Evaluation of how age modifies the risk for Parkinson's disease, based on stratified comparisons of descriptive data

This is a study of selected information on incidence and prevalence of Parkinson's disease (PD), mortality from Parkinsonism, and levodopa use (LDU). The concept of effect modification (EM) by age (EMA), a variation across age of incidence or odds ratios, was here extended to comparisons of prevalences, mortalities and drug use, and mathematically defined following a reported model. Age-specific EM from comparisons of empirical data was measured and combined with figures for calendar time and the annual infant mortality rates (IMRs) in the population of each observation, using linear regression analysis. Statistically significant associations of EM at ages 70-79 and 80+ were found: with IMR from prevalence comparisons; and with IMR and calendar time in comparisons of proportionate mortalities in longitudinal observations. A similar age-specific pattern of associations was observed with time from incidence in Rochester. The observations for LDU and PD incidence in Iceland and Turku, Finland, fitted well the age-specific regression lines of the IMRs with the age-specific EM components from prevalence data. We concluded that there is a universal age-specific pattern of variation of epidemiological characteristics of PD related to calendar time, and to local factors represented by the IMR. Moreover, this change can be perceived from different parameters under certain conditions. Comparisons of survey, death-records, and LDU data when used for generation of causal hypotheses for PD require that EMA be taken into account.