Background: the association between the rheological factors haematocrit and plasma viscosity and cognitive ability has not been extensively studied. It is possible that blood viscosity affects cerebral blood flow and cognitive function. This study tested the contemporaneous associations between these two markers of rheology and cognitive ability and estimated lifetime cognitive change in an elderly population with type 2 diabetes.
Methods: a cross-sectional cohort of 1,066 men and women with type 2 diabetes (Edinburgh Type 2 Diabetes Study) was used for the analysis. Plasma viscosity and haematocrit were measured in venous blood samples at baseline. Contemporaneously, a battery of seven cognitive tests was administered to all participants. These data were used to derive a general intelligence factor, g. A vocabulary-based test was also administered as an estimate of prior intelligence, and adjustment for scores on this test was used to estimate lifetime cognitive decline.
Results: increased plasma viscosity was associated with poorer age- and sex-adjusted scores on the cognitive domains of processing speed, mental flexibility and general intelligence, g, with standardised regression coefficients -0.092 (P < 0.01), -0.077 (P < 0.05) and -0.093 (P < 0.01), respectively. After adjusting for vocabulary, education level, cardiovascular dysfunction, duration of diabetes and glycaemic control, the associations remained significant for the measure of processing speed and g, with standardised regression coefficients -0.059 (P < 0.05) and -0.051 (P < 0.05). Increased haematocrit was significantly associated with better age- and sex-adjusted cognitive scores on the majority of the tests and with g. However, significant associations were not retained after adjustments for additional covariates.
Conclusions: increased plasma viscosity is associated with decreased cognitive ability and increased estimated lifetime cognitive decline. The relationship between haematocrit and cognitive ability requires further study.