Epidemiology

Hyponatraemia (serum Na <135 mmol/L or <135 mEq/L) is the most common electrolyte disorder encountered in clinical practice.[4][5]​​ It has been reported to occur in 15% to 28% of hospitalised patients.[6] However, the incidence of clinically significant cases of hyponatraemia (serum Na <130 mmol/L or <130 mEq/L) is between 1% and 4%.[7] The prevalence of mild hyponatraemia in people aged 75 years or older is around 16%.[8]​​

The incidence/prevalence of SIADH in particular is studied less thoroughly in the literature. SIADH was the most common cause of hyponatraemia in patients with cancer, accounting for around 30% of the total hyponatraemia cases.[6] Hyponatraemia is a well-recognised complication of neurosurgical conditions and in one study, SIADH was the underlying pathophysiology in 62% of cases.[9] In another study of older patients in hospital who had hyponatraemia, almost 25% of cases met the diagnostic criteria of SIADH.[10] The most common causes of SIADH were respiratory diseases (59%), followed by malignant tumours (29%), and central nervous system diseases (10%).[10]

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