Mild cognitive impairment: directions for future research.

Mild cognitive impairment (MCI), an intermediate state between normal aging and dementia, is characterized by acquired cognitive deficits, without significant decline in functional activities of daily living. Studies conducted on MCI have introduced new concepts regarding the possible distinctions between normal and pathologic aging of the brain. Neuroimaging and genetic testing have aided in the identification of individuals at increased risk for dementia. The measurement of change in cognitive and functional status in MCI remains challenging, because it requires instruments that are more sensitive and specific than those considered adequate for research in dementia. The authors provide an overview of the many methods that have been used to study MCI and directions that may help achieve greater uniformity in methodology. Considerable heterogeneity exists in research methodology used to study the epidemiology, thresholds for cognitive and functional impairment, rate of progression, risk factors, and defining subtypes of MCI. This article emphasizes the need for uniformity in the use of 1) appropriate and sensitive neuropsychological and functional measures to diagnose MCI, 2) reliable methods to determine progression or improvement of cognitive impairment, and 3) instruments in epidemiologic studies to establish population estimates for diverse ethnic and cultural groups. Greater consensus is needed to standardize definitions and research methodology for MCI, so as to make future studies more comparable and more useful for designing effective treatment strategies.