Mild cognitive impairment in a population-based epidemiological study

Background: It would be of considerable importance to be able to estimate the rate at which subjects with mild cognitive impairment (MCI) progress to dementia in a cohort of a population-based epidemiological study and to establish simple diagnostic methods for the identification of people at high-risk of dementia. Subjects in a community based elderly cohort of MCI were followed longitudinally. Methods: Subjects were selected from participants in the first epidemiological study conducted on all persons aged 65 years and older residing at home in Nakayama, Japan, using the Mini-Mental State Examination (MMSE). Mild cognitive impairment was defined as memory deficit with objective memory assessment, without dementia, impairment of general cognitive function or disability in activities of daily living. The conversion rate was calculated using the person-year method. Results: At baseline, the sample consisted of 104 subjects selected from 1162 community dwellers aged over 65 years of age. During the 5-year follow up, 11 subjects (10.6%) were diagnosed with Alzheimer's disease (AD), five (4.8%) were diagnosed with vascular dementia (VaD) and six (5.8%) were diagnosed with dementia of other etiology. In this survey, the annual conversion rate of MCI to AD was calculated as 8.5% per 100 person-years and shifted to dementia at a rate of 16.1% per 100 person-years. The proportion of subjects with MCI who progressed to develop dementia was the same as in previous reports. However, nearly 40% of subjects returned to normal. Conclusion: The MCI concept, as currently defined, is an unstable and heterogeneous group.