Dementia From Alzheimer Disease and Mixed Pathologies in the Oldest Old FREE

To the Editor: The oldest old (≥90 years of age) are the fastest growing segment of the US population and account for half of all persons with dementia. Alzheimer disease (AD) is the most common pathology underlying dementia in the old (ages 65-89 years). Recent community-based autopsy studies^1- 2 suggest the relationship between AD pathology and expression of dementia is attenuated in the oldest old.³ Studies may be complicated by the common coexistence of AD plus infarct and/or Lewy body (LB) pathology (mixed pathologies).⁴ Few data exist on mixed pathologies and dementia in the oldest old.⁵ We examined the relationship of AD and mixed pathologies to dementia in the oldest old compared with the old. We tested the hypothesis that the clinical expression of AD and mixed pathologies differs across age groups.

We included 804 persons from the Religious Orders Study (n = 456) and Rush Memory and Aging Project (n = 348), ongoing longitudinal clinical-pathological studies of aging started in 1994 and 1997, respectively.⁴ Both were convenience samples of community-dwelling older adults with high follow-up (>90%) and autopsy (87%) rates. All autopsied participants with complete data as of June 2, 2011 (93.7% of 858 autopsied) were included. Both studies were approved by the Rush University Medical Center institutional review board and all participants provided written consent.

Dementia was diagnosed using standard criteria (Table 1; last assessment: mean 7.1 months before death). Neuropathologic diagnoses were blinded to age and clinical diagnosis, using standard criteria (Table 1). Mixed pathologies included at least 2 of the following: a pathological diagnosis of AD, infarct (macroscopic or microscopic), or neocortical LB (identified on α-synuclein immunostaining) pathology. We report prevalence ratios from log-binomial regression models controlling for sex and education with interaction terms for pathology by age group. Analyses were performed using SAS version 9.2 (SAS Institute Inc). Testing was 2-sided with statistical significance threshold of α = .05.

The oldest old (n = 301; mean age = 94.3 years) were more likely to be demented and to have pathological diagnoses of AD or infarct than the old (n = 503; mean age = 83.8 years) (Table 1). A pathological diagnosis of AD was associated with a higher prevalence of dementia in the old and the oldest old; however, the association was attenuated in the oldest old (model 1 in Table 2). In a model including pathological diagnoses of AD, infarct, and LB, all 3 pathologies were associated with dementia in the old and the oldest old, but only AD showed an interaction with age (model 2 in Table 2). Mixed but not single pathologies were more common in the oldest old, specifically AD plus infarct (Table 1). In both age groups, a pathological diagnosis of AD plus infarct and/or LB (mixed AD pathology) was associated with a higher prevalence ratio for dementia than a diagnosis of AD alone; however, the association of mixed AD pathologies with dementia was attenuated in the oldest old (model 3 in Table 2).

We found that a pathological diagnosis of AD was more common in the oldest old and strongly related to dementia. The increase in AD pathology in the oldest old was primarily in the form of mixed AD and infarct pathology, which was related to a higher probability of dementia than AD pathology alone. Thus, the proportional effect of AD pathology is high in the oldest old. Nonetheless, the relationship between a pathological diagnosis of AD and dementia was significantly attenuated in the oldest old compared with the old even after accounting for known mixed pathologies, suggesting additional factors in the pathogenesis of dementia in the oldest old.

Limitations include data from a volunteer cohort agreeable to autopsy and few minorities, which may limit generalizability. These data suggest that research on dementia in the oldest old should focus on AD, mixed pathologies, and exploration of additional factors in the pathogenesis of dementia in the oldest old.