Dementia--epidemiological considerations, nomenclature, and a tacit consensus definition

Epidemiologic inquiry requires the definition of a "case." Dementia may be defined clinically or alternatively by inference of irreversible brain pathology. Several iterations of the Diagnostic and Statistical Manual of Mental Disorders and International Classification of Diseases have skirted this issue by using criteria that are at once syndromic and neuropathological. The limitations of this compromise are revealed by large discrepancies in case identification when the various published criteria are strictly applied. Despite this problem, neuroepidemiologists have produced convergent estimates of the prevalence and incidence of dementia and its association with risk factors. This progress has reflected the tacit reliance of investigators on a simple definition of dementia as the syndrome of substantial global cognitive decline not attributable to alteration in level of consciousness. Beyond this description, our knowledge of pathology and, ultimately, the etiology of individual cases is extremely variable. Whatever its antecedents, syndromically defined dementia presents a looming public health crisis.