(Hyponatremia in adults is discussed elsewhere.)
Hyponatremia is a serum sodium concentration < 135 mEq/L. Significant hyponatremia may cause seizures or coma. Treatment is cautious sodium replacement with IV 0.9% saline solution; rarely, 3% saline solution is required, particularly if seizures are occurring.
The most frequent cause of neonatal hyponatremia is hypovolemic dehydration caused by vomiting, diarrhea, or both when large GI losses are replaced with fluids that have little or no sodium (eg, some juices).
A less frequent cause is euvolemic hyponatremia caused by inappropriate ADH secretion and consequent water retention. Possible causes of inappropriate ADH secretion include CNS tumors and infection. 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, and coma; other symptoms include cramps and weakness. Infants with hyponatremic dehydration may appear quite ill, because hyponatremia causes disproportionate reductions in ECF volume. Symptoms and signs are related to duration and degree of hyponatremia.
Treatment of neonatal hyponatremia is with 5% D/W/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 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 of body weight or even 15 mEq/kg in young infants with severe hyponatremia) and include sodium maintenance needs (3 mEq/kg/day in 5% D/W solution). Neonates with symptomatic hyponatremia (eg, lethargy, confusion) require emergency treatment with 3% saline solution IV to prevent seizure or coma.