their findings were limited by lack of occupational-exposure information and by the separation of exposure into “any exposure” and “none.”

Summary and Conclusion

The six studies reviewed here offer conflicting results about the relationship between insecticides and PD. Three of the studies found no association (Seidler et al., 1996; Semchuk et al., 1992; Stern et al., 1991), one found a protective effect (Hertzman et al., 1994), and two found a positive association between insecticides and PD. Of the two positive studies, Butterfield and colleagues (1993) reported the highest odds ratios but was limited by low participation rates, which were considerably different between cases and controls. The best-designed study, Gorell and colleagues (1998), was hampered by its consideration of all exposures until the time of diagnosis, which could have included post-onset exposure.

The studies varied widely in reliability of their estimates of exposure. Investigators in some of the studies took more-detailed residential or occupational exposure histories and attempted to determine whether higher exposure led to increased risk of PD. Results, again, were mixed: two studies showed higher odds ratios at higher exposures (Butterfield et al., 1993; Gorell et al., 1998), and two found no association with duration of exposure to insecticides (Hertzman et al., 1994; Semchuk et al., 1992). Overall, because of the potential for bias, including confounding, these studies do not provide good evidence of an association between insecticide exposure and PD.

The committee concludes, from its assessment of the epidemiologic literature, that there is inadequate/insufficient evidence to determine whether an association exists between exposure to the insecticides under review and Parkinson’s disease.

Amyotrophic Lateral Sclerosis and Insecticide Exposure: Background and Epidemiologic Studies

Amyotrophic lateral sclerosis (ALS) is a rapidly fatal neurologic disorder characterized by progressive muscle weakness, muscle atrophy, and fasciculations. The disease is associated with degeneration of motor neurons in the spinal cord. Because ALS affects only motor neurons, the disease does not impair a person’s mind, personality, intelligence, or memory. Nor does it affect a person’s senses. About 20,000 people living in the United States are afflicted with ALS, and an estimated 5000 people are diagnosed each year. ALS is most commonly diagnosed in people 40–60 years old, but younger people can also develop it. Men are affected slightly more often than women. Of all ALS cases, 90–95% are sporadic with no known risk factors and 5–10% are inherited. Although the etiology of ALS is unknown, some epidemiologic studies have suggested a relationship between lead exposure and ALS, because toxic lead concentrations can produce symptoms similar to those of ALS (Kamel et al., 2002). In North America, investigators generally use the term ALS in reference to three motor neuron diseases: ALS (the most common), progressive bulbar palsy (PBP), and progressive muscular atrophy (PMA). In Europe, investigators refer to ALS and classify the two other diagnoses as subtypes of the more generic term motor neuron disease (MND). PBP has the most rapidly fatal course, and PMA the most benign (Verma and Bradley, 2001). If the three diseases have different etiologies, differing case mixes within studies would render comparisons across studies difficult. To



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