Alzheimer's disease is the most common form of dementia, affecting 60 to 80 percent of cases. Symptoms can vary widely. The first problem many people with Alzheimer's notice is forgetfulness severe enough to affect their work, lifelong hobbies or social life. Other symptoms include confusion, trouble organizing and expressing thoughts, misplacing things, getting lost in familiar places and changes in personality and behavior..

Vascular dementia is the second most common type of dementia, after Alzheimer's. It occurs when clots block blood flow to parts of the brain, depriving nerve cells of food and oxygen. It can occur after a single major stoke or when a series of very small stokes, or infarcts, clog tiny blood vessels. Common symptoms can include forgetfulness, difficulty focusing attention and confusion.

In mixed dementia, Alzheimer's disease and vascular dementia occur at the same time. Decline may follow a pattern similar to either Alzheimer's or vascular dementia or a combination of the two. Experts suspect mixed dementia whenever a person has evidence of cardiovascular disease and dementia symptoms that get worse slowly.

In Lewy body dementia abnormal deposits of a protein called alpha-synuclein form inside the brain's nerve cells. Symptoms include memory problems, confusion, poor judgment, movement symptoms (stiffness, shuffling walk, shakiness, lack of facial expression, problems with balance and falls), excessive daytime drowsiness, visual hallucinations, and REM sleep disorder.

Lewy bodies may also be found in people with Parkinson's disease. The cells that are damaged and destroyed are chiefly in a brain area important in controlling movement. Symptoms include tremors and shakiness, stiffness, difficulty walking, muscle control, balance, lack of facial expression and impaired speech. Many individuals with Parkinson's develop dementia in later stages of the disease.