Incidence is defined as the number of new cases per year and is usually described per 100,000 people. Conducting an incidence study requires not only defining cases but also determining which are the new cases. Some new onset cases that ought to be included may not be recognized until sometime later. As well, the number of new cases in a community may vary from one year to the next. Consequently, incidence studies require a long period of observations in the same community using consistent case finding methods and case definitions.
The reported incidence rates of PD vary widely. The lowest PD incidence in Western countries was reported from Sardinia at 4.9/105 (25). A systematic review of 25 European studies identified five well-designed, similar studies of PD incidence (26). Four studies reported similar incidence of 16 to 19/105 but one Italian study reported a much lower incidence of 8.4/105, and age-adjusted PD incidence in Taiwan (Republic of China) is 10.4/105 (27).
In the Western countries, some of the most reliable incidence studies are those from Rochester Minnesota using the record system of the Mayo Clinic, which combines case ascertainment with diagnosis by neurologists. Four different incidence reports based on the Rochester population have been published (15-18). Since drug-induced parkinsonism was not recognized until the early 1960s (28), we excluded drug-induced parkinsonism from each study for the purpose of comparison. The rates for 1945-1954 (15), 1935-1966 (16), 1967-1979 (17), and 1976-1990 (18) were 20.5, 18.5, 18.2, and 20.5/105, respectively. There was no significant change in incidence over 55 years. The latest study (18) revealed overall incidence of parkinsonism of 25.6/105, including drug-induced parkinsonism.
Incidence increases with increasing age. The incidence of parkinsonism was 0.8/105 in those aged 0 to 29 years, 25.6/105 in those 50 to 59 years and, more than 11 times higher (304.8/105) in the 80- to 99-year age group (18). There has been no significant change in the age-specific incidence rates during the 55-year interval of these studies (29). Slightly higher overall incidence of parkinsonism in recent reports likely reflects longer life expectancy in the general population, more frequent use of neuroleptics and improved diagnosis (18,29).
Pathological studies (30,31) show a progressive increase in the rate of incidental Lewy body inclusions with advancing age. These cases are regarded as having preclinical PD. With every decade of life, there is a doubling of incidental Lewy bodies (31). The decline of parkinsonism and PD specifically, in the very old observed in some studies, is attributed to difficulty in ascertaining cases in the presence of comor-bid disorders (18). Age remains the single most important risk factor for parkinsonism. An Italian study noted a 9% increase in risk for every increasing year of age (i.e., a 60-year-old has a 90% greater risk than a 50-year-old), and men had higher risk than women (19). The current lifetime risk of parkinsonism from birth is estimated at 4.4% for men and 3.7% for women (32).
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