Written by Marcia J. Egles, MD. A study in 99 pairs of twins showed those with six months exposure to trichloroethylene had six times the risk of Parkinson’s disease.
A recent study of aging participants of the World War II Twin Registry has shown a strong link between long-term occupational exposure to the solvent trichloroethylene (C2C13H) and the development of Parkinson’s disease ( 1). Parkinson’s Disease (also known as Parkinson disease, PD) is a degenerative brain disease. Tremors, movement problems, and dementia are some of the afflictions of Parkinson patients.
Solvents are a broad category of substances and are present in fuels, paints, glues, lubricants and cleaning products. This study examined six specific solvents: trichloroethylene (TCE), tetrachloroethylene (also called PERC, or perchloroethylene ), carbon tetrachloride, toluene, xylene and n-hexane. These substances were chosen for investigation because of past case reports of Parkinson’s disease or Parkinson-like symptoms occurring in persons with exposure to one or more of these chemicals (2,3,4,5 ). Prior to this reported study, no population studies had investigated a possible association of solvents with Parkinson’s disease ( 1).
The study is case-control study enhanced by the use of a United States military twin registry started with World War II veterans. A total of 198 sets of male twins, with one twin unaffected and one twin having Parkinson’s disease, were available from this registry. Because twins are genetically very similar or identical, and share many lifestyle factors, twin-pair studies can be useful in minimizing variables that can confound typical case-control studies (6). Of the 198 twin sets, 85 sets were identical twins. The average age of Parkinson’s disease (PD) diagnosis was 66.2 years with a range of 35 to 85 years. A history of head injury involving loss of consciousness had occurred in 24% of the PD twins and 9% of the unaffected twins.
Occupational and hobby histories were completed for both twins in 99 pairs. Proxy informants such as a spouse or sibling were used for 46% of the twins with PD and 18% of the control twins. Rather than to ask subjects about exposure to specific substances, a method thought to be prone to “recall bias”, a detailed task–based occupational questionnaire was developed for the study. Participants were unaware of the study’s hypotheses. The questionnaires were then assessed by expert exposure classifiers who were unaware of any diagnosis in the participants.
The study found that a twin with a work exposure of six months or greater to the solvent trichloroethylene had six times the risk (odds ratio 6.1: 95% confidence interval 1.2 – 33, p=0.034) of Parkinson’s disease as compared to his unexposed twin (1). Two other chlorinated solvents, PERC and carbon tetrachloride, tended toward significantly increased risk, but the data was not sufficiently strong to demonstrate a clear association with PD. No risk was shown with the other solvents.
A major difficulty, and notable achievement, of the study was the recall requirements. Participants were asked histories spanning back to age 10, which was 70 years or more in some cases. This is an unavoidable limitation for a disease such as PD in which relevant exposures might be occurring decades prior to the appearance of clinical disease.
The study appears to be the first confirmation of a significant association between trichloroethylene exposure and Parkinson’s disease risk in a population study (1). Although the study was limited to work exposures, the researchers voiced concerns about more far reaching public health implications (1). Trichloroethylene is the most frequently reported organic contaminant in groundwater and is detectable in up to 30% of US drinking water (7,8).
Source: Goldman, Samuel M., et al. “Solvent exposures and Parkinson disease risk in twins.” Annals of neurology 71.6 (2012): 776-784.
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Posted on September 24, 2012.
Marcia Egles, MD, graduated from Vanderbilt University School of Medicine in 1986. Dr. Egles is certified in Internal Medicine and is a member of the American College of Physicians.
References:
- Samuel M. Goldman et al. Solvent Exposures and Parkinson Disease Risk in Twins Annals of Neurology 2012: 71: 76-784. Available free online at http://onlinelibrary.wiley.com/doi/10.1002/ana.22629/full
- Pezzoli G, Barbieri S, Ferrante C, et al. Parkinsonism due to n-hexane exposure. Lancet 1989; 2: 874.
- Tetrud JW, Langston JW, Irwin I, Snow B. Parkinsonism caused by petroleum waste ingestion. Neurology 1994; 44: 1051–1054.
- Uitti RJ, Snow BJ, Shinotoh H, et al. Parkinsonism induced by solvent abuse. Ann Neurol 1994; 35: 616–619.
- Gash DM, Rutland K, Hudson NL, et al. Trichloroethylene: parkinsonism and complex 1 mitochondrial neurotoxicity. Ann Neurol 2008; 63: 184–192.
- Hubinette A, Cnattingius S, Ekbom A, et al. Birthweight, early environment, and genetics: a study of twins discordant for acute myocardial infarction. Lancet 2001; 357: 1997–2001.
- United States Environmental Protection Agency. Toxic release inventory program. Available at:http://www.epa.gov/tri/index.htm
- Wu C, Schaum J. Exposure assessment of trichloroethylene. Environ Health Perspect 2000; 108( suppl 2): 359–363.
