Introduction

Mild cognitive impairment (MCI) is a clinical syndrome that represents a decline of cognitive functioning that is not sufficient to meet criteria for dementia. The cognitive decline is greater than that expected for normal aging but is not sufficient enough to interfere with most activities of daily living. In adults over 65 years of age, cognitive impairment without dementia is two to five times more common than dementia and occurs in approximately 11–27% of the population. Most studies suggest that the prevalence of cognitive impairment increases with age. For example, Unverzagt and colleagues reported that 29% of individuals between 65 and 74 years of age and as many as 55% of individuals over age 84 exhibited symptoms of cognitive impairment (without dementia). However, there is considerable variability in prevalence estimates for MCI, in part because of different methods for ascertaining and defining MCI.

Neurodegenerative disorders are proposed to account for the majority of cognitive decline in older adults. Numerous studies suggest that individuals with MCI have an increased risk for converting to dementia, particularly Alzheimer's disease (AD). However, the majority of these studies have focused on MCI with predominantly memory impairment. In the studies at the Mayo Clinic, individuals with MCI convert to dementia at approximately 12% per year, compared with healthy older adults, who convert at approximately 1–2% per year. It has also been proposed that most neurodegenerative diseases have an MCI stage; however, the nature of the prodromal stages of non-AD dementias is not yet well known.