Risk Factors

The vast majority of PD cases likely result from the complex interactions of multiple factors. Numerous genetic and environmental factors associated with increased - or in some cases reduced - occurrence of PD and related parkinsonism have been identified. In certain cases, environmental toxins, such as 1-methyl 4-phenyl 1,2,3,6-tetrahydropyridine (MPTP) (Langston et al., 1983), or specific genetic mutations (Belin and Westerlund, 2008) have been identified as causative factors for PD or related parkinsonism. However, despite a strong relationship between a number of risk factors and PD, how these factors act to cause PD is poorly understood.

Certain demographic factors have been linked to the risk of PD, but are not easily classifiable as genetic or environmental. These include:

• Aging: Aging is a major risk factor for PD. Although age of onset of the cardinal motor symptoms of PD does vary across patients and can occur prior to age 40 in ‘young onset’ PD, most cases generally occur after age 60. Prevalence of PD increases from about 1% of those over age 60 to about 4% of those over age 80 (de Lau and Breteler, 2006). The difference in age of onset may relate to the combination of risk factors underlying an individual’s particular disease, with genetic factors playing an apparently larger role in earlier onset PD (Klein and Schlossmacher, 2006).
• Gender: Male gender also appears to increase risk for PD, as the disease occurs more frequently in men than women. However, whether the gender disparity truly exists and whether it is driven by intrinsic biological differences or differential exposure to specific risk factors is unclear (de Lau and Breteler, 2006).
• Ethnicity: Some studies suggest a reduced risk of PD in people of African and Asian descent compared to Caucasians (Zhang and Roman, 1993; Van Den Eeden at al., 2003). However, results regarding ethinic differences are conflicting and require further validation (de Lau and Breteler, 2006).