What it is: About 1 in 5 people with Parkinson's disease develop dementia due to Lewy bodies in the brain and brain stem. The onset of Parkinson's disease itself involves damage to nerve cells that control muscle movement.

How the symptoms compare to Alzheimer's: The symptoms of Parkinson's are usually diagnosed first because the dementia develops in the disease's later stages. Parkinson's dementia does not typically involve problems with language.

How it's diagnosed and treated: When dementia occurs in someone with Parkinson's, a medical history, physical exam, and neurological exam are used to rule out other possible causes. There are no approved medications for treatment of dementia with Parkinson's disease, although symptoms can be managed as part of the overall therapy to manage the effects of the disease.