The prevalence rate is defined as the number of patients in the population at a given time. This can be difficult to ascertain, as approximately 15% of people in the community self-reporting a PD diagnosis do not actually have PD and 20% of people with PD have not been diagnosed (46). The two main factors that determine the prevalence rate are the incidence of new cases and life expectancy. Crude prevalence rates are greatly affected by the age distribution of the source population; age-adjusted rates are one way to permit comparisons between different populations but crude rates are most often reported.
Prevalence rates can be estimated by multiplying incidence rate and the mean survival. Most observers regard Rochester, Minnesota, incidence rates as representative for North America. The latest annual incidence of parkinsonism in Rochester is 25.6/105 (18). The survival in parkinsonism has increased substantially during the last three decades. A conservative estimate of mean survival in contemporary parkinsonism is 15 years, though an average PD case would survive longer. Thus, the minimum prevalence rate in the North American general population is estimated at 384/105.
Other methods have been used to determine prevalence rates for parkinson-ism. A state registry in Nebraska reported a prevalence of 329.3/105 (47). Some studies have used consumption of antiparkinsonian drugs as a surrogate for a diagnosis of parkinsonism (48,49), but these underestimate prevalence since many people with parkinsonism are untreated and the medications can be used for other diagnoses.
In the Caucasian population, the crude prevalence ratios vary from 84/105 to 775/105 population (50,51). Prevalence rates based on door-to-door surveys include 57/105 in China (People's Republic of China) (52), 371.5/105 in Sicily (53), and 775/105 in Australia (51). In a Parsi community from Mumbai, India, the prevalence rate was 328/105 (54). In a U.S. community-based study of Copiah County residents that included only persons over the age of 40 years, the prevalence rate was 347/105 (55). A Dutch study in the early 1990s found a prevalence rate of 1400/105 in those aged 55 to 64 years and 4300/105 in the 85 to 94 years age group (56).
In two Canadian studies using representative samples of residents aged 65 years and older, the prevalence rate in community residents was 3% (57) while in institutionalized persons the rate was 9% (58). Somewhat comparable figures were reported from Australia (51). Including only PD cases in persons aged 55 years and older, the prevalence rate of PD was 3600/105 in the community and 4900/105 in institutionalized persons. As discussed previously, incidence has remained relatively constant but life expectancy has increased; one would then expect overall crude prevalence rates to have increased over time.
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