Dehydration in children
In children, the most common cause of dehydration is diarrheal fluid loss. In the majority of children with the dehydration secondary to gastrointestinal loss, the serum sodium level is within the normal range (135-145 mEq/L). This is known as isotonic dehydration.
Electrolyte losses from diarrhea and vomiting range from iso- to hypo-osmolar. The tendency to hypernatremia from loss of hypo-osmolar fluid is partially countered by movement of fluid from the ICF to the ECF driven by the increase in ECF osmolality. This also helps maintain intravascular volume.
Hyponatremic (hypotonic) dehydration (serum sodium concentration < 135 /mEq/L) indicates body water content in excess of sodium content. One potential scenario for hyponatremic dehydration is replacement of fluid loss by electrolyte-free water such as apple juice or ice tea, or with hypotonic fluids such as D5 0.18% saline (fifth-normal saline). Because of anti-diuretic hormone (ADH) secretion stimulated by hypovolemia, water will be retained even in the face of a falling serum sodium level.
Hypernatremic (hypertonic) dehydration (serum sodium concentration
> 145 mEq/L) indicates excessive free water loss. For example, a child who is on gavage feeding with a fixed daily fluid intake and develops excessive fluid loss from tachypnea or fever will gradually develop hypernatremia. Hypernatremia is also seen in a small proportion of children with gastroenteritis and dehydration, presumably from excessive loss of water in relation to solute.