Hyponatremia and mortality risk: a Danish cohort study of 279 508 acutely hospitalized patients

OBJECTIVE: We aimed to investigate the impact of hyponatremia severity on mortality risk and assess any evidence of a dose-response relation, utilizing prospectively collected data from population-based registries. DESIGN: Cohort study of 279 ,508 first-time acute admissions to Departments of Internal Medicine in the North and Central Denmark Regions from 2006 to 2011. METHODS: We used the Kaplan-Meier method (1 - survival function) to compute 30-day and 1-year mortality in patients with normonatremia and categories of increasing hyponatremia severity. Relative risks (RRs) with 95% CIs, adjusted for age, gender and previous morbidities, and stratified by clinical subgroups were estimated by the pseudo-value approach. The probability of death was estimated treating serum sodium as a continuous variable. RESULTS: The prevalence of admission hyponatremia was 15% (41,803 patients). Thirty-day mortality was 3.6% in normonatremic patients compared to 7.3, 10.0, 10.4 and 9.6% in patients with serum sodium levels of 130-134.9, 125-129.9, 120-124.9 and <120 mmol/l, resulting in adjusted RRs of 1.4 (95% CI: 1.3-1.4), 1.7 (95% CI: 1.6-1.8), 1.7 (95% CI: 1.4-1.9) and 1.3 (95% CI: 1.1-1.5) respectively. Mortality risk was increased across virtually all clinical subgroups, and remained increased by 30-40% 1 year after admission. The probability of death increased when serum sodium decreased from 139 to 132 mmol/l. No clear increase in mortality was observed for lower concentrations. CONCLUSIONS: Hyponatremia is highly prevalent among patients admitted to Departments of Internal Medicine and is associated with increased 30-day and 1-year mortality risk, regardless of underlying disease. This risk seems independent of hyponatremia severity.