Extreme Temperatures and Stroke Mortality: Evidence From a Multi-Country Analysis

Abstract

Background: Extreme temperatures contribute significantly to global mortality. While previous studies on temperature and stroke-specific outcomes presented conflicting results, these studies were predominantly limited to single-city or single-country analyses. Their findings are difficult to synthesize due to variations in methodologies and exposure definitions.

Methods: Within the Multi-Country Multi-City Network, we built a new mortality database for ischemic and hemorrhagic stroke. Applying a unified analysis protocol, we conducted a multinational case-crossover study on the relationship between extreme temperatures and stroke. In the first stage, we fitted a conditional quasi-Poisson regression for daily mortality counts with distributed lag nonlinear models for temperature exposure separately for each city. In the second stage, the cumulative risk from each city was pooled using mixed-effect meta-analyses, accounting for clustering of cities with similar features. We compared temperature-stroke associations across country-level gross domestic product per capita. We computed excess deaths in each city that are attributable to the 2.5% hottest and coldest of days based on each city's temperature distribution.

Results: We collected data for a total of 3 443 969 ischemic strokes and 2 454 267 hemorrhagic stroke deaths from 522 cities in 25 countries. For every 1000 ischemic stroke deaths, we found that extreme cold and hot days contributed 9.1 (95% empirical CI, 8.6-9.4) and 2.2 (95% empirical CI, 1.9-2.4) excess deaths, respectively. For every 1000 hemorrhagic stroke deaths, extreme cold and hot days contributed 11.2 (95% empirical CI, 10.9-11.4) and 0.7 (95% empirical CI, 0.5-0.8) excess deaths, respectively. We found that countries with low gross domestic product per capita were at higher risk of heat-related hemorrhagic stroke mortality than countries with high gross domestic product per capita (P=0.02).

Conclusions: Both extreme cold and hot temperatures are associated with an increased risk of dying from ischemic and hemorrhagic strokes. As climate change continues to exacerbate these extreme temperatures, interventional strategies are needed to mitigate impacts on stroke mortality, particularly in low-income countries.

Keywords: climate change; extreme cold; hemorrhagic stroke; ischemic stroke; temperature.

Figure 1.

Pooled exposure-response relationships between overall temperature percentiles and relative risk (RR) of ischemic and hemorrhagic stroke mortality from 522 cities in 25 countries.

Figure 2.

Proportion of excess deaths for every 1000 hemorrhagic and ischemic deaths in A. extremely cold days (lower than the 2.5^th percentile of temperature) and B. extremely hot days (higher than the 97.5^th percentile of temperature)

Figure 2.

Proportion of excess deaths for every 1000 hemorrhagic and ischemic deaths in A. extremely cold days (lower than the 2.5^th percentile of temperature) and B. extremely hot days (higher than the 97.5^th percentile of temperature)

Figure 3.

Pooled exposure-response relationships between overall temperatures (in °C) and relative risk (RR) of ischemic and hemorrhagic stroke mortality after stratifying by the 25^th and 75^th percentile of country-level gross domestic product (GDP) per capita (75^th = higher GDP countries, 25^th = lower GDP countries), city-level mean summer temperature (75^th = warmer summers cities, 25^th = cooler summers cities), and city-level mean winter temperature (75^th = warmer winters cities, 25^th = cooler winters cities).