Parkinson’s disease (PD) is associated with genetic and environmental risk factors. Among these factors, the most common nongenetic risk factor for PD is traumatic brain injury (TBI).
TBI is damage to the brain caused by head injuries such as:
TBI can occur without significant signs of external injury. Symptoms of TBI — such as loss of consciousness, confusion, blurred vision, difficulties with memory or concentration — can begin immediately after injury or days or weeks later. TBI is classified as mild, moderate, or severe based on symptoms caused by the injury. The more severe the injury is, the more severe the symptoms will be.
Numerous studies have found a significantly increased risk of PD after TBI.
One study that examined PD and TBI in twins — that is, people who would have the same genetic risk factors — found that mild TBI (concussion) increased the risk of PD, even if the injury occurred decades before PD developed. The researchers found that people with TBI developed PD earlier, and the risk of PD was higher in people with more than one head injury.
Another group of researchers looking at the connection between TBI and PD published a meta-analysis (review) of 22 different research studies including nearly 100,000 participants. The researchers found that people with a history of TBI were more than 1.5 times as likely to develop PD than people with no history of TBI.
A more recent study examined the risk of PD in people who experienced head injuries later in life, at age 55 years or older. Five to seven years after the head injury, older adults with TBI were almost 1.5 times more likely to develop PD than their peers without TBI. This study also found that moderate to severe TBI and repeated TBI were associated with a greater increase in PD risk.
Finally, a study of PD in military veterans aged 18 and older revealed that TBI significantly increased the risk of developing PD by 71 percent. Using data from more than 300,000 people, the study’s researchers found that mild TBI increased the risk of PD by 56 percent, and moderate to severe TBI increased the risk of PD by 83 percent. The researchers also saw a higher rate of comorbidities (other diseases occurring along with PD) in people with TBI compared to those without TBI.
TBI is a risk factor for several neurodegenerative diseases, including PD, Alzheimer’s disease, amyotrophic lateral sclerosis, and chronic traumatic encephalopathy. The link between TBI and the risk of developing PD is clear: TBI affects the brain in some of the same ways as PD and other neurodegenerative diseases, causing the same types of damage to brain cells.
TBI causes metabolic disturbances, inflammation, and protein aggregation (buildup) in the brain cells, effects that can all cause immediate or long-term damage to the brain. All of these effects caused by TBI are also part of the disease processes seen in people with PD and certain other neurodegenerative diseases.
TBI can cause damage to the blood-brain barrier (BBB), the layer of cells in blood vessels in the brain that controls what substances can cross from the bloodstream into brain tissue. Damage to the BBB results in an inflammatory immune response and also disrupts normal metabolism in brain cells, causing long-term damage and cell death.
The immune system typically uses inflammation to help control and repair damage to tissue, but TBI can cause chronic inflammation in the brain. Chronic inflammation can lead to unintended consequences such as cell death and protein aggregation, the buildup of excess proteins in cells that can lead to cell dysfunction and cell death. TBI can cause aggregation of alpha-synuclein, amyloid-beta, tau proteins, and other proteins that are linked to neurodegenerative diseases including PD and Alzheimer’s disease.
Research has uncovered much about how PD occurs, but not why PD occurs in some people and not others. Not everyone who experiences a TBI will develop PD, and not everyone with PD has a history of TBI. It is most likely that a combination of inherited factors and acquired factors, such as TBI, are needed for PD to develop. It is believed that TBI may contribute to PD risk by accelerating the damage that leads to PD. TBI may cause some people to reach a tipping point where they have accumulated enough damage to lead to PD.
PD is a complex condition, but research is constantly expanding our knowledge of how and why it occurs. Understanding more about how TBI and related conditions cause damage to brain cells may lead to better treatments for PD and other neurodegenerative diseases. Additionally, knowing that TBI can contribute to the risk of developing PD can help doctors identify people who are at higher risk, potentially allowing for earlier diagnosis and intervention.
For people with Parkinson’s and other chronic diseases, understanding more about your condition can help you cope with it. Knowledge may not always give you more control over your disease, but it can be empowering to understand what is happening inside your body.
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