Familial hyponatremia conditioned by the R137C mutation with constitutive activation of the vasopressin receptor.
Hanna Szymanik-Grzelak, Maria Daniel, Małgorzata Pańczyk-Tomaszewska
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Case in brief
Two brothers presented with hyponatremia: a 2.5-year-old boy during a respiratory infection and his 7-month-old brother with reduced activity and muscle tone. Both had hypo-osmolality; the older child also had low uric acid and increased fractional sodium and uric acid excretion, with normal thyroid, adrenal, and renal function. Copeptin was low. Genetic testing identified hemizygous R137C mutations in AVPR2 in both boys, consistent with nephrogenic syndrome of inappropriate antidiuresis. They were advised to restrict oral fluids and take sodium supplements, targeting sodium levels of 133–140 mmol/L.
What made this case unusual
The report describes familial NSIAD in two brothers with the same hemizygous AVPR2 R137C mutation, presenting with markedly low sodium and low copeptin.
Diagnostic / clinical pearl
In a patient with hyponatremia, hypo-osmolality, normovolemia, and low copeptin, consider NSIAD and test for an AVPR2 mutation.