This study confirms that mild hyponatremia is a common condition in the PED, occurring in 17.6% of children who undergo a blood sampling, representing a risk factor for admission and longer hospital stay.
Several studies on the adult population, including thousands of patients with heart, liver, and pulmonary diseases, show that hyponatremia is strongly associated with a more severe disease course and higher overall mortality [4].
To date, studies in the pediatric population are mostly retrospective and focused on specific diseases. According to the literature, hyponatremia is associated with respiratory infections, such as pneumonia and bronchiolitis, urinary tract infections, gastrointestinal infections, meningitis [10], Kawasaki disease, sepsis, and malaria. The association between hyponatremia, respiratory infections, and pneumonia is well established. Hyponatremia is reported to occur in 13,5-45.4% of cases, being mostly mild and related to higher fever, elevated inflammatory markers, higher leucocyte count and hospitalization rates [2, 10, 11, 12, 13, 14]. The same applies to infants with bronchiolitis, in which hyponatremia can be as common as 57%, mainly occurring under six months. Among those infants who enter the ICU, sodium below 135 mEq/L is a risk factor for a more extended stay and mechanical ventilation. Data on the association between hyponatremia and age are controversial. Mazzoni studied 400 children with community-acquired infections and found that those with hyponatremia were younger than controls [15]. In contrast, Sung found the opposite in a larger sample of children with respiratory infections [12].
Lehtiranta et al. published a recent register-based cohort study involving 46518 children who accessed PED demonstrating that hyponatremia was an independent risk factor for hospitalization and the need for PICU treatment. In this series, only moderate to severe hyponatremia carried a higher risk of neurological symptoms and deaths [16].
Hyponatremia was also identified as a risk factor for complicated appendicitis and perforation in adults [17, 18] and pediatric populations [19]. Finally, up to 49% of children with urinary tract infections can present with low plasmatic sodium; it could be more often associated with other electrolyte abnormalities and correlates with longer hospitalization, increased rate of renal parenchymal involvement and higher CRP values [15, 20].
Our study confirms that hyponatremia in a PED is mostly mild and presents mainly in younger children with respiratory infections of viral origin. Males were more likely to have hyponatremia, although this may be explained by the higher incidence of viral infections in males in the first year of life [21].
The main finding of this study is that we found the same conclusions in the mild hyponatremia subgroup analysis. While moderate to severe hyponatremia carries a well-known risk of severe disease, mild hyponatremia is a common finding that may be easily over sought by emergency physicians. In fact, this series shows that a plasmatic sodium concentration of 134 − 130 mEq/L is associated with a higher risk of more severe infections with a higher hospitalization rate, suggesting that clinicians should carefully consider this laboratory sign. In their milestone study, Lethiranta and colleagues [16] showed that mild hyponatremia is related to worse outcomes, with even higher OR than our series. However, in that paper, 15% of children received an iv infusion of moderately hypotonic fluid therapy with 60–80 mmol/L of sodium. To our knowledge, our series is the first to refer to mild hyponatremia in a selected cohort of children not receiving any previous infusion.
This remainder has therapeutical application too, since physicians should consider the danger related to the use of hypotonic fluids in children with acute conditions [8]. Hyponatremia occurs more frequently in infants with respiratory infections such as bronchiolitis and pneumonia, which are well-known causes of a syndrome of inappropriate ADH secretion. In this setting, intravenous hypotonic fluids and overhydration should be carefully avoided since they can lower the sodium further to harmful levels.
The study's primary limits are the retrospective monocentric design and the fact that sodium levels were assessed according to the attending physician's clinical decision. Moreover, we didn't investigate mechanisms underlying hyponatremia since it was beyond the scope of our study.