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"Alzheimer disease"[MeSH Terms] AND treatment

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lecanemab donanemab Alzheimer disease FDA approved 2024 2025 treatment update

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Alzheimer's Disease - A Comprehensive Overview

1. Background and Definition

Alzheimer's disease (AD) was first described in 1906 by Alois Alzheimer, who identified the hallmark inclusions - amyloid plaques and neurofibrillary tangles (NFTs) - in a woman in her 50s with paranoia, memory loss, and aphasia. It is the most common cause of dementia, contributing to an estimated 60-70% of all dementia cases worldwide. Approximately 55 million people across the world currently live with dementia. In the United States alone, AD dementia care costs were estimated at $360 billion in 2024 (~$25,000 per patient).
The disease can manifest as early as the third decade of life, but is predominantly a disease of the elderly. Its definition has evolved: DSM-5 reclassifies it as "major neurocognitive disorder due to Alzheimer disease," and recent research frameworks now define AD biologically by the presence of both amyloid (A) and tau (T) biomarkers, irrespective of clinical symptoms.
  • Harrison's Principles of Internal Medicine 22E, p. 3519
  • Bradley and Daroff's Neurology in Clinical Practice

2. Epidemiology

Age GroupPrevalence
65-74 years~3%
75-84 years~11%
85+ years~32%
  • Incidence rises sharply with age: 2 new cases/1,000 (age 65-74) to 37 new cases/1,000 (age 85+)
  • Lifetime risk from age 45: ~10% in men, ~20% in women
  • While MCI is more common in men, AD dementia prevalence is higher in women, partly explained by longer female life expectancy
  • Projections estimate ~15 million U.S. cases by 2060
  • Bradley and Daroff's Neurology in Clinical Practice

3. Pathology and Histology

The histopathological hallmarks are:
Neuritic Plaques
  • Core of aggregated beta-amyloid (Aβ) peptide (39-42 amino acids) surrounded by degenerating neurites
  • Aβ is proteolytically derived from the larger amyloid precursor protein (APP)
  • Identified by decreased CSF Aβ42 or amyloid PET positivity
Neurofibrillary Tangles (NFTs)
  • Intracellular aggregations of hyperphosphorylated tau protein (a microtubule-associated protein)
  • Self-aggregate to form paired helical filaments
  • Identified by tau PET or elevated phospho-tau in CSF
For a histopathological diagnosis, at least 6 neuritic plaques and neurofibrillary tangles per low-power field in frontal, temporal, or parietal cortex are required.
  • Goldman-Cecil Medicine, p. 3863

4. Pathophysiology

The Amyloid Cascade
Beta-amyloidosis begins accumulating in the neocortex up to 20 years before dementia onset, detectable by PET imaging. Soluble oligomeric forms of Aβ are thought to be the key pathogenic molecules that induce neuronal injury. By the time clinical dementia appears, large numbers of neuritic plaques are present in the neocortex - but the amount of Aβ does not closely mirror dementia severity.
Tau Pathology and Spread
NFTs appear first in the medial temporal lobe and brainstem in cognitively normal persons as early as the fourth decade of life. The critical step in AD is transsynaptic spread of tangle pathology to cortical association areas:
  • Mild symptoms → tangles concentrated in entorhinal cortex and hippocampus (episodic memory impairment)
  • Moderate disease → tangles spread to frontal, parietal, and temporal association cortices
  • Severe/late disease → occipital lobes and primary motor/sensory cortices
This spatial pattern of NFT spread closely parallels clinical symptom progression.
Serial coronal MRI showing hippocampal atrophy progression over 11 years in a patient with Alzheimer disease. The right scan (11 years later) shows dramatically increased hippocampal atrophy and enlarged ventricles.
Serial coronal MRI: left = clinically normal; right = 11 years later with dementia. Hippocampal atrophy has dramatically increased. (Goldman-Cecil Medicine)
  • Goldman-Cecil Medicine

5. Genetics

Autosomal Dominant (Early-Onset) - Rare (<1% of AD)
GeneChromosomeProtein
APP21Amyloid precursor protein
PSEN114Presenilin-1
PSEN21Presenilin-2
Sporadic (Late-Onset) Risk Genes
  • ApoE ε4 allele (chromosome 19): the single most important genetic risk factor for sporadic AD
    • 1 ε4 allele: ~2-3x increased risk in women
    • 2 ε4 alleles (homozygous): 10-15x increased risk in both sexes
    • ~40-65% of AD patients carry at least one ε4 allele
    • ApoE ε4 leads to less efficient amyloid clearance and may promote tau aggregation
    • ApoE ε2 allele may be protective
  • TREM2 - inflammation-associated gene; heterozygous carriers have ~3x increased AD risk
  • Other GWAS-identified genes (CLU, PICALM, CR1) in pathways for innate immunity, lipid metabolism, and synaptic function
  • Harrison's Principles of Internal Medicine 22E

6. Clinical Manifestations

Typical Amnestic AD (most common)
  • Insidious onset with episodic memory loss progressing to multi-domain cognitive decline
  • Executive, language, and visuospatial deficits develop over years
  • Anosognosia (impaired awareness of own deficits) is common
  • Depression, social withdrawal, anxiety may precede cognitive symptoms
Atypical Presentations (~20% of AD)
  • Posterior cortical atrophy syndrome (visual processing dysfunction)
  • Logopenic aphasia (naming and repetition difficulties)
  • Corticobasal syndrome (asymmetric akinetic-rigid-dystonic)
  • Frontal/behavioral variant
Clinical Stages
StageDescription
Preclinical ADAmyloid biomarker positivity, no symptoms; precedes symptoms by up to 20 years
Subjective cognitive declineSelf-perceived memory worsening, not yet measurable
MCI / Early symptomatic ADMeasurable objective decline, functionally compensated; ~12% per year progress to dementia
AD dementiaCognitive impairment interfering with daily activities
  • Harrison's Principles of Internal Medicine 22E, pp. 3519-3523

7. Biomarker Framework (AT[N])

The current research framework classifies individuals by three biomarker groups:
BiomarkerPositive IndicatorMethod
A - Amyloid↓ CSF Aβ42 or amyloid PET positiveCSF, PET
T - TauElevated CSF phospho-tau or tau PET positiveCSF, PET
N - NeurodegenerationHippocampal atrophy on MRI, ↓ FDG-PETMRI, FDG-PET
A+T+ = Alzheimer disease (regardless of clinical stage) A+T- = Alzheimer pathological change only
  • Bradley and Daroff's Neurology in Clinical Practice

8. Diagnosis

Clinical Diagnosis of Probable AD requires:
  • Objective evidence of decline in memory AND at least one other cognitive domain
  • Insidious onset and gradual progression
  • No evidence of alternative etiology
  • The definitive diagnosis requires postmortem neuropathological confirmation
Cognitive assessments: Neuropsychological testing (more sensitive than MRI for disease tracking). Classic findings include impaired recognition memory, intrusion errors, absence of primacy effect in word-list recall.
Neuroimaging:
  • MRI: hippocampal atrophy (earliest reliable structural change)
  • FDG-PET: hypometabolism in posterior cingulate, parietal, temporal cortices
  • Amyloid PET and tau PET: detect pathological proteins in vivo

9. Treatment

Symptomatic Therapies

Cholinesterase Inhibitors (mild-moderate-severe AD)
These inhibit CNS acetylcholinesterase in the synaptic cleft, prolonging acetylcholine action. However, they do not halt neurodegeneration - benefit diminishes as presynaptic cholinergic neurons are progressively lost.
DrugTrade NameStageKey Notes
DonepezilAriceptMild-Moderate-SevereOnly ChEI approved for severe AD; noncompetitive reversible; t½=70h
RivastigmineExelonMild-ModerateTransdermal patch reduces GI side effects; dual AChE+BuChE inhibitor
GalantamineRazadyneMild-ModerateAlso modulates nicotinic receptors; competitive reversible
Memantine (moderate-severe AD)
  • NMDA receptor antagonist; reduces excitotoxic glutamate injury
  • Although commonly combined with a ChEI in clinical practice, trials have not consistently demonstrated cognitive efficacy in mild disease
  • Combined ChEI + memantine may provide better response than monotherapy
  • Kaplan & Sadock's Comprehensive Textbook of Psychiatry

Disease-Modifying Therapies (New - 2023-2025)

Anti-amyloid monoclonal antibodies represent a major advance:
  • Lecanemab (Leqembi) - FDA approved 2023; targets soluble Aβ protofibrils; slows clinical decline by ~27% in early AD in phase III trials
  • Donanemab (Kisunla) - FDA approved 2024; targets N-3pE-Aβ plaques; significant plaque clearance and slower decline in early AD
Key safety concern: ARIA (Amyloid-Related Imaging Abnormalities) - brain edema/microhemorrhages; risk is higher in ApoE ε4 carriers. ApoE genotyping is now recommended before starting these therapies.

10. Risk Factors and Prevention

Non-modifiable: Age (strongest), female sex, ApoE ε4 genotype, family history
Modifiable risk factors (Lancet Commission 2024): Hypertension, diabetes, obesity, smoking, physical inactivity, depression, low education, hearing loss, air pollution, social isolation, traumatic brain injury
Protective factors: Recent meta-analysis (PMID 39797935) found the Mediterranean diet significantly associated with reduced risk of cognitive impairment and AD. Physical exercise, cognitive engagement, and management of cardiovascular risk factors are also consistently supported.

11. Recent Evidence (2024-2026)

  • Updated amyloid and tau PET appropriate use criteria were released by the Alzheimer's Association and SNMMI in 2025 (PMID 39776249)
  • LATE (Limbic-predominant Age-related TDP-43 Encephalopathy) clinical criteria were published in 2025 (PMID 39807681) - an important AD mimic that co-occurs with AD and should be considered in older patients
  • Non-invasive brain stimulation + cognitive training shows promise as adjunct therapy in MCI/AD (meta-analysis, PMID 38937842)
Key References:
  • Harrison's Principles of Internal Medicine, 22nd Ed. (2025), Chapter on Alzheimer's Disease
  • Goldman-Cecil Medicine, International Edition (2-vol set), Chapter 371
  • Bradley and Daroff's Neurology in Clinical Practice, Chapters 44 & 95
  • Kaplan & Sadock's Comprehensive Textbook of Psychiatry, Chapter 55
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