Hypo- and Hypernatremia in Extremely Low Birth Weight Infants in the First 10 Days of Life: A Review

Background/Objectives: Sodium regulation is critical in extremely low birth weight (ELBW, <1000 g) infants. This study aimed to provide a comprehensive overview of sodium dynamics and related variables in ELBW infants in their first 10 days of life through a structured literature review. Methods: Applying PRISMA guidelines, six databases were searched (1 August 2023) on sodium measurements in ELBW cohorts, with quality assessment (RoB2, ROBINS-1, Newcastle Ottawa scale) of retained papers, and subsequent data extraction in line with these PRISMA guidelines to describe findings. Results: Only eight heterogeneous studies could be retained, including observational cohort studies (n = 5), case-control studies (n = 2, Tegaderm application yes/no, gestational age < 24 or 24-28 weeks), and only one randomized trial (sodium restriction versus no sodium restriction). Definitions of hyper- or hyponatremia were also heterogeneous, with incidence ranges for hyper- (8-92.2%) and hyponatremia (0-52.9%). Peak sodium values were observed on days 2-4 in the individual studies. When pooled and compared to the cohort mean sodium values, the highest increases in mean serum sodium values were observed on day 3 (+4, range, -0.6 to +8.6 mEq). Variables of sodium values were related to care factors [incubator settings (open/closed, double-/not double-walled, humidity), fluid regimens (water volume, sodium supplementation), occlusive skin care], as well as related maturational factors (postnatal age, gestational age, small versus appropriate for gestational age, SGA/AGA). Conclusions: Based on a structured literature review, patterns of sodium changes over postnatal age in ELBW cases were documented. Besides incubator settings, fluid regimens, or occlusive skin care, these patterns also depend on maturational factors of the ELBW infant (gestational age, postnatal age, SGA/AGA). These complexities emphasize the need for nuanced interpretation, the relevance of standardizing clinical practices and research definitions, and the need to report on additional datasets.