What is dementia?

Dementia is a syndrome characterized by:

1. impairment in memory,
2. impairment in another area of thinking such as the ability to organize thoughts and reason, the ability to use language, or the ability to see accurately the visual world (not because of eye disease), and
3. these impairments are severe enough to cause a decline in the patient's usual level of functioning.

Although some kinds of memory loss are normal parts of aging, the changes due to aging are not severe enough to interfere with the level of function. Many different diseases can cause dementia, but Alzheimer's disease is by far the most common cause for dementia in the United States and in most countries in the world.

What is Alzheimer's disease?

Alzheimer's disease (AD) is a slowly progressive disease of the brain that is characterized by impairment of memory and eventually by disturbances in reasoning, planning, language, and perception. Many scientists believe that Alzheimer's disease results from an increase in the production or accumulation of a specific protein (beta-amyloid protein) in the brain that leads to nerve cell death.
The likelihood of having Alzheimer's disease increases substantially after the age of 70 and may affect around 50% of persons over the age of 85. Nonetheless, Alzheimer's disease is not a normal part of aging and is not something that inevitably happens in later life. For example, many people live to over 100 years of age and never develop Alzheimer's disease.

Who develops Alzheimer's disease?

The main risk factor for Alzheimer's disease is increased age. As a population ages, the frequency of Alzheimer's disease continues to increase. Ten percent of people over 65 years of age and 50% of those over 85 years of age have Alzheimer's disease. Unless new treatments are developed to decrease the likelihood of developing Alzheimer's disease, the number of individuals with Alzheimer's disease in the United States is expected to be 14 million by the year 2050.
There are also genetic risk factors for Alzheimer's disease. Most patients develop Alzheimer's disease after age 70. However, 2%-5% of patients develop the disease in the fourth or fifth decade of life (40s or 50s). At least half of these early onset patients have inherited gene mutations associated with their Alzheimer's disease. Moreover, the children of a patient with early onset Alzheimer's disease who has one of these gene mutations has a 50% risk of developing Alzheimer's disease.
There is also a genetic risk for late onset cases. A relatively common form of a gene located on chromosome 19 is associated with late onset Alzheimer's disease. In the majority of Alzheimer's disease cases, however, no specific genetic risks have yet been identified.
Other risk factors for Alzheimer's disease include high blood pressure (hypertension), coronary artery disease, diabetes, and possibly elevated blood cholesterol. Individuals who have completed less than eight years of education also have an increased risk for Alzheimer's disease. These factors increase the risk of Alzheimer's disease, but by no means do they mean that Alzheimer's disease is inevitable in persons with these factors.
All patients with Down syndrome will develop the brain changes of Alzheimer's disease by 40 years of age. This fact was also a clue to the "amyloid hypothesis of Alzheimer's disease" (see section later in this article).