Prevalence of Parkinson’s Disease: SUMMARY AND CONCLUSIONS

2 Jan
2010

etiology

There are a number of possible explanations— some real, others factitious—for any observed differences in the prevalence of PD between groups of European origin and Africans or African Americans. First, we must consider that the differences in prevalence are real and the result of biological or environmental differences between populations. There is some evidence to suggest that PD may be milder or phenotypically different in African Americans. Specifically, there were fewer definite cases in the Copiah County study, while in South Africa, Cosnett found that though Parkinsonism was often seen, true PD was rare. Collomb suggested that PD in Senegal resembled PD in England but, in 25% of cases, deterioration was slower and patients survived for many years after diagnosis. Unmeasured environmental factors could also contribute to the differences in prevalence rates; however, none of the studies reviewed here attempted to address the role of environment.

On the other hand, there are a number of ways that observed differences in disease prevalence could be artifactual due to biased study designs rather the result of some hypothetical but as-of-yet unidentified biological or environmental factors. Hospital- or outpatient-based studies are clearly not a reliable means of ascertaining incidence or prevalence of PD, because socioeconomic and cultural factors can be major sources of bias in ascertainment. The only studies that systematically determined prevalence were the door-to-door studies in Copiah County and Nigeria. In an area of Mississippi where there is an equal percentage of whites as African Americans, the total incidence of PD (which includes definite, probable, and possible) was equal in both populations, although 58% of African Americans and 32% of Caucasians had not been diagnosed previously. Definite PD was more common in whites than blacks. These community-based studies suggest that PD is likely to be underdiagnosed in general and, most especially, among African Americans. Thus, PD may be one of many chronic diseases that have been shown through health disparities research to be both underdiagnosed and inadequately treated in racial minorities. Ironically, studies reporting a lower frequency of PD in African-American populations may lead to a reduction in the early diagnosis of the condition, as clinicians may not consider it as high on the list of differentials for this population. 

Another confounding factor of all the reports including the Copiah County study is the fact that the risk for PD increases with age. According to 1978 life tables, the average life expectancy for blacks was 68.1 years, compared with 74.1 years in whites. In a condition where age is a major risk factor, this is a noteworthy difference. Had African Americans lived longer at that time, the probable and possible cases may have become definite. The prevalence in Nigeria is reported to be much lower than in the United States. Once again, as prevalence is a function of incidence and survival, it is impossible to know whether Africans are less susceptible to PD or whether the higher mortality rates in Nigeria mean that people are dying before reaching an age where they would develop the condition. As long as African Americans have a shortened life expectancy in comparison to individuals of European ancestry, all prevalence studies must be corrected for the age profile of the populations being studied.

Recommendations for Future Studies

An accurate estimate of the prevalence of PD among African Americans relative to the prevalence in non-African Americans will require a community-based survey carried out by medically trained interviewers in a population with a similar proportion of black and white populations. Without a community-based study, there is the possibility that the prevailing idea that PD is less common among Africans or African Americans might bias ascertainment and serve as a self-fulfilling prophecy: as discussed previously, a cross-sectional study of a population such as this would only ascertain the prevalence of PD in these groups, while a long-term survey of the population would be needed to determine the incidence and phe-notypic picture of PD in the different ethnic groups. Aside from a detailed neurology questionnaire and a follow-up exam by a neurologist specializing in movement disorders, the study should also include a family history. Ideally, a postmortem would be performed on all individuals who died during the study period. While we acknowledge that a long-term study would have additional significant logistical difficulties due to loss to follow-up, etc., it is apparent that without such a thorough study, the true prevalence, incidence, and clinical course of PD among African Americans in the United States will remain unknown. Despite these challenges, the need to understand the phenotypic and epidemiological characteristics of PD and other neurodegenerative diseases among minority populations remains an important goal for clinical research. canada viagra online

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