What’s happening in the brain in Alzheimer’s disease and dementia?

The brains of people with Alzheimer’s disease contain abnormal proteins—amyloid-beta and phosphorylated tau. Over time, amyloid-beta plaques, which are first seen in the neocortex, increase in number and involve all brain regions. Dementia occurs only when the plaques involve the midbrain and medulla oblongata. How the dementia appears clinically is then influenced by other brain abnormalities commonly seen in elderly people, explained Professor Dietmar Thal, Leuven, Belgium, in a fascinating educational session at EAN 2021.

Abnormal amyloid-beta and phosphorylated tau protein deposits

Neurofibrillary tangles and senile plaques are hallmarks of Alzheimer’s disease1

Characteristic changes are seen in the brains of people with Alzheimer’s disease, explained Professor Thal. These changes are:

  • Neurofibrillary tangles, which are intraneuronal aggregates of abnormal phosphorylated tau protein
  • Senile plaques, which are extracellular aggregates of amyloid-beta (Aβ) protein1

The presence of Aβ protein plaques in the brain follows a distinct hierarchical sequence with gradual progression into regions that receive neuronal projections from regions already containing Aβ plaques.

Amyloid-beta deposition follows a distinct hierarchical sequence2

"Five phases can be identified as the plaques spread throughout the brain",2 said Professor Thal.

  • In phase 1, one or more Aβ plaques are found in the frontal, occipital, parietal and/or temporal neocortex
  • In phase 2, Aβ plaques are seen in the allocortex—the entorhinal cortex, the hippocampus, and the cingulate gyrus
  • In phase 3, the distribution of Aβ plaques includes the thalamus, the hypothalamus, and the basal ganglia, and by this phase, the entire neocortex is filled with Aβ plaques

Only the final two phases of amyloid-beta deposition are associated with dementia2

  • In phase 4, Aβ plaques are present in the midbrain and in the medulla oblongata
  • In phase 5, Aβ plaques are seen in the pons and cerebellum

"Dementia is only seen in patients with phase 4 or 5 Aβ plaques", added Professor Thal. "It is not usually seen patients with phase 1, 2, or 3 Aβ plaques".

 

Other brain pathologies contributing to the Alzheimer’s dementia phenotype

Alzheimer’s dementia is a heterogeneous disease3

Professor Thal highlighted that the brains of many elderly people show changes not only of Alzheimer’s disease, but also many other pathologies, including:

  • Cerebral amyloid angiopathy3
  • Small vessel disease3
  • Atherosclerosis of the circle of Willis3
  • Granulovacuolar degeneration4
  • Limbic-associated age-related transactive response DNA-binding protein (TDP)-43 encephalopathy (LATE)5
  • Lewy body disease6

"These brain pathologies can all contribute to the dementia phenotype", explained Professor Thal.

A diagnosis of dementia should include all brain pathologies

In addition, specific brain pathologies such as argyrophilic grain disease, which affects 5% of all patients with dementia, has additive effects on the cognitive symptoms associated with Alzheimer’s disease.7

"A diagnosis of dementia should therefore include all pathologies, and combined treatment strategies may be more effective than a treatment strategy targeting a single pathology", concluded Professor Thal. "This has implications for the design of clinical trials investigating drugs targeting Alzheimer’s disease".

 

EAN : European Academy of Neurology 
Aβ : amyloid-beta
LATE : Limbic-associated age-related transactive response DNA-binding protein (TDP)-43encephalopathy 

BE-NOTPR-0129; approval date : 2/2022 

Our correspondent’s highlights from the symposium are meant as a fair representation of the scientific content presented. The views and opinions expressed on this page do not necessarily reflect those of Lundbeck.

References
  1. Perl DP. Neuropathology of Alzheimer's disease. Mt Sinai J Med 2010;77:32–42.
  2. Thal DR, et al. Phases of A beta-deposition in the human brain and its relevance for the development of AD. Neurology 2002;58:1791–800.
  3. Thal DR, et al. Vascular pathology in Alzheimer disease: correlationof cerebral amyloid angiopathy and arteriosclerosis/lipohyalinosis with cognitive decline. J Neuropathol Exp Neurol 2003;62:1287–301.
  4. Thal DR, et al. Stages of granulovacuolar degeneration: their relation to Alzheimer's disease and chronic stress response. Acta Neuropathol 2011;122:577–89.
  5. Tomé SO, et al. Distinct molecular patterns of TDP-43 pathology in Alzheimer’s disease: relationship with clinical phenotypes. Acta Neuropathol Commun 2020;8:61.
  6. Tomé SO, Thal DR. Co-pathologies in Alzheimer’s disease: just multiple pathologies or partners in crime? Brain 2021;144:706–8.
  7. Thal DR, et al. The impact of argyrophilic grain disease on the development of dementia and its relationship to concurrent Alzheimer's disease-related pathology. Neuropathol Appl Neurobiol 2005;31:270–9.
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