The investigators, led by Walter Rocca, M.D., a Mayo Clinic epidemiologist, discovered the highest increase in Parkinson's risk in people with nine or more years of education. They also found that risk level rises as years of schooling increase. Occupationally, physicians had the greatest increased risk for Parkinson's compared to the general population, while those employed as construction and extractive workers (e.g., miners, well drillers), production workers (e.g., machine operators, fabricators), metalworkers and engineers had the lowest risk increase. The researchers also note that this study did not find farmers and other agricultural workers at increased risk for Parkinson's.
The Mayo Clinic investigators advise caution in interpreting this study. "Our findings for education and occupation are complex, and therefore they need to be interpreted with care," says Roberta Frigerio, M.D., the study's first author and former Mayo Clinic research fellow. "These factors may be surrogates for a variety of exposures, physical activity, personality or socioeconomic status. Further studies are needed to interpret our findings."
Demetrius (Jim) Maraganore, M.D., Mayo Clinic neurologist and study investigator, agrees. "We really can't say from this study that education and occupation are causal factors in Parkinson's disease; we can only say that they are associated," says Dr. Maraganore. "I don't think that schooling or wearing a stethoscope causes brain cells to degenerate or that digging holes with a digger protects your brain cells from atrophy, but I think that these are indirect indicators of factors that may relate to brain degeneration. And now what we need to do is use these clues to try and identify those molecular level events that differentiate these people."
Suggested Applications of Study Findings
The utility of the study's findings concerning occupation, education and Parkinson's disease risk is primarily informational rather than actionable for members of the public, according to Dr. Maraganore. This is especially true in light of the relatively low overall lifetime risk for Parkinson's for any given person, he says.
"Really, nobody should do anything differently based on these findings," says Dr. Maraganore. "These findings are not at all intended to change anybody's behaviors. I think that the bottom line is that we're talking about going from a baseline risk of 2 percent to develop Parkinson's disease during a lifetime to a risk of 4 percent if you are highly educated or a physician, or 1 percent if you are less educated or more physically active. So, I wouldn't change your schooling plans or your occupation based on these findings. I would just welcome these findings as new clues about possible causes of Parkinson's disease that will hopefully lead to the ultimate answers."
Dr. Maraganore also notes that the study's findings should be reassuring to farmers, welders or other metalworkers, who were not found in this study to be at increased risk for Parkinson's due to their occupations, in contrast to previous studies.
Potential Explanations of the Study's Findings
The researchers explain that the increased Parkinson's risk found for physicians and more educated individuals could be partly explained by earlier recognition and detection of the disease, in addition to better access to specialized medical care. However, nonphysicians and physicians in the study had a similar time between onset of symptoms and diagnosis of Parkinson's, which would speak against increased recognition of the disease due to education or profession.
The explanation for the four occupational groups found to have a reduced risk of Parkinson's compared to the general population also remains uncertain, according to the investigators. The findings may be due to chance, some bias due to higher nonparticipation rate in the telephone interview portion of the study among these occupational groups, or to confounding due to lower education and resulting decreased recognition of Parkinson's. Occupation may be a surrogate for physical activity level in the findings, leading to a higher risk in more sedentary professions such as physicians and lower risk in service occupations which involve greater motor skills use. The study investigators emphasize that these associations do not imply causality, however. Physical activity (recreation or work related) may protect against Parkinson's disease, but it is also possible that people predisposed to develop Parkinson's disease avoid strenuous activity earlier in life.
Dr. Maraganore explains that early Parkinson's disease could account for some of the educational and occupational risk findings. "It could be that people with Parkinson's disease have premorbid personalities that make them like education," he says. "For example, dopamine is the reward chemical in the brain and is deficient in the brains of people with Parkinson's disease. So, if you have a long-standing deficiency of dopamine, you may be less likely to party and more likely to sit at your desk and study. So, in these ways early, undetected disease can subtly shape your pattern of behaviors, giving the impression that education is a risk factor for the disease when in fact it's really an early manifestation."
How the Study Was Conducted
In this study, the researchers identified medical records of all individuals who had developed Parkinson's from 1976 to 1995 in Olmsted County, Minn., home of Mayo Clinic. All Parkinson's patients were then matched to someone similar in age and gender who did not have Parkinson's. The investigators collected information about education and occupations from a medical record review and also a telephone interview with the study individuals, using the 1980 Standard Occupational Classification to code each person's profession.
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