Pathophysiology of hyponatremia in preterm infants with relative adrenal insufficiency after the early neonatal period.

Objective Preterm infants often develop relative adrenal insufficiency (RAI) not only within the early neonatal period, but also beyond this period. RAI is commonly accompanied by hyponatremia, but the pathogenesis of hyponatremia with RAI has not been clarified. This study aimed to investigate the pathophysiology of hyponatremia in infants with RAI. Study design This is a single-centered retrospective cohort study. Preterm infants born at < 30 weeks of gestation or < 1000 g were enrolled. They were divided into the RAI group and the non-RAI group. The data of serum and urine examination, the amount of sodium intake, and fractional excretion of sodium were compared between the two groups. In the RAI group, data between before and after the administration of hydrocortisone were also compared. Results Sixteen infants in the RAI group and thirty-five infants in the non-RAI group were included in the analysis. In the RAI group, hyponatremia was common and preceded other clinical symptoms, such as oliguria and decreased blood pressure, therefore, hyponatremia with RAI was not likely to be caused by dilution due to oliguria. There was no difference in the fractional excretion of sodium between the two groups (adjusted for postconceptional age at examination), therefore, it is not likely that hyponatremia with RAI was mainly caused by excessive renal sodium loss. Since sodium intake was rather higher in the RAI group than in the non-RAI group, it is unlikely that insufficient sodium supplementation was the cause of RAI. Hyponatremia with RAI was considered to be likely caused by vascular hyperpermeability. Conclusion Hyponatremia is a common symptom among preterm infants with RAI and its pathogenesis can be vascular hyperpermeability.