The real and still ongoing challenge is the clinical definition of these conditions of slight cognitive deficit, and their distinction from normal aging; this is partially due to the fact that a change in cognitive performance is commonly an expected consequence of normal aging. The ability to identify the subgroup of elderly people who will develop dementia has therefore very important practical importance: in the short term the identification of these individuals would provide reliable prognostic information to patients and their families, in the long term it is the first step toward effective prophylactic and social medical intervention.
Many related and overlapping entities have been proposed during the last few years, and a profusion of terms and concepts currently exists in the field. Differences between the definitions of these conditions of minimal cognitive impairment reflect the controversial concept of "cognitive normality"in elderly persons.
Some researchers affirm that the goal is maintenance of the same performance levels shown by a young person, and one definition, that of "age-associated memory impairment (AAMI)", compares the performance of elderly subjects with that of younger persons ; up to 80% of individuals in their 80s will fall into the AAMI category by demonstrating memory performance at least 1 standard deviation (SD) below mean test values for younger subjects
, but longitudinal follow-up shows this group to be heterogeneous, consisting of both individuals preserving their cognitive functions and subjects deteriorating towards dementia
. This underlines the point of view that normality must be determined with respect to a homogeneous age group, and that cognitive aging is a normal phenomenon to be defined as cognitive performance at the same level as others of the same age.
In 2000, the Canadian Study of Health and Aging (CSHA) defined the concept of "cognitive impairment no dementia" (CIND) on the basis of a consensus conference of physicians, nurses and neuropsychologists . The CIND concept reflects essentially the presence of cognitive impairment in the absence of dementia, on the basis of clinical and neuropsychological examination, regardless of its causes (neurological, psychiatric or medical) and its degree ; "aging-associated cognitive decline (AACD)" was operatively defined as a history of cognitive decline during at least 6 months, with difficulties in several cognitive domains including, but not limited to, memory, and with low test scores in the relevant domains, in absence of dementia ; this concept reflect a somewhat different approach, focusing on patients' and families' complaints of memory and cognitive loss as starting point. It is well known that elderly subjects might complain of memory loss as a result of anxiety, mild depression or dementia in other family members or friends, but at the same time other studies show that memory complaints in elderly people deserve to be taken seriously, at least as early sign of actual decline, and investigated properly .
Other entities are based solely on test performance, and are called "age-consistent memory impairment" and "late-life forgetfulness" ; the stage called "questionable dementia" on the Clinical dementia Rating Scale (CDR) , rated as 0.5 on a scale of 0 to 3, represents the same concept of preclinical dementia, but based on history and clinical judgment, without considering neuropsychological test scores.
The most widely accepted concept to date is termed Mild Cognitive Impairment (MCI), as defined by Petersen et al in 1999 . Before the definition as an isolated memory deficit, the term had already been used to define an early stage on the Global deterioration Scale [15,16]. Having been broadened to include variants with impairments in other cognitive domains, MCI describes a cognitive state intermediate between normal aging and dementia; often with the implication that is a risk or prodromal state for AD or other dementias . The clinical validity of MCI concept has been demonstrated both with cross-sectional studies examining cognitive function  and longitudinal studies examining rates of decline in MCI subjects . However, some Authors argue that MCI cannot be a diagnostic entity, and that it seems to increase risk not because it creates a predisposition for AD but because 20% of those with MCI already have AD .
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