This protocol addresses patients with chronic hyponatraemia — documented to persist for at least 48 hours — with a serum sodium concentration below 135 mmol/l, in the context of syndrome of inappropriate antidiuresis (SIAD) with normal extracellular fluid volume, and without severe or moderately severe symptoms.
SIAD is a diagnosis of exclusion. It is characterised by a urine osmolality above 100 mOsm/kg, urine sodium concentration of 30 mmol/l or more, and normal extracellular fluid volume, with other causes of hyponatraemia excluded. The chronic nature of the hyponatraemia — defined as documented for at least 48 hours — is central to selecting the appropriate management strategy.
Management of this presentation involves solute-based interventions, applied as evidence-supported second-line options. The complete structured regimen — including which specific interventions qualify, how they are chosen, and under what conditions — is available in full below.
We define 'chronic' hyponatraemia as hyponatraemia that is documented to exist for at least 48 h.
SIAD is a diagnosis of exclusion. It fits the category of hyponatraemia with a urine osmolality >100 mOsm/kg, urine sodium concentration ≥30 mmol/l and normal extracellular fluid volume, but formal diagnosis requires exclusion of other possible causes of hyponatraemia.
In moderate or profound hyponatraemia, we suggest the following can be considered equal second-line treatments: increasing solute intake with 0.25–0.50 g/kg per day of urea or a combination of low-dose loop diuretics and oral sodium chloride (2D).
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