Urban Heat and Burden of Hyponatremia

Hyponatremia (sodium level <135 mEq/L), the most prevalent electrolyte disorder in clinical practice, is associated with considerable clinical and economic burdens. Despite its recognized effect and indication of seasonal patterns, there is a research gap regarding heat-related hyponatremia. Addressing this issue is crucial, especially regarding projected increases in environmental temperature, particularly in urban areas.

To comprehensively analyze the association of heat in an urban setting with hyponatremia prevalence.

This cross-sectional study using retrospective time series analysis was conducted among all adult patients (age ≥18 years) presenting to the Charité-Universitätsmedizin Berlin between March 1, 2000, and August 31, 2023, with a blood sodium measurement.

The daily heat index, which accounts for both outdoor air temperature and relative humidity.

The primary outcome measure was the daily number of hyponatremia cases, further categorized by severity. Differences in prevalence across age groups (adult patients, aged 18-65 years; and older patients, aged >65 years) and sexes were also examined.

A total of 7 135 688 sodium measurements from 2 028 537 hospital visits were analyzed. The mean (SD) age at admission was 57.8 (17.8) years, and 51.7% of patients were male. A clear seasonal pattern of heat-related hyponatremia was evident among older patients, especially for moderate (sodium level, 125-129 mEq/L) and severe hyponatremia (sodium level, <125 mEq/L), where the cumulative risk over a lag period of 5 days reached a maximum relative risk (RR) of 1.26 (95% CI, 1.07-1.48) when the heat index reached 30 °C, compared with the temperature at which hyponatremia occurred least frequently. Older women were disproportionately more likely to experience hyponatremia compared with older men, with a cumulative RR of 1.10 (95% CI, 1.03-1.18) at a heat index of 26 °C. The highest RR was on lag day 0 for all subgroups (older patients: RR, 1.04 [95% CI, 1.00-1.08]; moderate and severe hyponatremia in older patients: RR, 1.05 [1.01-1.10]; and older women: RR, 1.07 [95% CI, 1.01-1.12]) and significantly increased when the heat index exceeded 15 °C (RR, 1.01 [95% CI, 1.00-1.02]).

This cross-sectional study of patients with sodium measurements suggests that older people, especially women, were vulnerable to heat-related hyponatremia. Environmental heat was associated with an immediate exacerbation of hyponatremia. This finding highlights the importance of implementing prevention strategies to mitigate heat-related hyponatremia, as an increased burden in the future due to climate change is likely.