By American Geriatrics Society (AGS)

Dementia can be broken down into the “big four:” Alzheimer’s disease, vascular dementia, Lewy body disease, and frontotemporal dementia. Here is a quick guide to each.

Alzheimer’s disease

Alzheimer’s disease is the most common cause of dementia worldwide. In the United States, it’s the cause of dementia in 62 to 80 percent of cases, accounting for the symptoms of more than 5.3 million people in 2015. According to the Alzheimer’s Association, one in nine Americans older than age 65 have Alzheimer’s disease. Alzheimer’s disease is the sixth leading cause of death in the United States and the fifth leading cause of death in Americans age 65 and older.

Vascular dementia

After AD, vascular dementia is the next most common cause of dementia, affecting more than half a million people — roughly 10 percent of the total cases of dementia in the United States. It was previously known as multi-infarct or post-stroke dementia. It occurs because decreased blood flow from blood vessel blockage or bleeding from infarcts (strokes) in the brain, which limits oxygen supply to brain cells.

Vascular dementia symptoms are similar to those seen in AD, but depend on which parts of the brain the reduced blood flow affects — what parts of the brain have experienced oxygen deprivation from strokes and how many brain cells have been affected. A person who has experienced strokes may also suffer additional weakness or even paralysis of limbs and speech difficulties.

Circulation problems become more common as people get older and can affect people who already have AD. As a result, at least 10 percent of people have mixed dementia; that is, they have Alzheimer’s disease alongside vascular dementia, and a mix of symptoms of both.

Lewy body disease

A much rarer diagnosis, Lewy body disease makes up less than 4 percent of the number of dementia cases. Lewy bodies are protein deposits that damage brain cells. They’re also found in the brains of people with Parkinson’s disease, and as a result an overlap exists in the symptoms of people with these two conditions.

The symptoms of Lewy body dementia are also similar to those of Alzheimer’s, but in addition these sufferers also develop muscle stiffness, tremors, and shakiness in their limbs, and slower movement. They also frequently experience visual hallucinations, commonly seeing animals or people around them that aren’t really there.

Frontotemporal dementia

Frontotemporal dementia is the least common of the “big four,” affecting about 50,000 people in the United States and representing less than 2 percent of total dementia cases. It’s also the most likely of the four types of dementia to be diagnosed in people under the age of 65.

This type of dementia is named because of the areas of the brain that it affects most: the frontal and temporal lobes. These areas of the brain are involved in memory and personality. Thus frontotemporal dementia shares many of the features of AD, but has additional symptoms, including strange or sexually disinhibited behavior, lack of empathy, poor personal hygiene, apathy and loss of motivation, increased appetite for sweet or fatty foods, and repetitive and compulsive speech and actions.