ABSTRACT
BACKGROUND: Multiple systematic reviews have found that type 2 diabetes is associated with cognitive decrements. However, these reviews are heterogeneous in terms of methodology, quality and results, making it difficult for researchers and clinicians to build an informed overall picture. We therefore conducted a review of systematic reviews on the association between type 2 diabetes and cognitive decrements in relation to healthy controls. METHODS: Following a pre-registered research protocol, we searched four major databases. Nine systematic reviews met our inclusion criteria: seven were meta-analyses and two were narrative syntheses. We assessed the risk of bias in each review and reported all effect sizes and confidence intervals obtained. RESULTS: Type 2 diabetes was associated with cognitive decrements in all reviews, with small or negligible effect sizes obtained in the largest meta-analyses. The most studied cognitive domains were attention, executive functions, memory, processing speed and working memory. All reviews had methodological issues and were rated as having a high or an unclear risk of bias. CONCLUSIONS: Type 2 diabetes appears to be associated with lower cognitive performance in several cognitive domains and in different age groups. However, high-quality meta-analyses on the subject are still needed. Future reviews must follow the PRISMA guidelines and take into account the risk of bias of the original studies through sensitivity analyses and the heterogeneity of the studies by conducting subgroup analyses for example according to age group and disease duration. The meta-analyses that aim to study the entire type 2 diabetes population without excluding severe comorbidities, should assess concept formation and reasoning, construction and motor performance, perception, and verbal functions and language skills in addition to the cognitive domains that have been most frequently analysed in the reviews conducted so far.
ABSTRACT
INTRODUCTION: Type 2 diabetes has been associated with cognitive decrements already in middle-age. However, the sample sizes of the studies have been small and the neuropsychological tests used have been heterogeneous. In addition, only a few studies have matched the groups in terms of age, education and gender. In this cross-sectional matched pairs study, we investigated the cognitive performance of Finnish middle-aged type 2 diabetes patients compared to healthy individuals. METHOD: A neuropsychological test battery consisting of 16 tests and 21 outcome measures was applied to 28 patients and 28 age-, education- and gender-matched healthy individuals. Various exclusion criteria were applied to minimize the risk of cognitive dysfunction due to factors other than diabetes. RESULTS: We did not find between-group differences in any of the neuropsychological tests measuring attention, concept formation and reasoning, construction and motor performance, executive functions, memory, processing speed or working memory. In addition, there were no group differences in the frequency or severity of subjective cognitive symptoms, or in anxiety, depression, burnout, fatigue or alcohol use disorder symptoms. The effect sizes in this study were mostly negligible or small, with the mean effect size being -0.12. CONCLUSIONS: In a carefully matched sample of middle-aged type 2 diabetes patients and healthy individuals, we found no significant effects and no meaningful evidence of cognitive differences between the groups.
