Alzheimer's disease is a slowly progressive, irreversible brain disease that results in memory loss, impaired thinking ability, and ultimately changes in behavior or personality. The symptoms of Alzheimer's disease progress over time from mild forgetfulness to severely impaired mental function. These symptoms result from the death of brain cells and the lost connections between them. The course of the disease, including the range of symptoms and the speed at which mental function declines, varies widely from individual to individual. People with Alzheimer's disease live an average of eight to ten years after diagnosis, but some may live for twenty years or more.
Alzheimer's disease is not a normal consequence of growing older. Alzheimer's disease is more than simple forgetfulness. During normal aging, nerve cells in the brain are not lost in large numbers. However, in Alzheimer's disease, large numbers of nerve cells in the brain stop functioning, lose their connections with other nerve cells, and die.
Alzheimer's disease is the most common form of dementia, and the ninth leading cause of death in adults over 65 years of age. Dementia is the term used to describe a persistent, usually irreversible decline in mental abilities. Dementia can be caused by brain diseases such as Alzheimer's or Huntington's disease, or by conditions that affect the brain indirectly, such as clogged blood vessels leading to stroke, certain vitamin deficiencies (such as B12 deficiency), thyroid disorders, pituitary disease, HIV infection, or syphilis.
Alzheimer's disease is the most common cause of dementia among people over the age of 65. Approximately four million Americans live with the disease, and this number is expected to grow rapidly as the U.S. population ages.
The causes of Alzheimer's disease are not well understood, but the condition is most likely due to a destructive accumulation of a protein (called beta amyloid) outside brain cells. Genetic and environmental factors both contribute to the accumulation of beta amyloid in the brain Figure 01. These deposits or plaques are a hallmark feature of Alzheimer's disease. Plaques are a dense composite of beta amyloid protein and other cellular materials that accumulate around the nerve cells in the brain. It is not fully understood exactly how amyloid plaques destroy nerve cells.
A second abnormal structure, the neurofibrillary tangle, also has a role in Alzheimer's disease, although it is thought to play a secondary role. Neurofibrillary tangles are twisted fibers that build up inside brain cells. Tangles are formed from a type of protein (called tau) that normally constitutes part of the internal support structure of the cell. These intracellular tangles disrupt communication between brain cells and eventually destroy them.
Figure 01. Beta amyloid plaques and neurofibrillary tangles
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