Quality of diabetes care and risk of diabetes complications in individuals with severe mental illness and diabetes

Background
Individuals with severe mental illness (SMI) have a reduced life-expectancy of 10 – 20 years when compared to the general population. The increased mortality is largely due to physical diseases, especially cardiovascular disease (CVD). Type 2 diabetes is a major risk factor for CVD and the risk of type 2 diabetes is two - three times higher in individuals with SMI when compared to the general population. Clinical guidelines have been established to guide optimal diabetes care and prevent complications and premature mortality in individuals with diabetes. However, individuals with SMI may face challenges in accessing sufficient care, potentially increasing their risk of diabetes complications. The overall aim of this PhD thesis was to examine quality of diabetes care and risk of acute and long-term diabetes complications in individuals with SMI and diabetes compared to individuals with diabetes alone. 

Methods
We conducted four cohort studies using national register data. We included adults diagnosed with diabetes in Denmark (studies 1, 3, and 4) or in Scotland (study 2) and identified those with a SMI diagnosis, including schizophrenia, bipolar disorder, or major depression. Quality of care was measured as process of care indicators (for instance annual HbA1c assessments) and achievements of treatment targets. We compared quality of care indicators in individuals with and without SMI using generalized linear mixed effects models (studies 1 and 2). Long-term diabetes complications included diabetic kidney disease, retinopathy, amputations, or CVD, and acute complications included hospitalization for hypoglycemia or diabetic ketoacidosis (DKA). We used Poisson regression to estimate incidence rates and incidence rate ratios of first long-term diabetes complication and recurrent acute complications in individuals with and without SMI. All analyses were adjusted for confounders. 

Results
We found that in Denmark, individuals with SMI and type 2 diabetes were less likely to undergo assessments of HbA1c, LDL-cholesterol, and urinary albumin-to-creatinine ratio (UACR) and eye and footscreenings compared to those without SMI, with the most pronounced differences for UACR (OR 0.55, 95% CI 0.53–0.58). Furthermore, individuals with SMI had better achievement of HbA1c treatment targets and similar or lower achievement of LDL-cholesterol treatment targets. In Scotland, individuals with SMI and type 2 diabetes were similar or more likely to undergo assessments of HbA1c, LDL-cholesterol, UACR, serum creatinine, blood pressure, foot screening, body mass index (BMI), and smoking status during the first year after diabetes diagnosis when compared to individuals without SMI. Opposite these findings individuals with bipolar disorder and major depression were less likely to undergo eye screenings in the years following diabetes diagnosis (OR 0.81, 95 % CI 0.70–0.94 and OR 0.76, 95 % CI 0.72–0.81, respectively).

Furthermore, individuals with SMI and type 2 diabetes had a 10 – 40 % higher risk of diabetic kidney disease, amputations, and CVD, but a lower risk of retinopathy compared to individuals without SMI. The SMI disparities in risk were highest in the younger groups. Additionally, in both type 1 and type 2 diabetes, risk of first and subsequent hypoglycemia and first DKA hospitalization was higher in individuals with SMI compared to those without (the risk was 1.7- and 1.8-fold higher for first hypoglycemia and first DKA hospitalizations, respectively). Individuals with schizophrenia were particularly at risk of recurrent hypoglycemia hospitalizations. SMI disparities in risk of hypoglycemia and DKA were largest in younger groups but stable across calendar years.

Conclusion
This thesis shows, that in Denmark, the quality of diabetes care is markedly worse in individuals with SMI compared to individuals without SMI. Conversely, in Scotland, the quality of diabetes care is similar or better in individuals with SMI, with exception of eye screening which is worse. Moreover, individuals with diabetes and SMI have a higher risk of developing both long-term and acute diabetes complications and the risk is highest in the younger groups. 

Our findings highlight the need to develop effective interventions to reduce the marked inequalities in diabetes care between individuals with and without SMI in Denmark, and the inequalities in eye screening in Scotland. Furthermore, effective prevention of long-term and acute diabetes complications is needed, especially among the younger population and in those with schizophrenia.