ABSTRACT
The purpose of a diagnosis is to bring together illnesses that have the same characteristics. Ideally, a diagnosis should identify disorders having the same underlying pathology, the same causes, and the same likely response to treatment. However, such an ideal is rarely achieved. Thus a diagnosis of Huntington’s chorea identifies a necessary cause (an autosomal dominant gene), a pathology (cortical and extra-pyramidal degeneration), a clinical presentation (dementia and choreiform movements with onset usually between ages 30 and 50) and a course (progressive deterioration resulting in death). Ideally the diagnosis should also indicate an expected response to treatment, which in this example is at present limited to genetic counselling and symptomatic care. Although this diagnosis identifies a disease process, there are some limitations in its value, because not all those who suffer the disease will conform to this typical pattern. For example, some will show choreiform movements with little or no evidence of dementia. The age of onset will vary, and occasionally presentation may occur in childhood or old age. This illustrates the fact that all disease processes are subject to considerable variability.
