The results of this study indicate that elevated BP and DM in elderly people are associated with poor cognitive performance and a selective decline in executive function over 2 years. These findings support and extend previous cross-sectional studies showing that cardiovascular risk factors had a specific deleterious effect on executive measures in elderly people.8,9
Interestingly, when these subjects were followed longitudinally from baseline to first annual examination, performance on frontal-lobe mediated measures (reasoning tests, the DSST, and the UFOV) showed considerable improvement. Moreover, the rate of improvement in reasoning tasks was even higher for hypertensive subjects than normotensive subjects ( and ). Because baseline differences in measures of frontal cognitive abilities are large between groups, the magnitude of differences across exposure groups between baseline and first annual examination probably reflect, to some extent, a regression to the mean.43
Additionally, performance on many neuropsychological tests may be improved simply by prior exposure to testing stimuli and procedures.44,45
The duration of the beneficial practice effects after exposure to neuropsychological tests can vary from weeks to years.44
After the first annual examination, the results indicated that elevated BP and DM were associated with accelerated cognitive decline in frontal lobe-mediated cognitive functions but not in memory function in initially cognitively normal older adults. In addition, the adjusted means are more similar across exposure groups at the first annual examination, so regression effects are not likely candidates to explain the different magnitude of changes between the first and second examinations. The adverse effect of hypertension and DM on frontal cognitive abilities may have overcome any lingering practice effect. The present study has the strength of examining the cross-sectional and longitudinal relationships between cardiovascular risk factors and a variety of cognitive domains and suggests that hypertension and DM play a role in the decline of frontal lobe-mediated cognitive functions once residual practice effects are accounted for.
The study also demonstrated that hypertension and DM are related to deterioration in functional status and play an important role in the progression of functional disability in older adults. These findings are consistent with two recent longitudinal studies46,47
in which DM has been shown to predict functional decline in older people. However, these studies had problems with generalizability regarding their study populations: one analyzed 729 physically impaired older women,46
and the other focused its analysis on 1,789 older Mexican Americans.47
The present study has the advantage of examining the independent role of hypertension and DM in physical function using a group of relatively independent, geographically dispersed, and ethnically diverse older people. To the authors’ knowledge, this is the first study showing the predictive roles of both elevated BP and DM in the decline of functional status in elderly people.
The following mechanisms may explain why hypertension and DM are associated with decline of executive and physical function. Frontal lobes mediate executive abilities orchestrating complex planning, organizing, and multitasking activities. Integrity of the frontal-subcortical circuits, a series of pathways interconnecting various regions of the frontal lobes to subcortical structures, are essential to maintain gait and balance, as well as executive function,48
but these circuits are sensitive to increased loads of cardiovascular risks and are often interrupted by development of ischemic small-vessel injuries in cerebral watershed areas. Disruption of the frontal-subcortical systems may selectively impair executive function. In addition, the ischemic microangiopathic lesions may interfere with long loop reflexes mediated by deep white matter sensory and motor tracts and interrupt the descending motor fibers arising from medial cortical areas, which are important for lower extremity motor control,49
further affecting gait, balance, and physical function. In addition, executive dysfunction may interfere with some goal-directed abilities such as cooking, dressing, financing, and housework that are measured using ADL and IADL measures.50
This study has several limitations that deserve comment. The follow-up period of 2 years was short. There is often a crucial interaction effect between DM and hypertension on a variety of clinically important vascular outcomes such as stroke and myocardial infarction. Although this study assessed the interaction effect between BP and DM on changes of cognitive and physical functions, no evidence was found of a clinically meaningful or statistically significant interaction, possibly a consequence of the short follow-up period. An additional limitation of this study was that the questionnaire used for DM ascertainment did not include measurement of fasting glucose. Some subjects with fasting glucose levels 126 mg/dL or greater might be mis-classified as nondiabetic. Therefore, the effect of DM on cognitive and physical functions might have been underestimated.
In conclusion, this study found that elevated BP and DM are associated with increased risks for cognitive decline in executive measures and decline in physical function in otherwise nondemented, independent elderly subjects. There is no significant interaction between BP and DM on cognitive function, physical function, and their decline over 2 years. These findings have important clinical and public health implications because identification and management of those high-risk patients may help prevent or delay the development of cognitive and functional complications. Future research is needed to determine whether cardiovascular risks modification ameliorates cognitive and functional decline in elderly people.