Neurological disease casts a heavy shadow over the lives of the patient, their family and friends and over society. In a recent survey, in Europe about oneāthird of all burden of disease was caused by brain disease ā 23% of the years of healthy life is lost (YLL), 50% of years lived with disability (YLD) and 35% of disabilityāadjusted life years (DALYs). The aim of all neurological services must be to alleviate the suffering associated with the disease, and to realise this aim the rational planning of such health services requires epidemiological knowledge in five broad areas:
- Epidemiology of the condition ā its frequency and distribution within a population, its causation, mortality and coāmorbidity.
- Broad impact of the disease (the āburden of illnessā) on individuals, families, health services and societies and also its financial cost.
- Effectiveness and costāeffectiveness of diagnosis, investigation and treatment.
- Existing health care resources ā their distribution and priorities, and the potential for prevention.
- Prognosis and outcome, via cohort studies and caseācontrol studies.
The last three areas are outside the scope of this chapter; here an overview of selected issues related to the epidemiology and burden of neurological illness is given and, as this book is based on practice at Queen Square, here too I emphasise studies from the National Hospital for Neurology and Neurosurgery and the UCL Institute of Neurology. These set the scene for the more detailed consideration of neurological disease contained in the rest of the volume.
Epidemiology of neurological disease
It is selfāevident that knowledge of epidemiology is important to underpin any decision about the provision of health care resources. It is also clear that epidemiological data (on frequency, distribution, mortality, etc.) are of little practical value unless related to an intervention or therapeutic advance. Sadly, however, in practice, even where reliable data exist, these are used only inconsistently in planning health care. Neurological disease is one example of this depressing fact, for the amount of education and expenditure is far below its estimated impact. In many, indeed perhaps most, health care settings, the provision of facilities for neurological care is often surprisingly fragmented and inappropriately targeted, even where, as in the United Kingdom, there is a nationwide health service.
Frequency and distribution of neurological disease
Incidence and prevalence rates are the most common measures of frequency used in medicine.
Incidence is a measure of the rate at which new cases occur in a specified population during a specified period. The incidence rate is usually calculated as the number of new cases occurring per 100 000 of the general population per year.
Prevalence is defined as proportion of a population that are cases at a point in time. The prevalence rate is usually calculated as the number of existing cases per 1000 of the general population. Point prevalence is calculated as the number on a particular day (prevalence day) and period prevalence is calculated as the number in a population over a specified period of time. Lifetime prevalence is defined as the risk of acquiring the condition at any time during life and is another important figure.
For many neurological diseases, information on even these basic measures is incomplete. Furthermore, the frequency of many neurological disorders varies markedly in different geographical regions, differs in urban when compared with rural settings, may differ with ethnicity, and is often linked to lifestyle and socioāeconomic factors.
In most neurological illnesses there are also striking differences in frequency at different ages, and so the age distribution of the population will affect the frequency, and some diseases have marked gender differences. For these reasons, ageāspecific or sexāspecific rates, or frequency estimates in restricted age ranges, are generally more informative than crude rates. For instance, the annual incidence of stroke in a general population is about 190/100 000/year, but in the population over 65 years the rate is 1100/100 000/year. Similarly, the incidence and prevalence of Parkinsonās disease in the general population are 20/100 000/year and 2/1000, and in those over 65 years are 160/100 000/...
