Current and future burdens of heat-related hyponatremia – a nationwide register-based study

A seasonal variation in hyponatremia, with higher incidence rates during hot summer days has been demonstrated. Whether this applies to cool temperate regions is currently unknown.

The aim of this study was to investigate the influence of ambient temperature on hyponatremia in the Swedish population under current and future climate scenarios.

Design, setting and patients
Nationwide cohort study. All patients hospitalized with a first-ever principal diagnosis of hyponatremia between October 2005 and December 2014 were identified. Incidence rates for hyponatremia were calculated as number of hospitalizations divided by person-days at risk in the adult Swedish population at a given temperature, in increments of 1 ○C.

The incidence of hyponatremia was stable at 0.3 per million person-days from -10°C to 10°C, but increased rapidly at 24h mean temperatures above 15°C, with 1.96 hospitalizations per million days at the highest recorded temperature of 26°C. Women and elderly carried the greatest risk, with an incidence of 30 hospitalizations per million days in individuals ≥80 years old on the hottest days, corresponding to a 15-fold increase in incidence compared to cool days. A future 1°C or 2°C increase in mean temperature is expected to increase the incidence of hyponatremia by 6.3% and 13.9%, respectively.

The risk of hospitalization due to hyponatremia increases rapidly at temperatures above 15°C, indicating a threshold effect. Over the next decades, rising global temperatures are expected to increase the inpatient burden of hyponatremia by approximately 10%. Strategies for protecting vulnerable groups are necessary to reduce this risk.