Epidemiology seeks to prevent disease by identifying risk factors, be they genetic and/or environmental, that are of etiological relevance through the use of descriptive studies, natural experiments, and very occasionally randomized trials. In addition, clinical epidemiology can examine the utility of diagnostic tests, determine predictors of disease prognosis, and test whether therapies can modify disease progression. This chapter will focus on descriptive studies that have either measured disease frequency (prevalence or incidence) or estimated the risk associated with environmental exposures. We have chosen to focus solely on progressive supranuclear palsy (PSP) and multiple system atrophy (MSA) as there is hardly any data for other atypical parkinsonian disorders such as corticobasal degeneration or dementia with Lewy bodies. For any uncommon disease, undertaking epidemiologi-cal studies presents several major challenges: case identification, representativeness of cases, and obtaining adequate sample sizes. For both PSP and MSA, there are additional problems as both these conditions can be difficult to diagnose. As there are no adequate tests that are both highly sensitive and specific, the gold standard remains the diagnostic expertise of a movement disorders specialist (1).

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