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Risk factors

Age

The greatest known risk factor for Alzheimer's is increasing age. Most individuals with the disease are 65 or older. The likelihood of developing Alzheimer's doubles about every five years after age 65. After age 85, the risk reaches nearly 50 percent.

Family history

Another risk factor is family history. Research has shown that those who have a parent, brother or sister, or child with Alzheimer's are more likely to develop Alzheimer's. The risk increases if more than one family member has the illness. When diseases tend to run in families, either heredity (genetics) or environmental factors or both may play a role.

Genetics (heredity)

Scientists know genes are involved in Alzheimer's. There are two categories of genes that can play a role in determining whether a person develops a disease. Alzheimer genes have been found in both categories:

1) Risk genes increase the likelihood of developing a disease, but do not guarantee it will happen. Scientists have so far identified one Alzheimer risk gene called apolipoprotein E-e4 (APOE-e4).

APOE-e4 is one of three common forms of the APOE gene; the others are APOE-e2 and APOE-e3. APOE provides the blueprint for one of the proteins that carries cholesterol in the bloodstream.

Everyone inherits a copy of some form of APOE from each parent. Those who inherit one copy of APOE-e4 have an increased risk of developing Alzheimer's. Those who inherit two copies have an even higher risk, but not a certainty. Scientists do not yet know how APOE-e4 raises risk. In addition to raising risk, APOE-e4 may tend to make symptoms appear at a younger age than usual.

Experts believe there may be as many as a dozen other Alzheimer risk genes in addition to APOE-e4.

2) Deterministic genes directly cause a disease, guaranteeing that anyone who inherits them will develop the disorder. Scientists have found rare genes that directly cause Alzheimer's in only a few hundred extended families worldwide.

When Alzheimer's disease is caused by deterministic genes, it is called ìfamilial Alzheimer's disease,î and many family members in multiple generations are affected. True familial Alzheimer's accounts for less than 5 percent of cases.

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.