How long has the PD been present?

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Prior to the first evidence, or symptoms of Parkinson's disease, how long has the disease likely been present? Is there a period of dopamine loss or disease process going on before symptoms occur? (RH)
A:
What we do know is that approximately 80% of the dopamine-producing cells in the substantia nigra need to have died off before the first motor signs of Parkinson’s (tremors, slowness of movement, difficulty initiating movement, balance instability, etc.) appear. This has long suggested that the processes of PD may begin years before the symptoms become apparent so that a diagnosis CAN be made. Newer research is now suggesting that other areas of the brain may be affected as part of PD long before the substantia nigra and these changes are actually producing non-motor symptoms associated with PD such as depression, a loss of the sense of taste and/or smell, chronic constipation, etc. Keep in mind that these ideas are estimates based on research techniques with limited power. It is possible that the disease is present for only a few months before symptom onset. At this time, we have no definitive answers with regard to when the processes of Parkinson’s disease begin in the brain.