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Alzheimer's Disease Defined

Dementia is a brain disorder that seriously affects a person's memory, thinking, and reasoning skills. People with dementia often have trouble thinking and speaking clearly, remembering recent events, and learning new things. Over time, it becomes hard for them to handle everyday activities and take care of themselves. There are many causes of dementia, but Alzheimer's disease is the most common cause of dementia in older persons.

Scientists think that up to 4.5 million people in the United States suffer from Alzheimer's disease. The disease usually begins after age 65 and risk goes up with age. While younger people also may get Alzheimer's disease, it is much less common.

About 5 percent of men and women ages 65 to 74 have Alzheimer's disease, and nearly half of those age 85 and older may have the disease. It is important to note, however, that Alzheimer's disease is not a normal part of aging.

Alzheimer's disease is named after Dr. Alois Alzheimer, a German doctor. In 1906, Dr. Alzheimer noticed changes in the brain tissue of a woman who had died of an unusual mental illness. He found abnormal clumps and tangled bundles of fibers. The clumps are now called amyloid plaques and the tangles are called neurofibrillary tangles. Today, these plaques and tangles in the brain are considered signs of Alzheimer's disease.

Scientists also have found other brain changes in people with Alzheimer's disease. There is a loss of nerve cells and pathways in areas of the brain that are vital to memory and other mental abilities. There also are lower levels of some of the chemicals in the brain that carry complex messages back and forth between nerve cells.

Alzheimer's disease may disrupt normal thinking and memory by blocking these messages between nerve cells.