Read on to find out more about genetic factors and environmental exposures that might make Parkinson's disease (PD) more likely to appear in some people.
Some of these are unavoidable, but for others, early lifestyle choices and caution may help reduce the risk.
Age and sex
Older age and being male increase the risk of PD.
Two unavoidable factors that affect the risk of having PD are increasing age and whether a person is male or female.
Age: In most people who have PD, symptoms become noticeable at the age of 60 years or over.
However, in 5–10 percent of cases they appear earlier. When PD develops before the age of 50 years, this is called "early onset" PD.
Sex: Men appear to have a 50-percent higher chance of developing PD than women.
However, at least one study has found that, as women get older, their chance of developing it increases.
Researchers have suggested that this could be due to a variety of factors, including:
- lifestyle exposures
- genetic features
- hormonal and reproductive factors
- differences in the brain structures that relate to the production of dopamine
Genetic factors and family history
A person who has a close relative — such as a sibling or parent — with PD has a slightly higher risk of developing it, compared with others.
According to the Parkinson's Foundation, around 10 to 15 percent of cases are probably due to hereditary genetic factors.
The others are "sporadic." There is currently no way to predict that they will occur.
Autosomal dominant: In 1 to 2 percent of people with PD, the condition results from a change in just one copy of a specific gene. Genes that it can affect include Alpha-synuclein (SNCA) and leucine-rich repeat kinase 2 (LRRK2).
According to the Parkinson's Foundation, men of North-African Arab origin have a higher chance of having this trait.
Autosomal recessive: If changes occur in two copies of a certain gene, PD can occur. These changes may involve the genes known as PARK7, PINK1, and PRKN.
Risk-factor modifier genes: These genes affect the risk of developing PD, but they do not cause symptoms. The gene known as GBA is one of these. This gene makes the enzyme glucocerebrosidase.
Not everyone with a gene mutation will develop PD. Some people with a family history of PD choose to undergo genetic testing to have some idea of how likely they are to have this problem.
This can be useful to know, but it is not always a good idea. Some people have a genetic factor but never develop PD. This can lead to unnecessary anxiety.
Footballers must wear helmets for protection against a head injury. Traumatic brain injury (TBI) appears to be a risk factor for PD.
People who receive a blow to the head on at least one occasion may have a higher chance of developing PD.
A study published in 2018 found that, among military veterans, even a mild traumatic brain injury (TBI) can increase the risk of PD by 56 percent, even after taking psychological and other factors into consideration.
This link has raised concerns among sporting associations, as concussion is a common injury in football and many other activities.
To prevent this, people who participate in sports where a head injury is likely should wear protective headgear and seek medical help if they do experience a blow to the head.
Everyone should use a safety belt or other age-appropriate restraint when traveling in a motor vehicle.
Exposure to chemicals
People who work in certain professions may have a higher chance of developing PD because of exposure to certain chemicals.
There is some evidence that exposure to certain toxins can increase the chance of developing PD, according to the Parkinson's Foundation
Herbicides and pesticides
These may include:
- some herbicides, such as paraquat
- fungicides, such as maneb
- insecticides, such as the colorless, odorless rotenone
Chemicals that were present in the defoliant Agent Orange, used in the Vietnam War, have been linked to PD, although scientists have not definitively proven the link.
Studies have suggested that there is a higher rate of PD among people who work as farmers, those who are exposed to well water, and those who live in the countryside.
Although more studies are necessary to confirm any precise links, researchers believe that exposure to toxins, such as pesticides, increases the risk.
Researchers have noted a probable link between long-term exposure to certain metals and a higher risk of PD.
Exposure to chemicals, including solvents and metals, may increase the risk.
The metals that might do this are:
People who work in an environment where exposure is common may have a higher risk of PD.
However, the risk is difficult to measure, and there is no evidence to confirm an exact that any of these metals specifically pose this type of hazard.
Trichloroethylene (TCE) is a solvent that many industries use.
It is also commonly present in groundwater. People who experience long-term exposure to TCE may be more likely to develop PD later in life.
However, several studies, including a large cohort study in the United States, have not confirmed a link between PD and solvent exposure.
Polychlorinated biphenyls (PCBs)
Some scientists have found that people with PD have higher levels of PCBs in their bodies than those without the disease. This suggests that PCB exposure may increase the risk.
However, other studies have not found a link.
Medications and other drugs
Some medications — such as antipsychotics for treating severe paranoia and schizophrenia — can also cause Parkinsonism, or Parkinson-like symptoms.
There is evidence that the use of a synthetic heroin product, MPTP, can lead to Parkinson-like symptoms.
In April 2018, scientists published notes on case studies of seven young adults who had used the drug. The individuals showed symptoms of Parkinsonism after short-term use of the drug.
The researchers do not conclude that the people developed PD, but the symptoms suggest that they have undergone similar processes.
Impact of smoking
Interestingly, people who smoke cigarettes appear to have a smaller chance of developing PD than those who do not.
This does not mean that people should smoke in order to stave off PD, because smoking is responsible for a host of unwanted health problems.
However, researchers are interested in the role nicotine could play in a future treatment for the disease.
The Parkinson's Foundation note that people who do the following may have a lower risk of PD:
- drinking coffee or tea that contain caffeine
- having high levels of uric acid in the blood, although this can lead to gout
- using anti-inflammatory drugs
- using statins to reduce cholesterol levels
- having a higher level of vitamin D
It is important to note that research has not yet confirmed that these are beneficial.
Some of them may bring hazards of their own.
One point that may be worth noting, however, is that people who participate in physical activity early in life appear to have a lower risk of developing PD in their later years, according to the Parkinson's Foundation.
A review published in 2018 concluded that exercise can play a preventive role and it can help to treat symptoms and slow the progression of PD, especially in the early stages.
Anyone with a diagnosis of PD should speak to their health provider about a suitable exercise plan.
As with many conditions that can have both genetic and environmental causes, it may be neither one nor the other that produces symptoms.
In most cases, a person probably has a genetic tendency, or predisposition, to develop particular PD, but it takes an environmental trigger to cause it.
However, exactly which triggers activate the disease remain unclear.