Alzheimer's disease is a progressive disease that damages nerve cells (neurons) in parts of the brain involved in memory, learning, language, and reasoning. As the disease progresses, communication among the neurons breaks down. In early stages, short-term memory begins to fail. Over time, functions such as long-term memory, language, and judgment decline. Alzheimer's disease is the most common cause of dementia in older adults. Dementia is a loss of mental functions—such as thinking, memory, and reasoning—that is severe enough to interfere with a person's daily functioning.
There are two basic types of Alzheimer's disease: Early-onset Alzheimer's disease tends to strike people under age 65 and is more likely to run in families. Late-onset Alzheimer's disease, the much more common type, generally afflicts people after age 65. The exact cause of Alzheimer's is unknown, although researchers studying this puzzling disease are making progress.
Currently, about 4 million Americans suffer from Alzheimer's disease, and about 22,000 people die from Alzheimer's disease every year. The Alzheimer's Association estimates that 1 in 10 people over age 65 and nearly half of people over 85 have Alzheimer's disease. The number of Americans with Alzheimer's disease is expected to increase to 14 million by 2050, unless a cure or preventive measure can be found.
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The human brain is made up of billions of neurons, or nerve cells. Each one connects to thousands of other nerve cells, all of which communicate via chemical messengers. At any one moment, millions of these signals are speeding through pathways in the brain, allowing it to process and analyze information and send out instructions to various parts of the body.
Deep inside one portion of the brain, known as the temporal lobe, is a structure called the hippocampus, which is thought to be where short-term memories are converted into long-term memories. The hippocampus often is atrophied in the brains of Alzheimer's patients. Moreover, in these individuals, the hippocampus and other areas of the brain involved in thinking and decision making are filled with two types of abnormalities—beta-amyloid plaques and neurofibrillary tangles. Plaques are deposits outside and around the neurons, made up of dense globs of a protein called beta-amyloid, mixed with other cellular material. Tangles are made up of microtubules, part of the transport system inside nerve cells. Scientists are not sure whether these abnormalities cause Alzheimer's, but they are the focus of a lot of research because Alzheimer's patients have many more of them than healthy older people do.
Many doctors and researchers categorize Alzheimer's in two different ways: by the type of disease and by the stages people with Alzheimer's progress through.
Early-onset Alzheimer's: This is an uncommon form in which individuals are diagnosed with the disease before age 65. Fewer than 10 percent of all Alzheimer's disease patients have this type. Because of their genetic abnormality, people with Down syndrome are particularly at risk for a form of early-onset Alzheimer's disease. Adults with Down syndrome often are in their mid- to late 40s or early 50s when symptoms first appear.
Late-onset dementia: The most common form of Alzheimer's disease, late-onset dementia usually strikes after age 65. It occurs in almost half of all people over the age of 85 and may or may not be hereditary. Late-onset dementia is also called sporadic Alzheimer's disease.
Familial Alzheimer's disease (FAD): This form of Alzheimer's disease is known to be entirely inherited. FAD is extremely rare, accounting for fewer than 1 percent of all cases of Alzheimer's disease. It has a much earlier onset (often in the 40s) and follows clear patterns of inheritance.
In addition to age of onset, there are other differences among the types of Alzheimer's.
Younger people who develop Alzheimer's disease have more of the microscopic changes found in the brains of people with Alzheimer's disease. These changes include the twisted nerve cell fibers, known as neurofibrillary tangles, and a sticky protein called beta-amyloid, which forms structures called plaques. Plaques and tangles are associated with damage to healthy brain cells and result in brain shrinkage or atrophy. It may be, though, that younger brains simply need to be more damaged before a person starts to show symptoms, so this may not reflect a true difference in the nature of the disease.
A condition called myoclonus—muscle twitching and spasm—is more commonly seen in early-onset than late-onset Alzheimer's.
Some research suggests that people with early-onset Alzheimer's disease decline at a faster rate than do those with late-onset Alzheimer's disease.
Younger people who are diagnosed with the disease tend to be more physically fit and active, and many still have family and career responsibilities. As a result, they tend to feel more powerless, frustrated, and depressed upon diagnosis.
Studies show that chemical and structural changes occur in the brains of people with Alzheimer's disease. These changes interfere with a person's ability to process, store, and retrieve information. It is not known why these changes occur.
Scientists have found two significant abnormalities in the brains of people with Alzheimer's disease: twisted nerve cell fibers, known as neurofibrillary tangles, and a sticky protein called beta-amyloid, which forms structures called plaques. Plaques and tangles are associated with damage to healthy brain cells, causing the brain to atrophy and shrink.
Another characteristic of Alzheimer's disease is the reduced production of certain chemicals in the brain that are necessary for communication between nerve cells. These chemicals, called neurotransmitters, include acetylcholine, serotonin, and norepinephrine.
In the area of the brain responsible for memory, the hippocampus, there is a loss of nerve cells, and decreases occur in the levels of chemicals needed for carrying messages back and forth between the nerve cells.
Age: Your risk of developing Alzheimer's disease increases as you grow older. At 65 years, 1 percent of the population is affected; at age 85, 30 percent to 50 percent has the disease. While it has been suggested that everyone might develop dementia after living long enough, it should be emphasized that Alzheimer's is a disease and not simply an exaggeration of normal aging.
Family history: If you have relatives who have Alzheimer's, you are more likely to develop the disease, although a clear, inherited pattern of Alzheimer's disease exists for fewer than 1 percent of all cases. Alzheimer's disease strikes early and fairly often in certain families, often enough to be singled out as a separate form of the disease and given a label: early-onset familial Alzheimer's disease, or FAD.
Sex: Alzheimer's disease is more prevalent among women than among men.
Head injury: Some studies have shown an association between Alzheimer's disease and a history of significant head injury.
Education: Some studies have shown that low education levels are related to an increased risk for Alzheimer's disease, although why this might be the case is not clearly understood.
Down syndrome: People with Down syndrome often develop Alzheimer's disease in their 40s and 50s.
One of the most publicized and controversial theories concerns aluminum, which became a suspect in Alzheimer's disease when researchers found traces of this metal in the brains of patients with Alzheimer's disease. Many studies since then have either not been able to confirm this finding or have had questionable results. Aluminum does turn up in higher amounts than normal in some autopsy studies of Alzheimer's patients but not in all. Various studies have found that groups of people exposed to high levels of aluminum do not have an increased risk. On the whole, scientists can say only that it seems unlikely that exposure to aluminum plays a role in Alzheimer's disease.
Last reviewed on 10/21/2008
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