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Dementia is an acquired, generalized, and usually progressive impairment of the content of consciousness. Dementia differs from other disorders of cognitive function, such as coma (see Chapter 3, Coma) or confusional states (see Chapter 4, Confusional States), in that in dementia, the level of consciousness (wakefulness or arousal) is preserved.

Although the prevalence of dementia increases with advancing age (Figure 5-1), dementia is not an invariable consequence of aging, and results instead from diseases involving the cerebral cortex, its subcortical connections, or both. Normal aging may be associated with minor alterations in neurologic function (Table 5-1) and with neuroanatomic changes, such as enlargement of cerebral ventricles and cortical sulci seen on computed tomography (CT) or magnetic resonance imaging (MRI) scans. However, these alone do not imply cognitive deficits. The term mild cognitive impairment (MCI) is sometimes used to describe deficits that are more severe than are customarily seen with normal aging but are insufficient to warrant a diagnosis of dementia. Nevertheless, patients with MCI have an increased risk (approximately 10% per year) of developing dementia.

Figure 5-1.

Relationship between advancing age and incidence and prevalence of dementia. (Data from Lobo A et al. Prevalence of dementia and major subtypes in Europe. Neurology. 2000;54[suppl 5]:S4; Fratiglioni L et al. Incidence of dementia and major subtypes in Europe. Neurology. 2000;54[suppl 5]:S10.)

Table 5-1.Neurologic Changes in Normal Aging.

Whereas dementia affects multiple spheres of cognitive function, more limited cognitive disorders may also occur. These include deficits in language function (aphasia) or motor (apraxia) or sensory integration, which are considered in Chapter 1, Neurologic History & Examination. Memory disturbance (amnestic disorder or amnesia), another example of a circumscribed cognitive defect, is discussed later in this chapter. Memory may also be impaired in normal aging and in dementia, but in the former impairment is mild, and in the latter it is accompanied by defects in other spheres, such as reasoning, judgment, behavior, or language. Some causes of dementia, notably Alzheimer disease, produce early and disproportionate impairment of memory and, at least in the early stages of disease, may be difficult to distinguish from a pure amnestic disorder.


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