Journal of the New Zealand Medical Association, 17-November-2006, Vol 119 No 1245
Incidence of stroke in women in Auckland, New Zealand. Ethnic trends over two decades: 1981–2003
Lorna Dyall, Kristie Carter, Ruth Bonita, Craig Anderson, Valery Feigin, Ngaire Kerse, Paul Brown; on behalf of the Auckland Regional Community Stroke (ARCOS) Study Group
The lifetime risk of stroke in women (1 in 5) is greater than in men (1 in 6).1 Stroke is the third major cause of death in women in New Zealand after heart disease and cancer. Its impact on women as one of the major causes of mortality, morbidity, and ongoing disability is recognised but needs to be given more consideration with an increasing aging and ethnically diverse New Zealand population.2 In New Zealand, men have a higher rate of stroke events. In contrast, New Zealand women experience more stroke events in absolute terms and have poorer health outcomes following their stroke in comparison to men.2,3
On average, women in New Zealand have their first stroke event 5 to 10 years later than men.3 With an aging population it is expected that more women will experience stroke. Due to their age, other health problems and personal circumstances are likely to need assistance with daily life activities such as cooking and bathing, and some will require ongoing nursing or institutional care.2,4
As women often have their stroke later in relation to men they are also usually the most important family caregivers. Their state of health can affect the health and wellbeing of other family members and demands placed upon them in providing care to men and others within their social network can also increase their risk of stroke.5
Over the last 20 years, a modest decline in the standardised incidence rate of stroke in New Zealand has occurred, with gender and ethnic differences emerging.3 The Auckland Community Stroke Study suggests that Europeans have the highest crude incidence of stroke, followed by Pacific (i.e. mostly of Samoan, Tongan, Niuean, or Cook Islands origin) and Māori then Asian plus Others.
However, Māori and Pacific peoples have a greater age standardised incidence rate of stroke and more unfavourable trend in stroke incidence over the last 20 years.1 Furthermore, Māori and Pacific people also have their stroke 10 to 15 years earlier in comparison to Europeans.1 For subtypes of stroke, Māori, Pacific, and Asian have a higher age-adjusted risk of ischaemic stroke and primary intracerebral haemorrhage compared with New Zealand Europeans.6
This paper discusses the incidence of stroke as well as risk factors and 28 day case fatality for women in Auckland, New Zealand from 1981 to 2003. Emerging ethnic differences are discussed to expose the picture of stroke for women in New Zealand so as to assist future planning for the prevention of stroke, the provision of treatment services, and to consider the rehabilitation and support needs of women now and in the future.
Subjects and Methodology
Three population-based stroke incidence studies were conducted in Auckland, New Zealand, covering the following periods 1981–82, 1991–1992, and 2002–3, and have been reported elsewhere.1,3 These studies have been shown to meet the stringent criteria for ‘ideal’ stroke incidence studies, such as involving a large representative population using consistent methodology and definitions of stroke.7–9
The first study was conducted between 1981–1982, involved a sample 50% of Auckland’s primary care general practitioners (GPs), identified using a cluster sampling approach, to produce a random sample of half of all new stroke events in the population.10 The second study was conducted in 1991–1992 and used a register of all cases of acute stroke managed in hospital and a cluster sample of 25% of all GPs to estimate the number of ‘non-hospitalised, non-fatal’ events.11 The third study involved collecting all new cases of stroke in 2002–2003.3
During the course of the three studies, the size of the Auckland population has increased significantly and has become more ethnically diverse thus making Auckland one of the largest growing urban centres in Australasia. Auckland is a cosmopolitan city with a significant indigenous Māori population as well as growing Pacific and Asian populations due to increasing immigration.
Across all study periods, ethnicity was defined by self-identification and grouped according to NZ/European, Māori, Pacific peoples, and Asian plus Others as in each national Census. Each study followed shortly after each Census. The numerator and denominator for each ethnic group has been consistent, as defined by Statistics New Zealand, thus allowing for appropriate ethnic classification, for ethnic trends to be identified, and for miscalculation of ethnicity to be minimised.1
All original data from the 1981–1982 and 1991–1992 studies were reviewed and re-analysed, and rates re-calculated.3 The Cochrane-Armitage method was used to test for the significance of trends in the distribution of categorical variables and the Kruskal-Wallis non-parametric analysis of variance was used for continuous variables.
Crude annual stroke incidence (first-ever events) and attack (all events) rates per 100,000 people, together with 95% confidence intervals (CI), were calculated using Poisson distribution, with adjustments made for the sampling procedure in each of the first two ARCOS studies. Standardised rates were calculated by the direct method of adjusting to the age distribution of the WHO ‘world’ population.12 Standardised rate ratios (RR) were calculated to examine the change in rates between the study periods.
Demographic profile and characteristics of stroke in Auckland women—According to the Census data 2002–, 66% of the population self identified as New Zealand (NZ)/European (Pakeha), 9% Māori, 11% Pacific, and 14% Asian; other ethnic groups and women accounted for 53% of the total Auckland population. Since 1981, the Auckland population has aged, with a 43% increase in people aged over 75 years, has undergone a doubling in the proportion of Pacific people, and a 10-fold increase in Asian and Other groups due to immigration while the Māori population has not changed markedly in size.
The proportion of women experiencing stroke was stable across the three studies (Table 1). Over the course of the three studies, the mean age of onset of stroke in women increased overall from 73.8 years in 1981–1982 to 74.3 years in 1991–92 and finally to 75.8 years in 2002–2003 (p=0.004). The average age at onset of stroke increased across all ethnic groups except Asian and Other, although this latter group was complicated by small numbers. On average, Māori and Pacific women had their strokes 15 years younger than NZ/European women.
The ethnic profile of women experiencing stroke in Auckland changed over the course of the three studies, with the proportion of New Zealand European declining and the proportion of Pacific and Asian and Other populations greatly increasing (due to increases in immigration in these populations).
Approximately one-third of the women were partnered at the time of their stroke, thus suggesting two-thirds of women were unpartnered. In comparison, two-thirds of men were partnered (thus one-third on their own).
Over 50% of women who experienced a stroke event reported that they had high blood pressure, and one in four had previously had a stroke. These findings were stable across all three studies and also across ethnic groups. The number of women reporting that they had diabetes increased significantly from 8.6% in 1981–1982 to 15.4% in 2002–2003.
Smoking in women declined significantly: from 20% in 1981–1982 to 10% in 2002–2003 over the course of the three studies in all ethnic groups.
Demographic and risk factors—Table 2 shows demographic and risk factor changes for women recorded for all three studies across ethnic groups. Differences can be seen but due to low number of Māori, Pacific, and Asian plus Other women who have experienced stroke during the course of each of the three studies; most variations are not statistically significant.
High blood pressure for all ethnic groups of women, with the exception of Māori, has generally risen. A growing group of women have experienced a stroke event previously, although for Māori there has been a decline in 2002–3. Myocardial infraction has risen for all groups of women with the exception of Māori. The number of women reporting that they are a current smoker has declined although there is a significant difference in the number of Māori women, where over a third report that they smoke in comparison to Pacific 13%, Europeans 10%, and Asian plus Other <4%.
Diabetes mellitus has been variable across the three studies but there are ethnic differences beginning to emerge. With the exception of Pacific women in 2002–3, all other ethnic groups of women have experienced weight gain measured by BMI.
Incidence of stroke—Age standardised incidence rates for women were relatively stable across the three study periods with a rate ratio between the 1981–1982 and 2002–2003 studies of 0.93 (95%CI: 0.80–1.07) (Table 3). There was a significant decline in incidence rates between 1991–1992 and 2002–2003 studies (RR 1991:2002 0.86; 95%CI: 0.76–0.98), due to an increase incidence between 1981–1982 and 1991–1992.
Incidence of stroke for women of different ethnic groups varied in Auckland across the three studies (Table 3). NZ/European women experienced a significant decrease in the incidence of stroke from 1981–1982 to 2002–2003 for both first ever and total events. The rate in 1981–1982 for all stroke events for NZ/European women was 177 and which fell to 148 by 2002–2003 leading to a significant decline of 16% (RR 1981:2002 0.84 [95%CI: 0.73–0.96]).
For Māori women, the rate of first ever and total strokes increased from 1981–1982 to 2002–2003; this increase was not found to be significant. There was almost a three-fold increase in total stroke event rates for Pacific women across the study periods (RR 1981:2002 2.71; 95%CI 1.00–7.29). For Asian and Other women, the number of total stroke events varied with an overall decline in event rates from 1981–1982 to 2002–2003 (RR 0.45, 95%CI 0.21–0.98); the wide CIs reflect the small numbers for this population.
Case fatality after stroke—There were significant declines in 28-day case fatality in women, with a 39% relative decline, across the three studies (Table 4). These declines were also seen across the ethnic groups; the numbers were too small in the minority groups to reach levels of significant.
Table 4. 28-day case fatality in all female cases in Auckland, New Zealand, 1981–2003
*There are 24 female cases with missing ethnicity in the 2002–2003 study.
Overall the three studies provide convincing evidence that as women age and live longer, they are likely to have an increased risk of experiencing a stroke event especially for those aged over 75 years of age. Even so, the incidence rate of stroke for European women has declined over the 20-year period suggesting that interventions introduced to improve New Zealanders’ health, such as promoting a smokefree society are having an impact but not so dramatic for other populations.
Results from the Auckland studies are generalisable to New Zealand women as a whole. The studies provide an accurate account of the incidence of stroke within the defined populations and periods, by utilising multiple and overlapping sources of information. Changes in the definition of ethnicity over the course of the three studies have been managed by following Statistics New Zealand guidelines for the collection and management of ethnic data.
Individuals with a dual ethnic identity have been allocated to defined population groups enabling the numerator and denominator to be consistent for each study. Ethnic differences in women in 2002–3 provide an indication of future trends of stroke for women from different ethnic groups and supports previous studies which identify ethnic disparities for stroke within a population.13–15
Careful monitoring also needs to be in place to ensure that women receive appropriate care. Women have poorer outcomes than men for stroke, have higher total mortality and are treated differently when presenting with symptoms of stroke, such as not receiving appropriate investigations to ascertain the type, severity, and causes of their stroke.2,16–19 The situation in New Zealand is unlikely to be different from that reported in other developed countries, thus suggesting that more attention should be paid to acute stroke care and management in women.
When European women have their stroke in New Zealand they are likely to be in their mid 70s, to not have a partner, to have other health problems, likely to receive Government Superannuation, and may have limited social and economic resources to call upon to help them cope with their stroke event.
With changing family and work patterns in New Zealand, informal care may be unable to be provided by family members, who are usually women and this may become less common as women need to work. In the future, it is likely that there will be an increased demand for community care, such as home help and nursing care, as women age and have fewer people within their social network from whom to can call upon for help.
Support for informal caregivers through provision of information on stroke and basic training in nursing care would enable them to provide care for frail elderly female stroke survivors.20
Over the course of the 20-year period, the incidence rate of first-ever strokes for Māori women increased but the trend was not significant. Due to high prevalence of smoking amongst Māori women in the general population, heart disease is often a cause of death for Māori women and perhaps reduces the number to experience a stroke event later in their lives. However, in the future, more Māori women may experience a stroke event due to increasing rates of obesity and diabetes within the Māori population.21–23
The first onset of stroke for Māori women occurs, on average, at 60 years and is approximately 15 years earlier than for European or Asian and other women. This differential has significant socioeconomic and health implications for Māori women and their families as informal care often requires one or more family member giving up full or part time employment, resulting in loss of income. This situation for the survivor and caregiver can increase existing social, economic, and health disparities which exist between Māori and non-Māori in New Zealand. This disparity requires appropriate considerations by health and social care decision-makers.
The rate of total strokes for Pacific women has increased almost three-fold over the 20-year period but the causes of this unfavourable trend are not fully understood but may be related to population increase or some differences in case attainment over the 20-year period, such as sampling procedures used in the first two studies.
On average, Pacific people have their stroke close to 65 years of age and are eligible for Government Superannuation. For Pacific women there has been an increase in stroke events for 2002–2003, with almost a third having more than one stroke event in the year.
Overall, the Pacific population has a higher 28-day case fatality rate than other ethnic groups thus suggesting that management of stroke for Pacific people in New Zealand is an area which requires further investigation before the impact of increasing prevalence of obesity and diabetes becomes more profound in this population.
For Asian and other ethnic groups in New Zealand, their rate of stroke between 1991–92 and 2002–3 has increased, though statistically non-significant, and this reflects the changing ethnic and aging demographic population in New Zealand. Although the proportion of Asian and other populations in New Zealand is growing, this group currently has their stroke at a similar age as the Pacific population. The pattern of stroke for this population may change, however, as this population adopts lifestyles which increase their risk of stroke, such as increased smoking and weight gain.
Overall, the results from the three studies over two decades show differences in stroke incidence between ethnic groups of women in New Zealand. There are clear differences in the average age of stroke for women from different ethnic groups and this places different demands and stresses on their families and their ability to provide informal care.
The ability of family members in being able to provide informal care in the future will depend upon many different factors, including family members’ state of health, family circumstances, employment, location where family members live, and general financial position. Some family members may be unable to so female stroke survivors will then be dependent upon community services and health institutions.
It has been predicted, utilising information from the 1991–1992 study, that if no major changes occurs in relation to stroke prevention and management, New Zealand can expect to see a 10 to 50% increase in the incidence and prevalence respectively of stroke.22 However, if the incidence rate and one year case fatality rates for stroke falls by 1% across all ethnic populations the expected increase in stroke deaths would be reduced by two-thirds.22
The incidence of stroke and the 1-month fatality rate has fallen for European women over the period of observation. These changes are positive however with aging, the rising rate of diabetes, and other factors the effects of efforts towards reducing the burden of stroke will be limited.
The observed pattern in stroke incidence in New Zealand European women increased between 1981–82 and 1991–92 and decrease incidence between 1991–92 and 2002–3 is unlikely to be affected by methodological differences in case attainment between the studies.3
In conclusion, the three studies conducted over a 20-year period using similar methodology enable ethnic and gender trends to be analysed in order to improve interventions aimed at modifiable risk factors of stroke. As women make up the greatest proportion of caregivers, identifying support at an early stage may help alleviate the stresses and demands placed on the community and informal care givers who provide support to stroke survivors.
Conflict of interest statement: The authors have no potential conflicts of interest.
Lorna Dyall, Senior Lecturer Māori Health; Paul Brown, Senior Lecturer Health Services; Ngaire Kerse, Associate Professor, General Practice; Ruth Bonita, Professor Emeritus; Valery Feigin, Associate Professor, Clinical Trials Unit;
School of Population Health, Faculty of Medical and Health Sciences, University of Auckland, Auckland
Kristie Carter, Research Fellow; Craig Anderson, Professor; The George Institute for International Health, The University of Sydney and Royal Prince Alfred Hospital, Sydney, Australia
Funding: The Health Research Council of New Zealand funded the study but had no input into the design, performance, analysis, or reporting of the study, and did not see the manuscript prior to publication.
Acknowledgements: We are indebted to the research nurses for their dedication and performance; the support of staff at the Coroner’s Office in Auckland; the assistance of staff of the New Zealand Health Information Service; the help provided by staff at the Auckland office of the New Zealand Stroke Foundation; the support of many doctors, nurses, and administrative staff within and outside Auckland; and of course the ARCOS participants and their families and friends.
Correspondence: Dr Lorna Dyall, Division of Māori Health, School of Population Health, University of Auckland, Private Bag 92019, Auckland. Fax: (09) 303 5974; email: firstname.lastname@example.org
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