ROCHESTER, Minn. -- Mayo Clinic researchers have found that those who have experienced a head injury are four times more likely to develop Parkinson's disease than those who have never suffered a head injury. The risk of developing Parkinson's increases eightfold for patients who have had head trauma requiring hospitalization, and it increases 11-fold for patients who have experienced severe head injury.
The study is published in the Tuesday, May 20, issue of Neurology, http://www.neurology.org/.
Says James Bower, M.D., Mayo Clinic neurologist and the study's lead author, "The risk is elevated for people with more severe head injury -- longer loss of consciousness and brain bruising visible in a CT scan. We did not find mild head injury -- head injury with no or only a brief loss of consciousness -- to be associated with Parkinson's disease."
The investigators point out that caution is appropriate in interpreting one's risk, however. "By no means does it mean that if you have a severe head injury that you will definitely develop Parkinson's," says Dr. Bower.
A few of the study's findings were unexpected by the investigators. "I was surprised by the strength of the association," says Dr. Bower. "I also was surprised that the average head trauma was about 20 years before the start of the disease."
The exact link between head trauma and Parkinson's remains elusive, however. The investigators offer some possibilities, pointing out that while these theories are plausible, they are purely hypothetical.
"We're learning a lot about the molecular basis of Parkinson's," says Demetrius Maraganore, M.D., Mayo Clinic neurologist and one of the study's authors. "It's hard for me to know, however, how a blunt injury to the head would cause later Parkinson's disease."
The investigators offer three hypotheses -- ideas not yet tested in animal models -- about how head injury might lead to Parkinson's:
- A head injury kills brain cells at the time it occurs, and then normal aging causes loss of further brain cells to reach a certain threshold.
- The blood-brain barrier is disrupted, and certain proteins or poisons in the bloodstream enter the brain, triggering a slow process that takes 20 years to produce symptoms.
- Head trauma causes brain cells to make certain proteins. These proteins, which normally would not have developed, start a cascade leading to cell death.
Dr. Bower points out that Parkinson's disease is probably not caused by only one factor. Head trauma, then, may be one of various factors that may lead to Parkinson's.
The Mayo Clinic investigators initiated this study to help resolve conflicting findings in previous studies of the link between Parkinson's and head injury. They also had reason to suspect that Parkinson's and head injury might be related due to reports of a sports-associated disease.
"There is a disease seen in boxers called dementia pugilistica that looks a lot like Parkinson's disease," says Dr. Bower. "Although our patients did not have dementia pugilistica, it gave us a hint that there may be a link between trauma and parkinsonian symptoms."
Even though the association between head injury and Parkinson's is strong, the investigators caution that no direct causal link can be made between head injury and Parkinson's. Therefore, advice about what people can do with this new information is limited.
"It shows the risk is increased; it doesn't show causality," says Dr. Maraganore. "You can worry, but we don't encourage that. It would be enough of an increase in risk that I'd think, 'What can I do to reduce that person's risk?'"
As a precaution, Dr. Bower suggests using head protection while bike riding or playing other sports in which a person has an increased likelihood of head injury.
"Certainly, appropriate head gear is important in reducing the risk of serious head injury," says Dr. Bower. "Unfortunately, there is no other proven way to reduce the risk of getting Parkinson's disease once a head injury has occurred."
The study involved reviewing medical records of 196 patients participating in the Rochester Epidemiology Project, http://www.mayo.edu/research/mir/topic_1029.html, to look for preceding incidence of head trauma. Parkinson's cases were age-matched and gender-matched to someone who did not have Parkinson's disease.
The investigators point out that this study is the first population-based and medical records-based study of head trauma and Parkinson's.
"Previous studies showed conflicting results," says Dr. Bower. "Some suggested there was a link between Parkinson's and head trauma, and some suggested there wasn't a link. The problem with the previous studies is that they were based on patient memory. It is very common for those who have a disease to try to remember something that could explain the disease. Patients who do not have the disease don't go through that mental effort."
Dr. Maraganore concurs. "In our study, we relied on head trauma being recorded in the medical record before diagnosis of Parkinson's. That's why our study is stronger. It's less likely to suffer from recall bias," he says. "It's a confirmatory study, but confirmatory in a unique setting that renders this study more plausible."
The normal lifetime risk of Parkinson's for men and women combined is 1.7 percent. About 1.5 to 2 percent of the U.S. population has a history of head injury.
Lisa Copeland
507-538-0844 (days)
507-284-2511 (evenings)
e-mail: newsbureau@mayo.edu
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