Risk factors for symptomatic hyponatraemia: the role of pre-existing asymptomatic hyponatraemia

Background: Hyponatraemia is associated with substantial morbidity and mortality. Identification of the risk factors associated with the development of symptomatic hyponatraemia is important in determining preventive strategies. Methods: A retrospective analysis of the risks factors associated with the development of severe, symptomatic hyponatraemia requiring hospital admission over the past 3 years at our institution was carried out. Results: Forty-seven patients (26 women, 21 men) with a hospital admission serum sodium < 134 mmol/L were identified. Of these patients, 31 (65.9%) had associated changes in the mental status that improved with the treatment of the hyponatraemia suggesting causality. The average admission sodium level of this cohort was 118.8 mmol/L. Symptomatic hyponatraemia was associated with volume depletion (32.6%), congestive heart failure (26%), syndrome of inappropriate antidiuretic hormone (26%), thiazide diuretic use (26%) and selective serotonin re-uptake inhibitor use (26%). In 21.7% of cases, the cause was multifactorial (congestive heart failure, syndrome of inappropriate antidiuretic hormone or medication use with volume depletion). In 11% of cases, patients were taking both thiazide diuretics and serotonin re-uptake inhibitors. Most importantly, 70.9% of all patients admitted with symptomatic hyponatraemia had pre-existing hyponatraemia that was untreated and believed to be asymptomatic (P < 0.05). This was the most common risk factor identified. We next investigated the prevalence of presumed asymptomatic hyponatraemia in the outpatient setting. Out of 27 496 patients analysed, 14% had serum sodium levels less than or equal to 134 mEq/L and 4% had values less than 130 mEq/L. Conclusion: Pre-existing asymptomatic hyponatraemia is a common finding and is associated with a high risk for the development of worsening hyponatraemia with altered mental status.