Parkinson's disease is a chronic, degenerative neurological disorder that affects one in 100 people over age 60. While the average age at onset is 60, people have been diagnosed as young as 18. There is no objective test, or biomarker, for Parkinson's disease, so the rate of misdiagnosis can be relatively high, especially when the diagnosis is made by a non-specialist. Estimates of the number of people living with the disease therefore vary, but recent research indicates that at least one million people in the United States, and more than five million worldwide, have Parkinson's disease.
Parkinson's disease was first characterized extensively by an English doctor, James Parkinson, in 1817. Today, we understand Parkinson's disease to be a disorder of the central nervous system that results from the loss of cells in various parts of the brain, including a region called the substantia nigra. The substantia nigra cells produce dopamine, a chemical messenger responsible for transmitting signals within the brain that allow for coordination of movement. Loss of dopamine causes neurons to fire without normal control, leaving patients less able to direct or control their movement. Parkinson's disease is one of several diseases categorized by clinicians as movement disorders.
The exact cause of Parkinson's disease is unknown, although research points to a combination of genetic and environmental factors. If a continuum existed, with exclusively genetic causes at one end and exclusively environmental causes at the other, different Parkinson's patients would likely fall at many different places along that continuum.
In the past 10 years, researchers have identified a number of rare instances where Parkinson's disease appears to be caused by a single genetic mutation. In these cases, the mutated gene is passed from generation to generation, resulting in a great number of Parkinson's disease cases within an extended family. Mutations in the LRRK2 gene are the greatest genetic contributor to Parkinson's disease disovered to date.
On the opposite end of the continuum, in the early 1980s, a group of heroin users in California took drugs from a batch contaminated with a substance called MPTP. After ingesting this chemical, the drug users were stricken with a form of Parkinson's disease that was primarily, if not exclusively, "environmental" in origin.
For most Parkinson's patients, the cause lies somewhere in the middle. While many Parkinson's patients report one or more family members with the disease, it is not always clear that one or several genes are the cause. Similarly, while some patients suspect that exposure to one or another chemical or environmental toxin caused their Parkinson's disease, this also cannot be conclusively proved. Scientists currently believe that in the majority of cases, genetic and environmental factors interact to cause Parkinson's disease. Research into this subject continues aggressively every day. Unfortunately, however, it is generally impossible to determine what specifically caused an individual's Parkinson's disease.
Because the causes of Parkinson's disease are unknown, there is no scientifically validated preventive course to reduce the risk of its onset. The single biggest risk factor for Parkinson's disease is advancing age. Men have a somewhat higher risk than women.
That being said, a number of studies have highlighted factors that are associated with either greater or lesser risk of Parkinson's disease. For example, smoking and caffeine consumption have been associated with lower rates of Parkinson's disease, while head injury and pesticide exposure have been associated with higher risk. While such studies do not definitively link these factors with Parkinson's disease one way or another, they highlight areas where further research may guide us to risk-prevention or treatment strategies.