Late hyponatremia in very-low-birth-weight infants: incidence and associated risk factors.

Objective: To determine the incidence of late hyponatremia in very-low-birth-weight infants and to identify associated risk factors. Low serum sodium concentration in otherwise healthy premature infants beyond 2 weeks of life is referred to as the hyponatremia.

Design: Retrospective cohort review.

Setting/subjects: The intensive care nursery at St Luke's Hospital Perinatal Center, Kansas City, Mo. Criteria for subject selection were birth weight of 1,500 g or less; survival for more than 21 days; development of late hyponatremia or hospitalization for 42 days or more; and measurement of serum sodium concentration at least once between the 14th and 56th day of life. Of 515 infants admitted to the nursery for the 1992 calendar year, 124 had a birth weight of 1,500 g or less and survived for more than 21 days; however, 16 of these infants were discharged at 42 days of life or less, 11 did not have a serum sodium concentration measurement after the second week of life, and 1 did not have complete medical records. Thus, the final sample was 96 subjects.

Statistical Analyses: Percentages to determine incidence of late hyponatremia; t test of chi 2 test to determine differences between infants with and without late hyponatremia; multiple logistic regression to determine the strongest indicators of late hyponatremia.

Results: Incidence of late hyponatremia was 62.5%. Significant risk factors for late hyponatremia were birth weight of 1,000 g or less (P < .001), feedings of fortified human milk (P < .013), and occurrence of an intraventricular hemorrhage (P < .036). Fortified human milk feeding was a significant risk factor for late hyponatremia in both weight groups (ie, birth weight greater than or less than 1,000 g).

Conclusion: Despite standard fortification, human milk may contain an insufficient quantity of sodium to met the needs of very-low-birth-weight infants.