Journal of Epidemiology and Community Health 1997;51:121-126 121 Trends in stroke mortality in Greater London J Epidemiol Community Health: first published as 10.1136/jech.51.2.121 on 1 April 1997. Downloaded from and south east England-evidence for a cohort effect? Ravi Maheswaran, David P Strachan, Paul Elliott, Martin J Shipley Abstract to London to work in the domestic services Objective and setting - To examine time where they generally had a nutritious diet. A trends in stroke mortality in Greater Lon- study of proportional mortality from stroke don compared with the surrounding South suggested that persons born in London retained East Region of England. their lower risk of stroke when they moved Design - Age-cohort analysis based on elsewhere,3 but another study which used routine mortality data. standardised mortality ratios suggested the op- Subjects - Resident population aged 45 posite - people who moved to London acquired years or more. a lower risk of fatal stroke.4 Main outcome measure - Age specific While standardised mortality ratios for stroke stroke mortality rates, 1951-92. for all ages in Greater London are low, Health Main results - In 1951, stroke mortality of the Nation indicators for district health au- was lower in Greater London than the sur- thorities suggest that stroke mortality for rounding South East Region in all age Greater London relative to other areas may bands over 45. It has been declining in vary with age.5 both areas but the rate ofdecline has been The aim of this study was to examine time significantly slower in Greater London trends for stroke mortality in Greater London (p<0.0001). The differences in rates of de- compared with the surrounding South East cline were such that stroke mortality is Region in relation to age, period of death, and now higher in Greater London for people birth cohort. under 75. The crossover of age specific stroke mortality rates occurred at different periods in different age bands and is con- Methods sistent with a cohort effect, with similar Routine data on stroke deaths (International rates in Greater London and the sur- Classification of Disease 8th and 9th revisions rounding south east for men and women 430-438; 6th and 7th revisions 330-334) and born around 1916-21. This cohort effect Environmental mid-year population estimates were obtained Epidemiology Unit, does not appear to be consistent with past from the Registrar General' Statistical Reviews London School of maternal and neonatal mortality rates in ofEngland and Wales for the period 1951-1973 Hygiene and Tropical these areas, nor, within the limitations of http://jech.bmj.com/ Medicine, Keppel and from the Office of Population Censuses Street, London the data, with the ethnic composition of and Surveys annual Area Mortality series from WCIE 7HT cohorts. ' 6 R Maheswaran 1974-92. Conclusions - There seems to be a cohort The South East Region is currently one of Department of Public effect on stroke mortality which is not ex- eight standard regions in England based on Health Sciences, St plained by past maternal and neonatal the Registrar General's classification. Greater George's Hospital mortality. Ifthe decline in stroke mortality London and the South East Region were de- Medical School, continues at its current rate, the Health of London fined in 1931. Since then, the main boundary on September 28, 2021 by guest. Protected copyright. D P Strachan the Nation stroke target is unlikely to be have been in 1950, 1965, and 1974. achieved in Greater London. changes Department of However, these did not noticeably affect trends Epidemiology and in stroke mortality rates. Public Health, (7 Epidemiol Community Health 1997;51:121-126) Maternal (1901-30) and neonatal (1911-30) Imperial College School of Medicine at mortality were used as proxy measures of the St Mary's, London early-life environment.6 There were no data on P Elliott Standardised mortality ratios for stroke have neonatal deaths before 1911. The period in Department of been lower for Greater London than the sur- which these proxy measures were examined was Epidemiology and rounding South East Region for several years.' before Greater London and the surrounding Public Health, This is unusual as mortality from most other south east had been defined. Data on existing University College London Medical causes is higher in Greater London than the areas were therefore amalgamated to cor- School, London surrounding south east. respond to these two areas. M J Shipley The reasons for this lower mortality from The analysis required an indication of the Correspondence to: stroke in Greater London are not clear. It has ethnic composition of birth cohorts. However, Dr R Maheswaran, been suggested that many of the cohort of population ethnicity data were only available Department of Epidemiology and Public Health, Imperial people born in London in the late 19th and from the 1991 census, the first census in Eng- College School of Medicine early 20th centuries had a healthy intrauterine land and Wales to collect these data.7 These at St Mary's, Norfolk Place, London W2 1PG. environment which reduced their risk of stroke data were used to estimate the ethnic com- Accepted for publication in adult life.2 This was because they were the position of cohorts (eg, the Afro-Caribbean June 1996 children ofhealthy young women who migrated population in Greater London in the 50-54 122 Maheswaran, Elliott, Strachan, et al --------- Greater London J Epidemiol Community Health: first published as 10.1136/jech.51.2.121 on 1 April 1997. Downloaded from Surrounding southeast .----------------- England and Wales y 75+ y 140 45-64 800 r 2500 r 120 2000 600 , 100 I - -" - - - N a 80 1500 a) 400 I I >a, > 60 1000 40 200 _ C') 500 co 20 Ii iiliiiiii lllllllI lllllllllllIII 0) o 0. 19511 1961 1971 1981 1991 19 51 1961 1971 1981 1991 - 0 140 r- 800 r 2500 r 0 120 2000 C/) 600 . Cu 100 0C, co c 1500 a) 80 1- a) E XIsAN. 400 C] 0 60 1000 H 40 [ 200 [ 500 H 20 ,.l l lllll ll iiiiiiil iiiil f% Illllllllllllllllllllll U U 1951 1961 1971 1981 1991 1951 1961 1971 1981 1991 1951 1961 1971 1981 1991 Year of death Figure I Trends in stroke mortality for Greater London, the South East Region, and England and Wales between 1951 and 1992 inclusive. age band was 8.3% and this was taken to of decline between the two areas, was included. represent the percentage of Afro-Caribbeans in The percentages of ethnic minorities in each the 1937 to 1941 birth cohorts. This does not, cohort were then entered as continuous vari- however, make any assumptions about place of ables to see if they reduced the magnitude of http://jech.bmj.com/ birth). The census categories were amal- this area-by-cohort interaction term. Data on gamated into three groups: Afro-Caribbeans, men and women were analysed separately. Stat- Asians and others, and Irish born migrants. istical analysis was carried out using GLIM. ' The limitations of this method are addressed Some residual variation in excess ofthe Poisson in the Discussion. distribution (overdispersion) was observed. Adjustment for this overdispersion made little difference to the degree of significance of stat- STATISTICAL ANALYSIS istical tests. on September 28, 2021 by guest. Protected copyright. The patterns in stroke mortality rates for Greater London and the surrounding south east were examined graphically, using three Results year rolling averages (with two year averages TRENDS IN STROKE MORTALITY at the start and end of the time period ex- Trends in mortality from all cerebrovascular amined) to reduce the effect of random vari- diseases from 1951-92 in three age bands for ation. All rates for the surrounding south east men and women are shown in figure 1. The were calculated having excluded Greater overall pattern was similar for men and women. London. Stroke mortality was lower in Greater Lon- Log linear Poisson regression analysis was don than the surrounding south east in all three used to assess if the rate of decline in stroke age groups in 1951. The rate of subsequent mortality in Greater London in relation to birth decline was slower in Greater London than cohort was significantly different from that in in the surrounding south east, where the age the surrounding south east and to see if eth- specific rates of decline were closer to those in nicity could account for this difference.8 An England and Wales as a whole. Stroke mortality age cohort model was fitted.9"0 Age and area rates in Greater London and the surrounding were entered as categorical variables while co- south east crossed over around 1973 in the hort (ie, year of birth) was entered as a con- 45-64 year age group and around 1986 in the tinuous variable. An area-by-cohort interaction 65-74 year age group. No crossover has yet term, which represented the difference in rates occurred in the 75 + age group. Trends in stroke mortality in London and south east England 123 --------- Greater London J Epidemiol Community Health: first published as 10.1136/jech.51.2.121 on 1 April 1997. Downloaded from Surrounding south east 10 000 F Men 10 000 r Women Age bands 85+ y -a 'I-. 0) L- a)> 1000 F- 1 000 e 0)n 75-84 y CL _. 0a 0 0 0 65-74y 0 1oo F- 100 e C,) a) C] \; 55-64 y 45-54 y 101 1887 1897 1907 1917 1927 1937 1887 1897 1907 1917 1927 1937 Central year of birth Figure 2 Age specific stroke mortality rates in relation to year of birth in Greater London and the surrounding South East Region, 1887-1942.