A longitudinal examination of attention, working memory and executive function deficits in subtypes of mild cognitive impairment

Mild cognitive impairment (MCI) has emerged as a classification for a prodromal phase of cognitive decline that may precede the emergence of Alzheimer's disease (AD). Studies of MCI show elevated rates of conversion to dementia at the group level. In contrast, longitudinal examination of individual outcomes in MCI samples reveal great heterogeneity, with up to 44% of persons remaining stable or improving in cognitive functioning. Recent research suggests that attention, executive and working memory deficits may appear at the earliest stages of AD and these deficits may be more predictive of the emergence of AD than memory processing deficits. In the present longitudinal study, 48 multiple-domain amnestic-MCI (a-MCI+), 12 single-domain amnestic-MCI (aMCI), 29 non-amnestic MCI (na-MCI), and 26 age- and education-matched controls undertook neuropsychological assessment of visual and verbal memory, attentional processing, executive functions, working memory capacity, and semantic language at 10 month intervals. The results indicate that both a-MCI+ and na-MCI display a decline in simple sustained attention with the a-MCI+ group also displaying a significant decline on a task of divided attention. However, both a-MCI+ and na-MCI were found to display stable deficits to all other measures of attention, working memory and executive functioning, with a-MCI+ also being associated with stable impairments to visual and verbal memory. Subsequent analyses indicated that outcome classification at 20 months varied considerably between MCI subgroups. Of the a-MCI+ group 25% progressed to AD, with none of the a-MCI or na-MCI groups progressing to AD. Among the different MCI subclassifications 12-42% improved to normal levels of function, 17-55% remained stable; and 10-24% transitioned to the a-MCI+ subtype. Discriminant function analysis identified that a combination of measures of attention, working memory, and visual, verbal and semantic memory at baseline correctly identified the outcome of 84% of participants at 20 months. The results of the present study raise questions regarding the specificity of existing criteria for the subtypes of MCI, with the results of this study indicating a high degree of instability in MCI classification over time