Who Has Parkinson's?

From James Beck Ph.D., Director of Research Programs

When battling a disease of unknown origin, like Parkinson’s disease (PD), knowledge is power. Knowing something even as simple as the number of people living with the disease, or its prevalence, is very important. With Parkinson’s, knowledge of prevalence and incidence can help not only to appropriately advocate for scarce healthcare resources and research dollars but, when tied with other information like location, age, and gender, it can also shed light onto possible causes of the disease itself, e.g., environmental factors.

However, determining the prevalence of PD is not simple. While some other diseases are counted through a national registry, there isn’t yet one for Parkinson’s. (More on that later). Without a registry, the recognized gold standard method for assessing the number of people living with Parkinson’s is the door-to-door survey. This is an arduous process and the most recent survey conducted in the US occurred in 1978 and was published in 1985. It took place in the rural county of Copiah, Mississippi (population 23,842). This frequently cited survey has been used by some to estimate the number of people living with PD in the US - an estimate which stood at 340,000 in 2005 for people over the age of 50. Since this estimate neither includes people under the age of 50, nor those who have not yet been diagnosed, PDF and other organizations estimate a higher prevalence for Parkinson's. For instance, PDF estimates that there are nearly one million people in the US living with Parkinson's.

It makes one wonder…how accurately can data from a small rural county in the South be applied to the rest of the geographically diverse US? Well, a recent study has measured a more diverse group, leveraging the extremely large US Medicare database. This database includes medical information about 98 percent of the population age 65 or older, i.e., greater than 29 million people. This study has found different results:

• The prevalence of PD for those 65 and older is 1.6 percent (vs. 0.75 percent as reported in the 1985 study), meaning there are actually an estimated 483,000 people living with PD who are 65 and older vs. the 276,000 estimated using the earlier Copiah data for the same age group.
  


• The prevalence of Parkinson’s shows a racial distribution with about 1.5 times the level observed in whites than in blacks and Asians, though Hispanics had nearly the same prevalence as whites.
  


• There is a geographic distribution, or a ‘PD Belt’ as the authors describe it, of both the numbers living with and diagnosed with Parkinson’s. The highest levels of Parkinson’s are found in the northern Midwest and Northeast.
  


• Along these same lines, there is a higher prevalence and incidence of Parkinson's in urban environments vs. rural environments.
  


• Interestingly, the Medicare data for Copiah County demonstrated one of the lowest rates of prevalence -- about 1.1 percent -- not too far off the 1985 mark.
  

Are These Numbers Correct?

The results of this study are not perfect. One issue is that the study relied entirely upon diagnosis codes, which were not validated with spot checks. Diagnoses are recorded on a person’s medical records, and thus can be helpful in assessing who has Parkinson’s. But there is room for error. One study shows that even Parkinson’s experts who see hundreds of people with PD a year are only right in their diagnoses 90 percent of the time, so it is likely community-based physicians who only see 3-4 cases a year may have erred too.

Still, the results from this study provide a newly revised estimate of the number of people living with PD in the US…more inline with the anecdotal evidence that we at PDF hear from people living with PD. The PD Belt that researchers report is also interesting because the areas with the highest incidence and prevalence are also areas with the highest pesticide use and pollution levels—environmental factors hypothesized to play a role in PD. With so little understanding as to the cause of idiopathic PD, perhaps this information will guide future epidemiological research.

Although compelling, the PD community is still left with imperfect information about the nature of this disease. One possible solution would be the formation of a national registry for PD. A bill for creating exactly that is before both houses of Congress. I encourage everyone to get involved and help see this bill passed so that we can get the information we need about this disease.