Alzheimer Disease

Alzheimer disease is the most common neurodegenerative cause of dementia, a progressive disorder marked by the accumulation of amyloid-beta plaques and tau neurofibrillary tangles in the brain and by the gradual loss of memory and other cognitive abilities. It typically begins with impairment of recent memory and advances over years to affect language, reasoning, orientation, and the capacity for independent living.

Definition

Alzheimer disease is a progressive neurodegenerative disorder defined neuropathologically by extracellular amyloid-beta plaques and intracellular neurofibrillary tangles of hyperphosphorylated tau, accompanied by synaptic and neuronal loss, and clinically by an amnestic-predominant (or, less commonly, atypical) dementia syndrome.

Scope

This entry covers the defining neuropathology of Alzheimer disease, the amyloid and tau hypotheses, the clinical syndrome and its typical and atypical presentations, the shift toward biomarker-based biological definitions, and the epidemiology of the disease. It is a reference overview of how Alzheimer disease is understood and classified, not a guide to diagnosis or treatment of any individual.

Core questions

• How do amyloid-beta and tau pathology relate to each other and to symptoms?
• Why does pathology often precede clinical symptoms by many years?
• How should the disease be defined: by clinical syndrome, or biologically by biomarkers?
• What explains the heterogeneity of clinical presentations and rates of progression?

Key concepts

• Amyloid-beta plaques
• Neurofibrillary tangles and hyperphosphorylated tau
• AT(N) biomarker classification
• Amnestic and atypical (e.g. posterior cortical, logopenic) presentations
• Preclinical, prodromal (mild cognitive impairment), and dementia stages
• APOE epsilon-4 risk allele
• Mixed pathology with cerebrovascular and other changes

Key theories

Amyloid cascade hypothesis

The accumulation and aggregation of amyloid-beta is proposed as an early, initiating event that triggers a downstream cascade including tau pathology, neuroinflammation, synaptic loss, and neurodegeneration, though the precise causal sequence and therapeutic implications remain debated.

Biological definition by biomarkers

The NIA-AA Research Framework proposes defining Alzheimer disease in living people by biomarkers of amyloid, tau, and neurodegeneration (the AT(N) system) rather than by clinical syndrome alone, treating it as a biological continuum.

Mechanisms

Alzheimer disease involves the aggregation of amyloid-beta into extracellular plaques and of hyperphosphorylated tau into intracellular neurofibrillary tangles, accompanied by synaptic dysfunction, neuroinflammation, and progressive neuronal loss, particularly in the medial temporal lobe and association cortices. Tau pathology spreads in a stereotyped topographic pattern that correlates with the evolution of cognitive deficits, while amyloid burden accumulates earlier and more diffusely. The amyloid cascade hypothesis frames amyloid as an upstream trigger; biomarker frameworks formalise the separable contributions of amyloid, tau, and neurodegeneration, and recognise that mixed pathologies are common, especially in older patients (Scheltens et al., 2021; Jack et al., 2018; Dugger & Dickson, 2017).

Clinical relevance

Alzheimer disease is the leading cause of dementia and a major reason that cognitive assessment, neuroimaging, and increasingly fluid and imaging biomarkers are used to characterise cognitive decline. Diagnostic frameworks distinguish preclinical, prodromal, and dementia stages and recognise atypical presentations. This entry explains how the disease is conceptualised and assessed; it is not a basis for individual diagnostic or treatment decisions.

Epidemiology

Alzheimer disease accounts for an estimated majority of dementia cases worldwide and its prevalence rises steeply with age. Because it is strongly age-related, the absolute number of affected people is increasing as populations age, making it a leading and growing contributor to the global burden of neurological disease (Scheltens et al., 2021).

History

Alois Alzheimer described the clinicopathological features of the disease in 1906-1907 in a patient with progressive dementia, plaques, and tangles. Over the twentieth century the disease was recognised as a common cause of dementia rather than a rare presenile condition, and the identification of amyloid-beta and tau as the molecular constituents of plaques and tangles, together with the amyloid cascade hypothesis, reframed it in molecular terms. More recently, diagnostic criteria have moved from purely clinical definitions (McKhann et al., 2011) toward biomarker-based biological definitions (Jack et al., 2018).

Debates

How central is amyloid to causation and treatment?

The amyloid cascade hypothesis has guided much research and therapeutic development, but the modest and contested clinical effects of amyloid-lowering interventions have kept the relative causal roles of amyloid and tau, and the best therapeutic targets, under active debate.

Should Alzheimer disease be defined biologically or clinically?

Defining the disease by biomarkers identifies it before symptoms and sharpens research, but raises questions about labelling people with abnormal biomarkers yet no symptoms as having a disease, balancing biological precision against clinical meaning.

Key figures

• Alois Alzheimer
• John Hardy
• Clifford R. Jack
• Dennis Dickson
• Philip Scheltens

Seminal works

• scheltens-2021
• jack-2018
• mckhann-2011

Frequently asked questions

Is Alzheimer disease the same as dementia?

No. Dementia is a clinical syndrome of acquired, progressive cognitive decline that has many causes; Alzheimer disease is the most common single cause of that syndrome, but other diseases (vascular, frontotemporal, Lewy body, and mixed pathologies) also cause dementia.

What are the hallmark changes in the Alzheimer brain?

The two defining microscopic features are extracellular amyloid-beta plaques and intracellular neurofibrillary tangles made of hyperphosphorylated tau, accompanied by synaptic and neuronal loss that is most pronounced in memory-related and association cortex regions.