Results of a 30-year study suggest that Parkinson’s disease (PD) and related disorders are on the rise. Scientists led by Walter Rocca of the Mayo Clinic in Rochester, Minnesota, report in the June 20 JAMA Neurology that the number of new cases of parkinsonism has jumped every 10 years since 1976, particularly in men older than 70. This finding is in stark contrast to a measurable decrease in Alzheimer’s disease in Europe and the United States, which researchers attribute to healthier lifestyles of late (May 2013 news). “To our knowledge, [this] study is the first to examine long-term trends in PD incidence,” wrote Honglei Chen, National Institute of Environmental Health Sciences, North Carolina, in an accompanying editorial. If PD incidence has been growing at the rates they propose, it could present an enormous challenge to health care systems, he added.
Past studies that have looked at changes in PD incidence suggest that new cases have occurred at a relatively stable or even slightly declining rate (see Horsfall et al., 2013; Akushevich et al., 2013; Liu et al., 2016). However, many of these studies span a decade or less. In the new study, first author Rodolfo Savica wanted to use the rich data set available in the Rochester Epidemiology Project to look over a longer time frame. This population-based study, funded by the National Institute on Aging and the Mayo Clinic, tracks the medical records of almost all people who have lived in Olmstead County, Minnesota, since 1966.
Savica and colleagues scanned the database of records collected between 1976 and 2005 for those that contained diagnostic codes related to parkinsonism. Two movement disorders specialists then looked in detail at the 906 identified cases and diagnosed the type of movement disorder and when it developed. A disorder was labeled parkinsonism if a person had two of four cardinal signs: resting tremor, bradykinesia, rigidity, or impaired postural reflexes. Parkinson's disease was diagnosed if these symptoms had no other cause such as stroke or head injury, responded to levodopa, and didn’t appear to affect the wider nervous system.
The researchers found that for each consecutive 10-year block since 1976, the number of new cases of parkinsonism in men rose by 17 percent, while PD jumped by 24 percent. These differences were mainly driven by those over the age of 70, who experienced a 24 percent increase for parkinsonism, and a 35 percent hike for PD. For women, incidence appeared relatively stable over the time period. When the researchers analyzed disease incidence by birth year, men born between 1915 and 1924 were most likely to have the disease.
What could explain the trend? The authors note that if it were due to improvements in diagnosis, the same increases should apply to the women in the study. They suggest instead that the decline in smoking over the past 60 years could be responsible. Previous epidemiological studies have reported that cigarette smoking may prevent PD, but that has been controversial, not least because of the myriad reasons smoking is unhealthy (Ritz et al., 2014; Thacker et al., 2007; Morozova et al., 2008; Mar 2004 news). In the United States, smoking has been on the decline since its peak in the 1950s, especially in men, and could explain the increase in Parkinson’s, the authors wrote.
However, Chen noted that the authors had no data on smoking behavior or other possible risk factors for PD, and hence couldn’t analyze possible reasons. He also cautioned that these results come from a single county of mainly white people, so scientists should look for patterns in other populations. The authors agreed that the results need to be confirmed in other cohorts.—Gwyneth Dickey Zakaib
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