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Duchesne S, Rousseau LS, Belzile-Marsolais F, Welch LA, Cournoyer B, Arseneau M, Lapierre V, Poulin SM, Potvin O, Hudon C. A Scoping Review of Alzheimers Disease Hypotheses: An Array of Uni- and Multi-Factorial Theories. J Alzheimers Dis 2024; 99:843-856. [PMID: 38788067 PMCID: PMC11191496 DOI: 10.3233/jad-230772] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Accepted: 03/27/2024] [Indexed: 05/26/2024]
Abstract
Background There is a common agreement that Alzheimers disease (AD) is inherently complex; otherwise, a general disagreement remains on its etiological underpinning, with numerous alternative hypotheses having been proposed. Objective To perform a scoping review of original manuscripts describing hypotheses and theories of AD published in the past decades. Results We reviewed 131 original manuscripts that fulfilled our inclusion criteria out of more than 13,807 references extracted from open databases. Each entry was characterized as having a single or multifactorial focus and assigned to one of 15 theoretical groupings. Impact was tracked using open citation tools. Results Three stages can be discerned in terms of hypotheses generation, with three quarter of studies proposing a hypothesis characterized as being single-focus. The most important theoretical groupings were the Amyloid group, followed by Metabolism and Mitochondrial dysfunction, then Infections and Cerebrovascular. Lately, evidence towards Genetics and especially Gut/Brain interactions came to the fore. Conclusions When viewed together, these multi-faceted reports reinforce the notion that AD affects multiple sub-cellular, cellular, anatomical, and physiological systems at the same time but at varying degree between individuals. The challenge of providing a comprehensive view of all systems and their interactions remains, alongside ways to manage this inherent complexity.
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Affiliation(s)
- Simon Duchesne
- Department of Radiology and Nuclear Medicine, Université, Laval, Quebec City, QC, Canada
| | - Louis-Simon Rousseau
- CERVO Brain Research Centre, Quebec City, QC, Canada
- School of Psychology, Université, Laval, Quebec City, QC, Canada
| | - Florence Belzile-Marsolais
- CERVO Brain Research Centre, Quebec City, QC, Canada
- School of Psychology, Université, Laval, Quebec City, QC, Canada
| | - Laurie-Ann Welch
- CERVO Brain Research Centre, Quebec City, QC, Canada
- School of Psychology, Université, Laval, Quebec City, QC, Canada
| | | | | | - Véronick Lapierre
- CERVO Brain Research Centre, Quebec City, QC, Canada
- School of Psychology, Université, Laval, Quebec City, QC, Canada
| | | | - Olivier Potvin
- Quebec Heart and Lung Research Institute, Quebec City, QC, Canada
- CERVO Brain Research Centre, Quebec City, QC, Canada
| | - Carol Hudon
- CERVO Brain Research Centre, Quebec City, QC, Canada
- School of Psychology, Université, Laval, Quebec City, QC, Canada
- VITAM Research Centre, Quebec City, QC, Canada
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Abstract
An aging-related reduction in the brain's functional reserve may explain why delirium is more frequent in the elderly than in younger people insofar as the reserve becomes inadequate to cover the metabolic requirements that are critically increased by stressors. The aim of this paper is to review the normal aging-related changes that theoretically compromise complex mental activities, neuronal and synaptic densities, and the neurocomputational flexibility of the functional reserve. A pivotal factor is diminished connectivity, which is substantially due to the loss of synapses and should specifically affect association systems and cholinergic fibres in delirious patients. However, micro-angiopathy with impaired blood flow autoregulation, increased blood/brain barrier permeability, changes in cerebrospinal fluid dynamics, weakened mitochondrial performance, and a pro-inflammatory involution of the immune system may also jointly affect neurons and their synaptic assets, and even cause the progression of delirium to dementia regardless of the presence of co-existing plaques, tangles, or other pathological markers. On the other hand, the developmental growth in functional reserve during childhood and adolescence makes the brain increasingly resistant to delirium, and residual reserve can allow the elderly to recover. These data support the view that functional reserve is the variable that confronts stressors and governs the risk and intensity of and recovery from delirium. Although people of any age are at risk of delirium, the elderly are at greater risk because aging and age-dependent structural changes inevitably affect the brain's functional reserve.
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