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(Stroke. 1996;27:370-372.)
© 1996 American Heart Association, Inc.

Articles

The Enigma of the Decline in Stroke Deaths in the United States

The Search for an Explanation

Ruth Bonita, PhD, MPH Robert Beaglehole, MD, Dsc

From the University Geriatric Unit, Department of Medicine (R. Bonita), and the Department of Community Health (R. Beaglehole), School of Medicine, University of Auckland (New Zealand).

Correspondence to Ruth Bonita, MPH, PhD, University Geriatric Unit, North Shore Hospital, PB 93-503, Takapuna, Auckland 9, New Zealand.

	Introduction

Top Introduction References

 
Approximately 1 in 12 of all deaths worldwide are due to stroke; the majority occur in poorer countries.¹ From the limited data on national cause of death (available for only about one quarter of the world's population), it is clear that there are striking variations among countries in both stroke death rates and their direction and rate of change. Age-standardized death rates in central and eastern European countries are four to five times greater than the rates in the United States and are increasing.² Stroke death rates have been declining in the United States at least since the 1950s, with an increase in the rate of decline since the early 1970s. This decline, which has also occurred in many other industrialized countries, is real and has contributed to the increasing life expectancy of middle-aged and older people. Stroke is also a leading cause of disability, with poor countries carrying much of the global burden of stroke disability.³ With the aging of all populations, the global burden of stroke will inevitably increase.

Well-designed studies, which prospectively measure the incidence and case fatality of stroke in large, well-defined populations, are required to understand the reasons for these trends. The change in death rates could be due to a decline (or an increase) in incidence as a consequence of the success (or the failure) of primary preventive efforts, to an improvement (or a deterioration) in case fatality as a consequence of treatment, or to a change in the natural history of the disease. Unraveling these competing explanations is a major challenge for epidemiologists. The decline in incidence rates is the most likely explanation for the striking declines in stroke mortality over most of the last four decades because in this period there have been few advances in the acute treatment of stroke that are widely applicable. The evidence is, however, inconsistent. Some studies have shown a decline in stroke incidence for different time periods,^{4 5 6 7} others show no overall change^{8 9} or an in-crease in incidence,^{10 11 12} and the trends are not always consistent for all age groups or for both men and women. Other studies point to improved case fatality as the more likely explanation for the trends.^{6 8 9 13}

The best known of the long-term studies comes from Rochester, Minn, where stroke rates have been monitored since 1955. The strengths of the Rochester study are due to the well-established medical records and record-linkage system at the Mayo Clinic and the affiliated medical institutions that allow retrieval of the records of residents of Rochester with a stroke diagnosis. However, this population is not entirely typical of the population of the United States; the residents of Rochester are more educated, more homogeneous, more affluent, and younger on average than the US population, and they have greater access to a sophisticated health system. Stroke mortality rates in Rochester are lower than elsewhere in the United States, and the mortality decline has been more rapid than in the United States as a whole.¹⁴

Until the end of the 1970s, successful primary prevention (declining incidence of stroke) was the explanation for the reduction in mortality from stroke in the Rochester population.¹⁵ Although this was attributed to improvements in the treatment of persons with hypertension,^{16 17} the majority of strokes occurred in those in the middle range of the blood pressure distribution.¹⁸ The lack of further reduction in incidence in the early 1980s, despite continuing efforts to improve the treatment of individuals with high blood pressure with newer and more costly drugs, suggests that the impact of antihypertensive medication on population blood pressure levels may have been minimal.¹⁹

Since the apparent end of the decline in the incidence of stroke in the Rochester population at the beginning of the 1980s was noted,²⁰ an update has been eagerly awaited for the light it might shed on explanations for the decline in stroke deaths. The article published in this issue reporting stroke incidence rates and case fatality in Rochester for the quinquennial period of 1985 through 1989 suggests that the early leveling off of incidence rates has continued and rates are now as high, if not higher, than 10 years previously.²¹ These results, together with a lack of evidence of any improvement in early case fatality, are sobering and, if real, disturbing. They also present an enigma: they offer little understanding of the substantial improvement (around 5% each year) in stroke mortality rates that has occurred in the United States since the beginning of the 1970s.

There are a number of reasons that might explain an apparent increase in incidence rates in the 1980s. First, the population of Rochester is small: fewer than 100 stroke events are registered each year, and precise estimates of incidence rates are difficult. Although not stated in the article, extrapolation from data provided in Table 1 suggests that the average annual population in the 1985 to 1989 quinquennium was 67 000, only 10% of whom were over the age of 65 years. Second, although essentially based on clinical criteria, definitions appear to have changed during the time period, reflecting increasing access to neuroimaging techniques. Trends in subtypes are particularly difficult to measure in a meaningful manner when diagnostic capability has changed so dramatically over the study period. Third, changes in case finding are likely to have increased the number of mild stroke patients identified. For example, the increase in the number and proportion of all events in women in the oldest age group (85 years and over) may have been a consequence of the expanded Medical Index and greater access to CT; in 1985 through 1989, one third of all events in women registered were in this age group, compared with only 17% in 1975 through 1979, despite the fact that there was little population increase during this time. An increase in the proportion of stroke patients receiving CT scans (from 6% in the early 1970s to over 80% in the 1980s) will have had an impact on case finding, especially because of the reliance on retrospective assessment of medical records. A decline in autopsy rates may also have had an impact on case finding because in earlier years autopsy reports that confirmed a diagnosis of stroke were included, regardless of the presence or absence of clinical signs.

So where does this leave us? There has been a welcome increase in the number of population-based registers in many countries in recent years. These studies have been accompanied by improvements in design, with much more attention paid to the use of standardized data.²² However, only a few are able to report comparable data over time, and many of the new stroke registers are not based on sufficiently large populations to provide trend data with any confidence. It is regrettable that the Rochester study is the only long-term study of stroke trends in North America.

Perhaps the most promising effort to understand the decline in stroke mortality, and to link this with changes in stroke risk factors, is the World Health Organization MONICA Project, which has investigated 21 populations in 12 countries, mostly European, over a 10-year period beginning in the early 1980s.²³ In addition to monitoring incidence and case fatality with strict definitions and guidelines, all official mortality data are validated against the stroke registers. The study shows that geographical differences are real; the variation in mortality rates between countries is also reflected in incidence rates,²⁴ indicating the enormous potential for prevention. The trend data from the MONICA Project are yet to come.

We need to build on the best of the existing population-based studies of stroke that are of sufficient size for incidence rates to be estimated with confidence. Parallel studies monitoring changes in risk factor levels in the population are equally important. Lessons from earlier studies, including the Rochester study, need to be incorporated into new initiatives; adherence to strict criteria and standardized methods will be essential. Because most strokes occur in low- and middle-income countries, documenting changes in the stroke profile must become an international effort, with researchers in the wealthy countries assisting those in the poorer countries. Without this effort, the enigma will remain.

The recent results from the Rochester study serve as a timely warning. If it is true that stroke incidence rates are no longer declining, it is a clear signal that primary prevention is failing. If it is also true that there is no improvement in survival in the acute phase, it suggests that the widespread adoption of neuroimaging techniques has not improved outcomes. The results from the Rochester study suggest that the efforts currently directed at secondary prevention have been at the expense of attempts to prevent strokes from occurring in the first place. Primary prevention strategies need greater emphasis. The high-risk approach should focus only on individuals at highest absolute risk of stroke,^{25 26} but the greatest gains will come from the population strategy that aims to reduce the average level of risk in entire populations.

	Footnotes

 
The opinions expressed in this editorial are not necessarily those of the editors or of the American Heart Association.

	References

Top Introduction References

 

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