Reviewed by Joseph Maloney, MDAlzheimer's disease (AD) is a disorder marked by certain brain changes and dementia that usually affects older people. Alzheimer's disease is not a normal part of aging - it is not something that inevitably happens in later life. Rather, it is one of the dementias, a group of brain diseases that lead to the loss of mental and physical functions. Five to 6 percent of older people are afflicted by Alzheimer's disease or a related dementia—which means that approximately 3 to 4 million Americans have one of these debilitating disorders.
Research indicates that 3 percent of the population aged 65 to 74 has severe dementia, increasing to 15 percent of those aged 75-84 and to 47 percent of those 85 or older. At least half the people in U.S. nursing homes have Alzheimer's disease or a related disorder. In 1998, the annual cost of caring for individuals with Alzheimer's disease and related dementias in institutional and community settings was estimated at $61 billion for direct costs alone and is probably closer to $200 billion per year. As our population ages and the number of Alzheimer's patients increases, costs of care will rise as well. It is estimated that by the middle of this century about 14 million Americans will have Alzheimer’s disease.
The onset of Alzheimer's disease is usually very slow and gradual, seldom occurring before age 65. Over time, however, it follows a progressively more serious course. The main risk factor for Alzheimer's disease is increased age. The rates of the disease increase markedly with advancing age, with 40 percent of people over 85 suffering from Alzheimer's or other severe dementia.
Some investigators, describing a family pattern of Alzheimer's disease, suggest that in some cases heredity may influence its development. A genetic basis has been identified through the discovery of several genetic markers on chromosomes 21 and 14 for a small subgroup of families in which the disease has frequently occurred at relatively early ages (beginning before age 50). Also, according to Kumar, Pathologic Basis of Disease, pathologic changes identical to those observed in Alzheimer's disease occur in almost all individuals with trisomy 21 who survive beyond 45 years of age. Some evidence points to chromosome 19 as implicated in certain other families that have frequently had the disease develop at later ages.
At the same time, data indicate that the likelihood that a close relative (sibling, child, or parent) of an afflicted individual will develop Alzheimer's disease is low. Over 90 percent of cases are sporadic, not familial. In most cases, such an individual's risk is only slightly higher than that of someone in the general population, where the lifetime risk is below 1 percent. And, of course, many disorders have a genetic potential that is never expressed—that is, despite being at risk for a certain illness, one might go through life without ever developing any symptom of the disease.
