Introduction

The study of Parkinson's disease (PD) in populations – the epidemiology of PD – is an arduous and difficult undertaking. Descriptive epidemiology of a disease provides a picture of the prevalence – the amount of disease present in a given population; the incidence – how frequently new cases develop in a given population over time; the mortality – the risk of death associated with the disease; and the natural history of the disease. The risk of disease is best measured by incidence as prevalence figures can be distorted by differential survival between study populations. Analytical epidemiology investigates the associations, exposures, risk factors, and comorbidities of a disease in an effort to determine the aetiology. There are also increasing attempts to describe the disability, handicap, quality of life and health and social service provision resulting from specific diseases (see Chapter 8).

The major causes of neurodegenerative disease, Alzheimer's disease (AD), PD and motor neurone disease (MND) all share a strong age associated risk and are likely to be an issue of increasing public health concern as a result of the worldwide aging of populations. Certified deaths from AD, PD and MND have been projected to overtake cancer as the second commonest cause of death in the USA by the year 2040 (Lilienfield et al. 1990).

A major difficulty in interpreting the many and varied epidemiological studies of PD is that PD is a relatively rare disorder, the diagnosis of which, for practical purposes, relies solely on the clinical skills of history taking and bedside examination.