(Hyponatremia in adults is discussed elsewhere.)
The most frequent cause of neonatal hyponatremia is hypovolemic dehydration caused by vomiting, diarrhea, or both. When fluid loses are replaced with fluids that have little or no sodium (eg, some juices), hyponatremia can result.
A less frequent cause is euvolemic hyponatremia caused by inappropriate ADH secretion and consequent water retention. Possible causes of inappropriate antidiuretic hormone secretion include intracranial hemorrhage, central nervous system (CNS) infection, and rarely CNS tumors. Also, overdilution of infant formula can lead to water intoxication.
Finally, hypervolemic hyponatremia occurs in the setting of water retention and excess sodium retention, such as in heart failure or renal failure.
Symptoms and signs of neonatal hyponatremia include nausea and vomiting, apathy, headache, seizures, hypothermia, weakness, and coma. Infants with hyponatremic dehydration may appear quite ill, because hyponatremia causes disproportionate reductions in extracellular fluid volume. Symptoms and signs are related to duration and degree of hyponatremia.
Treatment of neonatal hyponatremia is with 5% D/0.45% to 0.9% saline solution IV in volumes equal to the calculated deficit, given over as many days as it takes to correct the sodium concentration by no more than 10 to 12 mEq/L/day (10 to 12 mmol/L/day) to avoid rapid fluid shifts in the brain. Neonates with hypovolemic hyponatremia need volume expansion, using a solution containing salt to correct the sodium deficit (10 to 12 mEq/kg [10 to 12 mmol/kg] of body weight or even 15 mEq/kg [15 mmol/kg] in young infants with severe hyponatremia) and include sodium maintenance needs (3 mEq/kg/day [3 mmol/kg/day] in 5% dextrose solution). Neonates with symptomatic hyponatremia (eg, lethargy, confusion) require emergency treatment with 3% saline solution IV to prevent seizure or coma.