Definition Of Dementia

Current scientific definitions of dementia focus on a loss of ability to comprehend information or one's environment as well as a lack of ability to act in a fashion that is appropriate to one's circumstances. In addition to incorporating the concept of a loss of cognitive functions from a presumably normal premorbid state due to a biological cause, most current definitions indicate that the affected subject retains relatively normal consciousness. In this respect, the definition offered by the International Neuropsychological Society dictionary (Loring, 1999), is typical: "Generalized loss of cognitive functions resulting from cerebral disease in clear consciousness (i.e., in the absence of confusional state)."

Consistent with the contemporary emphasis on quality of life in medical research and practice, many current definitions also note that the deterioration in mental functions disrupts the ability of the affected individual to carry out everyday life functions. Thus, according to Bondi et al. (1996, p. 165), dementia is "a syndrome of acquired intellectual impairment of sufficient severity to interfere with social or occupational functioning that is caused by brain dysfunction." Similarly Mesulam (2000, p. 444-445) defines dementia as "a chronic and usually progressive decline of intellect and/or comportment which causes a gradual restriction of customary daily living activities unrelated to changes of alertness, mobility, or sensorium." In contrast to dementia, MCI (see previous section) does not significantly affect everyday life functions. The most widely used definition of dementia in North American clinical practice is that offered by the American Psychiatric Association (2000) in the Diagnostic and Statistical Manual-IV-TR. The DSM-IV defines dementia as a syndrome characterized by multiple cognitive deficits including memory impairment (an inability to learn new material or a loss of previously learned material) and at least one of the following deficits: aphasia (a disturbance of language), apraxia (an inability to carry out learned movements despite intact sensorimotor functioning and comprehension of the task), agnosia (an inability to recognize something despite intact sensation), or executive dysfunction (problems with higher-order functions such as planning, initiation, sequencing, monitoring, stopping, and organizing). These cognitive deficits must be severe enough to cause significant impairment in occupational or social functioning and must represent a decline from a previously higher level of functioning. A diagnosis of dementia is not made if the cognitive deficits only appear during the course of a delirium. The DSM-IV requires that a dementia be related to a general medical condition or central nervous system (CNS)-active substance, and several etiologies of dementia are specified.

While widely used in clinical work and research, this definition of dementia is not without criticisms. A requirement of functional impairment decreases the sensitivity of the DSM-IV diagnosis of dementia. In highly functioning individuals, dementing illnesses often do not cause significant functional impairment early in their course, and even poorly functioning individuals in supported living situations may not show functional impairment until a dementing illness is severe. The DSM-IV requirement that memory impairment be present also decreases the sensitivity of the definition. Patients who do not develop obvious memory problems until late in the course of their illness (e.g., frontotemporal dementia patients) would not be regarded as demented in the early or middle stages of illness.

In an attempt to circumvent these problems, Cummings and Benson (1992) have put forth an alternative definition of dementia. They suggest that dementia is a syndrome characterized by acquired and persistent abnormalities in at least three of the following five domains: memory, language, visuospatial skills, personality or mood, and executive functioning. This definition of dementia does not require memory impairment, functional impairment, or a medical/neurological etiology and therefore has a higher level of sensitivity than the DSM-IV definition. A lower threshold for diagnosing early dementia may be important now because treatments are available that can slow the progression of some of the more common dementing illnesses discussed below.

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