May be due to deposition of amyloid in the parenchyma of the brain which is toxic to neurons and oligodendroglia. An alternative hypothesis is that deposition of amyloid is a secondary effect. The amyloid deposition is thought to be due to an abnormality in the amyloid precursor protein. Alzheimer's disease is the most common senile dementia in patients greater than 65 years old. Alzheimer's disease is seen before 65 years of age and is called presenile dementia.
Most familial cases are presenile.
Since the 85 years of age and older group is increasing rapidly, Alzheimer' disease is becoming an important disease. The diagnosis is made by counting the number of senile plaques per 20x field and the number of neurofibrillary tangles in the same area, using the Bielschowsky stain. The patient is finally bedridden with many neurological signs and usually dies of pneumonia, heart attack, or stroke. Lifespan with the disease is usually 5 Years after diagnosis but may be 10 years if the patient is otherwise healthy. Toward the end the patient is unable to recognize anyone and may become hostile.