In our study, disability was associated with an increased risk of heart disease in people with diabetes. The more severe the disability, the higher the risk. By the type of disability, mobility, and communication disability, but not mental disability, significantly increased the incidence of heart disease in people with diabetes. Disability increased the risk of heart disease in people with diabetes even after adjusting for socio-demographic and healthcare-related factors. In subgroup analysis, the difference in heart disease incidence between people with and without disability tended to be greater in men than in women, those with a younger age, those with a higher income, and those with a lower MPR.
The ‘Framingham Disability Study’ 17 has identified a relationship between disability and risk factors for cardiovascular disease in people without a diagnosis of cardiovascular disease. Disability was associated with several cardiovascular risk factors, including hypertension, diabetes, body mass index, and smoking in different ways depending on sex and age. Although that study had a different population and used risk factors for heart disease as the outcome variable rather than heart disease itself, its results were somewhat consistent with ours. A previous study has also shown that disability can predict the development of cardiovascular disease in a population aged 55 years and more 18. Another study has reported increased prevalence of hypertension and ischemic heart disease in traumatic spinal cord injured patients compared with their controls and increased prevalence of diabetes and ischemic heart disease in amputees 19. Disability progression in patients with multiple sclerosis is also associated with cardiovascular disease comorbidities 20. However, to the best of our knowledge, few studies have comprehensively examined associations between different types of disability and heart disease per se, especially in populations at risk of cardiovascular disease such as people with diabetes.
While much of the research to date has focused on the association between disability itself and risk factors for heart disease, it is important to note that disability can also lead to heart disease in a population with risk factors such as diabetes. The reason why we focused on a population with risk factors for cardiovascular disease was to see if disability might have an impact not only on risk factors themselves, but also on progressing to heart disease. Given the bidirectional association between disability and diabetes in previous studies 11,12, preventing risk factors in people with disabilities is insufficient to prevent cardiovascular disease in people with disabilities. Furthermore, there is an association between disability and heart disease in people with diabetes even after adjusting for treatment-related variables, suggesting that additional interventions beyond risk factor prevention are needed.
The mechanisms by which disability increases the incidence of cardiovascular events in people with diabetes need to be interpreted differently depending on the type and severity of disability. In the present study, mobility disability, communication disability, and disabilities classified as others, including mental retardation and developmental disability, significantly increased the incidence of heart disease. However, mental disability did not significantly increase the incidence of heart disease. We used the classification of the Korean Disability Welfare Act to define mental disability, which included bipolar affective disorder, schizophrenia, schizoaffective disorder, recurrent depressive disorder, obsessive-compulsive disorder that would not improve with continued treatment, temperamental mental disorder due to neurological damage to the brain, Tourette's disorder, and narcolepsy.
We speculate that the reason why certain disabilities could lead to the development of cardiovascular disease in diabetes is due to differences in insulin resistance or reduced muscle mass due to low levels of physical activity. Main mechanisms by which cardiovascular disease develops in diabetes are atherosclerosis and vascular inflammatory response 4,21–24. Given that insulin resistance may contribute to cardiovascular disease 25,26 and that studies have linked low muscle mass in diabetes with inflammatory responses and various forms of atherosclerosis 27–29, it is possible that certain disabilities might have negatively impact obesity, overall muscle mass, and/or insulin resistance in people with diabetes. Mobility disability, communication disability, and those classified as others in our study are often associated with a high likelihood of being unable to perform basic activities of daily living independently 30,31. Mental disability, on the other hand, is not expected to cause problems with activities of daily living per se, but only difficulties with complex social activities. Control of mental symptoms by medication is expected to allow a variety of physical activities. However, in our study, there were no specific measurements of variables that could prove our speculations, such as level of daily activities or muscle mass for each disorder. Further research into the mechanisms of physical pathology is therefore needed.
Based on subgroup analyses, it is likely that male, younger, and higher-income people without disability are more proactive in managing complications of diabetes and therefore have lower heart disease incidence, whereas people with disability in the same conditions have poorer diabetes control than people without disability. This suggests that younger, higher-income people with disability need more supportive policies to prevent them from having poorer diabetes control than people with other conditions. The difference in heart disease rate between people with and without disability was particularly large in the group with poor MPR, suggesting a great need for support for people with disability having poor diabetes control due to poor access to healthcare. This suggests that supportive policies are needed to reduce the gap in heart disease rate between people with and without diabetes by ensuring no gaps in ongoing care and treatment in healthcare facilities.
Our study has several strengths. First, we used nationwide data to analyze a 1:1 matched sample of people with and without disability. Second, the incidence of outcomes was examined by disability severity and type, suggesting that prevention and management of heart disease in people with diabetes should be approached differently according to disability severity and its type. Third, it was noteworthy that disability was associated with an increased incidence of heart disease in diabetes even in subgroup analyses after adjusting for access to or quality of care, such as income, region, and MPR. This means that the association between disability and heart disease in diabetes is not determined by access to care alone. The severity and type of disability itself might have effects. However, this only suggests that other mechanisms are at work even after adjusting for access to care. The importance of these factors should not be underestimated. As noted in the explanation for subgroup analyses, our findings did not contradict this.
The study has the following limitations. First, although a follow-up period of nine years is not typically short in studies, there might not have been enough time between the diagnosis of diabetes and the development of heart disease. However, this should be considered in the context of the risk of underestimating the incidence of heart disease. It might be a minor limitation to results of this study suggesting an increased risk. Second, this study did not look at various factors that could influence the development of heart disease, such as detailed health status and behaviors. However, the fact that the risk of heart disease did not vary significantly with medication adherence, such as MPR, suggested that the type and severity of disability could increase the risk of heart disease in people with diabetes, without confounding by healthcare access. Further research is needed to identify specific mediating or moderating factors between the severity and type of disability and the incidence of heart disease in high-risk populations such as people with diabetes.
We found that disability status and severity per se could increase the risk of heart disease in people with diabetes even after adjusting for socio-demographic or healthcare access factors and that different types of disability might have different effects on the risk. Therefore, disability should be carefully considered as an important factor contributing to cardiovascular disease in high-risk groups such as people with diabetes. Policy discussions should focus on solutions to improve existing health conditions or health behaviors associated with disability itself, such as obesity and physical activity, beyond simply improving access to health care. In addition, more intensive policy support might be needed for groups with a high incidence of heart disease by disability, such as males, those with young age, and those with relatively high-income levels. Future research is needed to identify specific mediators or modifiers between disability and cardiovascular disease in high-risk groups.
In conclusion, disability was associated with an increased incidence of heart disease in people with diabetes. The incidence of heart disease was increased with increasing severity of disability. By the type of disability, the incidence of heart disease was increased in those with mobility, communication disability, and other disabilities. However, the increase in mental disability was not statistically significant. This was true even after adjusting for health care access variables such as income, region, and medication possession ratio. To prevent heart disease in people with disability and diabetes, policy interventions should be targeted according to the severity and type of disability.