Risk of diabetic complications and subsequent mortality among individuals with schizophrenia and diabetes - a population-based register study

Background
Individuals with schizophrenia often develop diabetes, but little is known about their risk of diabetic complications. We aimed to study incidence of registered diabetic complications and subsequent mortality in individuals with schizophrenia and diabetes compared to individuals with diabetes only.

Methods
We conducted a cohort study using nationwide registers and followed all individuals in the entire Danish population diagnosed with diabetes from 1997 to 2017. Incidence rate ratios (IRR) of diabetic complications, all-cause and cause-specific mortality rate ratios (MRR) were estimated by Cox regression comparing individuals diagnosed with schizophrenia and diabetes to individuals diagnosed only with diabetes.

Results
In a cohort of 239,118 individuals with diagnosed diabetes, the incidence of any diabetic complication was similar in females with schizophrenia and diabetes compared to females with diabetes only; IRR = 0.93 (95%CI: 0.84–1.02), and significantly lower in males; IRR = 0.85 (95%CI: 0.78–0.92). The all-cause mortality for individuals with a diagnosis of a diabetic complication was higher in individuals with schizophrenia and diabetes than in those with diabetes only; MRR = 1.92 (95%CI: 1.65–2.23) for females and MRR = 1.69 (95%CI: 1.49–1.92) for males. Among those without diabetic complications, schizophrenia was also associated with a higher mortality.

Conclusions
Individuals with schizophrenia and diabetes had similar or lower rates of diabetic complications, compared to those with diabetes only. Among those with diabetic complications, schizophrenia was associated with higher mortality. Similar, among those without diabetic complications, schizophrenia was also associated with higher mortality. Hence, diabetic complications do not seem to explain the excess mortality seen in individuals with schizophrenia.