A new study supports the hypothesis that Head injury increases the risk of Parkinson’s disease (PD). In the study, which included twin pairs in which one twin had PD and the other did not, head injuries of mild-to-moderate severity were associated with a threefold increased risk of PD developing several decades later.

“Because PD patients were matched in the study with their unaffected twin, the study results are particularly robust, and strongly suggest that the association is truly causal and not just coincidental,” Dr. Samuel M. Goldman from The Parkinson’s Institute in Sunnyvale, California noted in comments to Reuters Health.

Head injury is an “inconsistently” reported risk factor for PD, note Goldman and colleagues in the latest issue of Annals of Neurology. “Because twins share many environmental and genetic characteristics, risk factors identified in investigations of twins discordant for PD may be less likely to be spurious,” they further point out.

In the 93 twin pairs studied, a prior Head injury with amnesia or loss of consciousness was associated with a 3.8-increased risk for PD. Similar to other studies, there was a long latency between Head injury and PD. Head injuries occurred, on average, more than 30 years before the development of PD, Goldman’s team reports.

“The clinical significance of this observation is that if an insult to the brain takes 30 years to cause clinical disease, this means there is a lengthy period during which time we might be able to intervene and stop or slow the degenerative process,” Goldman said.

In the study, having two prior head injuries was associated with greater risk for PD than having one Head injury.

It’s also notable, according to the team, that the association between Head injury and PD was somewhat stronger in genetically identical or “monozygotic” twins than in genetically similar or “dizygotic” pairs.

In a subanalysis of 18 twin pairs in which both twins had PD, the twin with “younger onset PD” was more likely to have suffered a Head injury. This suggests to investigators that Head injury might hasten the development of PD in susceptible individuals.

Goldman told Reuters Health “there are many biological reasons” through which Head injury may lead to a neurodegenerative process such as PD. “Head injury might cause long-term inflammation in the brain,” he explained. If so, “then the possibility to intervene would likely apply to many types of early life environmental insults in addition to Head injury,” Goldman said.

SOURCE: Annals of Neurology July 2006.