Background:
Dehydration describes a state of negative fluid balance that
may be caused by a number of disease entities. Diarrheal illnesses are the
most common etiologies. Worldwide, dehydration secondary to diarrheal
illness is the leading cause of infant and child mortality.
Pathophysiology: The negative
fluid balance causing dehydration results from decreased intake, increased
output (renal, gastrointestinal, 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.
Dehydration is often categorized according to
serum sodium concentration as 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.
Isonatremic (isotonic) dehydration occurs when
the fluid that is lost 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 fluid that is lost contains more sodium than the blood (loss of
hypertonic fluid). Relatively more sodium than water is lost. Since the
serum sodium is low, intravascular water will shift to the extravascular
space, exaggerating intravascular volume depletion for a given amount of
total body water loss.
Hypernatremic (hypertonic) dehydration occurs
when the fluid that is lost contains less sodium than the blood (loss of
hypotonic fluid). Relatively less sodium than water is lost. Since the
serum sodium is high, extravascular water will shift to the intravascular
space, minimizing intravascular volume depletion for a given amount of
total body water loss.
Neurologic complications can occur in
hyponatremic and hypernatremic states. Rapid correction of chronic
hyponatremia (>2 mEq/L/hour) 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, with
cerebral edema being the most devastating consequence. Slow rehydration
over 48 hours generally minimizes this risk.
Frequency:
- In the US: Diarrheal
illnesses in children caused 3 million physician visits, 220,000
hospitalizations (10% of all children who require hospitalization), and
400 deaths per year.
- Internationally: Diarrheal
illnesses with subsequent dehydration account for nearly 4 million
deaths per year in infants and children.
Mortality/Morbidity: Mortality
and morbidity are generally dependent upon the severity of dehydration and
the promptness of oral or intravenous rehydration. If treatment is rapidly
and appropriately obtained, morbidity and mortality are low.
Age: Children younger than 5
years are at the highest risk.
History:
- Intake of fluids, including the volume, type
(hypertonic or hypotonic), and frequency
- Urine output, including the frequency of
voiding, presence of concentrated or dilute urine,
hematuria
- Stool output, frequency of stools, stool
consistency, presence or blood or mucus in stools
- Emesis, including frequency and volume,
bilious or non-bilious, hematemesis
- Contact with ill people, especially others
with gastroenteritis
- Underlying illnesses, especially cystic
fibrosis, diabetes mellitus, hyperthyroidism, renal
disease
Physical:
- A complete physical exam is essential to
determine the underlying cause of the patient’s dehydration and to
define the severity of dehydration. The clinical assessment of severity
of dehydration will determine the approach to
management.
-
Table 2: Estimated Fluid
Deficit
|
Severity |
Infants (weight < 10
kg) |
Children (weight > 10
kg) |
|
Mild Dehydration |
5% or 50 mL/kg |
3% or 30
mL/kg |
|
Moderate
Dehydration |
10% or 100
mL/kg |
6% or 60
mL/kg |
|
Severe Dehydration |
15% or 150
mL/kg |
9% or 90
mL/kg |
Causes: Determination of the
cause of dehydration is essential. Poor fluid intake, excessive fluid
output and increased insensible fluid losses all may cause intravascular
volume depletion. Successful treatment requires identification of the
underlying disease state.
- Gastroenteritis: This is the most common
cause of dehydration. If both vomiting and diarrhea are present,
dehydration may progress rapidly.
- 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. 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.
- 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
- Gastrointestinal obstruction: This often is
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.
- Cystic fibrosis: This results in excessive
sodium and chloride losses in sweat, 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.
