Alzheimer’s disease is an age-related, irreversible brain disorder that develops over many years. In the very early stage, people experience memory loss, which can be mistaken for memory changes that happen in normal aging. As the disease progresses, these symptoms gradually lead to dementia, a condition characterized by marked memory loss and behavior and personality changes. Eventually, the disease also leads to a decline in other cognitive abilities (such as decision making and language skills), an inability to recognize family and friends, and a severe loss of mental function. These losses are related to the breakdown of the connections between neurons (nerve cells) in the brain and the eventual death of many of these cells.
The course of AD varies from person to person, as does the rate of decline. In most people, symptoms first appear after age 60. AD and other dementing disorders are caused by diseases that affect the brain, although age-related brain and body changes also can affect the development of dementia.
Dr. Alois Alzheimer, a German psychiatrist and neuropathologist, first described what we now know as AD when he reported the case of 51-year-old Auguste D. in 1906. He described the unusual features of her brain tissue—the numerous globs of sticky proteins in the spaces between neurons (what we now call beta-amyloid plaques) and tangled bundles of fibrils within neurons (what we now call neurofibrillary tangles). However, it was not until the 1960s and 1970s that scientists began to recognize AD as a pathology associated with aging (Katzman, 1976).
The public AD research enterprise began in earnest in the 1970s. Initially, investigators focused on understanding the manifestations and natural progression of the disease. Findings from these studies, combined with advances in many scientific areas—diagnostic imaging, genetic analysis, molecular and cellular biology, and the development of animal models, to name just a few—have led to an explosion of knowledge about AD.
Today, some 30 years later, the maturing field of AD research encourages the creativity and insights of individual investigators and targets special areas for emphasis. Enormous progress has been made, and many promising avenues lie ahead. However, if one theme predominates today, it is the growing appreciation of the pathological and clinical complexity of AD. It is clear that AD has no single cause but develops from multiple factors that interact over many years. A variety of scientific disciplines must collaborate if we are to understand the genetic and environmental influences, including lifestyle and health factors, that may increase or decrease an individual’s risk of developing cognitive decline, amnestic mild cognitive impairment, or AD. (Amnestic mild cognitive impairment, or aMCI, is a condition of greater-than-normal memory problems but no impairment in multiple cognitive domains.)
The National Institutes of Health’s (NIH’s) broad program of AD research taps into those varied disciplines, with a growing emphasis on bringing what scientists learn in the laboratory to the clinical arena as rapidly as possible.
An Urgent National Health Priority
The urgency of the AD research task cannot be underestimated. AD is the most common form of dementia. Its impact on the Nation is illustrated by studies estimating its prevalence (the number of people with the disease at any one time). For example, recent estimates from a nationally representative sample in the Aging, Demographics, and Memory Study (part of the ongoing NIA-supported Health and Retirement Study) suggest that among Americans age 72 and older, one in seven has dementia and about 2.4 million have AD (Plassman et al., 2007). Other investigators, using projections from community-based studies, estimate that the number of Americans aged 65 or older with AD will be 5.1 million in 2010 (Evans et al., 1990; Hebert et al., 2003).
Despite the differing methodologies and results of their studies, experts agree that the number of people with AD will increase significantly if current U.S. population trends continue and no prevention methods emerge (Hebert et al., 2003).
Our aging society makes AD an especially critical issue because the number of people with the disease doubles for every 5-year age interval beyond age 65. The U.S. Census Bureau estimates that the population age 65 and older is expected to double to about 72 million people in the next 25 years. Moreover, the 85-and-older age group is now the fastest growing segment of the U.S. population.
About This Report
The 2007 Progress Report on Alzheimer’s Disease describes NIH’s important AD research effort. It begins with a brief primer on AD that reviews the main features of the disease, discusses the causes, and describes how AD is diagnosed and treated. The next section, “Progress in AD Research Continues,” highlights recent advances in nine major areas. The Progress Report concludes with an outline of the diverse ways in which NIH is building on the momentum of 3 decades of groundbreaking AD research.
| How Many People Have AD? |
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Published estimates of the prevalence of AD vary. That’s because investigators use different methods for counting, and each method has its own strengths and weaknesses.
NIA continues its research interest in studies to estimate AD prevalence because having an as-accurate-as-possible number indicates the scope of AD as a public health problem. This helps scientists, policy makers, health care providers, and health care insurers determine the costs of AD to the Nation and develop appropriate services and resources to address the problem.
Equally important, the ability to reliably track trends in AD prevalence and incidence (the number of new cases in a specified time period) helps investigators correlate these trends with changes in environmental and biological factors. Results from these correlations may provide insights into potential risk and protective factors and help inform the design of prevention and treatment interventions.
Some scientists have obtained a prevalence count by using a random sample of older adults that reflects characteristics of the overall U.S. population. Sample findings are then extrapolated to the entire population. Other scientists calculate a national estimate by projecting results from one or more studies of particular communities within the United States. No matter which method scientists use, getting a complete and accurate count of people with AD is a daunting challenge.
Measuring AD in a population requires a different approach than diagnosing the disease in an individual at, for example, a major AD clinical center. In populations, researchers try to identify all the people in a selected group with AD, ruling out those who do not have the condition. There is no simple test to do this, and the count can be further compromised if individuals or their caregivers decline to participate in a study.
To make population assessments easier and more cost-efficient to conduct, investigators often use screening tests and abbreviated cognitive test batteries. These study instruments may help, but they risk overlooking milder cases of AD or diagnosing AD in individuals who have another type of dementia. Further, prevalence studies generally test people at only one point in time, and the tests may not be able to pick up the disease in its early stages. Better diagnostic tools are needed to capture cognitive impairment and AD at their earliest stages, both in population studies and studies conducted in major AD clinical centers, so that we can get a better measure of the scope of the problem.
One of the toughest issues is who to include in the count, a decision that can be driven by budget and staff limitations as well as by study design considerations. For example, including only people older than 65 will miss younger people who may have the disease. If scientists exclude people in nursing homes and assisted living facilities, then they also are likely to end up with an underestimate. Also, many people choose not to participate in these kinds of studies because they don’t want to find out if they have AD or because participation is inconvenient. |
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