Parkinson's disease patterns
Descriptive epidemiology
Descriptive epidemiology is the study of distribution, course and patterns of a disease. It is a tool giving information about the natural history of a disease and can be used to help in the search for causes. It is also useful in planning health services.
The figure for annual incidence (the number of new cases per year) of Parkinson's is quoted at 20/100,000 worldwide, which means that, in Australia, there would be about 3,500 new cases a year.
The prevalence (the number of people with Parkinson's at any one time) varies depending on the study, but figures quoted seem to range from 160 to 300/100,000. Which would mean somewhere between 27,000 and 50,000 people living in Australia with Parkinson's at any one time in the total population. The prevalence for people over 65 is about 1 per cent.
Some studies indicate males have higher risk with a male to female ratio of 3:2, but when all reports are gathered together, it seems that there is no difference in the incidence in male and female.
There do not seem to be any race-related differences, although there have been some studies which seemed to indicate that whites may have a slightly higher risk.
Parkinson's usually begins between ages of 40 and 70 and the mean (average) age of onset is 61.6 years, with only 5-10 per cent being diagnosed before the age of 40.
In the days before treatment with levodopa, the average survival was 9.5 years and today the mean life expectancy for treated Parkinson's is 15 years.
The accuracy of figures for incidence and prevalence is made more difficult because there is no definitive test for Parkinson's and post mortem studies have shown that up to 25 per cent of clinically diagnosed Parkinson's is inaccurately diagnosed. This is becoming more of a problem as more and more parkinsonian conditions are being recognised as different from idiopathic Parkinson's disease.
Analytic epidemiology
Analytic epidemiology looks at patterns of disease in an attempt to find out about causes. These are usually genetic or environmental or a combination of genetic susceptibility with environmental factors.
Genetic factors
Looking for a genetic link for Parkinson's is difficult and the probability of having a family member with Parkinson's is about 5 per cent even if there is no genetic factor. There has been a positive family history in 16 to 41 per cent of cases, but historical information about family members is inaccurate in an estimated 25 per cent of cases. For example, when a person is diagnosed with Parkinson's it is easy for family members to remember that a grandparent had a tremor and to report that they had Parkinson's. The genetic story is further complicated by the fact that many families have been exposed to similar environmental factors, so what could appear to be genetic could actually be environmental.
Twins studies generally show no difference between identical and fraternal twins in the rates of both twins having Parkinson's, although there does seem to be a difference in young-onset Parkinson's.
Even though Parkinson's does not seem to be a simply inherited condition, there are several large families with inherited Parkinson's. Genetic studies of these families have found genetic abnormalities. In one family, there is a mutation in a gene called alpha-synuclein on chromosome 4, which has not been found in most other people with Parkinson's.
Environmental factors
Over the years there have been reports of various environmental factors (such as rural/urban lifestyle, prior exposure to herbicides/pesticides and smoking) influencing the possibility of getting Parkinson's. There is, however, no conclusive evidence that any of these factors is significant. For instance, a recent study showed that there appeared to be no protective effect from smoking, whereas 30 previous studies had shown that there was!
The occurrence of Parkinson's caused by a toxin (poison) called MPTP gave rise to speculation that there was an environmental cause and that exposure to similar chemicals, such as some pesticides, could cause Parkinson's but there have been no clearly documented clusters of Parkinson's relating to these chemicals.
Risk factors
The only definite risk factor for Parkinson's is age, with, at this stage, no real proof of the role played by other environmental or genetic risk factors.
More study is needed into risk factors and some of the factors that may be associated with increased risk are:
- age;
- male gender;
- Caucasian ancestry;
- herbicide/pesticide exposure;
- rural residence;
- higher intake of dietary fats;
- conservative pre-Parkinson's personality;
- metal exposure;
- family history of Parkinson's;
- extreme stress; and
- head trauma.
- cigarette smoking;
- anti-oxidants in the diet; and
- early life measles infection.
References:
1. 'Epidemiology of Parkinson's.' A. H. Rajput, Saskatoon, Canada; Carolyn M. Tanner, Sunnyvale, California. Seminar presentation, 5th International Congress of Parkinson's Disease and Movement Disorders, New York, 1998.
2. 'Smoking and Parkinson's disease: a case-spouse control study.' R. Kao, S. Fahn, W. Tsai, P. Greene, New York Poster presentation at the 5th International Congress of Parkinson's Disease and Movement Disorders, New York, 1998.
Last Reviewed: 23 July 2002
