To the Editor In a study published in JAMA Neurology, Savica et al1 aimed to investigate the incidence of Parkinson disease (PD) through a retrospective study related to the 30-year period between 1976 and 2005. Interestingly, they found a statistically significant increment of the incidence of PD in men 70 years or older. However, the increment was not significant in the women’s group. This finding is in line with the previous prediction of an increase of incidence of PD in the United States as a consequence of the reduction of smoking considered a protective risk factor.2 However, Rocca et al3 did not find a particular trend in a previous US PD population investigated over a 15-year period (1976-1990). On the contrary, Horsfall et al4 found a 6% decline in incidence in a UK PD population between 1999 and 2009. However, the authors were cautious in the interpretation of their surprising results because there is the possibility that they represented changes in diagnosis and/or coding rather than a true decline in incidence. Of note, the only epidemiological data consistent over these different studies is the sex difference with the higher representation of male patients. The conflicting results in the same country as well as in a different population clearly show to scientists and clinicians that other potential environmental and genetic factors may be responsible for the incidence of PD and the explanation cannot be restricted just to smoking. Indeed, a number of environmental risk factors have been previously pointed out including alcohol, elevated urate level, pesticides, and head injury.5
Macerollo A, Chen J. Trends in the Incidence of Parkinson Disease. JAMA Neurol. 2016;73(12):1497–1498. doi:10.1001/jamaneurol.2016.3850
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