Alzheimer's Disease

What is Alzheimer's Disease?

According to the Alzheimer's Association Alzheimer's is the most common form of dementia, a general term for memory loss and other intellectual abilities serious enough to interfere with daily life.^[2] Alzheimer's disease (AD) is a disease of unknown cause that results in declining brain function.^[1] There is no known cure for the disease. Over time, memory and thinking skills are destroyed. Symptoms usually appear after age 60. Alzheimer's disease is the most common form of dementia, or decreased brain function, in older people.
In the United States, an estimated 2.4 to 4.5 million people have Alzheimer's disease. It is the seventh leading cause of death in the United States.^[2] Alzheimer's disease strikes men and women equally.^[3] Risk for Alzheimer's disease increases with age. Higher levels of education are associated with reduced risk for Alzheimer's disease.

Causes

Evidence has accumulated that Alzheimer's disease is multifactorial— that is, it is caused by a combination of several genetic and environmental factors.
GENETIC. Early-onset AD is caused by a defect in one of three genes known as APP, presenilin-1, and presenilin-2, found on human chromosomes 21, 14, and 1, respectively. Early-onset AD is also associated with Down syndrome, in that people with trisomy 21 (three forms of human chromosome 21 instead of a pair) often develop this form of Alzheimer's. The brains of people with Down syndrome age prematurely, so that those who develop early-onset AD are often only in their late 40s or early 50s when the symptoms of the disease first appear.

NEUROBIOLOGICAL. Investigators since Alois Alzheimer's time have studied the abnormalities found at autopsy in the brains of patients with AD. One abnormality is plaques, or clumps, of a starchy protein called beta amyloid. Beta amyloid is formed when a substance called amyloid precursor protein, or APP, fails to be metabolized properly in the body. APP is a substance found in many parts of the body, but its precise function is not yet known. Following the formation of beta amyloid, pieces of it then stick to one another and gradually build up into plaques. The other abnormal finding is neurofibrillary tangles, which are twisted threads formed from parts of the dying nerve cell called the tau protein, which was discovered in 1986. If the tau protein is damaged by the addition of molecules of phosphorus, a process called hyperphosphorylation, it forms filaments that twist around each other to form the neurofibrillary tangles. As the plaques and tangles accumulate in the brain, they cause the nerve cells to wither and eventually die. As the nerve cells die, the affected parts of the brain start to shrink in size. It is not known as of 2002, however, whether the plaques and tangles are causes of AD or results of it. The relationship between the plaques and the tangles is another question that has not yet been answered. Although the plaques usually appear in brain tissue before the tangles, it is not clear that they cause the tangles. There are other brain disorders, such as Pick's disease, in which tangles appear in the brain cells without plaques.

ENVIRONMENTAL. Researchers have been studying the possibility that certain chemicals or other toxins in the environment may have a role in causing or triggering AD. The environmental factors that have been considered include aluminum, zinc, toxins in contaminated food, and viruses. Although there is little evidence as of 2002 that AD is caused by a virus or other infectious agent, the possibility cannot be completely excluded.