NIEHS epidemiologist Honglei Chen, M.D., Ph.D., asked whether society is ready for a potential increase in Parkinson’s disease (PD), in an editorial published online June 20 by JAMA Neurology. He was commenting on a new time-trend study, published the same day, on the incidence of PD. Excerpts from Chen’s editorial were featured in news reports on the study, published in the U.S., Europe, and South America.
The authors of this population-based study, a team of scientists funded by the National Institute on Aging and working with the Mayo Clinic Rochester Epidemiology Project, found a surprising but steady increase in the incidence of parkinsonism and PD among residents in Olmstead County, Minnesota, between 1976 and 2005, particularly among men, whose rate of PD increased approximately 24 percent per decade.
According to Chen, explanations for this trend may not be straightforward. The team, led by Walter Rocca, M.D., speculated that the increase could be tied to the decreased rate of smoking in the past fifty years. Many epidemiological studies have found that smokers have a significantly lower risk of PD, although it is debatable whether cigarette smoking itself decreases PD risk.
Although Chen acknowledged the importance of this study, he expressed caution about the results and study limitations, as did the authors. Chen pointed to the relatively small study in the overwhelmingly white population of Olmstead County, and the need for independent confirmation of study findings in other populations and locations.
At the same time, Chen also warned about potential public health implications of the findings. “If, indeed, PD incidence has been increasing at the rates suggested by the current study,” he wrote, “we would need to revise the estimate [of a doubling by 2030] to an even larger number, presenting an economic challenge to our health care system. Effective planning for this possibility will therefore be needed.”
Many questions and few definitive answers
Like several other neurodegenerative diseases, PD develops over decades and is often clinically diagnosed at an older age, based on the presence of motor dysfunctions, such as rest tremor, slow body movement, and rigid muscles. A conclusive diagnosis requires a brain autopsy.
Increasing attention has been paid in recent years to various nonspecific symptoms, such as constipation and loss of the sense of smell, which may develop before clinical observation of motor dysfunction. Researchers hope to explore novel ways to identify the disease early.
Among the many unanswered questions about PD are the causes of the disease, reliable estimates of prevalence and incidence, whether these rates have changed over time in response to environmental exposures, and whether there will ever be a good animal model for studying the disease.
Epidemiological studies have pointed to an association between a number of environmental factors, including smoking, exposure to pesticides and related toxins, early life infections, and consumption of caffeine. “The debate over whether the association [with smoking] is causal seems never to be resolved,” Chen wrote.
Both the Mayo team and Chen described the study as the first of its kind to examine long-term trends in PD incidence and an important step toward a better understanding of the incidence of the disease. “The current study attests to the importance of understanding fundamental epidemiology of neurodegenerative diseases, and highlights the unique contributions that are possible by well-designed analyses of high-quality longitudinal electronic medical records,” Chen concluded.
Chen H. 2016. Are we ready for a potential increase in Parkinson incidence? JAMA Neurol [Online 20 June 20; doi:10.1001/jamaneurol.2016.1599].
Savica R, Grossardt B, Bower JH, Ahlskog JE, Rocca WA. 2016. Time trends in the incidence of Parkinson Disease. JAMA Neurol [Online 20 June 2016; doi:10.1001/jamaneurol.2016.0947].
(Eddy Ball, Ph.D., is a contract writer with the NIEHS Office of Communications and Public Liaison.)