Dementia is a disorder of the brain with deterioration of intellectual or thinking abilities severe enough to interfere with social and/or occupational functioning. Alzheimer's disease (AD) accounts for more than half of all cases of dementia. Other causes include vascular dementia, Lewy body dementia, frontotemporal dementia, and dementias associated with Parkinson's disease and Huntington's disease.

After AD, vascular dementia is the second most common cause of dementia. Vascular dementia is caused by decreased or interrupted blood flow to parts of the brain. Risk factors that can be controlled are the same as those for heart and blood vessel disease in other parts of the body. These include high blood pressure, smoking, high blood fats or lipids including cholesterol, elevated homocysteine (an amino acid), little exercise, and being overweight.

The onset of symptoms in vascular dementia is usually sudden, differing from the gradual onset of symptoms in AD. Reduced ability to think, remember and process information is usually sudden and often follows an event where blood flow is diminished or blocked. The most common cause of poor blood flow is a stroke where muscles or other functions such as speech or vision controlled by a part of the brain are affected. Sometimes parallel (or collateral) circulation restores blood flow before brain tissue dies, thereby restoring brain functions. Often the progression of vascular dementia occurs in steps as the brain experiences further blockage of blood flow over time. This pattern is described as multi-infarct dementia, and that term is sometimes substituted for the more modern term of vascular dementia.

Vascular dementia, like Alzheimer's disease, is more common with advancing age. Vascular dementia can result from strokes and a stroke can unmask underlying AD. It is not surprising that some people have dementia caused by both diseases. While AD is commonly progressive, treatment of risk factors for vascular dementia can reduce the risk of further injury to the brain for those with vascular dementia. There is evidence that vascular dementia responds to AD medication treatments.

Lewy body dementia is caused by progressive destruction of brain cells by protein deposits called Lewy bodies. Visual hallucinations (seeing things that aren't present) are a hallmark symptom of Lewy body dementia, but the person usually also has problems with concentration and other symptoms of dementia. About one-third of individuals with Parkinson's disease (tremors, stiffness of limbs and facial muscles that cause difficulty with walking and speech) develop dementia. Lewy bodies are often found in the brain regions associated with Parkinson's disease. Lewy bodies are also present in some people with AD. Lewy body dementia is sometimes treated with cholinesterase inhibitors.

Frontotemporal dementia (FTD) used to be called Pick's disease. This dementia involves primarily the frontal and temporal lobes of the brain. FTD is very uncommon and usually begins with substantial personality and behavioral changes that precede problems with memory and language. There are no treatments in common use.

There are other causes of dementia, but those discussed above are the most common after AD.

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