The incidence of parkinsonism and Parkinson’s disease in a Minnesota county may have increased over a 30-year period, primarily in men age 70 or older, according to a study published online ahead of print June 20 in JAMA Neurology. The increased incidence may be due to changes in smoking behavior during that time or other factors, the researchers said. The trend needs to be confirmed in other populations, they added.
“The decline in smoking rates in men may explain in part the increasing incidence of parkinsonism and Parkinson’s disease. However, other environmental or lifestyle risk or protective factors that are related to sex may also be involved such as pesticide use, head trauma, and coffee consumption,” Walter A. Rocca, MD, MPH, of the Mayo Clinic in Rochester, Minnesota, and coauthors said.
Walter A. Rocca, MD, MPH
Previous studies have found that smoking is associated with reduced risk of Parkinson’s disease, but whether the relationship is causal remains uncertain. Morozova et al suggested that smokers have a 74% reduction in risk of Parkinson’s disease, possibly attributable to nicotine or other tobacco elements. Researchers have speculated that a decline in smoking frequency after its peak in the 1940s and 1950s may have caused an increase in Parkinson’s disease.
To study this question, Dr. Rocca and coauthors investigated time trends and birth cohort trends for the incidence of parkinsonism and Parkinson’s disease in Olmsted County, Minnesota, from 1976 to 2005.
Parkinson’s Disease and Parkinsonism Definitions
The researchers used medical records from the Rochester Epidemiology Project to identify the frequency of Parkinson’s disease and other types of parkinsonism in Olmsted County during the 30-year period. A movement disorder specialist classified all the medical records based on diagnostic criteria. The researchers defined parkinsonism as the presence of at least two of four cardinal signs (ie, rest tremor, bradykinesia, rigidity, and impaired postural reflexes). They defined Parkinson’s disease as parkinsonism with no other cause, no documentation of unresponsiveness to levodopa at doses of at least 1 g per day in combination with carbidopa, and no prominent or early signs of extensive nervous system involvement.
Researchers analyzed 906 incident cases of parkinsonism with onset between January 1, 1976, and December 31, 2005. The median age of onset was 74, and 501 of the patients with parkinsonism were men. Of the 464 patients with Parkinson’s disease, the median age at onset was 73, and 275 of the patients were men. The investigators evaluated changes in incidence rates for men and for women using two age classes: patients younger than 70 and patients age 70 and older. The investigators used negative binomial regression models to evaluate time trends.
Men and Higher Incident Rates
Overall, men had higher rates of parkinsonism and Parkinson’s disease than women. The incidence rate of parkinsonism in men increased from 38.8 cases per 100,000 person-years between 1976 and 1985 to 56.0 cases per 100,000 person-years between 1996 and 2005. The incidence rate of Parkinson’s disease in men increased from 18.2 cases per 100,000 person-years between 1976 and 1985 to 30.4 cases per 100,000 person-years between 1996 and 2005. Compared with that in men younger than 70, the increase in incidence rates was greater for men age 70 or older. There was not a statistically significant increase in incidence rates of parkinsonism or Parkinson’s disease in women, although there was a nonsignificant increase in the incidence rate of Parkinson’s disease in women age 70 or older.
In addition to lifestyle and environmental factors, increased awareness of symptoms, improved access to care, and better recognition of parkinsonism by physicians also could be responsible for the increased incidence observed, the researchers noted.
Limitations of this study include its small population size. In addition, there were no data on potential risk factors for Parkinson’s disease. As a result, researchers were unable to confirm whether decreased smoking or environmental factors were responsible for the increased incidence rates, according to Honglei Chen, MD, PhD, Head of the Aging and Neuroepidemiology Group at NIH, in an accompanying editorial.
—Erica Robinson