Objective: To estimate the prevalence and stability over time of Mild Cognitive Impairment (MCI) according to clinical criteria (Petersen et al 1999) within a representative community population.
Design: Retroactive application of operationalized version of MCI criteria to data collected at three successive biennial data collection waves in the course of a prospective epidemiological study.
Materials and Methods: The Monongahela Valley Independent Elders Survey followed a cohort of individuals aged 65+ years over a total of 15 years. At each wave, participants were screened with a cognitive test battery including but not limited to the Mini Mental State Examination (MMSE) and Delayed Recall of the 10-item CERAD Word List (DRWL). Functional impairment was assessed using the OARS scale for Instrumental Activities of Daily Living IADLs). Participants were asked for subjective reports of their own memory functioning. Based on clinical evaluation, subjects were rated on the Clinical Dementia Rating Scale (CDR).
Based on available data, the MCI criteria were operationalized as follows: (1) Impaired Memory: DRWL one standard deviation below mean; (2): Normal Mental Status: MMSE score 25+; (3) Normal Daily Functioning: 0 IADL impairments; (4) Memory Complaint: subjective report of memory being worse than it used to be; (5) Not Demented: CDR score less than 1. These criteria were applied to MoVIES cohort members who survived and participated in all data collection cycles from the second to the fifth biennial wave of followup.
Results: At Wave 5, the cohort had a mean (SD) age of 78.7 (5.0 years) and included 633 individuals without dementia (CDR less than 1). Among them, 23 persons (3.6%) met all criteria for MCI. Looking at stability/transitions over time from the second to the fifth wave (Wave 2- Wave 3, Wave 3- Wave 4, and Wave 4 -Wave 5), between 16.7% and 20.6% of those with MCI at one wave were still classified as MCI at the next wave. Between 35% and 50% of those with MCI had reverted to normal (i.e. no longer met criteria for MCI) by the next wave. Between 8.6% and 13.3% had developed dementia by the next wave, significantly higher (p < 0.05) than those who were normal at each wave, between 2.1% and 3.4% had developed dementia by the next wave.
Conclusion: Current clinical criteria for MCI , when applied retroactively to epidemiological data, identify a subgroup of approximately 3.6% of those without dementia. These individuals are at significantly higher risk of progressing to dementia, within two years, than normal individuals. However, only about a tenth of them progress to dementia, about a fifth remain stable, and a third to half revert to normal over two years, suggesting that MCI as currently defined is a high-risk but unstable and heterogenous group. Further empirical refinement of these criteria is recommended.
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