Epidemiologic investigation plays an important role in health planning, by providing estimates of disease frequency, including regional and temporal variations. In the 1950s, Kurland and his colleagues1 in Rochester, Minnesota, and Gudmundsson2 in Iceland provided the first community-based estimates of Parkinson's disease (PD) prevalence, finding it to be one of the most common neurodegenerative disorders of the elderly. This chapter reviews the worldwide incidence and prevalence of PD and provides the reader with tools for interpreting such studies.
Epidemiology is also a powerful method for investigating disease etiology. Observational studies have revealed numerous factors that are associated, positively or negatively, with the occurrence of PD. These associations include both environmental and genetic factors. Prevailing thought as to whether genetic or environmental factors are more important in causing PD has fluctuated multiple times since PD was first described in 1817.3 The etiology remains largely unknown, and this may reflect a complex interaction of environmental and genetic factors that combine to cause disease. These factors could also vary from individual to individual or population to population. Epidemiologic studies thus continue to be a cornerstone of our search for the determinant(s) of PD. We review the associations, both direct and indirect, of PD with demographic and environmental factors. We also consider familial factors, as these have been revealed using observational study designs. First, we emphasize some methodologic considerations important to the interpretation of this continually expanding literature.
The diagnosis of PD is currently based on clinical examination. The cardinal signs of PD (bradykinesia, resting tremor, cogwheel rigidity, and postural reflex impairment) are not unique to PD, but are seen in several possibly related but clinically distinct disorders such as progressive supranuclear palsy, multiple system atrophy, or diffuse Lewy body disease. Particularly early in the course, clinical distinction among these disorders is difficult. The overlap in clinical features between multiple disorders that do not have premortem biological markers creates a challenge for epidemiologic studies. Attempts to discover the determinants of disease will be confounded if samples of “cases” include patients with clinically similar disorders but with different risk factors. In one series, 20% of cases diagnosed in life as having PD had some other diagnosis, usually some form of atypical parkinsonism, at autopsy.4 Because cases in which the clinical diagnosis is in question are more likely to be referred for autopsy, this rate is likely higher than would be found if all cases could be evaluated pathologically.5 Nonetheless, these results highlight the possibility that some cases of atypical parkinsonism may be erroneously diagnosed as PD. Inclusion of these cases in etiologic studies may obscure an association between risk factors and PD. In epidemiologic ...