PREVALENCE RATES OF MILD COGNITIVE IMPAIRMENT SUBTYPES AND PROGRESSION TO DEMENTIA (2006)

To the Editor: Zanetti and colleagues reported the results of 400 community-dwelling subjects aged 65 and older who came freely to the geriatric unit as part of a comprehensive geriatric assessment program and were evaluated for predementia syndromes. The authors reported a prevalence rate of mild cognitive impairment (MCI) of 16.2%, including amnestic MCI (aMCI) (7.8%) and multiple cognitive domains slightly impaired MCI (mcdMCI) (8.5%). Twenty of the 65 subjects with MCI (31%) progressed to dementia within 3 years. All patients who progressed to Alzheimer's disease (AD) had been classified with aMCI at baseline, whereas all patients who progressed to subcortical vascular dementia (VaD) had been classified with mcdMCI at baseline, suggesting that this MCI subset could be an early stage of subcortical VaD.1 In the Italian Longitudinal Study on Aging, a population-based study with a 3.5-year follow-up involving 2,963 individuals aged 65 to 84, we found a prevalence rate of 3.2% for MCI and a progression rate to dementia of 3.8 per 100 person-years. In addition, of those who progressed to dementia, 60% progressed to AD and 33% to VaD.2 We used a more general concept of MCI, selecting these MCI patients on the basis of the memory loss. Therefore, many of them may have had aMCI, but we did not subdivide MCI into different subtypes. Unfortunately, in our study, we did not report a lower educational level or any difference in sociodemographic features for mcdMCI subjects, as mentioned in the Discussion section of the study of Zanetti and colleagues. Moreover, our findings suggest that a predementia syndrome with a cognitive pattern similar to AD and with a central role for memory decline may also progress to VaD.2,3 Population-based studies estimate that the prevalence of MCI is more than double that of dementia.4 However, the diagnostic criteria and referral patterns used vary between studies, causing discrepant prevalence estimates.4 Prevalence estimates of amnestic variants of MCI in other worldwide population-based studies are consistent with those reported in our study: 3% in France, 3.1% in Germany, 3.02% in Canada, and 3.2% in the United States (Monongahela Valley Independent Elders Survey), with one notable exception.4 In fact, the Cardiovascular Health Study Cognition Study found a prevalence of aMCI of 6% of 599 participants, whereas the overall prevalence of MCI, including mcdMCI, was 19% of 2,470 participants.5 Other population-based studies reported higher prevalence rates, more similar to those reported in the study of Zanetti and colleagues but with MCI constructs different from the amnestic variants. A prevalence of 6.1% was reported in an elderly Finnish population aged 65 to 79,6 but MCI was defined according to criteria that were more similar to those of age-associated cognitive decline or mcdMCI.3 Frisoni and colleagues found a prevalence rate of 15% in a population-based study conducted on subjects aged 75 to 95 by defining MCI based only on a score 1.0 standard deviation below the mean of age- and education-specific norms on the Mini-Mental State Examination.7 Finally, in the Leipzig Longitudinal Study of the Aged, in a community sample of 1,045 dementia-free individuals aged 75 and older, in which the criteria for MCI subtypes were codified and validated, the prevalence rates ranged from 1% to 15% depending on the subset employed.8 In conclusion, the higher prevalence and progression rates of MCI and aMCI in the study of Zanetti and colleagues, more similar to those of clinical-based samples,9 could be due to the referral pattern of these community-dwelling older adults, who freely chose to come to a geriatric unit as part of a comprehensive geriatric assessment program. The program of the study of Zanetti and colleagues was not dedicated to the recognition of memory impairment, but the patients' self-referral, as acknowledged in the report by the authors, could strongly influence the prevalence and progression rates of this study. Persons who have been referred to a memory clinic and other clinical research centers are likely to be at a different stage of cognitive decline than cognitively impaired individuals from community samples, because they noticed a memory problem and actively sought help for it.4,10 In these clinical research settings, individuals with MCI have been shown to progress to dementia at a rate of 18% per year.9 Findings of population-based studies have shown that up to 44% of patients with MCI at their first visit were estimated to return to normal a year later.4 In the study of Zanetti and colleagues, 70% of the MCI patients were stable at a 3-year follow-up, but no patient showed any objective improvement in cognitive function. Financial Disclosure: This work was supported by the Italian Longitudinal Study on Aging (Italian National Research Council—CNR-Targeted Project on Aging—Grants 9400419PF40 and 95973PF40) (Dr. Panza, Dr. D'Introno, Dr. Colacicco, Pr. A. Capurso, and Dr. Solfrizzi). Author Contributions: Dr. Panza contributed to concept, interpretation, and manuscript preparation. Dr. C. Capurso, Dr. D'Introno, Dr. Colacicco, Pr. A. Capurso, and Dr. Solfrizzi contributed to interpretation and manuscript preparation. Sponsor's Role: The funding agencies had no role in design or conduct of the study.