Understanding Variations in Incidence of PD Key to Policy, Research, and Care

Among adults aged 65 years and older, the annual incidence of Parkinson disease (PD) is 50% higher compared with previous estimates of 60,000 diagnoses annually. This highlights the need for improved estimates regarding the incidence of PD, which will aid in care delivery planning and future health care policy, but also in improving the understanding of PD risk. These are the findings of a retrospective study published in the journal NPJ Parkinson’s Disease.

For the study, researchers sought to investigate the incidence of PD across 5 epidemiologic cohorts in North America in 2012. The cohorts included the following:

• Honolulu-Asia Aging Study (HAAS; Hawaii, USA);
• Kaiser Permanente Northern California (KPNC; California, USA)
• Rochester Epidemiology Project (REP; Minnesota, USA);
•  Ontario Health Care (Ontario, Canada); and
• Medicare program (USA).

These 5 cohorts included data on 6.7 million person-years in adults aged 45 years and older and 9.3 person-years in individuals aged 65 years and older. In any given population, disease frequency can be measured as the annual prevalence (ie, the proportion of individuals currently diagnosed with a particular disease) or as the incidence (ie, the proportion of individuals newly diagnosed with a particular disease).

It is well known that disease prevalence is influenced not only by new cases being reported in the population, but also by the survival of the established cases. In contrast, incidence is an important complementary statistic because it is a more direct reflection of the impact of risk factors for a particular disease (in the absence of changes in diagnostic efficiency). Improved estimates of disease incidence and mortality are needed for enhancing our understanding of disease risk; planning health care capacity and delivery; anticipating and addressing health care disparities; and identifying unwarranted variations in the delivery of health care.

In the current analysis, the researchers found that estimates of age-adjusted and sex-adjusted PD ranged from 108-212 per 100,000 among individuals aged 65 years and older, and from 47-77 per 100,000 among individuals aged 45 years and older.

The incidence of PD was higher among men and increased with age. A persistent spatial clustering of incident PD diagnoses was observed in the United States. Estimates of PD varied across data sources, which was due, in part, to case ascertainment and diagnosis methods, but also to the impact of population factors (ie, the prevalence of genetic risk factors or protective markers) and geographic locale (ie, exposure to environmental toxins).

Several study limitations should be noted. Because of its retrospective design, the current analysis is prone to measurement error that can be due to factors such as confounding, selection bias, miscoding, and misclassification. The estimates were derived primarily with the use of data from completed epidemiologic studies, which thus limited the ability to apply new or varying diagnostic and ascertainment criteria across datasets.

The researchers concluded that their “study highlights the need for more work throughout the translational epidemiology continuum and the potential benefits of harmonizing case ascertainment and diagnostic criteria across data sources.” They added that “Alternative approaches and data sources, such as those included here, can provide high-quality estimates for both the prevalence and incidence of disorders such as Parkinson’s disease.”