Stability of age-associated memory impairment during a longitudinal population-based study.

Abstract

To the Editor: The National Institute of Mental Health established criteria (NIMH) for "age associated memory impairment"(AAMI) to describe memory loss that may occur in healthy elderly subjects.1 We examined the stability of AAMI and whether aging-related decrease in cognition is stable or progressive in subjects with AAMI compared with subjects without AAMI. We also examined whether age, education, or depressive mood contribute to the AAMI. A random sample of 1300 subjects born between 1912 and 1921 was drawn from the Kuopio population register.2 In the baseline examination, 122 subjects fulfilled the NIMH criteria for AAMI. One and one-half years later, 88 of the AAMI subjects (72.7%) participated in the follow-up examination. Education, age, or vocabulary3 did not differ between those AAMI subjects who participated in the follow-up examination and those AAMI subjects, who did not. We selected a control group from study subjects including subjects who did not fulfill the criteria of subjective or objective memory impairment for AAMI using the same exclusion criteria. Altogether, 96 subjects met these criteria in the baseline and follow-up examinations. The examinations also included further assessment of memory with a Word List Learning Test4 and Heaton Visual Memory Test5 with immediate and 30-minute delayed reproduction of the figures and copying the stimulus figures. The Verbal (VFT) and Category Fluency Test (CFT)6 were used. Of the original 88 AAMI subjects, 56 (63,6%) could be classified as AAMI in the follow-up examination: they exhibited stable AAMI (SAAMI group). Seven subjects (12.1%) no longer showed subjective memory complaints according to the Memory Complaint Questionnaire (MACQ).3 Twelve subjects (21.5%) no longer showed the objective memory impairment according to the Benton Visual Retention Test (BVT)3 or the Paired Association Test (PAL).3 These 19 subjects exhibited unstable AAMI (UAAMI). Four subjects (6.4%) had a Mini -Mental State3 score of 23 or less. Six subjects (10.6%) had illnesses that were exclusion criteria. Three subjects could not be classified because of incomplete data. Altogether 32 (36.3%) of AAMI subjects did not fulfill the criteria of AAMI in the follow-up. Some subjects had changed their opinion in self-rated memory impairment, and objective memory impairment disappeared in many cases at follow-up. The re-test reliability of the Memory Complaint Questionnaire (MACQ)3 at 4-week intervals has previously been fairly good (.64), but in the present study, at the 1.5-year it was only .46. Changes in mood may also contribute to changes in subjective and objective memory impairment. Multiple forward stepwise regression analysis, at the .05 significance level, was performed for age, education, the baseline scores of the MACQ, and the Geriatric Depression Scale (GDS)7 using the MACQ, the BVT, and the PAL separately as a dependent variable separately for each group. The relationship with depressive mood explained 7% of the variance in the MACQ (F(df = 1,53)5.1, P < .05) in the SAAMI group. Depressive mood explained 44% of the variance in BVT in the UAAMI group (F(df = 1,17)15.3 P < .001). Improved affective state may explain at least partly, why 22% of AAMI subjects no longer showed objective memory impairment at follow-up. To examine aging-related decrease in cognition, multivariate analysis of variance (MANOVA) was used. MANOVA showed an over time change in the immediate and delayed reproduction part as well as the copying of the figures of the Heaton Visual Memory Test (Table 1), but this change did not differ between groups. Scores of all groups deteriorated in immediate recall and improved in delayed recall of figures during follow-up. All groups performed more poorly at copying the figures. The AAMI and control subjects were divided into two groups according to apolipoprotein E (APOE) genotype: a group with one or two ε4 alleles and a group without any ε4 alleles. Frequencies of alleles did not differ between groups. The SAAMI subjects with ε4 alleles were more impaired in the CFT than SAAMI subjects without ε4 alleles (Table 2). Previously, AD patients were found to be impaired in producing abstract category exemplars, but they encountered few, if any, problems in the letter fluency tasks.8,9 In conclusion, at the 1.5-year follow-up, 36% of AAMI subjects no longer met the criteria of AAMI. This may be attributable to improved affective state, to the practice effect in objective memory testing, and to low reliability of subjective memory rating over long intervals. Age-related decrease in visual function was found, but no systematic deterioration of memory function. It may be possible to find subjects at risk for dementia by combining apolipoprotein E ε4 genotype and cognitive decline with repeatedly confirmed AAMI.