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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 AIDS 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 modern medicine offers the means of treating several of these conditions, and in some instances, of restoring the patient to normal mental competence. Moreover, modern diagnostic technologies now allow earlier recognition of the underlying pathologic process, thus improving the chances of recovery or of preventing the disease’s progression.

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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 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, it is illogical to set apart this constellation of cerebral symptoms on the basis of speed of onset, rate of evolution, severity, reversibility, or duration; in other words, dementia can be the result of a static encephalopathy such as head trauma or cerebral anoxia. We propose that 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.

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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.

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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). It is global because it characterizes an individual’s behavior as a whole; it is an aggregate in the sense that it is composed of a number of independent and qualitatively distinguishable cognitive abilities. This topic should be of interest ...

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