Article by Rachel Dolhun, MD, June 17, 2016, michaeljfox.org
Head Trauma and Parkinson's Disease
The possible connection between head trauma and Parkinson's is illustrated — perhaps nowhere more prominently — by Muhammad Ali's diagnosis of young-onset Parkinson's disease (PD) following a career in boxing. Many have wondered whether repeated hits to the head caused his PD. While it's true that environmental factors — including head injury — have been associated with an increased risk of Parkinson's, few (if any) have been determined to be definitive causes of the disease. Environmental factors and genetics may interact to cause disease, and this complex interplay makes it virtually impossible to point to the exact cause(s) in any individual.
Genetics and Environmental Factors May Contribute to Parkinson's
A number of genetic mutations are associated with Parkinson's disease — some simply increase risk for the disease and others appear to be more direct causes. Many environmental factors (e.g., head trauma, pesticides and solvents) are also linked to an elevated risk of Parkinson's. Generally though, these environmental risks are designated as "associations" rather than "causes" (i.e., for some reason they contribute to an increased risk but they are not sufficient to cause PD in and of themselves).
When faced with these types of associations, researchers look to see if there are other variables or reasons — such as general trauma (to the body rather than specifically to the head or brain) — that are a truer or more important basis for the connection. They also aim to exclude "reverse causation" (i.e., undiagnosed Parkinson's disease leading to falls that cause head trauma — rather than vice versa). Genetics may play a more significant role in some cases, particularly in people who have a younger age of disease onset and/or a family history of PD. However, since family members share genes as well as environmental exposures, the latter cannot be excluded as a strong contributing factor.
To summarize, head trauma and traumatic brain injury are associated with an increased risk of Parkinson's but the mechanisms for how they are connected are unclear and direct causation (head or brain injury actually causing Parkinson’s) has not been established. More work is necessary to fully understand this relationship.
Full original article can be found here: https://www.michaeljfox.org/foundation/news-detail.php?ask-the-md-head-trauma-and-parkinsons-disease&et_cid=596288&et_rid=61543063&et_lid=Headerem_cid