Alzheimer Disease

Amyloid-Beta Cascade Hypothesis

• Aβ production and accumulation: Amyloid precursor protein (APP) is sequentially cleaved by β-secretase (BACE1) and γ-secretase to produce Aβ42, which aggregates into oligomers and deposits as senile plaques in the extracellular space
• Oligomeric toxicity: Small soluble Aβ oligomers are more neurotoxic than plaques themselves, triggering synaptic dysfunction, mitochondrial dysfunction, and oxidative stress
• Neuroinflammation initiation: Plaques activate microglia and astrocytes through pattern recognition receptors, releasing pro-inflammatory cytokines (IL-1β, TNF-α, IL-6) that amplify neuronal damage

Tau Pathology and Neurodegeneration

• Tau hyperphosphorylation: Kinase dysregulation (increased GSK-3β, CDK5 activity; decreased phosphatase activity) causes pathologic phosphorylation of microtubule-associated protein tau
• Neurofibrillary tangle formation: Hyperphosphorylated tau self-assembles into paired helical filaments and tangles within neurons, disrupting axonal transport and causing neuronal death
• Tau propagation: Tau pathology spreads trans-synaptically in a prion-like manner, progressing from transentorhinal cortex (Braak stage I) through limbic regions to neocortex (Braak stage VI)

Additional Neurodegenerative Mechanisms

• Synaptic loss and dendritic spine retraction: Early loss of dendritic spines and synapses precedes neuronal death, correlating better with cognitive decline than plaque or tangle burden
• Neuronal loss: Particularly affects the hippocampus, entorhinal cortex, and temporal lobes, with 30–50% neuronal loss in severe AD
• Metabolic dysfunction: Impaired glucose metabolism, mitochondrial dysfunction, reduced ATP production, and increased oxidative/nitrosative stress
• Protein aggregation and autophagy failure: Accumulation of other pathogenic proteins (α-synuclein, TDP-43) and impaired lysosomal-autophagy clearance pathways
• Vascular dysfunction: Blood-brain barrier disruption, reduced cerebral blood flow, and increased amyloid angiopathy contribute to neurodegeneration

Genetic Risk Factors

• APOE4 genotype: APOE4 allele is the strongest genetic risk factor for late-onset AD; presence of one APOE4 copy increases risk ~3-fold, two copies ~8-fold; APOE4 impairs Aβ clearance and enhances tau pathology
• Autosomal dominant mutations (early-onset AD): APP, PSEN1, PSEN2 mutations cause familial AD (FAD) with disease onset typically before age 60; PSEN1 mutations account for ~80% of familial cases

Non-Modifiable Risk Factors

• Advanced age: Exponential increase in risk after age 65; age is the strongest risk factor
• Female sex: Women represent ~65% of AD cases; longer life expectancy and possible reduced estrogen-neuroprotection post-menopause
• Family history: First-degree relatives of AD patients have increased lifetime risk

Modifiable Risk Factors

• Cardiovascular risk factors: Hypertension, diabetes mellitus, dyslipidemia, atrial fibrillation, obesity
• Cognitive inactivity and low educational attainment: "Cognitive reserve" hypothesis suggests mentally stimulating activities reduce risk
• Physical inactivity: Sedentary lifestyle increases risk; aerobic exercise shows protective effects
• Sleep disturbance: Chronic sleep deprivation and obstructive sleep apnea associated with increased amyloid accumulation
• Depression and psychological stress: Chronic stress elevates cortisol and promotes neuroinflammation
• Traumatic brain injury: Moderate to severe head trauma increases AD risk, particularly with loss of consciousness
• Apolipoprotein E4 status: Modifiable factors have greatest impact in APOE4 carriers

Environmental Exposures

• Air pollution: Fine particulate matter (PM2.5) penetrates to brain and triggers neuroinflammation
• Pesticide/heavy metal exposure: Epidemiologic associations with agricultural exposure and aluminum
• Midlife hypertension and metabolic syndrome: Vascular risk factor management becomes increasingly important as modifiable prevention strategy

Cognitive Manifestations

• Insidious memory loss: Initially affects short-term memory (recent events) while long-term memory preserved early; progressively becomes global; family often notes 1–3 year history before clinical recognition
• Dysexecutive dysfunction: Impaired planning, judgment, reasoning, problem-solving; difficulty managing finances or medications
• Language disturbance (aphasia): Early anomia (difficulty naming objects), progressing to reduced output and comprehension
• Visuospatial dysfunction: Difficulty with directions, getting lost in familiar places, impaired driving ability
• Apraxia: Inability to perform learned purposeful movements (dressing, grooming) despite intact motor strength and comprehension

Behavioral and Psychiatric Symptoms

• Personality changes: Behavioral disinhibition, increased irritability, emotional lability, apathy
• Depression: Present in 30–40% of AD patients; may precede cognitive decline
• Anxiety and agitation: Particularly common in later stages
• Delusions and hallucinations: Visual hallucinations more common than auditory; often non-threatening content
• Wandering and sundowning: Increased agitation, confusion, and behavioral disturbance in evening hours
• Aggression and physical acting-out: Especially with frustration, sensory stimuli, or care transitions

Physical Examination Findings

• Preserved early-stage motor function: Initially intact strength, tone, reflexes, gait (distinguishing from other dementias like vascular or Lewy body where motor signs prominent)
• Myoclonic jerks: Occur in advanced disease; associated with more rapid progression
• Primitive reflexes: Grasp reflex, snout reflex emerge in moderate-to-advanced stages
• Neuropsychological deficits: Poor performance on Mini-Cognitive Assessment (Mini-Cog), Montreal Cognitive Assessment (MoCA), Mini-Mental State Examination (MMSE) with specific pattern of deficits
• Gait disorder and incontinence: Late-stage manifestations; magnetic gait (slow, broad-based shuffling) and urinary incontinence

Clinical Staging

• Mild stage (2–4 years): Memory problems become noticeable, mild language/executive dysfunction, largely independent with activities of daily living (ADLs)
• Moderate stage (2–10 years): Increased memory loss, behavioral changes, require assistance with ADLs (bathing, dressing, toileting), may become disoriented to time/place
• Severe stage (1–3 years): Profound cognitive decline, loss of communication, loss of physical abilities, require total care, death often from aspiration pneumonia or other complications

Clinical Diagnosis Criteria

• Probable AD dementia (McKhann criteria 2011):
• Cognitive impairment documented by clinical evaluation and cognitive testing affecting at least two cognitive domains
• Gradual onset over months to years
• Progressive decline
• Absence of evidence for other causes (structural, vascular, psychiatric, or other degenerative conditions)

• Possible AD dementia: Atypical presentation (acute onset, focal deficits) or etiologically mixed pathology suspected

Cognitive Testing & Biomarkers

• Neuropsychological testing: Comprehensive battery assessing memory, language, executive function, visuospatial abilities; helps differentiate AD from other dementias and normal aging
• Mini-Cognitive Assessment (Mini-Cog): 3-minute screening tool combining 3-word recall and clock-drawing; sensitivity 89%, specificity 86%
• Montreal Cognitive Assessment (MoCA): More sensitive than MMSE, scores <26 suggest cognitive impairment
• Cerebrospinal fluid (CSF) biomarkers (from lumbar puncture, increasingly used):
• ↓↓ Aβ42 (<500 pg/mL): reflects brain amyloid pathology
• ↑ Phosphorylated tau (p-tau181, p-tau217): reflects tau pathology and correlates with neurodegeneration severity
• ↑ Total tau: general neurodegeneration marker; CSF tau:Aβ42 ratio >0.31 supports AD diagnosis

• Positron emission tomography (PET) imaging (research/specialized settings):
• Amyloid PET: Shows fibrillar amyloid accumulation; positive in asymptomatic individuals with genetic risk
• Tau PET: Regional tau tangles correlate with cognitive symptoms and disease progression
• FDG-PET: Shows hypometabolism in temporal and parietal regions characteristic of AD, sparing occipital cortex; also useful to differentiate from frontotemporal dementia (frontal/temporal predominance) or Lewy body dementia (occipital hypometabolism)

• Blood biomarkers (emerging, increasingly important):
• Phosphorylated tau variants (p-tau181, p-tau217, p-tau396): Correlate with amyloid and tau PET; excellent sensitivity/specificity for AD pathology; increasingly used to identify at-risk individuals
• Plasma phospho-tau/Aβ42 ratios: May identify preclinical AD before symptoms
• Neurofilament light (NfL): General neurodegeneration marker; elevated in AD but not disease-specific
• APOE4 genotyping: Identifies genetic risk; discussed before testing given implications for testing cascade

Structural Imaging

• MRI findings in AD:
• Medial temporal lobe atrophy: Particularly hippocampal atrophy most characteristic finding; may be absent in early disease
• Cortical atrophy: Diffuse cortical thinning with relative preservation of primary motor and visual cortices
• Excludes other causes: mass lesions, stroke, normal pressure hydrocephalus, subdural hematoma, white matter disease

• CT brain: When MRI contraindicated; less sensitive for early changes but rules out acute processes

Diagnostic Approach Algorithm

1. 1.Clinical assessment: History (insidious onset, progressive decline, functional impact), neuropsychological testing establishing cognitive impairment in ≥2 domains, detailed history from informant
2. 2.Neuroimaging: Structural MRI (or CT) to exclude other causes; FDG-PET or amyloid/tau PET if diagnostic uncertainty
3. 3.Laboratory evaluation: TSH, vitamin B12, folate (exclude reversible causes), blood biomarkers (phospho-tau) increasingly used for confirmation
4. 4.Cognitive assessment: Standardized instruments (MoCA preferred over MMSE), informant-based functional assessment
5. 5.Exclusion of other diagnoses: Lewy body dementia (visual hallucinations, parkinsonism, REM sleep behavior disorder), frontotemporal dementia (behavioral/language changes, age <65 typical), vascular dementia (abrupt onset, focal deficits, vascular disease burden), depression (pseudodementia)

Cognitive Enhancement - Disease-Modifying Agents

First-Line Cognitive Enhancers (Acetylcholinesterase Inhibitors)

• Mechanism: Inhibit breakdown of acetylcholine at synapses, improving cholinergic neurotransmission; provide modest symptomatic benefit particularly early-moderate disease
• Donepezil: Initial 5 mg nightly; increase to 10 mg after 4–6 weeks; maximum 23 mg (higher-dose formulation); half-life allows once-daily dosing; most frequently used
• Rivastigmine: Dual cholinesterase and butyrylcholinesterase inhibitor; dosing 1.5–6 mg twice daily; transdermal patch 4.6–13.3 mg/day option
• Galantamine: 4–8 mg twice daily; additional nicotinic receptor agonism; similar efficacy to donepezil
• Efficacy and timeline: Slow cognitive decline by ~30–50% for 6–12 months; benefits plateau then decline resumes; benefit modest and patient-dependent; continued use supported even when minimal apparent effect (withdrawal may worsen cognition acutely)
• Adverse effects: Nausea, diarrhea, vomiting (dose-dependent, often mitigated by slow titration), bradycardia/syncope (caution with cardiac conduction disease), muscle cramps, vivid dreams

NMDA Receptor Antagonist

• Memantine: Non-competitive N-methyl-D-aspartate (NMDA) receptor antagonist; proposed mechanism reduces excitotoxicity from excessive glutamate
• Dosing: Initial 5 mg daily, titrated weekly by 5 mg increments to target 10 mg twice daily over 3–4 weeks; also available as extended-release 20 mg daily
• Efficacy: Slows cognitive decline in moderate-to-severe AD; can be combined with acetylcholinesterase inhibitors; smaller effect size than cholinesterase inhibitors
• Adverse effects: Dizziness, confusion, hallucinations; generally well-tolerated; renally cleared (reduce dose if CrCl <30)
• Use in renal impairment: Contraindicated if CrCl <5 mL/min

Novel Anti-Amyloid Monoclonal Antibodies (Disease-Modifying Therapy)

• Aducanumab (FDA-accelerated approval 2021, withdrawn 2023): Resulted in clinical controversy; modest amyloid reduction but unclear cognitive benefit; approval withdrawn after post-hoc analyses questioned efficacy

• Lecanemab (Leqembi): First monoclonal antibody with demonstrated clinical benefit in early symptomatic AD (MCI or mild dementia stage) with confirmed amyloid pathology
• Mechanism: Targets protofibrils of amyloid-beta; reduces amyloid plaques on PET imaging; slows cognitive decline ~27% over 18 months (5% absolute slowing of decline)
• Dosing: Bi-weekly IV infusion 10 mg/kg after loading phase escalation; requires amyloid PET or CSF biomarkers confirmation before initiation
• Black box warning: Amyloid-related imaging abnormalities (ARIA) risk—brain microhemorrhages (ARIA-H) and microinfarcts/edema (ARIA-E); pre-treatment MRI required, genetic testing for APOE4 status recommended (higher risk in APOE4 homozygotes)
• Monitoring: MRI at 7, 18, 35 weeks then annually; clinical assessment for cognitive/behavioral decline, headache, confusion suggesting ARIA
• FDA approval: June 2023 for symptomatic AD with mild cognitive impairment or mild dementia stage and documented amyloid pathology

• Donanemab (Kisunla): Second anti-amyloid monoclonal antibody; targets different epitope of amyloid; similar efficacy (~35% slowing) with potentially lower ARIA risk in early trials
• Phase 3 trial: Shown ~35% slowing of cognitive decline in early symp

Complications of the disease

• Aspiration pneumonia: end-stage loss of swallowing coordination plus impaired cough; the most common terminal event. Signaled by new hypoxia, tachypnea, or a right-lower-lobe infiltrate in a bedbound patient — a medical emergency.
• Delirium superimposed on dementia: reduced cholinergic reserve makes the AD brain exquisitely vulnerable to any systemic insult. Signaled by acute fluctuating inattention on a baseline of chronic decline; an emergency requiring a hunt for infection, hypoxia, retention, metabolic derangement, and offending drugs.
• Wandering and elopement: visuospatial and topographic disorientation; an emergency given exposure and traffic risk. Falls with hip fracture and subdural hematoma follow from gait apraxia plus poor judgment.
• Cerebral amyloid angiopathy: vascular Aβ deposition (same peptide, different compartment) predisposes to lobar intracerebral hemorrhage and cortical superficial siderosis — sudden focal deficit or headache is an emergency.
• Weight loss, malnutrition, pressure ulcers, incontinence with recurrent UTI: consequences of apraxia, apathy, and immobility.
• Caregiver burnout, depression, and elder abuse/neglect: routinely screened for; also loss of decision-making capacity, requiring early advance directives and surrogate designation.

Complications of therapy

• Cholinesterase inhibitors: excess muscarinic tone causes nausea, diarrhea, anorexia with weight loss, and bradycardia/AV block with syncope — obtain an ECG and reconsider the drug rather than placing a pacemaker reflexively.
• Prescribing cascade: treating drug-induced urge incontinence with an anticholinergic (oxybutynin) directly antagonizes therapy and worsens cognition; the AGS Beers Criteria advise avoiding anticholinergics in dementia.
• Anti-amyloid monoclonal antibodies: ARIA-E (vasogenic edema) and ARIA-H (microhemorrhage/siderosis) from antibody-mediated plaque clearance at amyloid-laden vessels. Most are asymptomatic on surveillance MRI, but headache, confusion, focal deficit, or seizure means emergent MRI and drug hold; risk is highest in APOE4 homozygotes and with anticoagulation.
• Antipsychotics for agitation: carry an FDA boxed warning for increased mortality in elderly patients with dementia; APA guidance restricts them to non-emergent use after non-pharmacologic measures fail.

• The cholinergic lesion is localized: degeneration of the nucleus basalis of Meynert (plus locus ceruleus noradrenergic loss) explains why acetylcholinesterase inhibitors help and why anticholinergics are catastrophic in these patients.
• Histology buzzwords: extracellular neuritic (senile) plaques of Aβ42, intracellular neurofibrillary tangles of hyperphosphorylated tau, plus granulovacuolar degeneration and Hirano bodies (actin-rich, hippocampal). Tangle burden, not plaque burden, tracks with clinical stage.
• The association examiners test: Down syndrome. Trisomy 21 means three copies of APP, and essentially all patients develop AD pathology by the fourth decade. Corollary: APOE4 raises risk in a dose-dependent way while APOE2 is protective; APOE is a risk allele, not a determinative gene, whereas PSEN1/PSEN2/APP mutations are autosomal dominant and cause early-onset disease.
• Single best next step in a new dementia workup: exclude reversible and structural mimics before anchoring on AD — TSH, vitamin B12, and non-contrast head CT or MRI. Amyloid PET, CSF Aβ42/p-tau, and plasma p-tau217 confirm pathology but are not the first move in a straightforward stem.
• Distinguish the mimics: visual hallucinations + parkinsonism + REM sleep behavior disorder + neuroleptic sensitivity = Lewy body dementia; magnetic gait, incontinence, and dementia with ventriculomegaly out of proportion to atrophy = normal pressure hydrocephalus (do a large-volume LP); disinhibition or progressive aphasia before age 65 = frontotemporal dementia.
• Common distractor — treatment expectations: cholinesterase inhibitors and memantine are symptomatic only; they do not halt neurodegeneration. Anti-amyloid antibodies are restricted to biomarker-confirmed MCI or mild dementia, not moderate–severe disease.
• Common distractor — screening: the USPSTF finds current evidence insufficient to recommend routine cognitive screening in asymptomatic older adults; testing follows a patient or informant complaint.
• For agitation, non-pharmacologic environmental measures come first; antipsychotics are a last resort carrying an FDA mortality warning.