Severe impairment of memory and thinking, along with confusion and disorientation, may develop with dementia, the worst of the nonmotor Parkinson's symptoms. It can render the patient incapable of taking on daily activities of living. Although research studies vary widely on the numbers, experts generally estimate that 30 percent of Parkinson's patients suffer from dementia.

Usually, dementia arises in later stages of Parkinson's. If major cognitive trouble arises early, another illness is likely to blame, such as Alzheimer's disease or a Parkinson-like disorder called dementia with Lewy bodies (which actually looks similar to very late-stage Parkinson's dementia).

The dementia of Parkinson's disease is somewhat different from that of Alzheimer's, although there are similarities, says Friedman. Alzheimer's patients don't record new memories and wouldn't remember a grocery list of five items. People with Parkinson's dementia would store the memory of the items but may not recall them until you give hints or cues. Patients struggle more with language and forgetting of words and names in Alzheimer's than in Parkinson's. On the other hand, Parkinson's dementia patients are, earlier on, "very bad at judging spatial relationships between objects, which can be a problem with their driving," Friedman says. "They have trouble organizing, they have trouble multitasking."

The Alzheimer's drug Exelon (rivastigmine) was federally approved for treating dementia in Parkinson's in 2006, but its benefits are relatively modest, says Friedman. The same appears true of other Alzheimer's drugs in the same category, called cholinesterase inhibitors.