Ten-year risk of dementia in subjects with mild cognitive impairment.

We read with great interest the study by Visser et al. The authors show that the majority of their patients (40-85 years old) with mild cognitive impairment (MCI) did not progress to dementia in a 10-year follow-up. In addition, the dementia risk in subjects with MCI was strongly dependent on age and varied with the definition of MCI used. [1] The authors discuss these results in relation to studies that included subjects with MCI across a smaller age range and with a shorter follow-up. [2,3] In the Italian Longitudinal Study on Aging (ILSA) [2], we evaluated 2,963 individuals from a population-based sample and found a progression rate to dementia of 3.8/100 person-years in a 3.5-year follow- up. We used a more general concept of MCI, selecting these MCI patients on the basis of the memory loss. Therefore, they may be well represented by amnestic MCI (aMCI), and in the study by Visser et al this was the only predementia syndrome subgroup that developed dementia in the long term. [1] Compared to population-based studies with a 10-year follow-up that used the definition of aMCI and were conducted in subjects of on average 75 to 77 years, [3] the 10-year dementia risk in the group of subjects with aMCI of 70 to 85 years old from the study by Visser et al was 0.55 to 0.72 higher. [1] This finding suggests that the dementia risk of subjects with MCI is substantially higher in a memory clinic setting than in population-based settings, confirming the heterogeneity in progression to dementia of MCI and other predementia syndromes of population-based studies with shorter follow-up. [2-4] Previous data suggested that, in contrast with clinical-based studies where progression is more uniform, in population-based studies the MCI classification is unstable. The apparent homogeneity of MCI and its progression to dementia in clinical-based samples may reflect referral patterns and selection criteria suggesting that MCI is a heterogeneous descriptor and that the outcome at follow-up depends on which population is studied and how MCI is defined. [3,4] Furthermore, the study by Visser et al demonstrates that the risk for dementia in the long term was dependent on the definition of MCI. The highest risk for dementia was observed when the criteria of aMCI and mild functional impairment (a score of 3 on the Global Deterioration Scale) were used. [1] In particular, several recent studies have suggested that subjects classified as having MCI could have visual, auditory, or muscoloskeletal disabilities impairing their activities of daily living (ADL), in the absence of functionally disabling cognitive impairment. [3,5] In the early stages of cognitive impairment, subtle but important changes in everyday functional competence are evident. In the ILSA, while we generally adhered to the diagnostic criteria for MCI as defined by Petersen et al, [2,3] we allowed for the presence of noncognitive disabilities and comorbid illnesses. In conclusion, we suggest that a predementia syndrome with higher predictive diagnostic value may have a cognitive pattern with a central role for memory decline (aMCI) and with the category of normal "ADL/instrumental ADL" criterion suggesting that the diagnosis of MCI should include subtle functional changes in everyday life activities (mild functional impairment).