http://www.pdf.org/en/parkinson_research_highlights_10

In a new analysis of current Parkinson’s research, Robert E. Burke, M.D., and his colleagues at Columbia University, with funding from a PDF center grant, found that only about 30 percent of a person’s dopamine neurons have died by the time that individual is diagnosed with the disease. These new findings contrast with a widely-cited number from previous studies that 50 percent or more of these cells have been lost at time of diagnosis.