Beta-Amyloid Accumulation Precedes Alzheimer Disease

In patients with Alzheimer disease, the most common cause of dementia, deposits of an abnormal protein called beta-amyloid accumulate around neurons in the brain. It is thought that these beta-amyloid deposits choke off neurons, eventually killing them. When there are too few neurons left, cognitive impairment and eventually dementia set in. But when exactly do these beta-amyloid deposits begin to accumulate, and could their accumulation and the subsequent development of Alzheimer disease be prevented? Attempts to find drugs that would prevent the development of Alzheimer disease have been hampered by the fact that it is difficult to find people to test the drugs on; that is, people who are still seemingly normal but who are likely to develop Alzheimer disease later in life. By the time Alzheimer disease becomes evident, it's too late to prevent it; all one can do is treat the symptoms.

Now a new study offers hope. It's a meta-analysis (remember, a meta-analysis is a study of previous study; like a summary) of 55 previous studies, involving over 7,500 persons who were either normal or had subjective or mild cognitive impairment - none had reached the level of Alzheimer disease yet. All of the studies looked at the prevalence of beta-amyloid pathology, either by positron emission tomography (PET) scan or by analysis of cerebrospinal fluid.

The study found that the prevalence of beta-amyloid pathology increased with age and decreasing level of cognitive function, ranging from 10% in 50-yr-olds with normal cognition to over 70% in 90-yr-olds with mild cognitive impairment.

Most importantly, the study suggests that beta-amyloid pathology begins to develop as early as 20 to 30 years before the onset of dementia. This is good news for researchers seeking to find a drug to prevent Alzheimer disease. By testing a lot of seemingly normal individuals and then enrolling only persons with evidence of beta-amyloid pathology (persons at high risk for Alzheimer disease) in their research studies, the researchers are more likely to find a potential Alzheimer prevention drug that works.

For those of us unlikely to enroll in a research study, however, it's not a particularly helpful finding. Yes, we could be tested to see if we had signs of beta-amyloid accumulation yet, but what would be the point, if there is no cure?