According to Harrison's Principles of Internal Medicine, dementia is a syndrome with many causes. The term is applied when a "deterioration in cognitive abilities impairs the previously successful performance of normal activities" in your daily life. Memory is the most common and most important cognitive ability that is lost.
The most common cause of dementia is Alzheimer's disease (AD), making up 50% of all cases. The other 50% is made up of vascular dementia caused by multiple strokes, alcoholism, Parkinson's disease, and drug intoxication. The term Alzheimer's disease originally meant presenile or early onset dementia but it is now used to describe any progressive dementia with the characteristic pathological changes.
What Causes Alzheimer's Disease?
AD (Alzheimer's disease) is characterized by a progressive process that kills brain cells and destroys synaptic connections between nerve cells in the brain. The disease is traditionally characterized by the presence of what are called "neuritic" or "senile" plaques, neurofibrillary tangles, and the loss of nerve cells that rely on acetylcholine as a neurotransmitter as explained below.