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Alzheimer's: Causes and Diagnosis

Factors linked to Alzheimer's and how doctors diagnose the disease

By National Institutes of Health

Scientists don’t yet understand what causes Alzheimer’s disease, but it is clear that it involves a complex series of events that take place in the brain over a long period of time.

It is likely that the causes include genetic, environmental and lifestyle factors. Because people differ in their genetic make-up and lifestyle, the importance of these factors for preventing or delaying Alzheimer’s differs from person to person.

The Mysteries of Alzheimer’s

Scientists are conducting studies to learn more about plaques, tangles and other features of Alzheimer’s disease. They can now visualize plaques by imaging the brains of living individuals. They are also exploring the very earliest steps in the disease process.

One of the great mysteries of Alzheimer’s disease is why it largely strikes older adults. Research on how the brain changes normally with age is shedding light on this question. For example, scientists are learning how age-related changes in the brain may harm neurons and contribute to Alzheimer’s damage. These age-related changes include atrophy (shrinking) of certain parts of the brain, inflammation and the production of unstable molecules called free radicals.

What Role Do Genes Play?

In a very few families, people develop Alzheimer’s disease in their 30s, 40s and 50s. Many of these people have a mutation, or permanent change, in one of three genes that they inherited from a parent. We know that these gene mutations cause Alzheimer’s in these “early onset” familial cases. Not all early-onset cases are caused by such mutations.

Most people with Alzheimer’s disease have “late onset” Alzheimer’s, which usually develops after age 60. Many studies have linked a gene called APOE to late-onset Alzheimer’s. This gene has several forms. One of them, APOE ε4, increases a person’s risk of getting the disease. About 40 percent of all people who develop late-onset Alzheimer’s carry this gene. However, carrying the APOE ε4 form of the gene does not necessarily mean that a person will develop Alzheimer’s disease, and people carrying no APOE ε4 forms can also develop the disease.

Most experts believe that additional genes may influence the development of late-onset Alzheimer’s in some way. Scientists around the world are searching for these genes. Researchers have identified variants of the SORL1, CLU, PICALM and CR1 genes that may play a role in risk of late-onset Alzheimer’s. For more about this area of research, see the Alzheimer’s Disease Genetics Fact Sheet, available at

Does a Healthy Lifestyle Reduce the Risk of Alzheimer's?

A nutritious diet, physical activity, social engagement and mentally stimulating pursuits all can help people stay healthy. New research suggests these acticities also might help to reduce the risk of cognitive decline and Alzheimer’s disease. Scientists are investigating associations between cognitive decline and vascular and metabolic conditions such as heart disease, stroke, high blood pressure, diabetes and obesity. Understanding these relationships and testing them in clinical trials may determine whether reducing risk factors for these diseases helps with Alzheimer’s as well.

How Doctors Diagnose Alzheimer’s


Alzheimer’s disease can be definitively diagnosed only after death by linking its clinical course with an examination of brain tissue and pathology in an autopsy.  Doctors now have several methods and tools to help them determine fairly accurately whether a person who is having memory problems has “possible Alzheimer’s disease” (dementia may be due to another cause) or “probable Alzheimer’s disease” (no other cause for dementia can be found). To diagnose Alzheimer’s, doctors:

  • Ask questions about the person’s overall health, past medical problems, ability to carry out daily activities, and changes in behavior and personality.
  • Conduct tests of memory, problem solving, attention, counting, and language.
  • Carry out medical tests, such as tests of blood, urine, or spinal fluid.
  • Perform brain scans, such as computerized tomography (CT) or magnetic resonance imaging (MRI).

These tests may be repeated to give doctors information about how the person’s memory is changing over time.

Early Diagnosis Can Prolong Brain Function

Early diagnosis is beneficial for several reasons. Having an early diagnosis and starting treatment in the early stages of the disease can help preserve function for months to years, even though the underlying disease process cannot be changed. Having an early diagnosis also helps families plan for the future, make living arrangements, take care of financial and legal matters, and develop support networks.

What is Alzheimer's disease?  
Common drugs for treating Alzheimer's
Genes offer clues to Alzheimer's disease

Part of a series on understanding and managing Alzheimer's Disease based on editorial content provided by the Alzheimer's Disease Education and Referral (ADEAR) Center and the National Institute on Aging.

National Institutes of Health
By National Institutes of Health

The National Institutes of Health, a part of the U.S. Department of Health and Human Services, is the nation's medical research agency — making important discoveries that improve health and save lives. NIH is the largest single source of financing for medical research in the world, seeking new ways to cure disease, alleviate suffering and prevent illness. By providing the evidence base for health decisions by individuals and their clinicians, NIH is empowering Americans to embrace healthy living through informed decision-making. NIH is made up of 27 institutes and centers, each with a specific research agenda, focusing on stages of life, like aging or child health, or particular diseases or body systems.

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