Midlife vascular risk factors and Alzheimer's disease in later life: longitudinal, population based study

Examination and re-examination

The protocol for the midlife examination is described in greater detail elsewhere.5 In brief, the survey included a self administered questionnaire on medical history, cerebrovascular and cardiovascular events, and vascular conditions diagnosed by a physician. Height and weight were measured. Blood pressure was measured in the right arm after participants had been seated for five minutes. Venous blood specimens were taken for determination of serum cholesterol concentrations.5

During the re-examination in 1998, the survey methods followed those of the previous surveys in all aspects. Dementia was diagnosed in a three phase study design: a screening phase (phase 1), a clinical phase (phase 2), and a differential diagnosis phase (phase 3) (figure on website). The diagnosis of dementia was based on the Diagnostic and Statistical Manual of Mental Disorders, fourth edition,6 and the diagnosis of Alzheimer's disease was made according to the criteria of the National Institute of Neurological and Communicative Disorders and Stroke and the Alzheimer Disease and Related Disorders Association.7

A total of 57 (4%) participants were diagnosed as having dementia, of whom 48 fulfilled the diagnostic criteria for probable or possible Alzheimer's disease. All patients with Alzheimer's disease showed generalised or medial temporal atrophy, and none showed appreciable vascular pathology on magnetic resonance scans. Isolated minor lacunae or periventricular white matter signal changes were not considered as exclusion criteria for Alzheimer's disease. All patients with Alzheimer's disease scored four or less on the Hachinski ischaemia scale.8

Four patients were diagnosed as having vascular dementia, two as having Parkinson's disease with dementia, two as having alcoholic dementia, and one as having frontotemporal dementia. Since the focus of this study was Alzheimer's disease and the number of participants with non-Alzheimer dementia was small, these nine cases were excluded from further analyses.

Statistical analyses

Differences between the study groups were analysed with Student's t test and the χ² test as appropriate. Midlife blood pressure values were classified into normal (<140 mm Hg), borderline (140-159 mm Hg), and high (≥160 mm Hg) for systolic pressure; and normal (<90 mm Hg), borderline (90-94 mm Hg), and high (≥95 mm Hg) for diastolic pressure. Midlife serum cholesterol concentrations were classified into high (≥6.5 mmol/l) and normal (<6.5 mmol/l). The association between midlife blood pressure and cholesterol concentration and subsequent Alzheimer's disease was investigated with multiple logistic regression analyses, using normal blood pressure and cholesterol as the reference category (model 1). As blood pressure and cholesterol concentration are influenced by age and body mass index, the analyses were replicated controlling for these factors (model 2). Further analyses were carried out controlling also for education, history of myocardial infarction and cerebrovascular symptoms (used as indicators of vascular events), and smoking status and alcohol consumption (model 3).

Sociodemographic and clinical characteristics

Table 1 shows the sociodemographic and clinical characteristics of the study population. Mean age was 50.4 years (SD 6.0, range 40–64 years) in the original survey and 71.3 years (4.0, 65-80) at re-examination. Patients with Alzheimer's disease were significantly older, and they also had less formal education than participants without dementia. Patients with Alzheimer's disease had higher body mass index, systolic blood pressure, and serum total cholesterol in midlife than did participants without dementia, but at re-examination these values were similar in the two groups. Diastolic blood pressure was similar in patients with Alzheimer's disease and participants without dementia both in midlife and at re-examination.

Blood pressure, cholesterol, and risk of Alzheimer's disease

High systolic blood pressure in midlife was a significant risk for Alzheimer's disease in later life (table 2), and this remained true after all the adjustments (models 2 and 3). Borderline high systolic blood pressure in midlife also increased the risk (model 1), but after adjustments (models 2 and 3) this association was no longer significant. Midlife diastolic blood pressure had no significant effect on the risk of Alzheimer's disease in any of the models. High serum cholesterol concentration in midlife was a significant risk for Alzheimer's disease, even after all the adjustments (models 1-3).

Because high systolic blood pressure and cholesterol concentration were significant risk factors for Alzheimer's disease, we evaluated the added risk related to possessing both these risk factors. Participants with both risk factors in midlife had significantly higher risk for Alzheimer's disease than did those with either of these risk factors alone (table 2).

Analyses included control for age at the time when risk factor data were collected, because the effects of the midlife variables were the focus of this study. The length of follow up did not differ between participants who developed Alzheimer's disease and those who did not; thus controlling for age at re-examination did not change the results (data not shown).

Medical history and Alzheimer's disease

Table 3 shows the medical history of the participants. Patients with Alzheimer's disease were more likely to have been treated with antihypertensive drugs in midlife, but at re-examination later in life there was no difference between the groups. At re-examination, patients with Alzheimer's disease were significantly more likely to have a history of myocardial infarction and cerebrovascular symptoms (almost invariably expressed as transient ischaemic attack) and less likely to be alcohol users than participants without dementia.

The role of blood pressure

Two previous longitudinal, population based studies have shown an association between raised blood pressure and subsequent Alzheimer's disease. 1 2 These studies, however, suggested that the risk was related to raised diastolic blood pressure rather than raised systolic pressure. Differences in the study settings and populations could account for the discrepancy. For instance, treatment patterns may have contributed to our finding. Participants with Alzheimer' s disease were more likely to have received antihypertensive drug treatment in midlife but, despite the treatment, still had higher systolic blood pressure in midlife than their counterparts without dementia. This is in line with findings that a considerable proportion of treated hypertensive patients do not achieve the target blood pressure and the fact that diastolic blood pressure has traditionally been the main indication for antihypertensive treatment. 2 9 10

Our study, including both treated and untreated participants, may underestimate the risk related to diastolic blood pressure. Notably, in Japanese-American men the raised diastolic blood pressure in midlife predicted Alzheimer's disease only in participants never treated with antihypertensive drugs.2 From this perspective, our data should not be interpreted as discounting the potential risk of Alzheimer's disease related to raised diastolic blood pressure but rather as emphasising the importance of raised systolic blood pressure, even in people with normal diastolic blood pressure. Our findings are partly corroborated by the only drug trial to date showing that blood pressure control may prevent dementia, which was carried out in patients with isolated systolic hypertension.11

High serum total cholesterol concentration in midlife also predicted Alzheimer's disease in later life. This finding is consistent with the findings in elderly Finnish men.3 Our study extends these findings to younger age groups and both sexes, making these data more representative.

The role of cholesterol

In this study, clinical indicators of atherosclerosis were more common in patients with Alzheimer's disease than in participants without dementia. A population based, cross sectional study has previously indicated an increased risk of Alzheimer's disease in patients with atherosclerosis.12 Hypertension and hypercholesterolaemia may increase the risk of dementia by inducing atherosclerosis and impairing blood flow, but they may also directly induce the neurodegeneration of Alzheimer's disease.13 Controlling for vascular events did not change the association between midlife high systolic blood pressure and cholesterol concentration and subsequent Alzheimer's disease, suggesting that hypertension and hypercholesterolaemia themselves pose a risk for Alzheimer's disease. Furthermore, the combination of hypertension and hypercholesterolaemia in midlife was a particularly strong predictor of Alzheimer's disease; these factors may accelerate the development of Alzheimer's disease partly through different pathophysiological mechanisms.

Apolipoprotein E genotype may influence the observed associations, but more than 85% of the variation in serum cholesterol concentrations is thought to be independent of the apolipoprotein E genotype.14 Accordingly, the findings in elderly Finnish men suggested that high serum cholesterol concentration was an independent risk factor for Alzheimer's disease regardless of the apolipoprotein E genotype.3 However, more research is needed to determine if apolipoprotein E genotype modulates the effects of risk factors for Alzheimer's disease.

Study methodology

The design of our study as a population based, longitudinal study with a large cohort of participants and substantial response rate increases the credibility of these findings. No autopsy data were available to confirm the clinical diagnosis, although the accuracy of the clinical diagnosis of Alzheimer's disease at Kuopio University Hospital, verified by neuropathology, has been reported to be 96%.15

We investigated the possibility of selection bias due to non-participation. The prevalences of both raised systolic blood pressure and hypercholesterolaemia in midlife were higher among non-participants. People with cognitive decline are less likely to participate in clinical studies.16 Hence, if non-participants were at an increased risk of cognitive impairment and dementia, our results would represent an underestimate of the true effect of the risk factors rather than the opposite. Only participants scoring ≤24 in the mini-mental state examination in the screening phase underwent the exhaustive examinations needed for the diagnosis of dementia. Some dementia cases may have been lost because of this cut-off score, and this may also have resulted in underestimation of the prevalence of dementia.