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.

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A search of the 1988 mortality files, created by the National Center for Health Statistics (NCHS), looking for individuals who died with an ICD-9 diagnosis of PD revealed racial differences. Whether PD was the underlying cause of death, a contributing cause, or just a diagnosis that people carried, whites had significantly higher rates than blacks. This study confirmed the findings of earlier reports that had mainly looked at census data from 1959—1961. These studies were obviously limited by the fact that they relied both on a person carrying the diagnosis at the time of death and that death certificates were completed in a consistent fashion throughout the country.

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Of the 750 inpatient and outpatient cases which Harries had seen in his neurology practice in Kenya over a five-year period, he observed 27 cases of PD (-4% of neurology patients) with an age range of 45-60 years. A survey of patients admitted to the hospital over a one-year period in what was formerly known as Rhodesia revealed three patients with a diagnosis of PD, accounting for less than 1% of all neurological admissions. Both these percentages were similar to those reported by Haddock in Ghana among 280 admissions and 192 outpatients. Col-lomb who saw inpatients and outpatients over a 10-year period in Senegal also reported that less than 1% of neurological admissions were due to PD. Interestingly, his observations of outpatients over time suggested that among patients with classical PD in 25% of cases, the disease seemed to progress more slowly in comparison to his previous clinical experience in England. Conversely, Cosnett’s experience in Natal (South Africa) led him to conclude that though Parkinsonism was common, true PD was rare. For his summary on Nigerian patients, Osuntokun reviewed 13 years of records and found 107 cases of Parkinsonian syndrome (1% of neurological admissions). He observed that the peak incidence was in sixth decade and commented that liver disease was the cause of many of the early-onset cases. A later study by the South African neurologists Cosnett and Bill involved reviewing records from all patients seen for neurological consultation during a seven-year period (1979-1985). Patients with Parkinsonian symptoms were classified as having PD or Secondary Parkinsonism (SP). They found PD in 0.15% of blacks and 2.3% of whites, whereas SP occurred in 0.5% of blacks and 1.15% of whites. Interesting is the fact that only 22.8% of black patients were over the age of 50 as opposed to 47.8% of white patients.

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INTRODUCTION

Parkinson’s disease (PD) is a common progressive neurodegenerative disorder characterized clinically by bradykinesia, resting tremor, rigidity, and postural instability. Symptomatic response to L-DOPA therapy is also often used as an additional diagnostic criterion. It affects 1-2% of the population over the age of 60 years, although the disease is seen in younger individuals as well. A recent meta-analysis of studies to date suggests an overall annual incidence of 16-19/100,000. In 1993, a review of the worldwide occurrence of PD reported that Asian and black African populations have the lowest prevalence rates of this disease in the world. Reports to date have not only suggested that the prevalence rate is lower in black Africans but, in addition, that this rate is also lower in African Americans as compared to Caucasians. We have noted a deficiency of black families among volunteers for our ongoing studies on the genetics of PD. We, therefore, undertook a literature review in order to ascertain whether a reduced prevalence of PD among African Americans might be a contributing factor to there being fewer black individuals and families affected with PD in our study.

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