Dementia is a syndrome characterized by deterioration in clear consciousness of multiple cognitive functions, which together constitute intelligence. 66 Dementia may be acute as after stroke, subacute as in prion diseases (such as Creutzfeldt-Jacob disease) or immunodeficiency syndrome (AIDS), or chronic as in Alzheimer's disease or hydrocephalus. In most cases, social function is affected, but some patients maintain their role because of assistance from family or friends. Dementia is differentiated from delirium, mental retardation, and specific cognitive impairments. Mental retardation is lifelong impairment. Delirium is cognitive impairment with an alteration of consciousness. Focal impairments include the amnestic disorders (e.g., Korsakoff's psychosis), in which memory is impaired out of proportion to other cognitive defects; and aphasia, a disorder of language comprehension and expression. This article discusses the clinical process by which a diagnosis is reached. For Alzheimer's disease, as it is the most common dementia disorder, this article discusses the several ways in which it can present depending on how far advanced it is, and whether or not various noncognitive symptoms and additional medical or psychiatric conditions also are present. The process of the differential diagnosis of dementia varies with geographic location, as it is affected by the prior probability or prevalence of any condition at a particular site. Several autopsy series from the United States indicate that most cases of dementia are caused by Alzheimer's disease, with vascular disease next. Ten percent or less of cases are reversible. 70 Through the use of modern techniques of imaging, the diagnostic accuracy of Alzheimer's disease during life is approximately 85%. 61 Although a treasured medical rule of differential diagnosis is to look for a horse not a zebra at the sound of hoofbeats, a doctor whose practice is in the African plains might very well diagnose a zebra and be right. Furthermore, if there were special value to identifying a zebra, as in the case of uncovering a rare but curable disease, zebras should always be considered even if unlikely. In addition to the clinical process, the physician must differentiate those cases that fall into particular social designations such as disabled or not, 55 competent or not, 51 responsible or not, and requiring hospitalization or not. 13,28 These judgments depend largely on the presence of a particular syndrome, its severity, and the accompanying behaviors that affect the person's social function, not on the basis of pathology or etiology.

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