillary leak and shock.
  • Heat stroke: Hyperpyrexia, dry skin, and mental status changes may occur. [8]
  • Cystic fibrosis: This results in excessive sodium and chloride losses in sweat, placing patients at risk for severe hyponatremic hypochloremic dehydration.
  • Diabetes insipidus: Excessive output of very dilute urine can result in large free water losses and severe hypernatremic dehydration.
  • Thyrotoxicosis: Weight loss is observed, despite increased appetite. Diarrhea occurs.
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Epidemiology

United States data

Diarrheal illnesses in children causes 3 million physician visits, 220,000 hospitalizations (10% of all children who require hospitalization), and 400 deaths per year. Children younger than 5 years are at the highest risk. On average, North American children younger than 5 years have 2 episodes of gastroenteritis per year.

International data

Diarrheal illnesses with subsequent dehydration account for nearly 4 million deaths per year in infants and children. The overwhelming majority of these deaths occur in developing nations.

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Prognosis

The prognosis is excellent if the child is promptly and effectively treated. However, the child with severe dehydration and hypovolemic shock can have significant morbidity and mortality if treatment is delayed.

Mortality/morbidity

Mortality and morbidity generally depend on the severity of dehydration and the promptness of oral or intravenous rehydration. If treatment is rapidly and appropriately obtained, morbidity and mortality are low.

Complications may include irreversible shock, sagittal or other venous sinus thrombosis, intractable seizures, and renal failure.

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