About 25% of those with Parkinson's disease will develope dementia in the later stages of the illness. Typically the sufferer will become confused, have hallucinations and show other mental and emotional disorders. One common sign is the disruption of normal sleeping patterns where they sleep all day and are awake all night

Research shows that in some people there is a link with the use of Sinemet, where the drug is thought to either bring on the dementia or worsen the condition http://www.pdf.org/Ask/kb.cfm?category=66

Dementia with Parkinson's disease can be of two distinct types, with 3/4 of people showing Alzheimer's type symptoms & the remainder displaying Lewy Bodies associations

Diagnosis is by referal to a Dementia Specialist who will carry out tests for blood, neuroimaging and MMSE (Mini-Mental Status Examination), before determining treatment in conjunction with that required for Parkinson's

There is no recommended course of treatment for the dementia aspect, though recent trials suggest that drugs such as tacrine HCl, (Cognex®) or donepezil (Aricept®), approved for use in the Alzheimer population, may help to treat dementia associated with Parkinson's disease

More information is available here:

http://www.parkinson.org/med31.htm

http://www.parkinson.org/pddement.htm