Alzheimer disease pathology in subjects without dementia in 2 studies of aging: the Nun Study and the Adult Changes in Thought Study

Ascertainment bias in dementias: a secondary to tertiary centre analysis in Central Italy and conceptual review

BACKGROUND AND AIMS

Ascertainment bias (AB) indicates a bias of an evaluation centre in estimating the prevalence/incidence of a disease due to the specific expertise of the centre. The aim of our study was to evaluate classification of different types of dementia in new cases appearing in secondary and tertiary centres, in order to evidence possible occurrence of AB in the various (secondary to tertiary) dementia centres.

METHODS

To assess the mechanism of AB, the rates of new cases of the different forms of dementia reported by different centres were compared. The centres involved in the study were 11 hospital-based centres including a tertiary centre, located in the University Department of Clinical Neurology. The tertiary centre is endowed with state-of-the-art diagnostic facilities and its scientific production is prominently focused on dementia with Lewy bodies (DLB) thus suggesting the possible occurrence of a bias. Four main categories of dementia were identified: Alzheimer's disease (AD), DLB, fronto-temporal dementia (FTD), vascular dementia (VaD), with other forms in a category apart. The classification rate of new cases of dementia in the tertiary centre was compared with rates reported by secondary centres and rates of recoding were calculated during a follow-up of 2 years.

RESULTS

The study classified 2,042 newly diagnosed cases of dementia in a population of 1,370,000 inhabitants of which 315,000 were older than 65. AD was categorized in 48-52 % of cases, DLB in 25-28 %, FTD in 2-4 % and VaD in 17-28 %. During the 2-year follow-up the diagnosis was re-classified in 40 patients (3 %). The rate of recoding was 5 % in the tertiary centre, 2-8 % in referrals from secondary to tertiary centre, 2-10 % in recodings performed in secondary centres and addressed to tertiary centre. Recoding or percentages of new cases of AD or DLB were not different in the comparison between secondary or between secondary and tertiary centres. FTD and VaD were instead significantly recoded.

CONCLUSION

The results of the study suggest that in a homogeneous area, AB is not interfering with diagnosis of AD or DLB.

Neuropathological approaches to cerebral aging and neuroplasticity

Cerebral aging is a complex and heterogenous process related to a large variety of molecular changes involving multiple neuronal networks, due to alterations of neurons (synapses, axons, dendrites, etc), particularly affecting strategically important regions, such as hippocampus and prefrontal areas. A substantial proportion of nondemented, cognitively unimpaired elderly subjects show at least mild to moderate, and rarely even severe, Alzheimer-related lesions, probably representing asymptomatic preclinical Alzheimer's disease, and/or mixed pathologies. While the substrate of resilience to cognitive decline in the presence of abundant pathologies has been unclear, recent research has strengthened the concept of cognitive or brain reserve, based on neuroplasticity or the ability of the brain to manage or counteract age-related changes or pathologies by reorganizing its structure, connections, and functions via complex molecular pathways and mechanisms that are becoming increasingly better understood. Part of neuroplasticity is adult neurogenesis in specific areas of the brain, in particular the hippocampal formation important for memory function, the decline of which is common even in "healthy" aging. To obtain further insights into the mechanisms of brain plasticity and adult neurogenesis, as the basis for prevention and potential therapeutic options, is a major challenge of modern neurosciences.

Relation of neuropathology to cognition in persons without cognitive impairment

OBJECTIVE

A study was undertaken to examine the relation of Alzheimer disease (AD) pathology, cerebral infarcts, and Lewy body (LB) pathology to cognition in persons without cognitive impairment.

METHODS

Persons without dementia from 2 cohort studies of aging, the Religious Orders Study and the Memory and Aging Project, agreed to annual clinical evaluation and brain donation. The studies had 19 neuropsychological performance tests in common that assessed 5 cognitive domains. For 296 persons without cognitive impairment who died and underwent postmortem assessment, we quantified AD pathology as a global pathology score, and as amyloid load, paired helical filament tau-positive (PHFtau) tangle density, cerebral infarcts, and LB pathology. Linear regression was used to examine the relation of neuropathology to cognitive abilities, controlling for demographics.

RESULTS

Nearly all persons had AD pathology with >¾ exhibiting amyloid; 22% had macroscopic and 24% had microscopic infarctions, and 13% had LB pathology. The global measure of AD pathology was related to global cognition (p = 0.008), whereas infarcts and Lewy bodies were not. Amyloid load was related to global cognition (p < 0.05), with only a trend for tangles (p = 0.08). In analyses of cognitive domains, AD pathology (p = 0.006), PHFtau tangles (p = 0.03), and macroscopic infarctions (p = 0.02) were related to episodic memory, with a trend for amyloid load (p = 0.06); AD pathology (p = 0.02) and amyloid load (p = 0.03) were related to working memory. Findings for global cognition and episodic memory were stronger in additional analyses with neocortical amyloid and mesial temporal tangles.

INTERPRETATION

AD pathology and macroscopic infarctions are common in older persons without cognitive impairment and are related to episodic and working memory.

Prevalence of dementia subtypes in a developing country: a clinicopathological study

OBJECTIVES

To assess the distribution of dementia subtypes in Brazil using a population-based clinicopathological study.

METHOD

Brains from deceased individuals aged ≥50 years old were collected after the next of kin signed an informed consent form and provided information through standardized questionnaires. Post-mortem clinical diagnoses were established in consensus meetings, and only cases with moderate or severe dementia or without cognitive impairment were included in the analysis. Immunohistochemical neuropathological examinations were performed following the universally accepted guidelines. A diagnosis of Alzheimer's disease was made when there were at least both a moderate density of neuritic plaques (Consortium to Establish a Register for Alzheimer's disease B or C) and Braak stage III for neurofibrillary tangle distribution. For the diagnosis of vascular dementia, at least three zones or strategic areas had to be affected by infarcts, lacunae, or microinfarcts.

RESULTS

From 1,291 subjects, 113 cases were classified as having moderate or severe dementia, and 972 cases were free of cognitive impairment. The neuropathological diagnoses of the dementia sub-group were Alzheimer's disease (35.4%), vascular dementia (21.2%), Alzheimer's disease plus vascular dementia (13.3%), and other causes of dementia (30.1%). Small-vessel disease, which alone was not considered sufficient for a vascular dementia diagnosis, was present in 38.9% of all of the dementia cases and in 16.8% of the group without cognitive impairment (odds ratio = 2.91; 95% confidence interval, 1.53-5.51), adjusted for age, sex, and education.

CONCLUSIONS

The relatively high frequencies of vascular dementia and small-vessel disease in the dementia sub-group constitute relevant findings for public health initiatives because control of vascular risk factors could decrease the prevalence of dementia in developing countries.

On the issue of transmissibility of Alzheimer disease: a critical review

Results from recent experiments with rodents imply that Alzheimer disease might be inducible by seeding Aβ peptides into recipient animals. In respect to this new experimental data, public health aspects as well as epidemiological data have to be reevaluated. In this article, the available experimental and epidemiological data are reviewed.

Correlation of Alzheimer disease neuropathologic changes with cognitive status: a review of the literature

Clinicopathologic correlation studies are critically important for the field of Alzheimer disease (AD) research. Studies on human subjects with autopsy confirmation entail numerous potential biases that affect both their general applicability and the validity of the correlations. Many sources of data variability can weaken the apparent correlation between cognitive status and AD neuropathologic changes. Indeed, most persons in advanced old age have significant non-AD brain lesions that may alter cognition independently of AD. Worldwide research efforts have evaluated thousands of human subjects to assess the causes of cognitive impairment in the elderly, and these studies have been interpreted in different ways. We review the literature focusing on the correlation of AD neuropathologic changes (i.e. β-amyloid plaques and neurofibrillary tangles) with cognitive impairment. We discuss the various patterns of brain changes that have been observed in elderly individuals to provide a perspective for understanding AD clinicopathologic correlation and conclude that evidence from many independent research centers strongly supports the existence of a specific disease, as defined by the presence of Aβ plaques and neurofibrillary tangles. Although Aβ plaques may play a key role in AD pathogenesis, the severity of cognitive impairment correlates best with the burden of neocortical neurofibrillary tangles.