Heatwave and dementia: a systematic analysis for the Global Burden of Disease Study 2019.

Introduction: Climate change has increased extreme weather conditions, such as heatwave, but its effects on dementia have not been studied globally. This study aims to assess the global association between heatwave and dementia incidence and to examine its heterogeneity across different countries. Methods: Gridded daily maximum temperature from the European Centre for Medium-Range Weather Forecasts Reanalysis V.5 database and the age-standardised incidence of dementia from the Global Burden of Disease Study 2019 were derived for 153 countries from 1990 to 2019. The associations between heatwaves and annual growth in incidence of dementia were tested via mixed-effect models and two-stage meta-regressions. The relationships were further analysed in the high-income countries (HICs) and the low- to middle-income countries (LMICs) separately. Finally, supporting analyses at state level in the USA and individual level in the UK were conducted. Results: We found a rising trend of heatwaves but significant between-country variation in incidence of dementia. Heatwave was positively associated with the acceleration of within-country annual growth in incidence of dementia worldwide (0.0436 SD, 95% CI 0.0391 to 0.0481; p<0.0001). The association was significant in the HICs (0.0294, 95% CI 0.0178 to 0.041; p<0.0001), and in the LMICs (0.0482, 95% CI 0.0437 to 0.0528; p<0.0001). A better economic situation mitigated the association in the HICs but exacerbated it in the LMICs. These findings remained significant after sensitivity analyses and were consistent at the state and individual levels. Conclusions: This finding underscores the global association between heatwave and incidence of dementia and its heterogeneity, indicating the importance of public policy to address the heatwave-associated incidence of dementia, especially in the LMICs.