In a biracial urban community, individuals classified as having no cognitive impairment, MCI, or AD completed brief tests of cognitive function at 3-year intervals for up to 11 years. Compared to persons found to have no cognitive impairment, rate of cognitive decline during follow-up increased approximately twofold in MCI and fourfold in AD. The differences in rate of cognitive decline between AD and MCI underscore the clinical relevance of the progression to dementia. While MCI and dementia clearly exist on a continuum, the accelerating cognitive deterioration in dementia is highly relevant to clinicians counseling patients and families with dementia.
Progressive decline of cognitive function is the primary clinical manifestation of MCI and AD, but as noted above, quantification of this key disease consequence has proven difficult. The only previous population-based study of which we are aware found, like the present study, rapid cognitive decline in AD at about twice the rate seen in a subgroup with prodromal AD, with little to no decline evident in unaffected persons.28
Similarly, in 2 previous longitudinal cohort studies, cognitive decline was increased in MCI relative to no cognitive impairment,17,19
with an approximate doubling of the rate in 1 study.17
Overall, therefore, the present results confirm in a population-based setting the deleterious cognitive consequences of AD and its precursor, MCI, and provide quantitative estimates of the size of these effects.
In this population, we found no evidence of racial disparities in the cognitive consequences of MCI or AD. This is consistent with an earlier report of comparable mortality in affected African American and white subjects from this population.13
A previous study found reduced global cognitive decline in affected black persons compared to affected white persons,5
but in 2 other studies, this effect was only observed in a subset of cognitive outcome measures.4,6
This inconsistency may be due in part to the selected groups that were studied, making it difficult to form comparable subgroups of African American and white subjects, and the small number of African American participants (<130 per study).
Age was not related to rate of cognitive decline in the incident AD group. This is consistent with previous population data.28
By contrast, studies of persons identified in clinical settings have tended to find more rapid decline in younger affected persons,22,29,30
possibly due to age-related differences in factors that bring affected people to medical attention22,30
or to the inclusion of persons under the age of 65. In addition, gender was unrelated to cognitive decline in AD, consistent with previous studies in clinical settings.31–33
This study has several strengths. Participants were sampled from a geographically defined population. Clinical classification of MCI and AD was based on a structured uniform evaluation and widely accepted criteria applied by an experienced physician. The availability of a previously established composite measure of global cognition collected at regular intervals for up to 11 years with high follow-up participation enhanced our ability to reliably quantify individual differences in cognitive decline.
Several limitations should be noted. The robust difference between black and white participants in level of cognitive function complicates racial comparisons of rate of cognitive change, making it difficult to rule out the possibility of small racial differences in cognitive decline. All analyses were based on a single global cognitive outcome. Although it was able to accommodate wide individual differences in cognitive ability, differences in measurement sensitivity across the ability spectrum could have affected results. Also, results might vary across cognitive domains, and nonlinear models might improve estimation. For these and other reasons, multiple studies will be needed to securely determine the cognitive impact of AD. Finally, including prevalent cases of MCI may have introduced error into the estimate of decline in that group because change prior to study onset is unknown.