Common risk factors do not explain greater stroke burden in low-income countries

February 20, 2009

New figures show that stroke mortality rates and burden vary greatly by country and between regions, with low-income countries being the most affected. However, known risk factors for stroke, including diabetes and alcohol consumption, are poor predictors of national stroke mortality and burden and do not explain the greater burden in low-income countries. These are the conclusions of an Article to be published Online First and in the April edition of The Lancet Neurology, and presented on Friday at the International Stroke Conference in San Diego, by S Claiborne Johnston from the University of California, San Francisco, which coordinated the study with the World Health Organisation (WHO).

A second study shows that, over the past forty years, the incidence of stroke in low-to-middle income countries has increased by more than 100%, and people living in these countries face a 20% greater risk of stroke than those living in high-income countries, where the incidence of stroke has declined by 42%. These are some of the main findings of a Review by Valery Feigin from the National Research Centre for Stroke in New Zealand and international colleagues, published Online first and in the April edition of The Lancet Neurology.

The authors state that the findings of these two studies further highlight the urgent need for improved prevention, treatment, and research funding for stroke in low-income countries.

In 2002, there were an estimated 15.3 million strokes worldwide. Although more than 85% of strokes occur in low-income and middle-income countries, most resources are allocated to stroke prevention and treatment in high-income countries. However, little is known about the global distribution of stroke or whether common risk factors for stroke--such as high blood pressure, diabetes, smoking, alcohol abuse, and obesity--have a similar or different impact in every region of the world, and whether they can explain the greater risk of stroke in low-income countries and provide potential targets for intervention.

The Article by Johnston and colleagues, assessed national differences in stroke mortality and disability-adjusted life years (DALYs) rates worldwide using vital statistics and data from systematic reviews of disease surveillance and models, as part of the WHO Global Burden of Disease Programme. They used similar methods to calculate whether differences in national income and national prevalence of known risk factors for stroke were predictors of regional differences in stroke incidence and mortality.

Findings showed large regional variations in age-adjusted mortality rates and in DALYs, with up to ten times as many stroke deaths and DALYs lost between the most-affected countries, in eastern Europe, north Asia, central Africa, and the south Pacific, compared with the least-affected countries, in western Europe and North America.

National income was the strongest predictor of stroke burden and mortality even after adjustment for known risk factors, with stroke-related deaths up to three and a half times greater in low-income than middle-to-high income countries. Indeed, most risk factors were found to be either not or only weakly associated with stroke deaths and burden, and were more prevalent in high-income countries. However, raised systolic blood pressure and low body-mass index were shown to predict stroke mortality, and greater prevalence of smoking predicted mortality and DALY loss.

The authors conclude that current methods of measuring and monitoring global determinants of stroke cannot explain regional variations in stroke burden and mortality. They suggest that: "Other known risk factors for stroke, such as rheumatic heart disease and atrial fibrillation, and emerging risk factors such as HIV/AIDS, could explain the burden of stroke in low-income and middle-income countries and should be monitored. Certainly, the need for interventions to reduce stroke risk should not be based solely on a review of the current metrics."

In the Review, Feigin and colleagues report that the incidence of stroke in low-to-middle income countries has reached an epidemic level with more than a two fold increase from 1970 to 2008. In addition, compared with high-income countries, people living in low-to-middle income countries are also disproportionally affected by haemorrhagic strokes (often the most severe strokes) and more often die from stroke within the first month after onset. The authors conclude that: "now is the time for action" and that any doubts as to whether stroke, as a health-management issue, should be high on government agendas in low-to-middle income countries has been dispelled.

In an accompanying Comment, Martin O'Donnell and Salim Yusuf, from McMaster University in Canada, say that the findings of the two studies "highlight the current deficits in the study of the determinants of global stroke incidence." They call for large international studies similar to those that have advanced the understanding of regional variations in the incidence and risk factors for coronary heart disease, to enable the tailoring of "population-based prevention and treatment strategies to such regional variation."
Professor S Claiborne Johnston, University of California, San Francisco, USA.
T) +1 415 279 0787 (mobile) E)

Professor Valery Feigin, National Research Centre for Stroke, AUT University, Auckland, New Zealand.
T) +64 9 921 9166 or +64 9 921 9174 E)

Dr Martin O'Donnell, McMaster University, Ontario, Canada.
T) +1 905 518 7622 (mobile) E)


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