Is type II diabetes associated with an increased risk of cognitive dysfunction? A critical review of published studies.

Type II (non-insulin-dependent) diabetes may be associated with impaired cognitive function. A detailed search of the literature has identified 19 controlled studies in which cognitive function in type II diabetes has been examined. The studies vary widely with respect to the nature of the diabetic populations studied and the psychological tests used. Thirteen studies demonstrated that the diabetic individuals performed more poorly in at least one aspect of cognitive function. The most commonly affected cognitive ability was verbal memory. Psychomotor ability and frontal lobe function were affected less consistently. The remaining six studies showed no differences in cognitive ability between subjects with type II diabetes and nondiabetic control subjects, but none had adequate statistical power to detect a between-group difference in cognitive ability of 0.5 of a standard deviation (a medium effect size). These findings are consistent with type II diabetes being associated with an increased risk of cognitive dysfunction. However, the widespread differences in methodology between the studies should lead to a cautious interpretation of their conclusions. The etiology of any cognitive decrement in type II diabetes is likely to result from an interaction between metabolic abnormalities intrinsic to diabetes, diabetes-specific complications, and other diabetes-related disorders.