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Increasingly, as the number of elderly in our population rises, the neurologist is consulted because an otherwise healthy person begins to fail mentally and loses his capacity to function effectively at work or in the home. This may indicate the development of a degenerative brain disease, a brain tumor, multiple strokes, chronic subdural hematomas, drug intoxication, chronic meningoencephalitis (such as caused by HIV or syphilis), normal-pressure hydrocephalus, or a depressive illness. Formerly, when there was little that could be done about these clinical states, no great premium was attached to diagnosis. But there are now effective means of treating several of these conditions, and in some instances, of restoring the patient to normal mental competence. Moreover, diagnostic technologies allow earlier recognition of the underlying pathologic process, thus improving the chances of recovery or of preventing the disease's progression.

The definitions of normal and abnormal states of mind were considered in Chap. 20, where it was pointed out that the term dementia denotes a persistent deterioration of intellectual or cognitive function with little or no disturbance of consciousness or perception. In current neurologic parlance, the term is used to designate a syndrome of failing memory and impairment of other intellectual functions as a result of chronic progressive degenerative disease of the brain. Such a definition may be too narrow. The term more accurately includes a number of closely related syndromes characterized not only by intellectual deterioration but also by certain behavioral abnormalities and changes in personality. Furthermore, dementia can be the result of a static encephalopathy such as head trauma or cerebral anoxia or of a progressive degenerative disease, but it differs from "encephalopathy" in its chronicity. Thus, it is not possible to determine if a confused, amnestic person is demented until some time has passed and the deficits have perished. Encephalopathies, by contrast, are largely reversible.

Beyond the need to properly define these terms, the two entities have different causes. There are several states of dementia of differing causes and mechanisms and that a degeneration of certain systems of cerebral neurons, albeit common, is only one of the many types. Thus, it is more correct to speak of the dementias or the dementing diseases.

To understand the phenomenon of intellectual deterioration, it is helpful to have some idea of how intellectual functions, particularly intelligence and memory, are normally organized and sustained, and the manner in which deficits in these functions relate to diffuse and focal cerebral lesions. The neurology of intelligence is considered in this chapter as a prelude to a discussion of the dementias and the neurology of memory.

Intelligence, or intelligent behavior, has been variously defined as a "general mental efficiency," as "innate cognitive ability," or as "the aggregate or global capacity of an individual to act purposefully, to think rationally, and to deal effectively with his environment" (Wechsler) in other words, the capacity to have ideas ...

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