G24(P) A salty allergy

<h3></h3> Hyponatremia, unless associated with extracellular fluid volume expansion, is an uncommon paediatric electrolyte imbalance. We report an infant presenting with chronic hyponatremia suggestive of a syndrome of inappropriate secretion of antidiuretic hormone (SIADH); however without ADH secretion. In this case, a gain-of- function of AVPR 2 was found to be responsible for a SIADH-like state. Only very few cases have been reported in the literature. A healthy 19 month old presented to his local hospital with a tonic-clonic seizure, demonstrating a sodium of 119 mmol/L. A few months prior, hyponatraemia was noted in the context of a lower respiratory tract infection, treated with a short course of sodium chloride supplementation with no further follow-up. Recently he was diagnosed with cow’s milk protein allergy. His mother described him being a poor water drinker with infrequent wet nappies. He was developing appropriately with no significant family or social history. He examined normally. Over 48 hours, he received six hypertonic (2.7%) saline boluses. His sodium remained refractory (116–122 mmol/L) and nephrology input was sought. Results showed serum osmolality of 250 mosml/l, urinary osmolality of 520 mosml/l and urinary sodium of 171 mmol/L. Chest radiograph was normal. On transfer to tertiary nephrology services he was normotensive with unremarkable examination. Investigations demonstrated: CT head, renal ultrasound and renin/aldosterone, cortisol, thyroid function were normal. Paired plasma and urine results suggested syndrome of inappropriate antidiuresis (SIADH). No secondary causes were apparent; thus nephrogenic syndrome of inappropriate antidiuresis (NSIAD) caused by gain of function mutation in AVPR2 was postulated. Analysis of the AVPR2 gene confirmed this x-linked dominant disorder (mutation (c.409C&gt;T)). Familial testing revealed his mother as a carrier, with subtle retrospective symptoms. We suspect the patient had chronic asymptomatic hyponatremia, acutely exacerbated by full switchover to oat milk in view of allergies, which has little protein content. This lowered his tubular osmotic load and coupled renal water loss. In conclusion, NSIAD can mimick SIADH and should be considered if no secondary cause found combined with a positive family history; though this was initially lacking in our case. This is the first case we have diagnosed in our centre.

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