The concept of disease

Disease as entity or as deviation from normal functioning

Kendell (1987) has pointed out that none of the four types of psychosis – schizophrenic, affective, good prognosis acute and chronic paranoid – discussed above has been clearly demonstrated to be a disease entity. Does it matterf? Kraepelin and Bleuler thought it did. But even so well 'validated' a disease as tuberculosis cannot be said to be an 'entity' in the sense that everyone with the bacillus in the bloodstream has the same (or any) symptoms, let alone the same course or outcome.

Cohen (1961) argued that the concept of disease that 'still dominates our textbook descriptions, as illustrated by the socalled classical pictures of typhoid fever, influenza, disseminated sclerosis and the rest, is little more helpful in diagnosis than would be a composite portrait of a football team in revealing whether any one individual is a member'.

Throughout medicine, but particularly during the past 50 years, rigid disease categories have been replaced by more useful concepts that are constantly evolving in the light of the experimental evidence. As disease concepts evolve in the light of the successes and failures of hypothesis testing, it becomes obvious that some diseases previously thought to be 'entities' are actually linked and that the fundamental processes involve deviations or blockages in the functioning of normal homeostatic cycles. Hypertension, diabetes and coronary heart disease are obvious examples, but so, increasingly, are most well-known diseases. Different defining formulae that use the same name have to fight against each other for survival. Which is more successful at any one time depends on the weight of evidence its protagonists can provide.

The evidence must also include the epidemiology of schizophrenia: the genetics, age and sex distribution, excess of births in the winter months and the possibility that the course and even the incidence varies both geographically and over time.

Hierarchy in psychiatric disorders

Some sense can be made of the relationships between psychiatric disorders if dimensional as well as categorical concepts are borne in mind. Both are useful so long as it is recognized that it is essential to move easily between them. The ancient hierarchies divided mental faculties into conative, cognitive and affective. If brain function is profoundly impaired, there can be no or only negligible function of will. Movement, thought and emotion will then be absent or distorted.

It is possible that each of the three faculties can be impaired independently of the others and that the conventional hierarchical system of diagnosis would have more than just practical use if based on three or four dimensions (the extra one representing motor functions). Positive and negative aspects would then be represented at every level.