The occurrence of dementia in patients with Parkinson's disease has been under discussion for a long time. Studies have reported incidences ranging from 0 to almost 100%. It is now, however, generally accepted that 15-20% of patients show signs of cognitive deterioration. There are two major draw-backs present in many of the studies. Firstly, patients with Parkinson's disease are often compared to patients with other neurodegenerative disorders, who have other major neurological symptoms. Secondly, tests for particular neurological deficits, which have been defined in relation to a specific neurological disease, are often used as though they had general validity. One has, therefore, to be very careful in interpreting isolated test results.
That aspects of any test used for Parkinson's disease are also hampered by motor disturbances goes without saying. The literature concerning dementia in Parkinson's disease remains, therefore, difficult to interpret.