Mild cognitive impairment (MCI) is an intermediate state between normal cognition and dementia. While specific changes in cognition are frequently observed in normal aging, there is increasing evidence that some forms of cognitive impairment are recognizable as an early manifestation of dementia . As the following topic review will highlight, MCI is a heterogeneous state and there remains controversy over aspects of the construct. However, the utility of this paradigm centers around the recognition that dementia is not a dichotomous state; thus refining our understanding of the layers of transition will improve the understanding of cognitive decline and ultimately benefit patients.
This topic will review the definition of MCI and related terms, as well as the epidemiology, pathology, and clinical assessment of MCI. The prognosis and treatment of MCI and topics related to dementia, including diagnosis, treatment, risk factors, and prevention of dementia, are discussed separately. (See "Mild cognitive impairment: Prognosis and treatment" and "Treatment of dementia" and "Risk factors for cognitive decline and dementia" and "Prevention of dementia" and "Clinical features and diagnosis of Alzheimer disease".)
Mild cognitive impairment (MCI) refers to cognitive impairment that does not meet the criteria for dementia. Several criteria for, and subtypes of, MCI have been proposed [1-3]. These criteria and subtypes differ somewhat, although there is considerable overlap. In general, these criteria include a measurable deficit in cognition in at least one domain, in the absence of dementia or impairment in activities of daily living.
As originally constructed, the concept of MCI emphasized memory impairment and its status as a precursor state for Alzheimer disease (AD). Subsequently, it was recognized that MCI can be heterogeneous in terms of clinical presentation, etiology, prognosis, and prevalence [3-5], and the construct was expanded to broaden the scope of MCI to other cognitive domains, thereby extending the early detection of other dementias in their prodromal stages [6-8]. Although these criteria were developed as a concept relating early changes in specific cognitive domains to those areas most commonly affected in the disorders (eg, memory problems and AD), pathological and long-term clinical follow-up data to support this paradigm are limited [8,9].
These criteria are imprecise. Considerable judgment is required in making the distinction between impairments that are normal for the elderly population and, on the other extreme, that do not represent dementia. What constitutes impairment in daily living is different for each individual. Such distinctions draw upon clinical expertise and do not rely on psychometric testing alone. These appraisals can differ between assessors and this may account for some of the conflicting results in studies of this disorder. Some clinicians and investigators have challenged specific aspects of the criteria including the requirements for subjective cognitive complaints and intact activities of daily living [10-12].