Dementia is a progressive (gradually worsening) decline of mental abilities that disturbs "cognitive" functions such as memory, thought processes, and speech as well as behavior, and movements. Dementia with Lewy bodies (DLB) is the name for a group of disorders in which dementia is caused by the presence of Lewy bodies in the brain. Lewy bodies are small round clumps of normal proteins that for unknown reasons become abnormally clumped together inside neurons (brain cells).Whether the Lewy bodies directly cause gradual degeneration (damage) to the brain cells, impairing their function and eventually killing them, or are only a marker of some other destructive process is not known. Lewy bodies are named after Frederich Lewy, the doctor who first described them in 1912. Dr. Lewy first found Lewy bodies in the brains of people with Parkinson's disease. Parkinson's disease is a condition best known for disrupting body movements. The most common of these "motor" symptoms are tremor (shaking or trembling) of the hands (that mainly occurs when the hands are at rest and not moving), rigidity (stiffness) of the trunk and limbs, slowness of movement, and loss of balance and coordination. Estimates vary from 30-60% about what percentage of people with Parkinson disease develop dementia. Scientists later discovered cases of Alzheimer's-type dementia linked to Lewy bodies. This was thought to be very rare, but as tests of brain tissue improved, it became clear that Lewy bodies were fairly common and were linked to several different types of dementia. A type of dementia similar to but different from Alzheimer's disease was recognized and called dementia with Lewy bodies. Dementia with Lewy bodies is now believed to be the second or third most common type of dementia after Alzheimer's disease. (There is controversy about whether dementia with Lewy bodies or vascular dementia is the second most common form of dementia.) The relationship between dementia with Lewy bodies and Parkinson's disease is not completely understood. When motor symptoms appear first and predominate over cognitive symptoms, the diagnosis is believed to be Parkinson disease. When cognitive impairment and behavioral disturbances are prominent early symptoms, dementia with Lewy bodies is believed to be the diagnosis. Dementia with Lewy bodies is a disease of aging. People affected by dementia with Lewy bodies are usually elderly or in late middle age.