Gurmeen Kaur, MBBS
It is projected that 13.8 million Americans will have dementia by the year 2050, making it a major public health epidemic. While the overall prevalence is on a rise, every individual’s chance of developing dementia per year is decreasing. The authors used the Framingham Heart Study (FHS) to demonstrate nearly a 20% decrease in developing dementia by a specific age over the past 30 years and have explored the temporal trends of this change.
Improved cardiovascular health and better management of stroke and vascular risk factors may be the reason for this observed decrease. Vascular risk factors have also been implicated in the pathophysiology of both vascular dementia and Alzheimer’s type dementia. A meta-analysis of 14,730 adults, including 862 with a history of stroke and 13,868 controls, demonstrated that a history of stroke increased the risk of AD dementia by 59%. Leukoariosis or increased burden of small vessel disease suggests silent ischemia. Many large databases show that the incidence of strokes is decreasing, which may be a contributing factor to decreased rates of dementia.
Blood pressure seems to have similar effects on dementia rates. The authors of the FHS report that the mean BP levels have been on a decline since the 1970s, with similar results from the NHANES study group. Hypertension has individually been linked to development of cognitive impairment. What remains unclear is the risk attributable to midlife versus late life hypertension, since late life presentation might be a harbinger of many years of untreated midlife hypertension — at which point BP control may not help; its contribution to the overall neurodegenerative process may already have ended.
Like other cerebrovascular risk factors, data from the U.S. Census suggests an overall decrease in smoking prevalence in the last 50 years. Additionally, current smokers, relative to never smokers and former smokers, have a 79% and 70% increased risk of AD, respectively. The proposed mechanism of increasing dementia among smokers may be chronic low grade inflammation and increasing oxidative stress and free radical injury.
Interestingly, the same association was not seen with hyperlipidemia. While there is some suggestion that midlife high triglycerides may hasten development of dementia, current evidence does not suggest that cholesterol-lowering therapy is an effective modifier in changing the course of dementia. Similarly, the FHS has shown an increase in prevalence of atrial fibrillation over 1958–2007, which is likely due to increased longevity. Diabetes and obesity seem to have a similar deleterious effect on promoting neurodegenerative illnesses. However, unlike other vascular risk factors, the rate of diabetes is rising in the U.S.
Cognitive reserve has been an upcoming concept in the past decade. Individuals with higher educational and occupational attainments have lower risks of vascular dementia and AD for the same degree of vascular insult.
The authors conclude that while trends seem to indicate that there are associations between improving vascular risk factors and decreasing rates of all cause dementia, we are still missing a piece of the puzzle that may have to do with sociocultural, early life changes or epigenetic influences.