Mild cognitive impairment: challenging issues.

The demographic imperative of global aging is now ubiquitously discussed, and research on cognition and dementia has burgeoned over the past decade. Ironically, the question of how to detect and diagnose dementia in its clinical and preclinical stages has become more, rather than less, complex over time, promoting endless debates, practice guidelines, and competing sets of criteria. This issue of AJGP brings together a diverse and intriguing group of articles on the topic of aging-related cognition and dementia. Three of the articles address the concepts and concomitants of cognitive functioning and decline, examined from very different perspectives. The fourth reports neuropathological validation of the current clinical criteria for Alzheimer disease (AD). While at first glance these studies may seem to resemble a collection of apples and oranges, or even bagels and oranges, there is in fact an important theme that connects them conceptually and potentially leads us forward. From the well-known Kungsholmen Project in Stockholm, Laukka et al. report prospective, population-level data on “terminal decline” in cognition as older adults draw closer to the end of their lives. In this cohort of over 500 individuals aged 75 and older, the key finding was that much of this accelerated decline proximal to death is in fact related to preclinical dementia. However, there were also some distinct changes that predict hastened mortality in those free of dementia. From a different kind of sample, a group of 24 normal-functioning older volunteers at the San Diego, CA, Veterans Administration Medical Center, Fine et al. demonstrate the potential value of “cognitive discrepancy” analysis. This approach essentially examines the difference between an individual person’s standardized scores on two cognitive tasks. This intraindividual technique is contrasted to the more typical approach of comparing individual’s performance on a given task with those of others on the same task, e.g., comparing observed to expected performance based on group norms (as when classifying individuals’ scores based on the number of standard deviations below the mean of the appropriate peer group). The discrepancy between individual’s performances on higher and lower level conditions of the same executive function test was a strong predictor of subsequent cognitive decline. In a cross-sectional study of 77 adults, in an university medical setting, who were either cognitively normal or classified as having mild cognitive impairment (MCI), Jefferson et al. compared a novel instrument. They measured subtle errors in selected complex instrumental activities, such as cooking, with standard questionnaire-based scales on functional ability. Only the error-based scale distinguished between the two groups, and only this scale was associated with an independent cognitive measure (verbal learning). This study reinforces the importance of developing appropriately sensitive measures to detect the functional deficits associated with very mild MCI. The above three studies shed light on different pieces of the puzzle. Together they help address critical nuances involved in studying and interpreting cognitive function test performance. They are