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Wednesday, July 27, 2016

Why Have Rates of Parkinson’s Disease Steadily Increased?

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Credit: Charlotte Eliopoulos

BY FRAN KRITZ

Cases of both parkinsonism and Parkinson's disease increased significantly in the 30 years between 1976 and 2005, especially in men 70 and older. That's according to a study by researchers at the Mayo Clinic in Rochester, MN, published in the June 20 issue of JAMA Neurology. The researchers, who claim their study is the first to show an increase, speculate that the rise may be linked to environmental and lifestyle factors such as pesticides and smoking.

Data from Medical Records

Using data from the Rochester Epidemiology Project (rochesterproject.org/), a collaboration between health care providers in Minnesota and Wisconsin, the researchers looked at medical records from birth to death of more than 1,000 people in Olmsted County, MN, who received at least one diagnosis related to parkinsonism. The researchers defined parkinsonism as an umbrella term that includes Parkinson's disease as well as other disorders that involve slowness of movement and at least one other symptom—a tremor while at rest, muscle rigidity, or a tendency to fall. A movement disorders specialist reviewed the records to confirm the diagnosis and to classify different types of parkinsonism, including Parkinson's disease (defined as having manifestations of parkinsonism but without any other known causes), which is the most common.

Risk Higher for Men

The researchers found that men of all ages had a 17 percent higher risk of developing parkinsonism and a 24 percent higher risk of developing Parkinson's disease for every 10 calendar years. Among men 70 and older, there was a 24 percent higher risk for parkinsonism and 35 percent higher risk for Parkinson's disease for every 10 calendar years. Thus, for men 70 and older, the risk more than doubled in 30 years, says Walter A. Rocca, MD, PHD, professor of neurology and epidemiology at the Mayo Clinic. The researchers also observed  an increase in the number of women with parkinsonism, but the rate did not reach statistical significance.

Smoking May Play a Role

The study authors theorize that there may be a link between a decrease in smoking during the last century and the rise in Parkinson's disease, based on earlier studies from a decade ago that Dr. Rocca highlighted that show an association between higher rates of smoking and lower rates of Parkinson's. For example, a 2008 article in the journal Movement Disorders that analyzed several studies on smoking and Parkinson's disease  determined that smoking decreased the incidence of Parkinson's disease by 74 percent. In another study published in Neurology in 2007, researchers looked at Parkinson's disease prospectively from 1992 to 2001 among 79,977 women and 63,348 men participating in a national cancer prevention study. Reseachers reviewed participants' responses to questions about smoking status and lifetime smoking history and found that, on average, participants  who smoked for more years, smoked more cigarettes per day, were at an older age when they quit smoking, and were part of the study for fewer years since quitting smoking, had a lower risk of Parkinson's. If smoking has a protective effect against Parkinons's disease, then the decline in smoking may be partly responsible for the rise in the incidence of the disorder, they say.

Pesticides May Be Culprits

During the last century there has been a dramatic increase in the use of herbicides and pesticides in agriculture. Dating back to 2007, in a study published in Movement Disorders, Mayo Clinic researchers, including some of the current study authors, identified everyone who developed Parkinson's disease in Olmsted County, MN, from 1976 through 1995, and matched them by age and sex to healthy controls. Through telephone interviews, they asked about exposure to chemicals and found that exposure to pesticides, but not other chemicals, was linked to Parkinson's disease in men. In the study, the researchers speculated that pesticides may interact with factors that are different in men and women.

Environmental Clues

"The trend cannot be explained by genetic factors because the genetic risk of Parkinson's disease is extremely low in the general population [so] the cause [of the increase] must be environmental or lifestyle," says Dr. Rocca. Exposure to environmental pollutants such as chemical toxins in the air, water, or soil, or infectious agents, such as viruses or bacteria, may be contributing factors. Lifestyle habits such as a poor diet, lack of physical exercise, unhealthy living conditions, or certain medications could be factors as well.

The higher incidence found in men who were born between 1915 and 1924  is important "because the people born in that particular decade may have been exposed to some environmental or other factor during their intrauterine life or soon after birth that increased the risk," says Rodolfo Savica, MD, MPH, a neurologist at the Mayo Clinic and an author of the study. "Changes in exposure to a number of risk factors may have caused [the incidence of] Parkinson's disease to rise."

Drs. Rocca and Savica hope other investigators will confirm this trend. In the meantime, they plan to study smoking and other potential risk factors to determine possible causes.