Instead, Alzheimer's disease is a type of dementia, or a loss of mental functioning. Specifically, it is a disorder involving the progressive degeneration of brain tissue. For an unknown reason, Alzheimer's causes neurons, or brain cells, to break down and decompose. Amyloid plaques are spheres of protein that are surrounded by cellular fragments. As the neurons break down and degenerate, pieces of neurons and other waste products attach themselves to protein, forming plaques.
Neurofibrillary tangles refer to a twisting and tangling of nerve fibers.
Prevalence and Diagnosis Azheimer's disease occurs in 2 to 4 percent of the population over the age of 65. Its prevalence increases with age so that at least one-third of adults over the age of 80 are thought to have Alzheimer's disease. Part of the difficulty in determining the prevalence of Alzheimer's is that it can only truly be diagnosed after death. A definitive diagnosis of Alzheimer's occurs after observing the amyloid plaques and neurofibrillary tangles during autopsy. Generally Alzheimer's is diagnosed by exclusion.