Causes and Risk Factors of Alzheimer’s Disease
Alzheimer's disease has no known single cause, but in the last 15 years scientists have learned a great deal about factors that may play a role.
Biology of Alzheimer's disease -- plaques and tangles
- Scientists believe that whatever triggers Alzheimer's disease begins to damage the brain years before symptoms appear.
- When symptoms emerge, nerve cells that process, store and retrieve information have already begun to degenerate and die.
- Scientists regard two abnormal microscopic structures called "plaques" and "tangles" as Alzheimer hallmarks.
- Amyloid plaques (AM-uh-loyd plaks) are clumps of protein that accumulate outside the brain's nerve cells.
- Tangles are twisted strands of another protein that form inside cells.
- Scientists do not yet know whether plaques or tangles cause Alzheimer's or are a byproduct of some other process. Clinical trials of experimental drugs targeting amyloid are under way and should help clarify the role plaques play.
- Most experts believe that the majority of Alzheimer's disease occurs as a result of complex interactions among genes and other risk factors. Age, family history and heredity are all risk factors we can’t change.
- Now, research is beginning to reveal clues about other risk factors we may be able to influence through general lifestyle and wellness choices and effective management of other health conditions.
Age and family history
- Late-onset Alzheimer's, which chiefly affects individuals over age 65, is the more common form of the illness that is most often associated with the term "Alzheimer's disease."
- The greatest known risk factors for late-onset Alzheimer's are increasing age and a family history of the disease.
- The likelihood of developing late-onset Alzheimer's rises dramatically with age after age 65.
- About one third (32 percent) of people age 85 and older have Alzheimer’s disease.
- There are two categories of genes that influence whether a person develops a disease: (1) risk genes and (2) deterministic genes. Researchers have identified Alzheimer's genes in both categories.
- Risk genes increase the likelihood of developing a disease, but do not guarantee it will happen. Researchers have found several genes that increase the risk of Alzheimer's. APOE-e4 is the first risk gene identified, and remains the gene with strongest impact on risk. APOE-e4 is one of three common forms of the APOE gene; the others are APOE-e2 and APOE-e3. Scientists estimate that APOE-e4 is implicated in about 20 percent to 25 percent of Alzheimer's cases (source).
- Deterministic genes directly cause a disease, guaranteeing that anyone who inherits one will develop a disorder. Scientists have found rare genes that cause Alzheimer's in only a few hundred extended families worldwide. These genes, which are estimated to account for less than 5 percent of Alzheimer's cases, cause familial early-onset forms in which symptoms usually develop between a person's early 40s and mid-50s (source).
- Genetic tests are available for both APOE-e4 and the rare genes that directly cause Alzheimer's. However, health professionals do not currently recommend routine genetic testing for Alzheimer's disease. Testing for APOE-e4 is sometimes included as a part of research studies (source).
- Head trauma: There may be a strong link between serious head injury and future risk of Alzheimer’s, especially when trauma occurs repeatedly or involves loss of consciousness.
- Heart-head connection: Growing evidence links brain health to heart health. Your brain is nourished by one of your body’s richest networks of blood vessels. Every heartbeat pumps about 20 to 25 percent of your blood to your head, where brain cells use at least 20 percent of the food and oxygen your blood carries.
- The risk of developing Alzheimer’s or vascular dementia appears to be increased by many conditions that damage the heart or blood vessels. These include high blood pressure, heart disease, stroke, diabetes and high cholesterol. Work with the doctor to monitor your heart health and treat any problems that arise.
- Studies of donated brain tissue provide additional evidence for the heart-head connection. These studies suggest that plaques and tangles are more likely to cause Alzheimer's symptoms if strokes or damage to the brain’s blood vessels are also present.
- Because Latinos and African-Americans in the United States have higher rates of vascular disease, they also may be at greater risk for developing Alzheimer’s. According to a growing body of evidence, risk factors for vascular disease — including diabetes, high blood pressure and high cholesterol — may also be risk factors for Alzheimer’s and stroke-related dementia.
- Some of the most exciting emerging evidence suggests that strategies for general healthy aging may also help reduce the risk of developing Alzheimer's. These measures include:
- Controlling blood pressure, weight and cholesterol levels (some studies indicate that the DASH and Mediterranean diets may reduce risk).
- Exercising both body (at least 150 minutes of moderate exercise per week) and mind (new and challenging learning has been suggested as being helpful).
- Avoiding tobacco and excess alcohol.
- Staying socially active.
For more information, please contact the Alzheimer’s Association at www.alz.org or call 24/7 at (800) 272-3900.