Loading ...

Extreme temperatures are linked to an increased risk for stroke mortality, and people who live in low-income parts of the world are disproportionately affected, results of a new multinational study of almost 6 million strokes suggested.

Of every 1000 ischemic or hemorrhagic stroke deaths, about 11 were attributable to extreme hot and cold days, the researchers concluded.

"Our findings are another step toward understanding the effects of climate change on stroke," lead author Barrak Alahmad, MD, MPH, PhD, research fellow at Harvard T.H. Chan School of Public Health, Boston, said in a news release.

"As temperatures become more extreme, we foresee an increase in fatal strokes and a widening disparity in stroke mortality between high- and low-income countries, as the latter are likely to bear the brunt of climate change," he added.

The study is the largest multinational investigation of stroke mortality risk and extreme weather to date and was published online on May 22 in Stroke.

Widespread Impact

Despite advances in stroke prevention and treatment, stroke was the second-leading cause of death in 2019, accounting for 11.6% of total deaths worldwide. In low- and middle-income countries, the stroke burden is higher, and patients are typically younger.

While the connection between extreme temperatures and global mortality is not new, previous work looking at heat, cold, and stroke outcomes has produced conflicting results, and the studies were usually limited to a single city or country.

Using the Multi-Country Multi-City Network, a global environmental health consortium, the research team behind this current study built a new stroke mortality database that allowed them to more broadly compare temperature and stroke outcomes and analyze those outcomes based on a country's wealth. The team computed excess deaths in each city that are attributable to the 2.5% hottest and coldest days based on each city's temperature distribution.

The research team collected data on about 3.4 million ischemic strokes and almost 2.5 million hemorrhagic stroke deaths from 522 cities in 25 countries from 1970 to 2019.

For every 1000 ischemic stroke deaths, they found that extreme cold and hot days contributed to 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.

Low-income countries, measured by gross domestic product per capita, were at an increased risk for elevated heat-related hemorrhagic stroke mortality compared with higher-income countries (P = .02).

Though evidence suggested that low-income countries may be more seriously affected by extreme cold as well, the findings were not conclusive. The study did not find a relationship between countries' gross domestic product and risk for temperature-related ischemic stroke mortality.

While this was a broader analysis than previously conducted, the study included limited data from South Asia, Africa, and the Middle East, and the data largely reflect urban, not rural, settings, the authors noted.

The study also did not evaluate relationships between extreme temperatures and nonfatal stroke, an area that should be explored, wrote Alahmad and his coauthors.

Worrisome Future

As temperatures throughout the world become more extreme, the researchers foresee an increase in stroke deaths and increasing disparity in mortality rates between high- and low-income countries, they wrote.

Better indoor temperature control systems and lower rates of outdoor work in high-income countries, as well as poorer access to quality healthcare in low-income countries, might explain the disparities.

"As climate change is driving more extreme temperatures and weather events, urgent attention to clinical care, adaptation, and mitigation is needed to minimize the risk of death from stroke, especially in low-income countries," wrote Alahmad and his coauthors.

The study was funded by the Kuwait Foundation for the Advancement of Sciences, the Medical Research Council-UK, the European Union's Horizon 2020 Project EXHAUSTION, the Swiss National Science Foundation, and the National Institutes of Health. Alahmad reported no conflicts. Other disclosures are included in the original article.