Exposure to the industrial solvent trichloroethylene increases a person's risk of developing Parkinson's disease nearly sixfold, California researchers said Sunday. Animal studies had suggested a potential problem with the solvent, but the new study by Dr. Samuel Goldman of the Parkinson's Institute in Sunnyvale is the first to quantify the risk.
Parkinson's disease, caused by the death of cells in the brain that secrete the neurotransmitter dopamine, is characterized by severe tremors, rigidity in the limbs and other symptoms. It strikes an estimated 100,000 Americans each year and is ultimately fatal. Genetics play a role in susceptibility to Parkinson's, but it has also been linked to head trauma, pesticides and illicit drugs.
Trichloroethylene, or TCE, is a solvent that was once widely used in dry cleaning and to clean grease off metal parts, and it was once used as an anesthetic, especially during childbirth. But concerns about its toxicity led to it being mostly abandoned and replaced by other anesthetics and solvents. There have been at least three reports of clusters of Parkinson's among workers exposed to TCE. Animal experiments following those reports showed that the chemical kills dopamine-producing cells in substantia nigra, the part of the brain affected in Parkinson's disease. It also impairs mitochondria -- the power sources of brain and other cells -- in the same locations that are affected by the illicit chemical MTPT, which is known to cause Parkinson's. "There's a lot of circumstantial evidence to show that it is relevant," Goldman said in an interview.
Goldman and his colleagues identified 99 sets of twins from the World War II Veteran Twins Cohort in which one twin had Parkinson's and the other didn't. They collected job histories for each subject and then had them analyzed blindly by an industrial hygienist and a preventive medicine specialist to assess exposure to occupational chemicals.
Goldman and his team found that exposure to the chemicals xylene, toluene and n-hexane was not associated with an increased risk of Parkinson's. Those exposed to TCE, however, were 5.5 times as likely to develop the disease as those who were not exposed. Those exposed to either TCE or tetrachloroethylene, known as PERC, had eight times the risk. Those exposed to carbon tetrachloride had 2.8 times the risk, and those exposed to PERC had nine times the risk; in both cases, however, the results fell short of statistical significance.
"Part of the problem is that the usage of the substances overlaps quite a bit," he said. Nonetheless, the "very high odds ratio" for TCE "is impressive, and certainly mandates that large population-based studies follow this up." Goldman will report his findings at a Toronto meeting of the American Academy of Neurology in April.
Those who were exposed to TCE had job histories primarily as dry cleaners, machinists, mechanics or electricians. Because the exposure estimates were not precise, Goldman said, the study needs to be replicated. To begin with, the team is looking at larger databases, and they will most likely try to find cohorts of people with high exposure to the chemicals to see how many have Parkinson's. "I think people will really move on this as quickly as possible now," he concluded.
-- Thomas H. Maugh II