Causes and Risk Factors
There are two types of Alzheimer’s disease—early-onset and late-onset. Early-onset Alzheimer’s is a rare form of the disease that occurs in people age 30 to 60. It occurs in less than 5 percent of all people with Alzheimer’s. Almost all people with Alzheimer’s disease have late-onset Alzheimer's, which usually develops after age 60.
Causes Not Fully Understood
Scientists do not yet fully understand what causes Alzheimer's disease in most people. In early-onset Alzheimer’s, a genetic mutation is usually the cause. Late-onset Alzheimer’s arises from a complex series of brain changes that occur over decades. The causes probably include a mix of genetic, environmental, and lifestyle factors. These factors affect each person differently.
Research shows that Alzheimer’s disease causes changes in the brain years and even decades before the first symptoms appear, so even people who seem free of the disease today may be at risk. Scientists are developing sophisticated tests to help identify who is most likely to develop symptoms of Alzheimer’s. Ultimately, they hope to prevent or delay dementia in these high-risk individuals.
Some risk factors for Alzheimer’s, like age and genetics, cannot be controlled. Other factors that may play a role in the development of the disease—such as how much a person exercises or socializes—can be changed.
Lifestyle factors, such as diet and physical exercise, and long-term health conditions, like high blood pressure and diabetes, might also play a role in the risk of developing Alzheimer’s disease. For more information, see the chapter entitled “Prevention.”
Older Age—The Biggest Risk Factor
Increasing age is the most important known risk factor for Alzheimer's disease. The number of people with the disease doubles every 5 years beyond age 65. Nearly half of people age 85 and older may have Alzheimer’s. These facts are significant because the number of older adults is growing.
Genetics appears to play a part in both early- and late-onset Alzheimer’s disease. In early-onset Alzheimer’s, most cases are caused by specific genetic mutations— permanent changes in genes that can be passed on from a parent to a child. This results in early-onset familial Alzheimer’s disease, or FAD.
Most people with Alzheimer’s disease have late-onset Alzheimer's, in which symptoms appear in a person’s mid-60s. No obvious family pattern is seen in most of these cases, but certain genetic factors appear to increase a person’s risk.
Many studies have linked the apolipoprotein E gene to late-onset Alzheimer’s. One form of this gene, APOE ɛ4, increases a person’s risk of getting the disease. But many people who get Alzheimer’s do not have the APOE ɛ4 gene, and some people with the gene never get Alzheimer’s.
Scientists have identified a number of other genes in addition to APOE ɛ4 that may increase a person’s risk for late-onset Alzheimer’s. Knowing about these genes can help researchers more effectively test possible treatments and prevention strategies in people who are at risk of developing Alzheimer’s -- ideally, before symptoms appear.