Senile dementia.

The Alzheimer type of senile dementia (SDAT) accounts for more than 50% of such cases, and is a very common disorder as well as being very costly in emotional, economic, and medical terms. It carries a markedly shortened life expectancy. Gray-to-white-matter ratios change and the brain shrinks slightly in the course of normal aging, but SDAT brains may not be significantly more atrophic than are normal controls. Cortical neurons are diminished in number in normal aging, but counts from frontal and midtemporal regions of SDAT specimens are not different from age-matched controls. There is loss of dendrites and of dendritic spines in both normal and abnormal aged specimens. Neurofibrillary tangles are made up of paired helical filaments that appear to be chemically and immunologically related to normal neurofibers. Neuritic plaques are made up of an amyloid core surrounded by abnormal axonal endings. Both plaques and tangles are to be correlated with the presence of senile dementia. There is some evidence for an infectious etiology of SDAT. Choline acetyltransferase is markedly reduced in cortical tissue of these patients, but the muscarinic receptors of acetylcholine are normal.