The concept of delirium.

The concept of delirium has a long and confusing history. This article outlines the development of ideas relating to core features of the syndrome: disturbance of consciousness, disturbance of cognition, its course and its external causation. The modern concept of delirium, and the diagnostic criteria found in current classifications are based upon a long tradition of clinical observation in younger patients, and their emphasis on positive symptoms and identifiable external causes may not be applicable to our ageing population.