Over the last two decades, increasing interest has been focused upon measuring the progression, impact and outcome of chronic neurological disease. The derivation of quantitative indices is important because such information has a wide range of applications. These include:

1. Exploration of pathogenesis. The time course of evolution has been a traditional tool for investigating the nature of the pathological mechanism underlying neurological diseases. The slow but inexorable advance of neurodegenerative processes contrasts substantially with the sudden onset of cerebrovascular disease or the fluctuating natural history of immunologically mediated neuropathology.

2. Evaluation of therapy. The need for quantification of symptoms, signs and functional impairment is self-evident in the fields of pharmacotherapy and surgical therapy. A comparison between active treatment and placebo, or between alternative forms of therapy, only becomes possible through the application of measurements of outcome.

3. Patient care. Acquisition of measures of disease progression is of value in the education of patients and their caregivers. The correlation of disease duration with disability provides a basis for estimating prognosis. This is of particular significance for planning the future practical needs of the patient.

4. Planning health care. Politicians, economists and sociologists must have information upon which to base their decisions when designing and implementing public and private health care systems. Quantitative assessment of outcome is a vital component of the equation that must be used to balance the requirements of society against the resources that are available.

General principles

Crude information on the outcome of disease, such as death, is useful in providing a coarse profile of events. More sensitive indices have been developed to generate a more detailed picture of the evolution of chronic illness.