Familial hyponatremia conditioned by the R137C mutation with constitutive activation of the vasopressin receptor

Hyponatremia can occur in endocrinological diseases, neoplasms, kidney diseases, and acquired or genetically conditioned disorders of antidiuretic hormone levels. Nephrogenic syndrome of inappropriate antidiuresis (NSIAD) is a rare X-linked disease caused by a point mutation of the type 2 vasopressin receptor (AVPR2) gene. This mutation results in constitutive activation of the AVPR2 and a low sodium level.We reported the first familial NSIAD in Poland in siblings with hyponatremia. Case 1. A 2.5-year-old boy, during a respiratory tract infection, showed the following laboratory test results: Na 125 mmol/L, serumosmolality 260 mOsm/kg H2O, low uric acid level, and increased fractional sodium and uric acid excretions. Thyroid, adrenal, and renal function were normal. Copeptin level was low. Case 2. A 7-month-old brother presented with reduced activity and muscle tone, a sodium level of 117 mmol/L, and a serum osmolality of 249 mOsm/kg H2O. They were both confirmed to be hemizygous for the R137C mutation on the AVPR2 gene. The boys were advised to restrict their oral fluid intake and supplement sodium orally, aiming for sodium levels of 133-140 mmol/L. Conclusions: Genetic testing for an AVPR2 mutation is crucial in patients with hyponatremia, normovolemia, hypoosmolality, and low copeptin level.