Severe Mental Illness and the Risk of Diabetes Complications: A Nationwide, Register-based Cohort Study

CONTEXT: Individuals with severe mental illness (SMI) are at increased risk of developing type 2 diabetes. OBJECTIVE: This work explores whether individuals with diabetes and SMI are also at increased risk of diabetes complications and the potential age-specific differences in development of these. METHODS: Using nationwide registry data, we followed the entire Danish population with type 2 diabetes from January 1, 1996 to December 31, 2018. Exposure was SMI (schizophrenia, bipolar, or depression disorders). Outcome was diabetes complications (nephropathy, retinopathy, lower limp amputations, and cardiovascular disease). We applied Poisson regression models to estimate overall incidence rate ratios (IRRs) and age-specific incidence rates (IRs) and IRRs of the first event of each complication in individuals with SMI compared to individuals without SMI. The models were adjusted for sex, age, diabetes duration, calendar year, education, and migration status. RESULTS: We followed 371 625 individuals with type 2 diabetes, of whom 30 102 had coexisting diagnosed SMI. Individuals with SMI had a higher IR of nephropathy (IRR: 1.15; 95% CI, 1.12-1.18), amputations (IRR: 1.15; 95% CI, 1.04-1.28), and cardiovascular disease (men: IRR: 1.10; 95% CI, 1.05-1.15, women: IRR: 1.18; 95% CI, 1.13-1.22) but a lower IR of retinopathy (IRR: 0.75; 95% CI, 0.70-0.81) when compared to individuals without SMI, after adjustment for confounders. For all complications except amputations, the difference in IR was highest in the younger age groups. CONCLUSION: Individuals with type 2 diabetes and SMI had a higher risk and an earlier onset of several diabetes complications diagnoses, emphasizing focusing on improving diabetes management in younger age groups with SMI.