The plaques found in the brains of people with Alzheimer's disease appear to be made, in part, from protein molecules--amyloid precursor protein (APP)--that normally are essential components of the brain. Plaques are made when an enzyme snips APP apart at a specific place and then leaves the fragments--beta amyloid--in brain tissue where they come together in abnormal deposits. It has not as yet been definitely determined how neurofibrillary tangles are formed.

As research on Alzheimer's disease progresses, scientists are describing other abnormal anatomical and chemical changes associated with the disease. These include nerve cell degeneration in the brain's nucleus basalis of Meynert and reduced levels of the neurotransmitter acetylcholine in the brains of Alzheimer's disease victims. But from a practical standpoint, the "classical" plaque and tangle changes seen in the brain at autopsy typically suffice for a diagnosis of Alzheimer's disease. In fact, it is still only through the study of brain tissue from a person who was thought to have Alzheimer's disease that a definitive diagnosis of the disorder can be made.


2. Brain Scans


Computer-Assisted Tomography (CAT scan) changes become more evident as the disease progresses--not necessarily early on. Thus a CAT scan performed in the first stages of the disease cannot in itself be used to make a definitive diagnosis of Alzheimer's disease; its value is in helping to establish whether certain disorders (some reversible) that mimic Alzheimer's disease are present. Later on, CAT scans often reveal changes characteristic of Alzheimer's disease, namely an atrophied (shrunken) brain with widened sulci (tissue indentations) and enlarged cerebral ventricles (fluid-filled chambers).

Several new types of instrumentation are enabling researchers to learn even more about the brain. Both positron emission tomography (PET scan) and SPECT (single photon emission computerized tomography) can map regional cerebral blood flow, metabolic activity, and distribution of specific receptors, as well as integrity of the blood-brain barrier. These procedures may reveal abnormalities characteristic of Alzheimer's disease. Another method, magnetic-resonance imaging (MRI), probes the brain by examining the interaction of the magnetic properties of atoms with an external magnetic field. MRI provides both structural and chemical information and distinguishes moving blood from static brain tissue (Taylor, 1990).


Clinical Features of Alzheimer's Disease

The "clinical" features of Alzheimer's disease, as opposed to the "tissue" changes, are threefold:

1. Dementia--significant loss of intellectual abilities such as memory capacity, severe enough to interfere with social or occupational functioning;

2. Insidious onset of symptoms--subtly progressive and irreversible course with documented deterioration over time;

3. Exclusion of all other specific causes of dementia by history, physical examination, laboratory tests, psychometric, and other studies.