STROKE OCTET Epidemiology of stroke RUTH BONITA Stroke is the third leading cause of death and an important of hospital admission and long-term disability in most industrialised populations. Epidemiological studies have lately addressed the causes and natural history of the disease and the need for hospital and community services. In this article I will review the epidemiological data and set the scene for the rest of the series. cause
Definitions and criteria The diagnosis of stroke in epidemiological studies initially relied on clinical observations; fortunately for such observations are reliable indicators of epidemiologists, incidence and prevalence.The World Health Organisation (WHO) definition of stroke is "rapidly developing clinical signs of focal (or global) disturbance of cerebral function, with symptoms lasting 24 hours or longer or leading to death, with no apparent cause other than of vascular origin".2 This definition includes subarachnoid haemorrhage but excludes transient ischaemic attacks, subdural haematoma, and haemorrhage or infarction caused by infection or tumour. Consequently, epidemiological studies are likely to underestimate the total burden of cerebrovascular disease. Criteria for classification of stroke subtypes have not been standardised and differ from one study to another. The ability to distinguish between haemorrhagic stroke and stroke due to infarction depends on computed tomography (CT scanning) and/or necropsy,l although some clinical scales can achieve a high positive predictive value.3 Since many episodes are treated at home it is difficult to achieve a high rate of CT scanning, but some studies have been successful in this respect.4,5 Clinical and diagnostic test criteria make subarachnoid haemorrhage a distinct epidemiological entity because in most cases these patients are admitted to hospital or die before medical attention can be sought.
Mortality Age-standardised mortality rates in people aged 40-69 years vary ten-fold from countries with high rates (24 0 and 14-4 per 10 000 population in men and women, respectively, in Bulgaria) to those with low rates (29 and 1 8 in Switzerland).6 Stroke accounts for 10-12% of all deaths in industrialised countries; about 88% of the deaths attributed to stroke are among people over 65 years. Death rates from stroke have fallen dramatically in recent decades in most industrialised nations, with average annual declines of up to 7% since 1970; Japan has experienced the most precipitous fall.6In the USA the decline began at the turn of the century; since 1970, the annual decrease has been 5 %. By contrast, in some eastern European countries (eg, Bulgaria, Hungary, and Czechoslovakia) stroke death rates have increased over the past two decades. We cannot fully explain trends in stroke mortality. Changing risk factor levels over time are the most likely explanation for the rapid alterations in stroke
mortality rates in recent years; risk factor changes could directly influence incidence or indirectly affect case-fatality by altering the natural history of the disease. Incidence The criteria for a well-designed stroke incidence study include use of the standard WHO definition, prospective ascertainment of cases from a large, well-defined, and representative population, and comprehensive case-finding methods to identify non-fatal cases treated out of hospital and patients dying shortly after an acute stroke event. In a 1987 review of sixty-six incidence studies, only nine met these criteria; these few studies form the basis of our knowledge about the incidence of stroke.7
Soderhamn (Swe) 1984
Age group (yrs) Fig 1-Average annual selected studies.
of stroke in
Incidence of stroke, strictly speaking, measures first-ever stroke (ie, with no history of stroke), although other measures of incidence are also used. Stroke incidence rates rise exponentially with increasing age, with a hundred-fold increase in rates from about 3 per 10 000 population in the third and fourth decades to almost 300 in the eighth and ninth decades.8 For comparison between studies and between countries, age-standardised rates provide the best measure. Age-standardised incidence rates (adjusted to the standard European population) from four studies conducted in the 1980s’ll-10 are shown in fig 1. The incidence rate in people aged 55-64 years varies from a low of 20 per 10 000 in Auckland, New Zealand, to a high of 32 in Soderhamn, Sweden; in the oldest age group (85 years and above), rates vary from 184 per 10 000 in Rochester, Minnesota, to 397 in Soderhamn. ADDRESS: University Geriatric Unit, Department of Medicine, North Shore Hospital, Private Bag 93-503, Takapuna, Auckland 9, New Zealand (Dr R. Bonita, MPH, PhD)
CUMULATIVE PROBABILITY (%) OF A PERSON AGED 45 YEARS HAVING AN ACUTE STROKE BY SELECTED AGES
The cumulative incidence of stroke-ie, the lifetime risk of having a stroke (assuming no competing risks)-is calculated by means of life tables from age-specific incidence data.8 The table shows that the risk of a person 45 years of age having a stroke within 20 years is very low (about 1 in 30). However, almost 1 in 4 men and nearly 1 in 5 women aged 45 years can expect to have a stroke if they live to their 85th year. Although the lifetime risk of having an acute stroke is higher in men than in women, the converse is true for the lifetime risk of dying of a stroke. Thus about 16 % of all women are likely to die of a stroke vs 8 % of all men;’this difference is largely attributable to the higher mean age at stroke onset in women, and to their greater life expectancy. Typically, first-ever strokes account for about 75 % of all acute events. The cumulative risk of recurrence over 5 years is high, ranging from about a third to almost a half of people who have a stroke.
Case-fatality Case-fatality measures the proportion of people who die within a specified period after an event; comparisons are based on the first-ever stroke in a lifetime since recurrent strokes have a higher case-fatality. 1-month case-fatality depends on the age structure and health status of the population studied and varies between 17 and 34% with an average of about 24%. About half of all deaths within the first month are due to the direct neurological sequelae of the stroke. 12 1-year case-fatality is about 42%.’" That most of the important prognostic factors for survival following a stroke (retention of consciousness, younger age, living at home at onset of the stroke, and no history of stroke)14 are not amenable to change testifies to the importance of primary prevention. Compared with the general population of the same age and sex composition, stroke patients do significantly worse irrespective of age or the length of follow-up; much of the difference is explained by the high early case-fatality. Fig 2 shows the 8-year survival of patients registered in the Auckland Stroke Study compared with the general population of the same age. Trends in incidence and case-fatality Have the trends in mortality been accompanied by similar trends in morbidity? Morbidity trends are often assessed by use of hospital discharge rates, but these data are limited because observed changes could reflect changes in natural history, in admission policies, or in management of acute stroke. Only four small community-based cohort studies are available to examine trends over time. In a small white rural population in Rochester, Minnesota, USA, the incidence of first-ever stroke declined by 54% between 1945-49 and 1975-79.9 Both sexes and all age groups experienced this favourable trend, with the largest decrease occurring in people aged 85 years and over. However, for 1980-84, the incidence of stroke seems to have increased in all age groups compared with 1975-79.9 This recent change may be explained by the introduction of CT scanning, which improves the detection of less severe strokes; greater clinical awareness may also lead to the inclusion of very mild strokes
Age group (yrs)
Fig 2-8-year age-specific survival of stroke patients compared with general population of the same age composition, Auckland 1981-89.
formerly not detected, irrespective of CT scanning. A changing pattern has also been reported from Soderhamn in Sweden, where the incidence in women rose by 38% between 1975-78 and 1983-85; the over-85 age group was responsible for most of this increase.lo The Hisayama study that began in Japan in 1961 has shown a decline in incidence of both cerebral haemorrhage (29%) and cerebral infarction (34%) between the periods 1961-66 and 1972-76.15 However, this study is based on only 1621 subjects and the small number of events precludes any assessment of trends in age-specific rates. Comparison of two prospective 8-year follow-up studies of random samples of the population aged 35-59 years from two provinces in eastern Finland, one beginning in 1972 and the other in 1977, showed a reduction in the incidence of stroke of about third. 16 Data on trends in case-fatality based on epidemiological studies are also scarce. A reduction of 50% in the 30-day case-fatality following stroke (all types combined) from 33 to 17% was observed in the Rochester study during the periods 1945-49 and 1980-84. Since some of the improvement in case-fatality occurred during the time when incidence was declining, increased detection of milder strokes is unlikely to be the only reason for the observed changes. In the Soderhamn study, 3-year survival improved by 15-4% and 5-5% for men and women, respectively, between the two study periods, 1975-78 and 1983-87; there was no difference in 1-month case-fatality. Most of the long-term improvement occurred between 2 and 6 months after the stroke and was attributed to fewer fatal complications rather than to a reduced risk of recurrent stroke." one
Burden of stroke
Extrapolation from the Auckland Stroke Study8 suggests that in a population of 1 million, 1250 people will experience their first-ever stroke each year, and an additional 350 people will have a recurrent episode. Of these 1600 people, only 880 will survive 6 months. 640 will be living in a private residence and the remainder, mostly heavily dependent elderly patients, will be in long-term care institutions. 6 months after the stroke, about two-thirds of stroke patients living at home regard themselves as independent and back to their pre-stroke functional status; the remainder,
representing about 15% of all acute events in a 1-year period, have residual difficulties with caring for themselves. 18 Apart from death, it is difficult to compare most outcome measures following stroke because of variable definitions of handicap and disability. Most epidemiological studies have tended to concentrate on handicap-ie, the disadvantage experienced as a result of impairment or disability resulting from stroke. However, many elderly people are already handicapped before their stroke and in institutional care; between 10 and 40% of all patients die before hospital admission can be arranged or are cared for in the
community. Hospital care in the acute phase after stroke is the most costly component of the care of stroke patients. The lengthy hospital stay of a small proportion of patients explains the high use and disproportionate share of hospital resources. The increasing burden of health care of stroke patients in the future will be in the acute stages rather than in the long-term management of elderly patients because first-ever stroke often kills people who have been handicapped by other causes, especially if they are elderly.19 Institutional care for about 20% of survivors likewise contributes to the costs. Nevertheless, the bulk of long-term care of stroke patients falls on family members and community services.18 The best measure of the total burden of stroke is prevalence, which provides information about the number of people at any one time in the population who have had a stroke, but reliable estimates of prevalence are especially difficult to come by. For health service planning and for the allocation of resources to help families cope with strokeimpaired survivors, we urgently need accurate information about the prevalence of stroke and about the prevalence of stroke disability. Total prevalence of stroke is estimated to be 5-8 per 1000 population over the age of 25 years.2o Unless a decline in stroke incidence can be achieved, the absolute numbers of stroke victims will rise significantly as the age of the population increases. The 1970s and 1980s were a time of expansion of stroke incidence studies, encouraged by WHO. Since 1984, about fifteen countries have registered stroke events in selected populations according to a standard protocol as part of the WHO MONICA projectand data collection will continue for 10 years. This collaborative effort will shed light on the changes in incidence because it is linked to studies of trends in risk factor levels in the same populations. REFERENCES
Molyneaux A, Warlow C. Value of computerised tomography in patients with stroke: the Oxfordshire Community Stroke Project. Br Med J 1985; 290: 193-97. 2. WHO MONICA Project, Principal Investigators. The World Health Organisation MONICA Project (monitoring trends and determinants in cardiovascular disease): a major international collaboration. J Clin Epidemiol 1988; 41: 105-14. 3. Allen CMC. Clinical diagnosis of the acute stroke syndrome. Q J Med
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1988; 19: 598-603. 11. World Health Statistics Annual 1989. Geneva: WHO, 1989. 12. Bamford J, Dennis M, Sandercock P, et al. The frequency, causes and timing of death within 30 days of a first stroke: the Oxfordshire Stroke Project. J Neural Neurosurg Psychiatry 1990; 53: 824-29. 13. Terent A. Survival after stroke and transient ischemic attacks during the 1970s and 1980s. Stroke 1989; 20: 1320-26. 14. Bonita R, Ford M, Stewart AW. Predicting survival after stroke: a three-year follow-up. Stroke 1988; 19: 669-73. 15. Ueda K, Omai T, Hirota Y, et al. Decreasing trend in incidence and mortality from stroke in Hisayama residents, Japan. Stroke 1981; 12: 154-60. 16. Tuomilehto J, Bonita R, Stewart AW, et al. Hypertension, cigarette smoking, and the decline in stroke incidence in eastern Finland. Stroke 1991; 22: 7-11.
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Primary prevention of stroke We do not clearly understand the most dramatic feature of the epidemiology of stroke-the striking decline in mortality that has been observed in many countries (see Bonita, p 342). Nevertheless, reductions in stroke mortality will probably continue; whether or not from our actions will be debatable. It is tempting to invoke dietary changes, with or without consequent alterations in blood pressure,’ or vigorous treatment of hypertension. Secular changes in average blood pressure are hard to document, not least because of the technical difficulties of conducting comparisons over time; and the reduction in stroke mortality was underway well before antihypertensive drugs were widely and successfully used. Such doubts do not detract from the body of knowledge that suggests that we can prevent stroke, and that preventive measures are worth trying. This article reviews what is known about risk factors for stroke and discusses possible strategies for prevention.
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Three types of evidence suggest that stroke is, in principle preventable. First, time trends: much of the rapid change in stroke mortality is likely to have resulted from change in incidence rates, so factors determining onset of disease must have changed. Second, there are large international ADDRESSES Department of Epidemiology and Public Health, University College and Middlesex School of Medicine, London, UK (Prof M G Marmot, FFPHM, N. R Poulter, MRCP); Department of Epidemiology and Population Sciences, London School of Hygiene and Tropical Medicine (Prof M G. Marmot); and Department of Clinical Pharmacology, St Mary’s Hospital Medical School, London (N. R Poulter). Correspondence to Prof M G Marmot, Department of Epidemiology and Public Health, University College and Middlesex School of Medicine, 66-72 Gower Street, London WC1 E 6EA, UK.