Severe Unexpected Hyponatremia in an Infant With Cystic Fibrosis Carriership

This report describes the development of severe hyponatremia in a 4-month-old infant, with known carriership of cystic fibrosis (CF; heterozygous delta F508 cystic fibrosis transmembrane conductance regulator (CFTR) mutation), in the course of a coronavirus disease 2019 (COVID-19) infection with mild respiratory symptoms and a urinary tract infection. Laboratory investigations were performed in the outpatient clinic because of persistent loss of appetite, fussiness and mild weight loss after a recent antibiotically treated urinary tract infection. The results showed severe hyponatremia of 115 mmol/L, mild hypokalemia of 3.0 mmol/L, with normal renal function, without other biochemical signs of dehydration and neither signs of fluid retention. Urinary fractional sodium excretion was extremely low, indicative of adequate renal sodium retention in the hyponatremic state. We hypothesize that the combination of disturbed extrarenal salt losses due to CF carriership, together with loss of appetite, relatively low sodium content of breastmilk and possibly COVID-19-associated mild syndrome of inappropriate antidiuretic hormone secretion (SIADH), resulted in the severe hyponatremia that was found in this infant. We believe it is important to report the findings in the current case to underscore the intricate balance of sodium homeostasis in the fully breastfed older infant and the alertness for checking sodium levels during infections/illness in heterozygote CF patients, as was previously reported in literature. In conclusion, this report describes the development of severe unexpected hyponatremia in the course of a mild infectious period in a fully breastfed 4-month-old infant with known CF carriership and underscores the importance of measuring sodium levels with low threshold during infections in CF carriers. Int J Clin Pediatr. 2023;12(2):45-50 doi: https://doi.org/10.14740/ijcp516