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An infant has a serum sodium of 127 mEq/L, below the normal accepted value of 135 mEq/L. The incidence of hyponatremia is greater than hypernatremia in premature infants. Evidence now shows it is a serious condition in very preterm infants (<33 weeks' gestation), who have large variations of the serum sodium concentration, as they are at risk for poor neuromotor outcome at 2 years. Preterm infants with an increased risk of hyponatremia from sodium restriction show impaired growth and worse neurodevelopment at 10–13 years of age. Hyponatremia is also a risk factor for sensorineural hearing loss, intracranial hemorrhage, and cerebral palsy. Hyponatremia in infants who experienced perinatal birth asphyxia are at risk for an increased mortality.
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II. IMMEDIATE QUESTIONS
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Is there any seizure activity? Seizure activity can be seen in patients with extremely low serum sodium levels (usually <120 mEq/L). This is a medical emergency, and urgent intravenous (IV) sodium correction is needed.
How much sodium and free water is the patient receiving? Is weight gain or loss occurring? Be certain that an adequate amount of sodium is being given and that free water intake is not excessive. The normal amount of sodium intake is 2–4 mEq/kg/d. Weight gain with low serum sodium levels is most likely a result of volume overload, especially in the first day or two of life, when weight loss is expected.
What is the urine output? With syndrome of inappropriate secretion of antidiuretic hormone (SIADH), urine output is decreased. If the urine output is increased (>4 mL/kg/h), perform a spot check of urine sodium to determine whether urinary sodium losses are high.
What medications is the infant receiving? Are renal salt-wasting medications being given? Diuretics such as furosemide may cause hypovolemic hyponatremia. Other medications that cause hyponatremia include theophylline, carbamazepine, chlorpromazine, indapamide, amiodarone, and selective serotonin reuptake inhibitors. Most of these cause SIADH (euvolemic hyponatremia). Morphine and barbiturates can also cause hyponatremia.
Did the mother receive hypotonic IV fluids or an excessive amount of oxytocin? Was the mother hyponatremic in the intrapartum period? If so, the infant can have hyponatremia at birth. Infants of mothers with hyponatremia can have low levels of sodium after delivery.
Is the infant <1 week old (early-onset hyponatremia) or is the infant in the third to fourth week of life (late-onset hyponatremia)? Early-onset hyponatremia may be due to free water excess from either increased maternal free water in labor or perinatal nonosmotic release of vasopressin (occurs in perinatal asphyxia, respiratory distress syndrome, bilateral pneumothoraces, intraventricular hemorrhage [IVH], and with certain medications). It can also occur from too much free water given or not enough sodium intake in fluids. Late-onset hyponatremia is usually from inadequate sodium intake or excessive renal losses. Late onset can also be from excessive antidiuretic hormone (ADH) release, renal failure, or edema causing retention of free water, but it is less common. Preterm infants >28 weeks have a high fractional excretion of sodium.
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III. DIFFERENTIAL DIAGNOSIS
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When considering the differential, determine whether the value is real. Certain conditions can cause pseudohyponatremia. Is the amount of sodium given adequate? Then you need to decide whether the hyponatremia is caused by deficit of total body sodium or an excess of free water. Is there an inability to excrete water? Is there an excessive sodium loss? Have medications caused the hyponatremia? The cause dictates the form of treatment. The most frequent cause of hyponatremia in the neonate is hypotonic hyponatremia (dilutional) caused by excessive fluid ...