The Association of Outdoor Temperature with Severe Hyponatremia: A Study Based on the Stockholm Sodium Cohort.

BACKGROUND Hyponatremia is a common condition with unspecific symptoms and a complicated etiology. The impact of outdoor temperature on hyponatremia is not well-studied and varies depending on the climate and location. This study aimed to investigate the association between outdoor temperature and the prevalence of severe hyponatremia. METHODS This retrospective register-based cohort study based on the Stockholm Sodium Cohort investigated the association between ambient temperatures and severe hyponatremia (<125 mmol/L) in adults. Prevalence rates of severe hyponatremia were calculated as the number of days of severe hyponatremia divided by person-days at risk for the same temperature, using mean daily temperatures at the area of residence of each study participant to calculate exposure time. A prediction model incorporating changes in demographics and climate was used to estimate the burden of severe hyponatremia in Stockholm by 2050. RESULTS We identified 51,143 episodes of severe hyponatremia in 21,924 adults. A near linear modest increase in prevalence with rising temperatures was observed up to 20°C, followed by rapidly increasing rates at higher temperatures. The prevalence was higher with older age, reaching >100 days of hyponatremia per million person-days among +80-years-olds at temperatures over 22°C. Women experienced twice the rate observed in men. Temperature rises of 1°C or 2°C by the year 2050 are expected to be associated with higher prevalence rates by 66% and 73%, respectively. CONCLUSIONS There was a strong association between high temperatures and severe hyponatremia. The higher prevalence of severe hyponatremia was most pronounced among elderly.