To figure out what was up with older people who consistently reported memory problems, the researchers performed MRI on 120 adults over 60 years old who had been extensively tested for memory impairment. Based on test results, information from self-reports and from informants, 40 of the group had been classified as completely normal (healthy control) and 40 were classified as mildly cognitively impaired (MCI). A group of 40 who tested normally but had more cognitive complaints than the healthy group were designated the cognitive complaint (CC) group. Though common factors causing memory complaints in older adults, like depression, medication, or white mater pathology, were ruled out in the CC group, they did have a lower composite score for verbal memory performance, albeit within the normal range. MRI scans were analyzed to measure hippocampal volume and grey matter density.

As expected, compared to healthy normal individuals the MCI group showed significantly lower gray matter in the medial temporal lobe and in distributed cortical regions, but surprisingly, the CC group also displayed a loss of gray matter density. Gray matter densities in the CC group were intermediate between the normal and MCI groups, suggesting that the CC subjects might represent a stage between normal aging and MCI, a clinical prodrome of AD (see ARF related news story).

Changes in gray matter density were more sensitive to the CC state than measurements of hippocampal volume made in the same subjects. In accordance with many studies, the MCI group showed significantly smaller hippocampal volume compared to healthy controls, while the CC group did not show a visible change in volume. Thus, density changes may be a better measure of the early preclinical stages of brain disease, and could be useful for early detection.

The changes in tissue density faithfully tracked with cognitive function: in the whole sample, lower verbal memory performance was associated with lower gray matter density in the left mediotemporal regions. Interestingly, higher levels of cognitive complaints correlated with decreased hippocampal gray matter density.

The results suggest that current neuropsychometric testing regimens are not sensitive enough to pick up subtle changes in mental function related to early brain atrophy, particularly among highly educated, high-functioning elders like those who made up the study group. In this regard, Saykin’s results recall a fascinating report last year on an aging chess player who complained of cognitive impairment because he could only plan four moves ahead in his game, where previously he could plan seven (Archer et al., 2005). Eventually, his prodigious mental capacities declined enough to be classified as MCI. When he died of unrelated causes shortly thereafter, an autopsy revealed severe AD.

That case, and this work, highlight the clinician’s difficulty in detecting subtle memory problems in people with high baseline functioning. Since the criteria for MCI are based on achieving a level of cognitive function relative to the population mean, people who start higher will necessarily have to decline more to reach criteria. The present work suggests that by taking measure of subjective complaints, and by following patients carefully over time, physicians may be able to catch pathology earlier.

“These findings highlight the importance of CCs in the clinical evaluation of older adults and suggest that those who present with significant CCs warrant evaluation and close monitoring over time,” Saykin and coauthors write. Taking note of patient-perceived, pre-MCI changes in mental function could open a door to early intervention with existing and future treatments.—Pat McCaffrey.

Reference:
Saykin AJ, Wishart HA, Rabin LA, Santulli RB, Flashman LA, West JD, McHugh TL, Mamourian AC. Older adults with cognitive complaints show brain atrophy similar to that of amnestic MCI. Neurology. 2006 Sep 12;67(5):834-42. Abstract