Parkinson’s Incidence On the Rise, but Source is Unclear

A new study published in JAMA Neurology suggests that the incidence of parkinsonism and Parkinson’s disease may have increased from 1976 to 2005 in a cohort of patients. The authors hypothesize that this trend might be associated with smoking behavior changes that have occurred in the latter half of the 20^th century.¹

Previous studies have linked tobacco use to a lower risk of Parkinson’s disease. With that, there is speculation that the incidence of Parkinson’s disease would increase along with the decrease in tobacco use since the 1950s.

To test this hypothesis, Rodolfo Savica, MD, MSc, PhD, of the Department of Health Sciences Research at the Mayo Clinic in Rochester, Minn., and colleagues analyzed data from the Rochester Epidemiology Project to estimate the incidence of Parkinson’s disease and parkinsonism over a 30-year period. Potential cases were identified in the database by a computerized screening tool and then confirmed with a review of the medical record.

Participants were classified as having parkinsonism if they demonstrated 2 of 4 signs (impaired postural reflexes, rigidity, bradykinesia, and resting tremor). Participants with parkinsonism and no prominent or early evidence of a more extensive neurological disease, no other known cause, and no history of unresponsiveness to levodopa/carbidopa were classified as having Parkinson’s disease.

The investigators identified 906 participants (501 men) with incident parkinsonism of all types and 464 participants with Parkinson’s disease (275 men) with a median age of onset of 73 years.

During the 30-year study time frame, the age-adjusted incidence of parkinsonism was stable for women. Men, however, were observed to have an increase in incidence from 38.8 cases per 100,000 person-years (1976-1985) to 55.9 cases (1996 -2005). This increase in incidence for men was found to be significant by negative binomial regression (RR 1.17 per decade, 95% CI: 1.03-1.33, P=.01). Further, the incidence of Parkinson’s disease increased in men from 18.2 per 100,000 person-years (1976-1985) to 30.5 (1996-2005), which was significant with negative binomial regression (RR 1.24 per decade, 95% CI: 1.08-1.43, P=.003). The increased incidence of parkinsonism and Parkinson’s disease was greater in men over 70 years of age (RR 1.24 and RR 1.35, respectively).The researchers observed a trend towards increased risk in both men and women born between 1915-1924, however the risk was only significant in men with Parkinson’s disease.

“These results, while interesting, are not very robust,” Serg Przeborski, MD, PhD, of the Department of Neurology at Columbia University, told Neurology Advisor, noting that the authors encourage the results to be “interpreted with caution,” as they may be due in part to increased symptom awareness, improved diagnosis, and changes in medical coding.¹

In an accompanying editorial, Honglei Chen, MD, PhD, of the National Institutes of Environmental Health Sciences, wrote that the results could have important public health consequences if replicated. Dr Chen goes on to suggest that the results may provide indirect evidence for causality for the relationship between smoking cessation and Parkinson’s disease risk.²

“Where I disagree with the authors is where they present that as a support of causality,” Dr Przeborski said. “Epidemiology does not report on causality, but on association. Such results are inspiring but must be experimentally validated and, if not possible, at least be independently confirmed in distinct cohorts.”

The study was supported by a grant from the National Institute on Aging and by the Mayo Foundation for Medical Education and Research.

References

1. Savica R, Grossardt BR, Bower JH. Time Trends in the Incidence of Parkinson Disease. JAMA Neurol. 2016; doi:10.1001/jamaneurol.2016.0947.
2. Chen H. Are We Ready for a Potential Increase in Parkinson Incidence? JAMA Neurol. 2016; doi: 10.1001/jamaneurol.2016.1599.