Time Trends in the Incidence of Parkinson’s Disease: a 30-year Study

IMPORTANCE Changes over time in the incidence of parkinsonism and Parkinson’s disease (PD) remain uncertain. OBJECTIVE To investigate secular trends (period effects) and birth cohort trends in the incidence of parkinsonism and PD over 30 years in a geographically defined American population. DESIGN, SETTING, AND PARTICIPANTS We used the medical records-linkage system of the Rochester Epidemiology Project (REP) to identify incidence cases of PD and other types of parkinsonism in Olmsted County, MN, from 1976 to 2005. All cases were classified by a movement disorder specialist using defined criteria through the review of the complete medical records within the system. MAIN OUTCOMES AND MEASURES Incidence rates of parkinsonism and PD over 30 years. We tested for secular trends (period effects) using negative binomial regression models and for birth cohort effects using age-period-cohort models. RESULTS The overall incidence rates increased significantly over 30 years in men for both parkinsonism (RR 1.17 per decade, 95% CI 1.03-1.33) and PD (RR 1.24 per decade, 95% CI 1.08-1.43). These trends were driven primarily by the older age groups. In particular, for men ≥ 70 years, incidence rates increased for both parkinsonism (RR 1.24 per decade, 95% CI 1.07-1.44) and PD (RR 1.35 per decade, 95% CI 1.10-1.65). The secular trends were not significant for women overall or in age strata. We observed an increased risk for both men and women born in the 1920 cohort (1915-1924). However, this birth cohort effect was significant only for PD and only in men. CONCLUSIONS AND REVELANCE Our study suggests that the incidence of parkinsonism and PD may have increased in recent decades particularly in men 70 years old and older. These trends may be related to the dramatic changes in smoking behavior that took place in the second half of the 20th century. However, the trends could be spurious, and need to be confirmed in other populations.