Public Health (1992), 106,343-350
(~) The Society of Public Health, 1992
Trends in Mortality from Stroke in Scotland, 1950-1986 M. Janghorbani, PhD, A.J. Hedley, MD FRCP, R.B. Jones, PhD, W.H. Gilmour, MSc and M. Zhianpour, ME) MPH Department of Public Health, University of Glasgow, Glasgow G 12 8QQ
The Scottish Registrar General's Annual Reports have been used to study trends in mortality from stroke in Scotland during 1950-1986 in those aged 45 to 74. In 1950 the age-adjusted mortality rate was 347.4 per 100,000 population for men and 360.8 for women, falling to 199.6 for men and 155.8 for women in 1986. This downward trend has increased from 1976 for males. The average annual decline in age-adjusted mortality from stroke over the 37-year period was 4.0 per 100,000 in males and 5.5 in females. This reduction in death rates was proportionally higher for women compared with men in all age groups over 55 years. As with cardiovascular deaths, mortality from stroke was lower in the east than in the central region and west of Scotland. The reduction in mortality resulted in a substantial 'saving' of lives, estimated at 12,500 between 1980 and 1984.
Introduction Cerebrovascular diseases are now the third leading cause of death in most developed countries and in addition lead to serious disability in the survivors. Analysis of mortality statistics undertaken in several countries during the past two decades suggests that deaths due to stroke are on the decline. 1-4 This trend accelerated in the mid-1960s in the U S A 5 and early 1970s in Japan 6 but the reasons have not yet been fully explained. The decline could have been caused by a decrease in the incidence of stroke or improved survival rates or both. The most plausible explanation is that the incidence has fallen, although the evidence is, as yet, incomplete. 6-8 Many workers attribute the decline to advances in the treatment of high blood pressure. 9-I1 Antihypertensive drugs have been available since the early 1950s and the range of agents and their acceptability have increased considerably over the past 15 years. 12 In several countries the detection, treatment and control of hypertension have improved considerably over the past two decades. 1°,13 Other factors which may explain the observed trend include changes in lifestyle and general improvements in medical care. 9 This report presents data on the secular trends in stroke mortality in Scotland during the period of 37 years from 1950 to 1986, and discusses possible causal factors.
Subjects and Methods Subjects Between 1950 and 1986 a total of 71,031 men and 81,445 w o m e n in Scotland aged 45 to 74 died from stroke. Death rates for persons aged 4 5 - 7 4 were examined to assess Correspondence to: Dr R.B. Jones, Department of Public Health, 2 Lilybank Gardens, Glasgow G12 8RZ.
344
M. Janghorbani et al.
the impact of premature mortality from stroke. Deaths amongst people over 75 years old have been excluded because of the problems of diagnostic reliability in the elderly. People aged under 45 have also been excluded since the number of deaths is too small at these ages to form the basis of a reliable statistical analysis.
Methods The data on deaths for the years from 1950 to 1986 were obtained from the Registrar General's Annual Statistical Report for Scotland (Registrar General 1950-86). During these years the Sixth (1950-58), Seventh (1959-67), Eighth (1968-78) and Ninth (1979-86) Revision of the International Classification of Diseases and Injuries and Cause of Death (1CD) had been used for the classification of cause of death. In the Sixth and Seventh Revision of the ICD, the rubric for stroke included ICD Nos. 330-334, (vascular lesions affecting the central nervous system) and these were transferred, in the Eighth and Ninth Revision, to ICD Nos. 430-438 (disease of circulatory system, cerebrovascular diseases). The comparability ratio for the change of classification from the Seventh to Eighth and Eighth to Ninth Revisions was estimated as 0.988 and 1.043 respectively by the Registrar General in Scotland. 14 As the ICD was revised three times over the period of this study, it was not practicable to use the comparability ratio for this analysis and the data were considered as a part of a continuous trend. However, the comparability ratio for stroke mortality in Scotland was close to unity. The denominator population used was that calculated by the Registrar General on the basis of census data, updated yearly by the use of birth and death notifications and information on migration.
Regional variation To examine regional variations in mortality trends from stroke, data from 15 Scottish health board areas were subdivided into four geographical divisions. These are made up as follows. North Division: Highland, Orkney Islands, Shetland Islands; West Division: Argyll and Clyde, Ayrshire and Arran, Western Isles and Greater Glasgow; East Division: Grampian, Tayside, Fife, Lothian, Borders; Central Division: Forth Valley, Lanarkshire, Dumfries and Galloway. Regional variations were examined from 1974 when a new regional structure was introduced.
Analysis Mortality rates were adjusted for age and gender by the direct method using the 1981 Scottish population as standard. Age adjustment was based on three age categories: 45-54, 55-64, 65-74. The entire study period of 37 years was divided into seven quinquennia and one two-year period to investigate the rate of change in the observed trends. A least-squares linear regression line was fitted for each gender, for the entire 37-year period and for each quinquennium.~5 T-tests were performed to ascertain whether the estimated slope of the line was significantly different from zero.15 Absolute and percentage changes in mortality were calculated by expressing the difference between first and last quinquennia as a percentage of the first quinquennia. The estimation of lives saved was made by multiplying the difference between the 1950-54 and 1980-84 age-specific rates in 10-year intervals by the appropriate 1980-84 population and summing the number of deaths in each group.
Trends in Mortality Jrorn Stroke in Scotland 1950-1986
345
Results
Cerebrovascular disease mortality Figure 1 shows the secular trends, in the age-standardised and age-gender-specific mortality rates from stroke for men and women aged 45-74, on a semi-logarithmic scale so that proportionate changes can be compared between genders and age groups. In men the age-adjusted mortality rate from cerebrovascular disease, estimated by the slope of the linear regression, tended to increase slightly from 1950 to 1955; this increase was not significant and after 1956 it decreased. This downward trend accelerated in 1976. The rate in females shows a reduction during 1950-86 and a larger fall than males. The average annual decrease was 4.0 per 100,000 males and 5.5 for females. Table I shows the decline in the six a g e - g e n d e r categories as indicated by the value of the regression slope and t-test. The age and gender-specific 1000-
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Figure 1 Secular trends in the age-specific and age-adjusted mortality rates (per 100,000) from stroke, persons aged 35-74, in Scotland, by gender
M. Janghorbani et al.
346 Table I
Trends in age-gender-specific death rates (per 100,000) and measure of change over the period 1950-54 to 1980-84 from cerebrovascular disease, Scotland, 1950-86
Year 1950 -54 1955-59 1960-64 1965-69 1970-74 1975-79 1980-84 1985-86 1950-54 to 1980-84: % change Slope* t
45-54
Male 55-64
65-74
45 54
Female 55-64
65-74
67.8 57.1 56.3 51.3 53.8 51.2 42.0 36.5
255.8 240.5 228.4 213.7 209.0 184.4 149.0 134.3
868.5 890.6 861.0 778.0 764.2 669.6 576.4 517.3
78.8 69.8 60.6 57.0 57.2 48.5 40.2 31.3
257.6 236.0 200.3 184.0 163.9 151.6 116.9 104.1
845.1 812.2 736.2 687.6 588.5 501.9 431.4 390.8
-41.7 -3.3 -12.2"**
-33.6 -10.6 -8.8***
-54.6 -4.3 -21.4"**
-48.9 -13.8 -20.8***
-38.0 -0.7 -6.5**
-49.0 -1.3 -11.2"**
*Slope of regression line where the dependent variable is age-specific death rate and the independent variable is calendar year. **P < 0.00t; ***P < 0.0001. death rates increase with increasing age in both genders. A ten-year rise in age is accompanied by about a three- to fourfold increase in mortality. A p a r t from 1950-52 the rates are higher in males than in females. This difference is most p r o n o u n c e d a b o v e 55 years of age.
Change in trends Rates for every a g e - g e n d e r group decreased with females experiencing the larger fall and the size of the decrease was statistically significant for all groups. F r o m 1975 to 1986 the fall in mortality accelerated in males. The percentage changes for the whole period of 1950-86 were 56.8% in w o m e n and 42.5% in men.
Proportion of all causes of death Age-adjusted mortality rates from all causes also decreased over this period. In men the age-adjusted mortality rate tended to increase slightly until 1965 after which it started to decrease. The rate for females decreased consistently from 1950, showing a larger fall than for males. Nevertheless, cerebrovascular disease deaths have fallen in both males and females when expressed as a percentage of all deaths (Table II). This downward trend is highly significant in both genders ( P < 0.0001).
Avoidance of premature death The impact of the falling mortality rates from stroke on p r e m a t u r e death from 1950-54 to 1980-84 can be estimated by applying the 1950-54 age-specific rates to the a p p r o p r i a t e age categories in 1980-84. In the whole population, an additional 12,500 persons aged 4 5 - 7 4 would have died from cerebrovascular disease between 1980-84 if the 1950-54 rates had persisted.
Trends in Mortality from Stroke in Scotland 1950-1986 Table II
Cerebrovascular disease deaths as a proportion of all deaths
Cerebrovascular deaths (no.) Male Female
Year 1950 1955 1960 1965 1970 1975 1980 1985 1986
347
1985 2082 2109 2038 1957 2134 1745 1449 1348
2621 2700 2405 2540 2268 2083 1627 1404 1345
Age-adjusted death rate Male Female 347.4 371.1 361.4 335.5 296.6 308.6 251.8 214.9 199.6
Stroke as % of all deaths Male Female
360.8 357.9 303.4 303.4 257.5 228.5 180.6 162.1 155.8
11.5 12.2 11.8 10.7 10.0 11.0 9.6 8.6 8.2
18.0 19.4 17.7 18.6 16.5 15.8 12.7 11.5 11.4
Regional variation A g e - a d j u s t e d m o r t a l i t y t r e n d s in the f o u r g e o g r a p h i c r e g i o n s of S c o t l a n d f r o m 1974 to 1986 are p r e s e n t e d in T a b l e III. T h e a g e - a d j u s t e d m o r t a l i t y rates f r o m stroke were highest in c e n t r a l a n d west a n d lowest in the east r e g i o n for all 13 years of observation.
Table lII
Age-adjusted and percentage change mortality rate, cerebrovascular disease, 1974-86, four regions of Scotland, rate per 100,000
Year 1974 1975 1976 1977 1978 1979 1980 1981 1982 1983 1984 1985 1986 %changes: 1974-80 1980-85 1974-85
Central Deaths Death (No.) rate* 794 747 684 662 669 701 657 642 569 563 482 562 517
302.7 281.7 263.0 243.4 244.5 256.9 239.5 229.9 203.5 191.6 174.5 200.7 185.3 -20.9 - 16.2 -33.7
Region West Deaths Death (No.) rate 1670 1720 1496 1434 1448 1485 1270 1337 1266 1204 1135 1106 1044
262.8 276.0 258.8 232.1 235.9 242.2 208.9 226.7 216.3 208.9 201.1 181.0 177.0 -20.5 - 13.3 -31.1
*Age-adjusted rate using the 1981 Scottish population.
Deaths (No.) 1600 1400 1464 1358 1373 1417 1298 1281 1187 1095 1101 1074 1006
East Death rate 248.2 215.5 219.4 207.8 210.4 218.5 201.1 224.9 179.2 172.2 176.2 171.0 170.0 - 19.0 - 15.0 -31.1
North Deaths Death (No.) rate 179 194 174 151 150 159 147 150 163 128 129 111 126
265.7 285.8 256.7 219.7 219.6 235.6 216.2 203.9 231.1 182.8 188.1 160.6 171.9 - 18.6 -25.7 -39.5
348
M. Janghorbani et al. Discussion
The results of this study confirm the decline in mortality seen in other industrial countries. 9 This study showed that the decline in age-specific and age-adjusted mortality rates from stroke in Scotland over the years 1950-86 for people aged 45-74 has resulted in a substantial 'saving' of lives, estimated at 12,500 between 1980 and 1984. Similarly the strong relationship with age in both males and females and the higher age-specific mortality rates for males in Scotland agrees with some other studies. 16,17 The change in stroke mortality is more marked than the decline in all causes of death but could result from a general improvement in health and reduction of smoking rates. There has been an acceleration in the decline of tobacco consumption in Scotland since the mid-1970s, ~8 and a reduction in the use of cigarettes with high tar content. Environmental factors may also influence rates. For example, the higher mortality rates for stroke in 1979 may be attributed to a severe winter with the lowest temperature and heaviest snowfalls since 1962-63, ~4 which was associated with high mortality rates for many conditions. Similarly the mild winter of 1977 may account for the unexpectedly low stroke mortality that year. Effective treatment of hypertension reduces the incidence of stroke 19 and the decline in stroke mortality may be related to a reduction in untreated hypertension or lifestyle changes such as reduced consumption of food preserved in salt or more probably reduced cigarette smoking. Data from a cohort study in Renfrew and Paisley in the West of Scotland show that stroke mortality is strongly associated with age, blood pressure level, impaired glucose metabolism and cigarette smoking. 2° The regional analysis showed that, like cardiovascular disease, 21 mortality from stroke in people aged 45-74 was lower in the east than in central and the west of Scotland. The greater decrease in stroke mortality observed in women may be related to differences in the treatment rates between males and females. For example, in New Zealand more women than men are currently receiving antihypertensive treatment.13 Different risk factors may be involved in the pathogenesis of cerebrovascular disease. 22 The most important risk factors are ageing and hypertension. The risk of dying from stroke is most clearly related to hypertension with a smaller contribution from the other established major cardiovascular risk factors. 23,24 The continued decline in stroke mortality may be the result of more efficient treatment and effective control of hypertension, leading to reductions in the risk of stroke among persons with hypertension or a decline in hypertensive disease in the population secondary to changes in the risk factors for elevated blood pressure, e,25 Could these results be an artefact of changes in ICD coding, or fashions or new capabilities in diagnosis? Official mortality statistics may suffer some well=documented biases 21,26-29 which should be taken into account. However, if they are constant in magnitude between years and between the subgroups of the population under consideration, then the results remain valid. In Scotland the autopsy rate is relatively low 3° and the diagnosis of cerebrovascular accidents is based purely on clinical evidence with usually no attempt being made to distinguish between cerebral haemorrhage and thrombosis. On the other hand, it has been concluded that although the sub-diagnosis (type of stroke) was unreliable, stroke diagnosis on the whole was consistent and reliable. 3~,32 Epidemiological analyses of vital statistics data must always contend with the problem of their validity. During the period covered by this study the ante-mortem diagnosis of stroke rested primarily on the evaluation of signs and symptoms rather
Trends in Mortality f r o m Stroke in Scotland 1950-1986
349
than on laboratory measurements. 33 F u r t h e r m o r e , the observed clinical event localizes the site of the lesions rather than their cause. O t h e r forms of cardiovascular disease, particularly hypertension, often a c c o m p a n y stroke 34 and the designation of cause of death depends on the judgement of the certifying physician. Diagnostic fashions in death certification change over time. In the last 20 years m a n y new techniques have b e c o m e available in specialized units for the diagnosis of non-fatal stroke, including arteriography, ultrasound, radioisotope scanning and computerized axial tomography; their impact on the overall level of accuracy of diagnosis must depend on the proportion of stroke victims admitted to and investigated in these units. The possibility exists that the burden of stroke deaths could have b e e n shifted to other rubrics of cardiovascular disease classification. 2,35,36
Acknowledgements We wish to thank Dr G.C.M. Watt and Dr R.P. Knill-Jones for their helpful advice.
References 1. Uemura, K. & Pisa, Z. (1985). Recent trends in cardiovascular disease mortality in 27 industrialized countries. World Health Statistical Quarterly, 38, 142-162. 2. Haberman, S., Capildeo, R. & Rose, E. C. (1978). The changing mortality of cerebrovascular disease. Quarterly Journal of Medicine, 47, 71-88. 3. Acheson, R. M. (1960). Mortality from cerebrovascular accidents and hypertension in the Republic of Ireland. British Journal of Preventive and Social Medicine, 14, 138-147. 4. Metropolitan Life Insurance Co (1975). Recent trends in mortality from cerebrovascular disease. Statistical Bulletin of the Metropolitan Life Insurance Company, 56(11), 2-4. 5. Soltero, I., Liu, K., Cooper, R., Stamler, J. & Garside, D. (1978). Trends in mortality from cerebrovascular diseases in the United States, 1960 to 1975. Stroke, 9,549-558, 6. Tanaka, H., Tanaka, Y., Hayashi, M., Ueda, Y., Date, C., Baba, T., Shoji, H. & Horimoto, T. (1982). Secular trends in mortality for cerebrovascular diseases in Japan, 1960 to 1976. Stroke, 13,574-581. 7. Whisnant, J. P. (1984). The decline of stroke. Stroke, 15, 160-168. 8. Alfredsson, L., VonArbin, M. & DeFaire, U. (1986). Mortality from and incidence of stroke in Stockholm. British Medical Journal, 292, 1299-1303. 9. Anonymous (1983). Why has stroke mortality declined? [Editorial]. Lancet, i, 1195-1196. 10. Soltero, I. & Cooper, R. (1980). Improved hypertension control and decline in cardiovascular mortality. Comprehensive Therapy, 6, 60-64. 11. Levy, R. T. & Moskowitz, J. (1982). Cardiovascular research: decade of progress, a decade of promise. Science, 217, 121-129. 12. Bonita, R. & Beaglehole, R. (1986). Does treatment of hypertension explain the decline in mortality from stroke? British Medical Journal, 292, 191-192. 13. Jackson, R. T., Beaglehole, R. & Stewart, A. W. (1983). Blood pressure levels and treatment of hypertension in Auckland. New Zealand Medical Journal, 96,751-754. 14. HMSO (1968, 1979). Annual Report of the Registrar General for Scotland, 1968 and 1979. Edinburgh: HMSO. 15. Armitage, P. (1971). Statistical Methods in Medical Research. Oxford: Blackwell Scientific, Chs 9 and 12. 16. Fulton, M., Adams, W., Lutz W. & Oliver, M. F. (1978). Regional variations in mortality from ischaemic heart disease and cerebrovascular disease in Britain. British Heart Journal, 40,563-568. 17. Kannel, W. B. (1976). Epidemiology of cerebrovascutar disease. In: Russell, R. W. R. (ed.) Cerebral Arterial Disease. Edinburgh: Churchill Livingstone.
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18. Lee, R. N. (ed.) (1976). Statistics of Smoking in the United Kingdom. Tobacco Research Council, Research Paper No. 1, 7th edn. London: Tobacco Research Council. 19. Hypertension Detection and Follow-up Programme Cooperative Group (1982). Five year finding of the hypertension detection and follow-up programme. III: Reduction in stroke incidence among persons with high blood pressure. Journal of the American Medical Association, 247,633-638. 20. Janghorbani, M. (1988). Use and evaluation of a register for a well population in studies on cardiovascular disease and diabetes in the West of Scotland. PhD Thesis, University of Glasgow. 21. Yate, P. O. (1964). A change in the pattern of cerebrovaseular disease. Lancet, i, 65-69. 22. Garraway, W. M., Whisnant, J. P. & Drury, 2. (1983). The changing pattern of survival following stroke. Stroke, 14,699-703. 23. Kannel, W. B. & Gordon, T. (eds). (1970). The Framingham Study--an Epidemiological Investigation of Cardiovascular Disease. Section 30: Some characteristics related to the incidence of cardiovascular disease and death: Framingham Study; 18-year follow-up. Washington DC: DHEW, Public Health Service, National Institute of Health. 24. Council on Cerebrovascular Disease, American Heart Association (1971). Risk factors in stroke due to cerebral infarction (statement). Stroke, 2,423-428. 25. Haberman, S., Capideo, R. & Rose, F. S. (1979). Epidemiological aspects of stroke. In:, Greenhalgh, R. M. & Rose, F. C. (eds). Progress in Stroke Research. Tunbridge Wells: Pitman Medical, 3-14. 26. Lilienfeld, A. M, (1976). Foundation of Epidemiology. New York: Oxford University Press, Ch. 5. 27. Israel, R. A. & Klebba, A. J. (1969). A preliminary report on the effect of eighth revision ICDA on cause of death statistics. American Journal of Public Health, 59, 1651-1660. 28. Florey, C. D. V., Senter, M. G. & Acheson, R. M. (1967). A study of the validity of the diagnosis of stroke in mortality data. I: Certificate analysis. Yale Journal of Biological Medicine, 40, 148-163. 29. Heasman, M. A. & Liphorth, L. (1966). Accuracy of Certification of Cause of Death, Studies on Medical and Population Subjects No. 20. London: HMSO. 30. Cameron, H. M., McGoogan, E., Clarke, J. & Wilson, B. (1977). Trends in hospital necropsy rate: Scotland 1961-74. British Medical Journal, 1, 1577-1580. 31. Pedoe, H. T. (1982). Stroke. In: Miller, D. L. & Farmer, R. D. (eds). Epidemiology of Diseases. Oxford: Blackwell Scientific, Ch. 9, 136-145. 32. Hatano, S. (1977). Observer variations in the diagnosis of stroke. WHO collaborative study on the control of stroke in the community. Japanese Heart Journal, 18, 171-177. 33. Kurtzke, J. F. (1976). Epidemiology of cerebrovascular disease, cerebrovascular survey report for Joint Council Subcommittee on Cerebrovascular Disease, National Institute of Neurological and Communicative Disorders and Stroke and National Heart and Lung Institute (revised Jan.). Rochester, MN: Whiting Press, 213-242. 34. Berkson, D. M. & Stamler, J. (1965). Epidemiological findings on cerebrovascular diseases and their implications. Journal of Atherosclerosis Research, (Amsterdam), 5, 189-202. 35. Manton, K. G. & Baum, H. M. (1984). Cardiovascular disease mortality, 1968-1978: observations and implications. Stroke, 15,451-457. 36. Editorial (1978). Cerebral infarction and myocardial infarction: a similar aetiology? Lancet, i, 1239-1246.
(~) The Society of Public Health, 1992
Trends in Mortality from Stroke in Scotland, 1950-1986 M. Janghorbani, PhD, A.J. Hedley, MD FRCP, R.B. Jones, PhD, W.H. Gilmour, MSc and M. Zhianpour, ME) MPH Department of Public Health, University of Glasgow, Glasgow G 12 8QQ
The Scottish Registrar General's Annual Reports have been used to study trends in mortality from stroke in Scotland during 1950-1986 in those aged 45 to 74. In 1950 the age-adjusted mortality rate was 347.4 per 100,000 population for men and 360.8 for women, falling to 199.6 for men and 155.8 for women in 1986. This downward trend has increased from 1976 for males. The average annual decline in age-adjusted mortality from stroke over the 37-year period was 4.0 per 100,000 in males and 5.5 in females. This reduction in death rates was proportionally higher for women compared with men in all age groups over 55 years. As with cardiovascular deaths, mortality from stroke was lower in the east than in the central region and west of Scotland. The reduction in mortality resulted in a substantial 'saving' of lives, estimated at 12,500 between 1980 and 1984.
Introduction Cerebrovascular diseases are now the third leading cause of death in most developed countries and in addition lead to serious disability in the survivors. Analysis of mortality statistics undertaken in several countries during the past two decades suggests that deaths due to stroke are on the decline. 1-4 This trend accelerated in the mid-1960s in the U S A 5 and early 1970s in Japan 6 but the reasons have not yet been fully explained. The decline could have been caused by a decrease in the incidence of stroke or improved survival rates or both. The most plausible explanation is that the incidence has fallen, although the evidence is, as yet, incomplete. 6-8 Many workers attribute the decline to advances in the treatment of high blood pressure. 9-I1 Antihypertensive drugs have been available since the early 1950s and the range of agents and their acceptability have increased considerably over the past 15 years. 12 In several countries the detection, treatment and control of hypertension have improved considerably over the past two decades. 1°,13 Other factors which may explain the observed trend include changes in lifestyle and general improvements in medical care. 9 This report presents data on the secular trends in stroke mortality in Scotland during the period of 37 years from 1950 to 1986, and discusses possible causal factors.
Subjects and Methods Subjects Between 1950 and 1986 a total of 71,031 men and 81,445 w o m e n in Scotland aged 45 to 74 died from stroke. Death rates for persons aged 4 5 - 7 4 were examined to assess Correspondence to: Dr R.B. Jones, Department of Public Health, 2 Lilybank Gardens, Glasgow G12 8RZ.
344
M. Janghorbani et al.
the impact of premature mortality from stroke. Deaths amongst people over 75 years old have been excluded because of the problems of diagnostic reliability in the elderly. People aged under 45 have also been excluded since the number of deaths is too small at these ages to form the basis of a reliable statistical analysis.
Methods The data on deaths for the years from 1950 to 1986 were obtained from the Registrar General's Annual Statistical Report for Scotland (Registrar General 1950-86). During these years the Sixth (1950-58), Seventh (1959-67), Eighth (1968-78) and Ninth (1979-86) Revision of the International Classification of Diseases and Injuries and Cause of Death (1CD) had been used for the classification of cause of death. In the Sixth and Seventh Revision of the ICD, the rubric for stroke included ICD Nos. 330-334, (vascular lesions affecting the central nervous system) and these were transferred, in the Eighth and Ninth Revision, to ICD Nos. 430-438 (disease of circulatory system, cerebrovascular diseases). The comparability ratio for the change of classification from the Seventh to Eighth and Eighth to Ninth Revisions was estimated as 0.988 and 1.043 respectively by the Registrar General in Scotland. 14 As the ICD was revised three times over the period of this study, it was not practicable to use the comparability ratio for this analysis and the data were considered as a part of a continuous trend. However, the comparability ratio for stroke mortality in Scotland was close to unity. The denominator population used was that calculated by the Registrar General on the basis of census data, updated yearly by the use of birth and death notifications and information on migration.
Regional variation To examine regional variations in mortality trends from stroke, data from 15 Scottish health board areas were subdivided into four geographical divisions. These are made up as follows. North Division: Highland, Orkney Islands, Shetland Islands; West Division: Argyll and Clyde, Ayrshire and Arran, Western Isles and Greater Glasgow; East Division: Grampian, Tayside, Fife, Lothian, Borders; Central Division: Forth Valley, Lanarkshire, Dumfries and Galloway. Regional variations were examined from 1974 when a new regional structure was introduced.
Analysis Mortality rates were adjusted for age and gender by the direct method using the 1981 Scottish population as standard. Age adjustment was based on three age categories: 45-54, 55-64, 65-74. The entire study period of 37 years was divided into seven quinquennia and one two-year period to investigate the rate of change in the observed trends. A least-squares linear regression line was fitted for each gender, for the entire 37-year period and for each quinquennium.~5 T-tests were performed to ascertain whether the estimated slope of the line was significantly different from zero.15 Absolute and percentage changes in mortality were calculated by expressing the difference between first and last quinquennia as a percentage of the first quinquennia. The estimation of lives saved was made by multiplying the difference between the 1950-54 and 1980-84 age-specific rates in 10-year intervals by the appropriate 1980-84 population and summing the number of deaths in each group.
Trends in Mortality Jrorn Stroke in Scotland 1950-1986
345
Results
Cerebrovascular disease mortality Figure 1 shows the secular trends, in the age-standardised and age-gender-specific mortality rates from stroke for men and women aged 45-74, on a semi-logarithmic scale so that proportionate changes can be compared between genders and age groups. In men the age-adjusted mortality rate from cerebrovascular disease, estimated by the slope of the linear regression, tended to increase slightly from 1950 to 1955; this increase was not significant and after 1956 it decreased. This downward trend accelerated in 1976. The rate in females shows a reduction during 1950-86 and a larger fall than males. The average annual decrease was 4.0 per 100,000 males and 5.5 for females. Table I shows the decline in the six a g e - g e n d e r categories as indicated by the value of the regression slope and t-test. The age and gender-specific 1000-
800700 600 -
',
5oo-
~4"
s
-
~
400-
~-
.~
'/
"-.
65-74
---,
C
.2 -
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45-74
J o o~ 100"
"--
65-64 "'--'"
9od
80 70-
.-~ '--. , ,
50-
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4045 54 30-
'v
20-
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Male Female
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.... t0
,
,
~.
~
T
p
,
,
,
,
i
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50 5'2 5:~ 56 5860 62 64 66 68 70 72 74 76 78 80 82 84 86 Year
Figure 1 Secular trends in the age-specific and age-adjusted mortality rates (per 100,000) from stroke, persons aged 35-74, in Scotland, by gender
M. Janghorbani et al.
346 Table I
Trends in age-gender-specific death rates (per 100,000) and measure of change over the period 1950-54 to 1980-84 from cerebrovascular disease, Scotland, 1950-86
Year 1950 -54 1955-59 1960-64 1965-69 1970-74 1975-79 1980-84 1985-86 1950-54 to 1980-84: % change Slope* t
45-54
Male 55-64
65-74
45 54
Female 55-64
65-74
67.8 57.1 56.3 51.3 53.8 51.2 42.0 36.5
255.8 240.5 228.4 213.7 209.0 184.4 149.0 134.3
868.5 890.6 861.0 778.0 764.2 669.6 576.4 517.3
78.8 69.8 60.6 57.0 57.2 48.5 40.2 31.3
257.6 236.0 200.3 184.0 163.9 151.6 116.9 104.1
845.1 812.2 736.2 687.6 588.5 501.9 431.4 390.8
-41.7 -3.3 -12.2"**
-33.6 -10.6 -8.8***
-54.6 -4.3 -21.4"**
-48.9 -13.8 -20.8***
-38.0 -0.7 -6.5**
-49.0 -1.3 -11.2"**
*Slope of regression line where the dependent variable is age-specific death rate and the independent variable is calendar year. **P < 0.00t; ***P < 0.0001. death rates increase with increasing age in both genders. A ten-year rise in age is accompanied by about a three- to fourfold increase in mortality. A p a r t from 1950-52 the rates are higher in males than in females. This difference is most p r o n o u n c e d a b o v e 55 years of age.
Change in trends Rates for every a g e - g e n d e r group decreased with females experiencing the larger fall and the size of the decrease was statistically significant for all groups. F r o m 1975 to 1986 the fall in mortality accelerated in males. The percentage changes for the whole period of 1950-86 were 56.8% in w o m e n and 42.5% in men.
Proportion of all causes of death Age-adjusted mortality rates from all causes also decreased over this period. In men the age-adjusted mortality rate tended to increase slightly until 1965 after which it started to decrease. The rate for females decreased consistently from 1950, showing a larger fall than for males. Nevertheless, cerebrovascular disease deaths have fallen in both males and females when expressed as a percentage of all deaths (Table II). This downward trend is highly significant in both genders ( P < 0.0001).
Avoidance of premature death The impact of the falling mortality rates from stroke on p r e m a t u r e death from 1950-54 to 1980-84 can be estimated by applying the 1950-54 age-specific rates to the a p p r o p r i a t e age categories in 1980-84. In the whole population, an additional 12,500 persons aged 4 5 - 7 4 would have died from cerebrovascular disease between 1980-84 if the 1950-54 rates had persisted.
Trends in Mortality from Stroke in Scotland 1950-1986 Table II
Cerebrovascular disease deaths as a proportion of all deaths
Cerebrovascular deaths (no.) Male Female
Year 1950 1955 1960 1965 1970 1975 1980 1985 1986
347
1985 2082 2109 2038 1957 2134 1745 1449 1348
2621 2700 2405 2540 2268 2083 1627 1404 1345
Age-adjusted death rate Male Female 347.4 371.1 361.4 335.5 296.6 308.6 251.8 214.9 199.6
Stroke as % of all deaths Male Female
360.8 357.9 303.4 303.4 257.5 228.5 180.6 162.1 155.8
11.5 12.2 11.8 10.7 10.0 11.0 9.6 8.6 8.2
18.0 19.4 17.7 18.6 16.5 15.8 12.7 11.5 11.4
Regional variation A g e - a d j u s t e d m o r t a l i t y t r e n d s in the f o u r g e o g r a p h i c r e g i o n s of S c o t l a n d f r o m 1974 to 1986 are p r e s e n t e d in T a b l e III. T h e a g e - a d j u s t e d m o r t a l i t y rates f r o m stroke were highest in c e n t r a l a n d west a n d lowest in the east r e g i o n for all 13 years of observation.
Table lII
Age-adjusted and percentage change mortality rate, cerebrovascular disease, 1974-86, four regions of Scotland, rate per 100,000
Year 1974 1975 1976 1977 1978 1979 1980 1981 1982 1983 1984 1985 1986 %changes: 1974-80 1980-85 1974-85
Central Deaths Death (No.) rate* 794 747 684 662 669 701 657 642 569 563 482 562 517
302.7 281.7 263.0 243.4 244.5 256.9 239.5 229.9 203.5 191.6 174.5 200.7 185.3 -20.9 - 16.2 -33.7
Region West Deaths Death (No.) rate 1670 1720 1496 1434 1448 1485 1270 1337 1266 1204 1135 1106 1044
262.8 276.0 258.8 232.1 235.9 242.2 208.9 226.7 216.3 208.9 201.1 181.0 177.0 -20.5 - 13.3 -31.1
*Age-adjusted rate using the 1981 Scottish population.
Deaths (No.) 1600 1400 1464 1358 1373 1417 1298 1281 1187 1095 1101 1074 1006
East Death rate 248.2 215.5 219.4 207.8 210.4 218.5 201.1 224.9 179.2 172.2 176.2 171.0 170.0 - 19.0 - 15.0 -31.1
North Deaths Death (No.) rate 179 194 174 151 150 159 147 150 163 128 129 111 126
265.7 285.8 256.7 219.7 219.6 235.6 216.2 203.9 231.1 182.8 188.1 160.6 171.9 - 18.6 -25.7 -39.5
348
M. Janghorbani et al. Discussion
The results of this study confirm the decline in mortality seen in other industrial countries. 9 This study showed that the decline in age-specific and age-adjusted mortality rates from stroke in Scotland over the years 1950-86 for people aged 45-74 has resulted in a substantial 'saving' of lives, estimated at 12,500 between 1980 and 1984. Similarly the strong relationship with age in both males and females and the higher age-specific mortality rates for males in Scotland agrees with some other studies. 16,17 The change in stroke mortality is more marked than the decline in all causes of death but could result from a general improvement in health and reduction of smoking rates. There has been an acceleration in the decline of tobacco consumption in Scotland since the mid-1970s, ~8 and a reduction in the use of cigarettes with high tar content. Environmental factors may also influence rates. For example, the higher mortality rates for stroke in 1979 may be attributed to a severe winter with the lowest temperature and heaviest snowfalls since 1962-63, ~4 which was associated with high mortality rates for many conditions. Similarly the mild winter of 1977 may account for the unexpectedly low stroke mortality that year. Effective treatment of hypertension reduces the incidence of stroke 19 and the decline in stroke mortality may be related to a reduction in untreated hypertension or lifestyle changes such as reduced consumption of food preserved in salt or more probably reduced cigarette smoking. Data from a cohort study in Renfrew and Paisley in the West of Scotland show that stroke mortality is strongly associated with age, blood pressure level, impaired glucose metabolism and cigarette smoking. 2° The regional analysis showed that, like cardiovascular disease, 21 mortality from stroke in people aged 45-74 was lower in the east than in central and the west of Scotland. The greater decrease in stroke mortality observed in women may be related to differences in the treatment rates between males and females. For example, in New Zealand more women than men are currently receiving antihypertensive treatment.13 Different risk factors may be involved in the pathogenesis of cerebrovascular disease. 22 The most important risk factors are ageing and hypertension. The risk of dying from stroke is most clearly related to hypertension with a smaller contribution from the other established major cardiovascular risk factors. 23,24 The continued decline in stroke mortality may be the result of more efficient treatment and effective control of hypertension, leading to reductions in the risk of stroke among persons with hypertension or a decline in hypertensive disease in the population secondary to changes in the risk factors for elevated blood pressure, e,25 Could these results be an artefact of changes in ICD coding, or fashions or new capabilities in diagnosis? Official mortality statistics may suffer some well=documented biases 21,26-29 which should be taken into account. However, if they are constant in magnitude between years and between the subgroups of the population under consideration, then the results remain valid. In Scotland the autopsy rate is relatively low 3° and the diagnosis of cerebrovascular accidents is based purely on clinical evidence with usually no attempt being made to distinguish between cerebral haemorrhage and thrombosis. On the other hand, it has been concluded that although the sub-diagnosis (type of stroke) was unreliable, stroke diagnosis on the whole was consistent and reliable. 3~,32 Epidemiological analyses of vital statistics data must always contend with the problem of their validity. During the period covered by this study the ante-mortem diagnosis of stroke rested primarily on the evaluation of signs and symptoms rather
Trends in Mortality f r o m Stroke in Scotland 1950-1986
349
than on laboratory measurements. 33 F u r t h e r m o r e , the observed clinical event localizes the site of the lesions rather than their cause. O t h e r forms of cardiovascular disease, particularly hypertension, often a c c o m p a n y stroke 34 and the designation of cause of death depends on the judgement of the certifying physician. Diagnostic fashions in death certification change over time. In the last 20 years m a n y new techniques have b e c o m e available in specialized units for the diagnosis of non-fatal stroke, including arteriography, ultrasound, radioisotope scanning and computerized axial tomography; their impact on the overall level of accuracy of diagnosis must depend on the proportion of stroke victims admitted to and investigated in these units. The possibility exists that the burden of stroke deaths could have b e e n shifted to other rubrics of cardiovascular disease classification. 2,35,36
Acknowledgements We wish to thank Dr G.C.M. Watt and Dr R.P. Knill-Jones for their helpful advice.
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