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 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. 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. An estimated 30% to 60% of people with Parkinson's 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, or DLB. DLB is now believed to be the second or third most common type of dementia after Alzheimer's disease, accounting for about 10% to 20% of all dementias. (There is controversy about whether DLB or vascular dementia is the second most common form of dementia.)
The relationship between DLB 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's disease. When cognitive impairment and behavioral disturbances are prominent early symptoms, DLB is believed to be the diagnosis.
DLB is a disease of aging. People affected by DLB are usually elderly or in late middle age.