New study shows air pollution, high temperatures, and metabolic risk factors driving global increases in stroke

The findings of this major new analysis from the Global Burden of Disease, Injuries, and Risk Factors Study (GBD) are published in The Lancet Neurology journal and being presented at the World Stroke Congress in Abu Dhabi in October 2024.

Globally, the number of people having a new stroke rose to 11.9 million in 2021 (up by 70% since 1990), stroke survivors rose to 93.8 million (up by 86%), and stroke-related deaths rose to 7.3 million (up by 44%), making the condition the third leading cause of death worldwide (after ischaemic heart disease and COVID-19). More than three-quarters of those affected by strokes live in low- and middle-income countries (LMICs).

Additionally, estimates suggest that worldwide, the overall amount of disability, illness, and early death—a measurement known as disability-adjusted life years (DALYs)—lost to stroke increased by 32% between 1990 and 2021, rising from around 121.4 million years of healthy life lost in 1990 to 160.5 million years in 2021, making stroke the fourth leading cause of health loss worldwide after COVID-19, ischaemic heart disease, and neonatal disorders.

The burden of stroke is increasing in large part due to both population growth and the rise of aging populations worldwide, but also due to increasing contributions from preventable environmental, metabolic, and behavioural risk factors. Between 1990 and 2021, the global stroke burden linked to high body mass index (BMI; up by 88%), high temperatures (up 72%), high blood sugar (up 32%), diet high in sugar-sweetened drinks (up 23%), low physical activity (up 11%), high systolic blood pressure (up 7%), and diet low in omega-6 polyunsaturated fatty acids (up 5%) increased substantially.

However, if the impact of demographics is removed through age-standardisation (to allow comparisons between countries and over time), there has been a trend towards lower rates (age-standardised per 100,000 population) of incidence (down by 22%), prevalence (down 8%), deaths (down 39%), and DALYs (down 39%) worldwide, and across virtually all country income levels, since 1990.  But since 2015, improvements in global incidence rates have stagnated, while age-standardised rates of stroke incidence, death, prevalence and DALYs have got worse in Southeast Asia, East Asia, Oceania, and in people younger than 70 years.

Lead author Professor Valery L Feigin from Auckland University of Technology, New Zealand, an affiliate professor at the Institute for Health Metrics and Evaluation (IHME) at the University of Washington, USA, said: “The global growth of the number of people who develop stroke, and died from or remain disabled by stroke is growing fast, strongly suggesting that currently used stroke prevention strategies are not sufficiently effective. New, proven effective population-wide and motivational individual prevention strategies that could be applied to all people at risk of having a stroke, regardless of the level of risk, as recommended in the recent Lancet Neurology Commission on Stroke [4] should be implemented across the globe urgently.”

The current study builds on previous GBD analyses [2] to provide the most up-to-date and comprehensive analysis of stroke burden and risk factor estimates in countries on a global scale between 1990 and 2021, to help guide health-planning, prevention, and resource allocation.

Fast-growing burden of uncontrolled risk factors, mostly affecting LMICs

The study reveals striking differences in the overall stroke burden (as measured by age-standardised incidence, prevalence, death and DALY rates) between world regions and national income levels in 2021. In high-income North America and Australasia, and middle-income Latin America—regions with the lowest stroke burden—the age-standardised rates of incidence and prevalence were lowest in New Zealand (67.8 and 707.4 per 100,000 people respectively), death rates lowest in Canada (20.4 per 100,000 people), and DALY rates lowest in Australia (435.0 per 100,000) in 2021.

In contrast, in the regions of low- and middle-income East and Central Asia and sub-Saharan Africa, the rates of incidence, prevalence, death and DALYs were up to 2 to 10 times higher (over 248, 1458, 190, and 4320 per 100,000 people, respectively) in 2021.

Strikingly, half of all the disability and the lives lost to stroke globally (81 million healthy years of life lost) in 2021 were the result of haemorrhagic strokes—the deadliest form, mainly due to high blood pressure—despite being around half as common as ischaemic strokes (4.1 million new haemorrhagic strokes vs 7.8 million new ischaemic strokes). Most affected were people aged 70 and younger and those living in low-income countries, where the proportion of strokes that are intracerebral haemorrhage is double that of high-income countries (37% vs 18%).

“Stroke-related health loss disproportionately impacts many of the most disadvantaged countries in Asia and sub-Saharan Africa due to the growing burden of uncontrolled risk factors, especially poorly controlled high blood pressure, and rising levels of obesity and type 2 diabetes in young adults, as well as the lack of stroke prevention and care services in these regions,” explained co-author Dr Catherine O. Johnson, Lead Research Scientist at IHME. “The shift in stroke burden towards younger populations is likely to continue unless effective preventive strategies are implemented urgently.”

Increasing impact of environmental risk factors

The study estimates that the total number of stroke-related DALYs attributable to 23 risk factors [3] globally has risen from 100 million years of healthy life lost in 1990 to 135 million in 2021—presenting a public health challenge and an opportunity for action. The largest proportions of these risk factors are found in Eastern Europe, Asia, and sub-Saharan Africa.

Metabolic risk factors—especially high BMI, high systolic blood pressure, and high LDL cholesterol—contributed to the most stroke burden across all country income levels (ranging from 66-70%) in 2021, followed by environmental risk factors collectively (i.e., air pollution, low/high ambient temperature, lead exposure) in LMICs (35-53%).

In 2021, the five leading global risk factors for stroke were high systolic blood pressure, particulate matter air pollution, smoking, high LDL cholesterol, and location.

For the first time, the study suggests that ambient particulate matter air pollution is a top risk factor for subarachnoid haemorrhage, contributing to 14% of the death and disability caused by this serious stroke subtype, on a par with smoking.

In contrast, substantial progress has been made in reducing the global stroke burden from risk factors linked to poor diet, air pollution, and smoking, with health loss due to diets high in processed meat and low in vegetables declining by 40% and 30%, respectively, particulate matter air pollution by 20%, and smoking by 13%.

This suggests that strategies to reduce exposure to these risk factors over the past three decades, such as clean air zones and public smoking bans, have been successful.

“With 84% of the stroke burden linked to 23 modifiable risk factors there are tremendous opportunities to alter the trajectory of stroke risk for the next generation,” said Dr Johnson. “Given that ambient air pollution is reciprocally linked with ambient temperature and climate change, the importance of urgent climate actions and measures to reduce air pollution cannot be overestimated. And with increasing exposure to risk factors such as high blood sugar and diet high in sugar-sweetened drinks, there is a critical need for interventions focused on obesity and metabolic syndromes. Identifying sustainable ways to work with communities to take action to prevent and control modifiable risk factors for stroke is essential to address this growing crisis.”

Prevention must be top priority

The authors say that by implementing and monitoring the evidence-based recommendations set out in the 2023 World Stroke Organization-Lancet Neurology Commission on stroke [4], there is an opportunity to drastically reduce the global burden of stroke in this decade and beyond, as well as improve brain health and the overall wellbeing of millions of people around the world.

As Professor Feigin explained: “Additional and more effective stroke prevention strategies, with an emphasis on population-wide measures, such as task-shifting from doctors to nurses and health volunteers, and the wider use of evidence-based mobile and telehealth platforms, along with pragmatic solutions to address the critical gaps in stroke service delivery, workforce capacity building, and epidemiological surveillance systems must be urgently implemented across all countries.”

Writing in a linked Comment, Professor Ming Liu and Associate Professor Simiao Wu from West China Hospital, Sichuan University in China (who were not involved in the study) say: “Pragmatic solutions to the enormous and increasing stroke burden include surveillance, prevention, acute care, and rehabilitation. Surveillance strategies include establishing a national-level framework for regular monitoring of stroke burden, risk factors, and health-care services via community-based surveys and health records. Artificial intelligence and mobile technologies might not only facilitate the dissemination of evidence-based health services, but also increase the number of data sources and encourage participation of multidisciplinary collaborators, potentially improving the validity and accuracy of future GBD estimates. We hope that GBD analyses will continue to provide timely health data and inform action in the battle against stroke at the global, regional, and national levels.”