PD is a neurodegenerative disorder for which incidence and prevalence increase with age. Onset of symptoms for the majority of cases occurs after age 60. Evidence from twin, family and epidemiologic studies suggests a multifactorial etiology for PD comprised of genetic and environmental factors. Familial aggregation of the disease and an increased recurrence risk to siblings of people with PD supports a genetic susceptiblity to developing PD. As well, a rare form of early onset familial PD is caused by mutations in the ?-Synuclein gene. These mutations are found in a small fraction of families with multiple cases of PD, and therefore other PD genes likely remain to be discovered. Increasing age, smoking tobacco, pesticide use, heavy metals exposure, and rural residence have all been reported as potential risk factors for PD. Only the association with increasing age is consistent across studies. Variation in the effects of environmental factors on risk of PD may be due to differences in genetic susceptibility. Thus, dissecting the complex etiology of PD requires a comprehensive approach, jointly evaluating both genetic and environmental exposures.
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