Comment on Nunley et al. Clinically Relevant Cognitive Impairment in Middle-Aged Adults With Childhood-Onset Type 1 Diabetes. Diabetes Care 2015;38:1768–1776

Nunley et al. (1) examined the presence and correlates of clinically relevant cognitive impairment in middle-aged adults with childhood-onset type 1 diabetes as compared with adults without type 1 diabetes. The authors concluded that the prevalence of cognitive impairment was five times higher in adults with type 1 diabetes as compared with participants without type 1 diabetes (28% vs. 5%), independent of age, education, or blood pressure. Diabetes-specific factors related to cognitive impairment were chronic hyperglycemia and prevalent microvascular disease.

While the current study provides us with important insights regarding cognitive impairment in adults with type 1 diabetes, we regret that depression has not been taken into account. A systematic review and meta-analysis published in 2014 identified significant objective cognitive impairment in adults and adolescents with depression regarding executive functioning, memory, and attention relative to control subjects (2). Moreover, depression is two times more common in adults with diabetes compared with those without this condition, regardless of type of diabetes (3). There is even evidence that the co-occurrence of diabetes and depression leads to additional health risks such as increased mortality and dementia (3,4); this might well apply to cognitive impairment as well. Furthermore, in people with diabetes, the presence of depression has been associated with the development of diabetes complications, such as retinopathy, and higher HbA_1c values (3). These are exactly the diabetes-specific correlates that Nunley et al. (1) found.

We believe it is a missed opportunity that Nunley et al. (1) mainly focused on biological variables, such as hyperglycemia and microvascular disease, and did not take into account an emotional disorder widely represented among people with diabetes and closely linked to cognitive impairment. Even though severe or chronic cases of depression are likely to have been excluded in the group without type 1 diabetes based on exclusion criteria (1), data on the presence of depression (either measured through a diagnostic interview or by using a validated screening questionnaire) could have helped to interpret the present findings.

Determining the role of depression in the relationship between cognitive impairment and type 1 diabetes is of significant importance. Treatment of depression might improve cognitive impairment both directly by alleviating cognitive depression symptoms and indirectly by improving treatment nonadherence and glycemic control, consequently lowering the risk of developing complications. With regard to depression in people with diabetes, we wish to conclude, in line with van der Feltz-Cornelis et al. (5), that improvement of the general medical condition would ideally focus on both conditions simultaneously.