Mortality rates are lower in SIAD, than in hypervolaemic or hypovolaemic hyponatraemia: Results of a prospective observational study

Objective Hyponatremia is associated with increased mortality, but the mortality associated specifically with SIAD is not known. We hypothesised that mortality in SIAD was elevated, but that it was less than in hypervolemic(HEN) or hypovolemic(HON) hyponatremia. Design Mortality rates are presented as risk ratios(RR),with 95% confidence intervals (CI), and compared to normonatremic controls(NN). Methods Prospective, single center, non-interventional study of all patients with hyponatremia(≤ 130 mmol/l) admitted to hospital. Results 1323 admissions with hyponatremia were prospectively evaluated and 1136 contemporaneous NN controls. 431(32.6%) hyponatraemic patients had HON, 573(43.3%) had SIAD and 275(20.8%) patients had HEN. In patient mortality was higher in hyponatremia than NN (9.1% v 3.3%, p<0.0001). The RRs for in-hospital mortality compared to NN were: SIAD, 1.76 (95% CI 1.08-2.8, p=0.02), HON 2.77 (95% CI 1.8-4.3, p<0.0001) and HEN, 4.9 (95% CI 3.2-7.4, p<0.0001). The mortality rate was higher in HEN (RR 2.85; 95% CI 1.86-4.37, p<0.0001) and in HON, (RR 1.6; 95% CI 1.04-2.52; p=0.03), when compared to SIAD. The Charlson Comorbidity Index was lower in SIAD than in eunatraemic patients(p<0.0001). 9/121(7.4%) patients died with plasma sodium <125 mmol/l and 4(3.3%) with plasma sodium <120 mmol/l. However, 69/121(57%) patients died with a plasma sodium above 133 mmol/l. Conclusions We confirmed higher all-cause mortality in hyponatremia than in NN. Mortality was higher in SIAD than in normonatraemia, and was not explained on the basis of co-morbidities. Mortality was higher in HON and HEN than in SIAD. Mortality rates reported for all-cause hyponatremia in the medical literature are not applicable to SIAD. This article is protected by copyright. All rights reserved.