Parkinson’s disease (PD) is one of the most common neurologic disorders and the most common neurodegenerative diseases in the world. The pathophysiology of Parkinson’s disease is quite well understood, albeit not completely. Dopaminergic neurons particularly in the substantia nigra, a part of the brain, becomes damaged gradually die leading to lower than normal levels of the brain hormone dopamine. Although this is known, the exact cause of this damage and cell death is not clearly understood. Risk factors including certain genetic mutations have been identified. However, these risks account for only a small proportion of PD cases. Analyzing the incidence and distribution of any disease also helps to provide clues to those at risk based on demographics despite not having any of the known risk factors. The study of this incidence of a disease is known as epidemiology.
Most cases of Parkinson’s disease are seen in the elderly. In the United States about 1 out of ever 100 people over the age of 60 years has Parkinson’s disease. This incidence varies across the globe and even among certain communities as discussed below. The number of people living with Parkinson’s disease has increased over the years but this may not be necessarily due to a rise in the percentage of people developing PD. Instead this has to do with a longer life expectancy. Modern medicine, better living conditions and accessibility to health care services means that the “older live longer”. Naturally the number of people living with Parkinson’s disease has increased accordingly. This is further reflected in the difference of prevalence where there is now about 1 million more people living with Parkinson’s disease in the United States compared to 2005 when there was only about 500,000 Americans with PD.
Parkinson’s disease affects both men and women. It is, however, more common in men and in some countries like in the United States this may be as much as 1.5 times. This could correlate with certain risk factors like exposure to pesticides and industrial pollutants which may be associated with more male-dominated jobs. However, with more women entering the workplace and having a similar exposure, not to mention the non-occupational exposure, it seems that men are at greater risk for unknown reasons.
Nationality and Ethnicity
The incidence of Parkinson’s disease varies across the globe. However, this distribution may not be as simple as a geographical or ethnic factor. It is known that the Parkinson’s disease is more prevalent in North America and Europe than in Asia and West Africa. However, certain findings like that of the Parsi community in India indicates that this may not just be a geographical difference. The Parsi community in Mumbai has the world’s highest incidences of Parkinson’s disease where it affects about 328 out of every 100,000 people despite living in a country, India, with one of the world’s lowest incidence of PD (70 out of 100,000).
Family History and Genetic
Despite the focus on a family history and genetics, it is believed this accounts for less than 5% of cases. More genes are being identified in familial PD, like the recent VPS35 genetic mutation, but its role is sometimes overstated and hyped in the media. There is no evidence of any genetic mutation or strong family history in the majority of Parkinson’s disease patient although family history is an accepted risk factor. This is further supported by the fact that the incidence in monozygotic and dizygotic twins is about the same. Had genetics been a major factor, the incidence would have been higher in the monozygotic twins.
The Parkinson’s Patient
Based on these factors, once could surmise that the person who is at greatest risk would be :
- Over the age of 60
- Living in North America
- From a family with a strong history of PD
However, with Parkinson’s disease the incidence and risk factors are not as conclusive as one would think. It is safe to say that in the backdrop of the unidentified causes in the majority of PD cases, the epidemiology has not provided definitive risk factors apart from age.