Area of residence and risk of cardiovascular disease and mortality among adults with type 1 diabetes mellitus in Stockholm County

1 Background

According to the Institute of Health Matrix and the World Health Organisation (WHO), cardiovascular disease (CVD) remains the leading cause of non-communicable disease (NCD) morbidity and mortality in adults aged ≥ 30 years [1-3]. Although a considerable decline in global CVD morbidity and mortality especially in developed countries was registered in recent decades [2, 4, 5], in the year 2012, the WHO estimated that 17.5 million (37% of NCD) deaths in adults under 70 years of age were attributed to CVD [3] and case fatality is expected to rise to approximately 23.6 million by 2030 [6], making it a large scale global public health problem and underpins the need for increased public health efforts to combat its burden in society.

In Western Europe, Sweden is one of the countries with high CVD burden [7-9]. CVD accounted for 41% all cause mortality among adults aged 30-70 years in the year 2012 [10] and age- adjusted CVD death rates for men and women were 222.5 and 144.9 (per 100,000 people) respectively in 2010 [10]. This high burden of CVD was mainly attributed to the prevalence of biological and behavioural risk factors such as hypertension, smoking, physical inactivity etc [11-15] as well as surrogate contextual factors in the social and physical environments where people live and work, for instance socio-economic deprivation in area of residence [16-19]. However, interventions such as treatment with potent pharmacological drugs [20-22], surgical procedures e.g. vascularisation [23], dietary modifications [24, 25], increased physical activity [26, 27], tobacco control policies [28, 29] and control of hypertension and other risk factors [30, 31] have somewhat mitigated the high burden of CVD with notable decline in mortality due to stroke and coronary heart diseases reported in recent decades [16, 32, 33].

The WHO defined cardiovascular disease as a disorder of the blood vessels and the heart [6]. The main classifications and leading causes of CVD mortality and morbidity include coronary heart diseases (CHD), Cerebrovascular diseases, peripheral vascular diseases (PVD), deep venous thrombosis, hypertensive heart disease and inflammatory heart diseases as well as stroke and myocardial infarction which are some acute CVD events resulting mainly from impaired blood supply to the brain and myocardial tissues respectively. CVD exerts profound health, social and economic impact on individuals and societies. Apart from impairing quality of life and causing premature death [1, 9, 34], CVD incur enormous economic costs which exacerbate poverty and retards economic growth as a result of reduced labour force due to morbidity and premature deaths [34-36]. In the year 2009, the economic cost incurred  to CVD in Sweden revealed non-health care costs of approximately €1.6 million (2% of the total health care expenditure)  and  direct health care costs of €2.4million equivalent to 8% of the national health care expenditure [37]. This is a huge amount of money that would have done enormous work if it were invested in other national development initiatives.

The major CVD risk factors are known, the challenge remains with reduction of their prevalence in various populations through promoting healthy lifestyles and health seeking behaviours in a life-course perspective [6]. The proximal CVD risk factors are either modifiable or non-modifiable. The most common modifiable risk factors include hypertension, tobacco use, diabetes, physical inactivity, unhealthy diet, high blood cholesterol level and overweight/obesity. The non-modifiable risk factors include age, gender and family history of CVD (19). Exposure to proximal CVD risk factors tends to be largely influenced by surrogate contextual risk factors in the social and physical environments in which people live and work [38, 39]. Social conditions in which people live and work commonly measured using socio-economic status at individual or area level has been well documented as a social determinant of CVD morbidity and mortality [40, 41]. These conditions are known to reflect access to resources such as health knowledge, money, power as well as prestige and social connections that are beneficial [40] and largely influence the distribution of the proximal biological and behavioural risk factors for CVD, consequentially leading to inequalities in CVD morbidity and mortality in the general population [39, 40] as theorised by Link and Phelan in 1995 when they illustrated that “social conditions are the fundamental causes of disease” in society [42].

More recently, Diez Roux illustrated pathways linking residential environments to CVD morbidity and mortality [43]  In their description, social factors in residential neighbourhoods such as violence and safety, social cohesion and support may influence coping mechanisms for CVD biological precursors such as stress. Social norms in area of residence may also lead to adoption of unhealthy behaviours such as poor dietary habits, physical inactivity and smoking which increase the risk of hypertension, obesity, diabetes and subsequently CVD. The physical environment in residential areas also plays a role in CVD causality. Components of residential area`s physical environment such as access to recreational resources, sidewalk ways, bike lanes, street connectivity, aesthetic quality, access to healthy foods, food and tobacco advertisement, availability of tobacco, noise and air pollution play a big role in influencing detrimental health behaviours such as smoking, physical inactivity, stress coping mechanisms, sleep disturbance etc which are known biological risk factors for CVD [38, 41, 43-47]. The above components of the social and physical environments of residential areas were postulated not to act in isolation but also interact with each other resulting to potential multi-factorial causal network that link area of residence to CVD morbidity and mortality. This therefore underscores the role of residential areas in influencing inequalities in CVD risk and mortality and the need for it to be taken into consideration in the efforts to combat CVD morbidity and mortality.

Type 1 diabetes (T1DM) is a metabolic disorder which commonly occurs in children but also in adults. It occurs as a result of cell-mediated autoimmune destruction of the pancreatic beta-cells resulting into insulin deficiency, failure of blood glucose regulation and subsequently a myriad of complications [48]. It accounts for 5-10 % of all diabetes cases [48, 49]. Time trend analysis from 1983-2007 showed an increasing incidence of T1DM in young Swedish population aged 0-34years and it has been ranked as the 12^th leading cause of mortality and morbidity in the globe in the year 2010 [50, 51]. T1DM is also a risk factor for CVD, making patients with this condition more susceptible to CVD than the general population as described by de Ferranti et al in their recent systematic review [49]. Similarly some recent studies in Sweden have shown that the incidence of CVD in adults with T1DM is on rise [52, 53]. Simultaneous exposure to other proximal CVD risk factors will increase CVD risk profile and the probability of developing CVD among adults with T1DM [1, 49, 54] and yet the combined effect of CVD [55] and myriad of other complications in T1DM patients is demanding and requires regular contact with medical doctors / health care facilities [56]. Effective control of other CVD risk factors and a functional high quality and accessible health care is therefore paramount to prolong and improve quality of life and reduce inequalities in CVD morbidity and mortality in adults with T1DM. 

More encouraging is the equitable access to high quality health care and good performance of the Swedish health care system in comparison to other countries of the Organisation for Economic Cooperation and Development (OECD) in recent decades [57]. However, social differentials were suggested to be widening and isolated issues of access especially with regards to specialists care waiting time, rural-urban differences and user fee are still prevalent [58-61]. Critiques also argue that the recent changes in Swedish welfare state arrangements [61] and the shift to the market oriented healthcare [62] which might have resulted into withdrawal of extra resources from primary health care in disadvantaged areas [63] may likely favour socially advantaged groups and could further increase health and social inequalities [61, 64, 65]. CVD remains a major public health problem in Sweden as alluded to earlier on [66], its prevention and control is therefore a public health priority and essential for achievement of national public health targets and the realization of the WHO goal of reducing non-communicable diseases burden by 25% by 2025 [1, 67]. Targeting CVD vulnerable populations is therefore an important public health prevention strategy.

Although area disadvantage has been shown to increase the risk of CVD and CVD mortality in the general population, studies in adults with T1DM are rare to find in published literature. This study therefore aimed to deepen knowledge on residential area inequalities in CVD morbidity and mortality among adults with T1DM in Stockholm County. The expectation was that the risk of incident CVD and CVD specific mortality would be higher in adults with T1DM living in disadvantaged areas compared to other areas of Stockholm County. The study`s significance lies in adding to the growing knowledge of the residential area inequalities in the risk of incident CVD and CVD mortality in adults with T1DM and the award of my degree of masters in public health epidemiology. The findings may be of value to public health practitioners interested in designing targeted CVD prevention and control interventions and could also inform re-shaping of CVD prevention and control policies/strategies by public health policy makers in Stockholm County.

1.1 Area of residence

Area of residence intrinsically embodies components of the social and physical environment that influence health as described earlier on [38, 43]. Area disadvantage commonly referred to as area deprivation is a widely used concept with no single definition [68]. Some social epidemiologists claimed that it summarizes an area`s health risk potential based on geographical clustering of unemployment, poverty, social disorganization and economic divesture [69]. The choice of area of residence is influenced by several factors among which socio-economic status and culture play key role [40, 70]. A number of countries developed indices of area deprivation/disadvantage that suits the ecological characteristics of their populations. These indices remain the most widely used measures for assessing residential area inequalities in health and disease [71, 72]. Examples include the Swedish Care Need Index [73] and the English Index of Multiple Deprivation [74]. However in this study, we did not use deprivation index but rather dichotomised individuals into either disadvantaged or other areas of Stockholm County based on classification of residential areas in Stockholm County in 1998 for metropolitan development initiative[75]  

1.2 Area of residence and risk of CVD in adults with T1DM

Published longitudinal studies consistently showed increased risk of myocardial infarction[18, 76-78], coronary heart disease[46, 79, 80],stroke[81, 82], CVD [43, 82] and peripheral vascular diseases [83] among residents of disadvantaged areas compared to least disadvantaged areas in the general population. In most of these studies, after adjusting for contextual and individual level covariates, the effect remained, implying existence of an independent association between residential area disadvantage and CVD. However whether this association is causal is still a matter of debate as many critiques argue that limited empirical evidence exists to explain how residential area disadvantage can influence biological mechanisms for development of CVD. Presence of residual confounding and possible interaction of contextual factors in area of residence were some of the alternative explanations that might not be ruled out, given the use of different composite deprivation indices often derived from individual level risk factors [44, 68] as well as the possibility that the observed association between area of residence and risk of CVD might reflect the effect of risk accumulation over life-course [43, 45]. Though widely investigated in the general population, the association between area of residence and risk of incident CVD has been less investigated in subjects with T1DM who are more vulnerable to CVD than people without diabetes. Area deprivation is therefore more likely to increase their vulnerability, reduce their quality of life and chances of survival.

A cross-sectional study conducted in France found a 4-fold increase in the  risk of retinopathy in more deprived patients compared to the least deprived [84].Rawshani et al in a 6-year follow-up cohort study in Sweden also found that individuals in the lowest income quintiles had 2-fold increase in the risk of non-fatal/fatal CVD event, coronary heart disease and stroke when compared to those in the highest income quintile [17]. These few studies point to the importance of socio-economic deprivation in relation to CVD risk and may therefore be important to conduct more studies to demonstrate the consistency and temporality of this association relevant for causality judgment and making evidence based decisions by public health policy makers and practitioners for targeted CVD prevention and control interventions.

1.3 Area of residence and risk of cardiovascular disease specific mortality in adults with T1DM

A systematic review by Matteucci and Giampietro showed that adults with T1DM had 2-4 fold increase in CVD mortality when compared to the general population[85]. A longitudinal study from the UK that assessed the effect of material deprivation measured using the Townsend scores found an increased risk of CVD specific mortality among subjects with T1DM aged ≤35years. Those with the highest score of material deprivation had age standardised mortality ratio and 95% confidence interval of 1365 (727 to 2334) for female and 536 (332 to 819) for men[86]. A similar study among T1DM patients aged 17-65years also found an increased risk of CVD mortality in socio-economically deprived adults compared to those in higher social class; Odds Ratio 3.98 (95% CI 1.96-8.06) and a 6-fold increased risk of CVD mortality among the unemployed compared to the employed[87]. Likewise, a more recent Swedish study also found a socio-economic gradient in the risk of CVD mortality among subjects with T1DM[88].Although these studies have inherent weakness, especially with regards to statistical adjustment for contextual and some individual level risk factors, they still provide plausible explanation of material deprivation/socio-economic status (key components of social and physical environment of area of residence) as predictors of CVD specific mortality in this group of patients. Given the myriad of other medical complications experienced by patients with T1DM, their health and medical care needs surpass those of the general population. Therefore ascertaining residential area inequalities in CVD specific mortality in subjects with T1DM is essential for planning targeted CVD preventions and control interventions and policy modifications. Overall, previous studies consistently showed that material deprivation and socio-economic status often linked to area of residence is associated with increased risk of CVD and CVD specific mortality.