Danny R. Rose, Jr., MD
Vascular dementia is the second most common cause of dementia, but many aspects of the disease are poorly understood. In particular, there is conflicting evidence regarding the relationship between blood pressure and vascular dementia. Elevated blood pressure in midlife has been found to have a positive association with future development of dementia, but several other studies have found low blood pressure in old age to be associated with an increased risk of dementia. One possible explanation of these findings is that it represents “reverse causality,” meaning vascular dementia is responsible for low blood pressure by decreasing sympathetic drive. Blood pressure medication may also play a confounding role in this association. Emdin et al. sought to further clarify this association by conducting an analysis of 4.28 million individuals without vascular disease or dementia, supplemented with an analysis of a prospective population-based cohort of patients with TIA and stroke.
The study included patients from age 30 to 90 and excluded patients with pre-existing cardiovascular disease to minimize the potential of reverse causality related to advanced age and cardiovascular disease causing reduced blood pressure, respectively. The endpoint of the primary analysis was an inclusive definition of vascular dementia based on ICD 10 coding and was inclusive of patients with co-existing Alzheimer’s disease. Secondary analysis excluded these patients and excluded patients treated with medications commonly used to treat AD. The first four years of follow-up were excluded in the primary analysis to mitigate the effect of patients with undiagnosed dementia. Cox models, stratified by practice, were used to determine hazard ratios for the association for clustering of patients by practice. The primary analysis was adjusted for age, sex, body mass index and smoking status. The Oxford Vascular Study cohort was used to confirm findings independently.
Out of a cohort of 4.28 million individuals free of vascular disease and dementia, 14,934 cases were reported to have vascular dementia. After excluding for presentations during the first four years of follow-up, 11,114 cases were included. The association between usual SBP and risk of vascular dementia followed a linear progression within the age groups of 30-50 and 51-70. The age group of 71-90 did not show a significant association. The strength of association decreased with increasing age category. Overall for individuals aged 70 years or less at baseline, 20 mm Hg higher usual SBP was associated with a 26% higher risk of vascular dementia (HR 1.26 CI 1.17, 1.34). Significant negative associations with systolic and diastolic blood pressures were observed for the age group 71-90, but after excluding for the first eight years of follow-up, no significant association was observed. Adjusting for patients in the primary care cohort that had TIA and stroke events reduced the HR to 1.18, indicating that 30% of the excess risk of vascular dementia per 20mm Hg higher SBP is mediated through risk of future stroke and TIA. The OXVASC cohort did not show a relationship between the most recent SBP or DBP in patients relative to their diagnosis of new dementia, but did show significant positive associations with DBP and SBP in 5-9 years prior to the TIA/stroke and particularly 10-20 years prior.
This study supports prior positive associations between blood pressure in mid-life and vascular dementia and also suggests that elevated blood pressure attributes a significant risk for the development of vascular dementia at least until the age of 70. The authors’ rationale for excluding confounders in the cohort appears to be sound and had the intended effect of strengthening the associations studied. The study refutes previous reports of a negative association with blood pressure and vascular dementia in the elderly, likely in part due to the aforementioned adjustments, strengthening the authors’ hypothesis of reverse causality. This study represents by far the largest analysis of the association between blood pressure and risk of vascular dementia and although it is susceptible to limitations related to the diagnosis of dementia in a primary care setting, it represents a significant advancement in our understanding of the complex pathophysiology of vascular dementia.
