When you are diagnosed with Parkinson’s disease, the question “why me?” follows naturally. What research has shown so far is that in most cases it isn’t down to a single cause, but to several factors overlapping.
In most cases there is no clear cause
In 85% to 90% of people with Parkinson’s, the condition is idiopathic — meaning no clear cause can be identified. It can develop with no one else in the family affected, and it is not a condition you bring on by doing something wrong.
Genetics accounts for about 10% to 15%
The remaining 10% to 15% is known to involve specific genes. Variants in genes such as LRRK2The leucine-rich repeat kinase 2 gene. Variants in it are linked to Parkinson's that starts at a relatively later age.Learn more, GBAVariants in this gene raise the risk of Parkinson's and tend to come with cognitive decline that progresses relatively faster.Learn more and SNCAThe gene that makes the alpha-synuclein protein. Variants in it are linked to Parkinson's that starts at a relatively younger age.Learn more are being studied as factors that raise the risk of Parkinson’s. Carrying one of these variants does not mean you will definitely develop Parkinson’s, and genetic factors weigh more heavily when there is a family history or when the diagnosis comes at a relatively young age. If family history is on your mind, our article on whether Parkinson’s is hereditary covers it in more detail.
Age and environmental factors also play a part
Age is the biggest risk factor, because dopamine nerve cells naturally decline as you get older. Among environmental factors, pesticides have been studied the most — in particular, one study found that long exposure to a herbicide called paraquatA herbicide used in agriculture. Studies have reported that long-term exposure raises the risk of Parkinson's.Learn more raised the risk of Parkinson’s by up to 150%. Industrial solvents such as trichloroethylene (TCE)An industrial solvent used in dry cleaning and metal degreasing. It is one of the most common groundwater contaminants, and studies link long-term exposure to a higher risk of Parkinson's.Learn more, used in dry cleaning and metal degreasing, have also been reported to raise the risk with long-term exposure through groundwater, and air pollution is another environmental factor now under study. There is also research suggesting that Parkinson’s is more likely to appear in people who are genetically at risk when they are exposed to these environmental factors — in other words, genes and environment don’t act separately, but influence each other.
Some studies find it is less common in smokers and coffee drinkers
Several observational studies have found a lower risk of Parkinson’s among smokers and among people who regularly consume caffeine. This is not a result you can read as “smoking or caffeine prevents Parkinson’s” — smoking clearly raises other serious risks such as cardiovascular disease and cancer, so researchers think the correlation is driven not by nicotine or caffeine themselves but by some other component, or by a cause not yet identified, and they are still looking into it.
The hypothesis that the gut, not just the brain, is involved
The Braak hypothesisA hypothesis that in some people alpha-synuclein starts clumping in the gut or the olfactory nerve before it does in the brain, then slowly travels up to the brain along the vagus nerve.Learn more, which has drawn attention relatively recently, proposes that in some people the alpha-synuclein protein starts clumping in the nerves of the gut or in the olfactory nerve before it does in the brain, and then slowly travels up to the brain along the vagus nerveThe nerve that connects the brain to internal organs including the gut. In the Braak hypothesis it is identified as the route alpha-synuclein takes from the gut to the brain.Learn more. If the hypothesis holds, it could help explain why non-motor symptoms such as constipation and a reduced sense of smell so often show up before motor symptoms like hand tremor. Research continues to find that the makeup of gut bacteria differs in people with Parkinson’s, but this is not a settled conclusion yet.
What is certain, and what is still unknown
The process by which dopamine nerve cells are lost in the brain is known to involve a protein called alpha-synucleinA protein normally present in the brain. In Parkinson's, it is known to clump abnormally and build up inside nerve cells.Learn more clumping abnormally. Why this protein starts clumping, and why in some people and not others, is still not fully understood. Research to uncover the cause continues around the world today.
This article is not a substitute for medical diagnosis or treatment. If you have questions about genetic or environmental factors for you or your family, please talk with your care team.
