14 - Stroke Prevention - A Global Perspective
Editors: Norris, John W.; Hachinski, Vladimir
Title: Stroke Prevention, 1st Edition
Copyright 2001 Oxford University Press
> Table of Contents > III - Prevention: Policy and Practice > 14 - Stroke Prevention: A Global Perspective
14
Stroke Prevention: A Global Perspective
Ruth Bonita
Cerebrovascular disease (stroke) is the second leading cause of death worldwide, responsible for approximately 9.5% of all deaths.1 Despite almost 50 years epidemiologic research and increasing knowledge about appropriate intervention, the global burden of stroke is still rising, with most the deaths now occurring in the poorer regions of world. Although many lessons have been learned, the strategies adopted to date have been only partially effective in preventing and controlling the stroke epidemic.
Given the increasingly global nature of stroke, a global response is required. This chapter describes the dimensions of the global burden stroke, its evolution, and its several patterns; summarizes current understanding of the determinants of stroke at both the individual and population levels; identifies appropriate policies for the primary prevention and control of stroke, with a specific focus on developing countries.
The Global Burden of Stroke
Information on the global stroke burden that is of value for public health purposes comes from routine sources as well specific epidemiologic and clinical studies. Despite many decades of research into stroke and cardiovascular disease
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(CVD), there is still a surprisingly limited amount of reliable data on the global situation. In particular, there is a shortage of information from poorer regions and countries; most of the available data comes from developed countries. In addition, some difficulties remain in interpreting routine mortality data because of changes over time in national boundaries and in diagnostic, certification, coding practices. However, good evidence indicates that the overall secular trends apparent in these data are real.2
A particular shortage of routine mortality information exists for much Asia and Africa. Several eastern provinces of China are now collecting mortality data on a routine basis. A nationally representative system of 145 disease surveillance points covering about 1% of the Chinese population provides reliable statistics on death and disease.3 In several countries, including India, the verbal autopsy method has been used by health workers who interview families of the deceased to estimate cause-specific mortality.4 But for much of the world, it has been necessary to use extrapolation methods estimate overall, age, sex, and causespecific mortality rates. Most of the estimates global burden stroke come from the Global Burden of Disease (GBD) study,5 and the empirical data on which it is based are limited. However, less than one-third of the world's population is covered by national vital registration systems.
Mortality Patterns
Stroke was responsible for approximately 5.1 million of the total 16.7 million cardiovascular disease deaths in 1998.1 Of these deaths, 4.2 million occurred the poorer regions of the world, which are still experiencing a major burden communicable diseases, the so-called double burden of disease. In Asian countries, such as Japan and China, stroke is the leading cause of death, in contrast to most developed countries where stroke is the third leading cause of death. In absolute numbers, almost as many strokes occur in China in all developed countries combined.
The national burden of stroke and cardiovascular disease in general varies enormously depending on the social, economic, and cultural characteristics of country. The tremendous variation in stroke mortality rates at the national level for 15 selected countries is shown in Figure 14.1.
The highest total mortality rates for both men and women are in central and eastern European countries, especially those of the former Soviet Union. In all countries, the total mortality rates are higher in men than women. The ranking of countries is similar in men and women. The pattern remarkably similar to the total mortality league tables, suggesting that stroke mortality variations are likely to be real and not explained by a shift from one category another.
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FIGURE 14.1. Age standardized (per 100,000) stroke mortality rates, selected countries, women and men aged 40 69 years, 1997. |
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Incidence and Case Fatality
The best international stroke incidence data comes from the WHO MONICA Project, which has used standardized methods to monitor the trends and determinants of CVD in 37 defined populations in 21 countries, all of which, except China, are developed countries.6 As part of the MONICA Project, stroke incidence was monitored in 15 populations 10 countries the age group 35 64 years.7 In a 10-year period, 86,000 acute strokes were registered. The highest stroke rates were found in Russia and Finland, which were more than three-fold higher than in Friuli, Italy, the population with lowest rate. In half populations studied, the stroke incidence was twice as high in men as in women.8 For many MONICA populations, good agreement exists between stroke mortality statistics and fatal cases in the stroke register, although this is probably a reflection of false positive and negative' cases being roughly equal numbers in the routine mortality statistics.9 Case fatality at 28 days after a stroke in the MONICA populations ranged from 12% to 47%; higher case fatality rates are probably a reflection of under-ascertainment nonfatal strokes in the local register.
Disability
Various measures have been used to estimate the burden of stroke disability. Most recently, the GBD project used Disability Adjusted Life Years Lost (DALY) to
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estimate the global and regional burden of disease, including stroke. The DALY combines premature loss of life with years spent disabled to assess the overall and cause-specific burden of disease. The measured disease burden is the gap between a population's health status and that of an optimal, but perhaps unrealistic, reference population. The GBD study is a major advance, in that it has produced an internally consistent set of disease burden estimates for the entire world. At the same time, it has emphasized the extreme shortage of basic mortality and disease prevalence data for most of the world. In fact, because need to extrapolate from very limited data whole regions, for example, SubSaharan Africa, it appears that some regional estimates of CVD burden may be misleading.10 The GBD results have also given rise to criticism, especially concerning the use of DALYs rather than other types of quality adjusted life years.11 The GBD study is revising estimates as data become available.
TABLE 14.1. Distribution of Deaths Due to Stroke by Region, 19905 | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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Overall, stroke is estimated to be the sixth leading cause of DALYs worldwide, responsible for 3.0% of all DALYs globally in 1998. The distribution DALYs due to stroke by region for 1990 is shown in Table 14.2.12 The contribution to DALYs by stroke varies region, from 23% of the total in former socialist economies to 4% of the total in sub-Saharan Africa.
It has been projected that stroke will move from sixth place to fourth place as a cause of disability as measured by DALYs. These projections from the GBD study indicate that the coming epidemic of stroke disability will occur in the developing regions of the world.
As the CVD epidemic increases in developing countries, they will experience an economic burden comparable to that now experienced by developed countries. This economic burden will impose serious restrictions on the development capability of many developing countries, and they will be unable to afford the expensive, technological approach to CVD management already adopted by developed countries.
Evolution of Global Stroke Epidemics
Health Transition
The health transition is a useful framework for describing the spectacular shifts in the patterns and causes of death that have taken place in all countries over last few centuries. The health transition consists of three phases: the era of pestilence and famine (early), the era of receding pandemics (middle), the era of noncommunicable diseases (late). A fourth stage has also been suggested, described as the era of delayed noncommunicable (or degenerative) disease. The health transition describes the changes in both overall pattern of causes death and the changes within these broad categories. Four stages of the stroke transition are discernible.13 In the early phase, stroke is more common than heart disease,
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and hemorrhagic stroke more common than atherothrombotic. In the next stage, one sees an increasing percentage of atherothrombotic stroke and the emergence of heart disease. In the later phases health transition, heart disease becomes more common than stroke. And in the final phase, which is now being reached in developed countries, CVD death rates decrease, stroke becomes concentrated in the age group over 65 years, and life expectancy increases into the 80s.
TABLE 14.2. Distribution of DALYs Due to Stroke by Region, 1998 | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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Trends in Stroke Mortality
The best data on the recent evolution of CVD epidemic come from the WHO mortality database, which has reliable data for 35 (mostly developed) countries since the early 1950s. These data cover less than one-third of the world's population. Based on data from the WHO database, stroke mortality rates have been declining rapidly in most countries, especially since the mid-1970s (Figure 14.2). Spectacular declines in mortality rates have occurred Japan up to 7% on average per year. Stroke rates in Japan, which the 1950s were the highest world, are now similar to those in most western European countries.
Stroke rates in some eastern European countries were increasing until the 1980s. Within the region of the Americas, 15 countries for which trend data are available, stroke rates declined in all but four countries.14 Declines mortality are not necessarily associated with improvements in morbidity, disability, and quality of life. In most populations, the mortality trends were similar or larger
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than the corresponding attack rate trends. In both men and women, mortality decreased in most of the populations from western Europe, while nearly all populations from central and eastern Europe had an increasing mortality.
TABLE 14.3. Ten Leading Causes of DALYs, 2020 | ||||||||||||||||||||||||||||||||||||
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FIGURE 14.2. Change (% per year) in stroke rates, selected countries, men and women, 1970 1993. |
These declines in mortality most developed countries are such that fewer stroke deaths occur now than 25 years ago, despite the growth of the older population during that time. For example, approximately 110,000 fewer stroke deaths now occur each year in the United States than was projected based on the rates in the mid-1970s (Figure 14.3).
The profile of stroke patients has also changed in those countries experiencing a decline in stroke mortality: fewer premature stroke deaths has shifted the average age at time of stroke death upwards. The other impact these improvements in death rates is that an individual's risk of dying from a stroke has fallen considerably
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for both men and women; the lifetime risk of having a stroke is higher in women than in men because, on average, women live longer than men. The lifetime risk for men in the United States has declined from 11% in the 1950s to less than 6% in the 1990s. Although lifetime risk is higher among women, it has also decreased markedly, from 16% in the 1950s to 9% in the 1990s.
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FIGURE 14.3. Decline in absolute number of deaths the United States, 1975 1995. |
Trends in Incidence and Case Fatality
The major question concerning the trends in routine stroke mortality rates is the relative contribution to these trends of changes in incidence and case fatality rates. The WHO MONICA Project is the major international study investigating these trends. Stroke trends over a 7- to 10-year period across 15 MONICA populations in 10 countries indicate that annual stroke attack rates decreased in 8 populations in men and in 10 populations in women, with increased trends 7 in men and 3 in women.15 The average annual change in stroke attack rates was 1% in men and 1.3% in women. The average change mortality was 0.7% in men and 2.1% in women. In most populations, trends mortality were explained by both changes in incidence (one-third) and in case fatality (two-thirds).
The Demographic Transition
The changing age structure of populations will have an impact on the future burden of stroke. Great diversity exists in the pace and patterns ageing among countries. Some will remain young in the foreseeable future, for example, subSaharan Africa, and it may be difficult to get stroke other cardiovascular diseases on the agenda in the presence of continuing high levels communicable disease. Countries undergoing rapid aging especially those with large populations, such as China, India, and Indonesia face a double burden, coping with both communicable and noncommunicable diseases. Areas with an already relatively high percentage of the population aged over 65 years, such as Europe, Japan, and Australasia, are experiencing shifts towards the very old.
One impact of an increasing percentage people in the older age categories is the potential for an increasing burden of stroke. Population projections suggest that just seven countries China, India, Brazil, Russia, the United States, Japan, and Indonesia will dominate the world ageing scene in the future, with just two countries, China and India, making up almost half (44%) of the world's older population in the year 2020.
Figure 14.4 shows the projected increase in the older population between 1995 and 2025 for selected developing and developed countries. As the rate of increase in the latter is much slower, impact of stroke in the developed countries the future will be much less than in developing countries. Using the United States as an example, population-based incidence rates have been extrapolated to the
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entire U.S. population.16 Figure 14.5 shows that if there is no change in incidence rates, there will, not surprisingly, be a corresponding increase in the number of acute strokes from around 350,000 strokes each year in 1995 to 550,000 2030 because of the ageing population in that period.
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FIGURE 14.4. Projected percentage increase in the older population, 1995 2025. |
Other estimates show the impact of a small annual percentage either increase or decrease in stroke incidence rates. In the United States and most other developed countries, only a small annual percent decline in stroke mortality, of around 1% per year, is necessary to hold the numbers steady into foreseeable future. Small declines of around 2% to 3% per annum will lead to an ever-shrinking number of new stroke events each year. A future epidemic in the United States is therefore unlikely, even with a fall-off in the rate of decline, because up to a 6% decline each year has been achieved over the past few decades. The pattern is much the same for other developed countries: a continuing decline, even if not at the same level as in the past, seems achievable.
However, in countries where the pace of aging is much greater, as in China, a much greater preventive effort will be required to hold the numbers of new stroke events steady at around 2 3% (Fig. 14.6). Incidence rates based on a seven-county study have been extrapolated to the total Chinese population.17 A small increase in rates of only 1% per year will have a major impact, increasing the absolute numbers of stroke from just under 2 million in 1990 to 5 2020 and 7 million in 2030.
As per capita income increases, the social and economic conditions that allow for the widespread adoption of risky behaviors and habits gradually emerge. In particular, when disposable incomes increase sufficiently to produce a market for
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the multinational tobacco, food, and beverages, the basic causes of future epidemics of noncommunicable disease become established. Combined with the aging of the population, changes in nutrition, the adoption an atherogenic diet, and an increase in sedentary lifestyle are occurring much more rapidly than many Western, developed countries. It is likely that the stroke epidemic will emerge faster in these middle income countries. Such estimated projections of future stroke burden and the need to alter it through preventive efforts are most pressing challenge for the future.
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FIGURE 14.5. Projected number of new stroke cases in United States, 1990 2030, based on annual rate of change (%) in incidence. |
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FIGURE 14.6. Projected number of new stroke cases (in millions) in China, 1990 2030, based on annual rate of change (%) in incidence. |
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Prevention of Stroke
Traditionally, the focus of stroke research, prevention, and control has been on the relationship between risk factors and disease its management at the level of the individual. This body of research, much it epidemiologic, has produced a rich source of information that has provided evidence for the public health and health sector responses to stroke and other cardiovascular diseases.
Prevalence of Risk Factors and Trends
In addition to heart disease and stroke registers, the WHO MONICA Project collected standardized risk factor information over a ten-year period. In summary, the prevalence of the major risk factors varies considerably across the study populations. In general, levels are highest in those populations with the heart disease and stroke rates. Over the three risk factor surveys conducted at baseline and then after approximately five and ten years, smoking rates have usually declined in men, but the trends are mixed women; the mean diastolic blood pressure among populations differed by 15 mm Hg between the highest (Novosibirsk) and the lowest (Denmark). Smoking and elevated blood pressure explained 21% of the variations in stroke incidence among populations men and 42% in women.18 Obesity has generally increased in both men and women, particularly in men.
Risk Factor Levels in Developing Countries
Apart from the Beijing data collected as part of the MONICA Project, risk factor survey data are available from a number of other developing countries, including, Hong Kong, India, and Nigeria. In an Indian Council of Medical Research study conducted from 1989 to 1994, an urban population New Delhi was compared with a rural population from Haryana province. Although rural women in this province have an unusually high rate of smoking, general, risk factor levels are considerably higher in urban populations.4
Trend data in the level and proportion of standard risk factors is of most interest in predicting future patterns of stroke. In contrast to most developed countries in the MONICA Project, which risk factors have moved a favourable direction, the Beijing center registered increasing prevalence of smoking in both men and women.19
Effectiveness of Interventions to Reduce Risk Factor Levels
An increasing body of research indicates that interventions to reduce CVD risk factors is effective and in turn will reduce stroke mortality morbidity. The best evidence comes from randomized, controlled trials of pharmacologic interventions to control blood pressure and cholesterol levels in individuals. These trials
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have demonstrated that pharmacologic lowering of blood pressure reduces the risk of both stroke and coronary heart disease, although the relative effect is greater for stroke and is apparent in people both with without established disease. However, it has become increasingly obvious that this research has left the underlying social, economic, and cultural determinants of the epidemic largely unexamined, which has led to a rather narrow policy response to the epidemic.
Strategies to Reduce Risk
The debates on public health policy for CVD control have involved discussions of three issues: high-risk approach vs. population approach for prevention; single risk factor vs. multiple risk factor control strategies; and CVD control integrated noncommunicable disease control programs. The debates have served to highlight the strengths and limitations of each strategy. These approaches are not mutually exclusive. Public health policy must incorporate all of these into an overall CVD control program.
High-risk strategy
The high-risk approach aims to identify persons with markedly elevated risk factors who are therefore at the highest risk of disease. These individuals are then targeted by interventions that aim to reduce the risk factor levels. If successful, the benefits to individuals are large, because the individual absolute risk is large. However, since the number of persons in this high-risk category is proportionately much smaller than in the moderate-risk group, the overall benefits to society may be limited in terms of death or disability avoided. The strategy reduces, but does not necessarily minimize, the risk for the individuals concerned. Although a fall of blood pressure from 160/95 to 150/90 does indeed reduce the risk of a major CVD event, even this attained value poses greater risk than lower level of blood pressure. Further, this strategy may be difficult to sustain.20
Population approach for prevention
The population approach aims at reducing the risk factor levels in the population as a whole through community action. Because there is a continuum of risk associated with most risk factors, widespread change will result in a large benefit across a wide range of risk. While individual benefits are relatively small, the cumulative societal benefits are large ( the prevention paradox ).20
Blood pressure provides a good example because it is a key determinant of stroke risk, both in developed and developing countries. While the association appears to be stronger in Western populations, the Eastern Stroke and Coronary Heart Disease Collaborative Study Group, which analyzed data from 125,000 participants in 18 cohort studies from China and Japan, indicated that population-wide lowering of blood pressure has the potential to produce enormous declines
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in stroke eastern Asia.21 The study demonstrated that a small reduction in average diastolic blood pressure (by about 2%) would avert about a third of all strokes and one-sixth of all heart attacks. In the People's Republic China alone this would result in about 370,000 fewer stroke deaths each year, with most of these averted events occurring in people who were not considered hypertensive. If similar benefits could be achieved throughout the Asia-Pacific region by 2020, about one million deaths from cardiovascular diseases would be averted by a population-wide intervention (Table 14.4).
The relative benefits of both strategies are similar to estimates made for U.S. and U.K. populations.22,23 The absolute benefits, however, are many times greater due to the size of predicted CVD burden in Asia.24 Furthermore, the reduction in disability would be even more pronounced China, as stroke tends to be more disabling than coronary heart disease. A population-wide approach to reducing blood pressure by 2 % would avert the loss of 10 million healthy life years, about 1.5% of the total disease burden.
The population-based, lifestyle-linked risk reduction approach is particularly relevant in the context of developing countries, where it is necessary to ensure that communities currently at low risk are protected from acquiring factors (sometimes referred to as primordial prevention ). This is true for adults in the rural regions of most developing countries, as well for children in all populations. Population-wide changes that lower blood pressure, such as reduced salt intake,25,26,27 higher potassium intake,28 more exercise, and less obesity,29 all have the potential to produce large benefits. This approach is eminently applicable moderate risk groups in urban areas, where lifestyle-based modification will help avoid drug therapy, with its attendant economic and biologic costs.
Health professionals must recognise the societal benefits of the population strategy and play a strong advocacy role for promoting appropriate health behaviors
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in the community. Policy makers who desire a national impact on the CVD burden can ill afford to ignore the imperatives of investing in a population approach, which will pay large long-term dividends in the control of what are, in effect, lifestyle diseases. Risk factors for CVD tend to cluster in individuals due to common determinants. It is appropriate, however, adopt strategies that modify the total risk by reducing all or most of them.
TABLE 14.4. Estimated Number of Deaths Due to Coronary Heart Disease or Stroke Averted in Asia in 2020 by Two Blood Pressure Lowering Strategies24 | |||||||||||||||||||||||||||||||||||
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The decline of CVD mortality rates in industrial countries is the collective result of population-based prevention strategies that improve the risk profile communities, a high-risk approach of targeted interventions to protect individuals with markedly elevated risk factor levels, and case-management strategies to salvage, support, and sustain persons presenting with clinical problems.
Conclusion
Global prevention and control strategies need to be developed in tandem with surveillance systems to measure their impact on trends in stroke. The major challenge is to seize the opportunities and act now in the face of developing epidemics of stroke in particular and CVD in general. Greater emphasis on the public health dimensions of coming epidemics and the solutions necessary to contain them are amply illustrated by the case study of lowering blood pressure as a major preventive strategy. It is now obvious that not all strategies are equally relevant in all contexts.
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