Parkinson’s disease results from a progressive loss of brain cells that make chemicals important for movement and other functions. Most of the time, the reason this happens is unknown. Several risk factors make it more likely that a person will develop Parkinson’s disease. A major one is aging. According to the February 2013 American Family Physician, about 1 percent of Americans over age 60 and up to 4 percent over age 80 have Parkinson’s disease. Other risk factors include inheritance from relatives, environmental factors and head injury. For any given person, multiple factors may combine to cause the illness.
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Changes in the Brain
In people with Parkinson’s disease, certain brain cells accumulate an abnormal protein — alpha-synuclein — that forms clumps called Lewy bodies. Brain cells that have Lewy bodies don’t work as well and eventually die. The movement symptoms of Parkinson’s disease begin when at least half of the cells in the substantia nigra part of the brain are lost. These cells make the chemical dopamine, which the brain needs to send signals about proper movement. Brain cells that make chemicals other than dopamine also eventually break down in people with Parkinson’s disease, leading to symptoms like thinking, mood and sleep problems. Scientists think that the spread of Lewy bodies from one brain area to another may explain the progression of symptoms in Parkinson’s disease.
According to the December 2010 issue of the Journal of Geriatric Psychiatry and Neurology, about 20 percent of people with Parkinson’s disease have relatives with the disease. Scientists have found that some of them have changes in their DNA. These changes don’t guarantee that a person will develop Parkinson’s disease, but they increase the risk. In a much smaller number of people, changes in DNA directly cause disorders with symptoms similar to Parkinson’s disease. Known as familial Parkinsonism, some of these disorders are exactly like Parkinson’s disease, but others begin at a younger age or have different symptoms like muscle twitching and altered breathing.
Environmental and Toxic Exposures
Chronic exposure to certain environmental toxins has been associated with an increased risk of Parkinson’s disease. Among the pesticides that could increase the risk are the herbicides paraquat, dieldrin and 2,4-D — a component of Agent Orange — and the insecticides permethrin and rotenone. Chronic exposure to lead can increase the risk of Parkinson’s disease as well. People who live in rural areas, who are involved in farming and who consume well water are also at higher risk, potentially because of pesticide exposure. A syndrome similar to Parkinson’s disease has also been produced by the toxin MPTP, a contaminant that was created when people tried to make homemade heroin in the 1980s.
Head injury, with or without loss of consciousness, appears to be a risk factor for Parkinson’s disease. Animal studies show that brain cells that make dopamine are lost with head injury. Head injury may also cause inflammation, or swelling, in the brain, a problem that has also been noted in Parkinson’s disease. Some suggest that head injury is not enough by itself to cause Parkinson’s disease, but that additional risk factors — like pesticide exposure or DNA variants — are needed for the disease to occur. If a person has one of those risk factors, such as a DNA variant, a head injury could mean that Parkinson’s disease symptoms may start at an earlier age.