Alzheimer's Disease

• Amyloid-beta (Aβ) accumulation and amyloid hypothesis: The amyloid precursor protein (APP) is normally processed through nonamyloidogenic pathways by α-secretase, producing soluble fragments. However, alternative processing via β-secretase (BACE1) and γ-secretase generates amyloid-beta (Aβ42), a 42-amino acid peptide with high aggregation propensity. Aβ42 oligomerizes into neurotoxic protofibrils and eventually deposits as extracellular amyloid plaques. These plaques trigger inflammation, activate microglial cells, and release pro-inflammatory cytokines (IL-1β, TNF-α, IL-6). The amyloid hypothesis posits this cascade initiates downstream pathological events; genetic variants in APP, PSEN1, and PSEN2 (presenilin proteins—components of the γ-secretase complex) cause early-onset familial AD through accelerated Aβ production. However, amyloid pathology alone is insufficient for dementia, as cognitively normal individuals may have substantial plaque burden, indicating the multifactorial nature of AD.

• Tau pathology and neurodegeneration: Tau is a microtubule-associated protein stabilizing axonal microtubules. In AD, hyperphosphorylation of tau disrupts this stabilizing function, leading to microtubule destabilization and axonal transport impairment. Hyperphosphorylated tau polymerizes into paired helical filaments (PHF) forming neurofibrillary tangles (NFTs) within neuronal cell bodies and processes. Tau pathology progresses in a stereotypic manner through anatomically connected regions (Braak staging) beginning in the transentorhinal cortex and progressing to association cortices. The spatial and temporal dissociation of amyloid and tau pathology is critical: amyloid deposition precedes tau pathology by years; in some individuals, amyloid accumulates without significant cognitive decline until tau emerges. Tau pathology shows stronger correlation with cognitive decline than amyloid burden, suggesting tau represents a more proximal driver of neuronal dysfunction. Aβ oligomers facilitate tau phosphorylation through activation of kinases (GSK-3β, CDK5) and inhibition of phosphatases, mechanistically linking the two pathologies.

• Neuroinflammation and glial activation: Amyloid plaques and tau tangles trigger chronic neuroinflammation through pattern recognition receptors (TLRs, complement receptors) on microglia and astrocytes. Microglial activation produces pro-inflammatory mediators and reactive oxygen species (ROS) causing direct neuronal toxicity; chronically activated microglia may paradoxically impair clearance of pathological proteins. Astrocytes become reactive (astrogliosis) and contribute to neuroinflammatory milieu while potentially compromising synaptic support and glutamate clearance. Complement cascade activation (C1q, C3) marks synapses for destruction through microglial-mediated synaptic pruning, mechanistically contributing to synaptic loss observed in AD. This neuroinflammatory component explains why amyloid-lowering alone may be insufficient and why combination approaches targeting multiple pathways show greater promise.

• Synaptic dysfunction and loss: Early cognitive dysfunction in AD precedes significant neuronal death, implicating synaptic dysfunction as an initiating event. Aβ oligomers disrupt synaptic transmission through multiple mechanisms: direct binding to synaptic NMDA and AMPA receptors, disruption of long-term potentiation (LTP) through effects on calcium signaling, and impaired neurotransmitter release. Tau pathology similarly disrupts synaptic function through effects on axonal transport and dendritic spine stability. Synaptic loss (reduced dendritic spine density and synaptic bouton numbers) correlates strongly with cognitive decline in AD. This synaptic phase offers a therapeutic window before irreversible neuronal death.

• Neuronal death and brain atrophy: Progressive accumulation of pathological proteins drives neuronal death through excitotoxicity (glutamate-mediated calcium overload), mitochondrial dysfunction (reduced ATP production, increased ROS), endoplasmic reticulum stress, and activation of apoptotic pathways. The hippocampus and entorhinal cortex show earliest and most severe atrophy, explaining initial memory impairment. Subsequently, widespread cortical atrophy develops (temporal, parietal, frontal regions), correlating with progressive cognitive and functional decline. Neuroimaging shows progressive ventricular enlargement and cortical sulcal widening; volumetric MRI demonstrates hippocampal atrophy early in disease. Neuronal loss in AD averages 10-15% in the cerebral cortex overall, but reaches 60% or greater in severely affected regions.

• Genetic and molecular risk factors: APOE ε4 allele is the strongest genetic risk factor for late-onset AD (LOAD). APOE protein participates in Aβ clearance and aggregation; APOE ε4 is associated with increased Aβ accumulation, reduced Aβ clearance, and impaired tau clearance. APOE ε4 homozygotes have ~12-fold increased lifetime risk of AD compared to APOE ε3 homozygotes; heterozygotes have ~3-fold increased risk. Beyond APOE, genome-wide association studies (GWAS) have identified multiple susceptibility loci (SORL1, BIN1, PICALM, CLU, CD2AP, EPHA1, MS4A, ABCA7, CD33, INPP5D, etc.) involved in amyloid metabolism, inflammation, and lipid metabolism. Rare mutations in APP, PSEN1, and PSEN2 cause autosomal dominant early-onset AD (EOAD); PSEN1 mutations are most common (~50% of EOAD cases). These autosomal dominant mutations uniformly produce pathogenic Aβ and produce disease onset by age 65, often in the 40s-50s.

• Cerebral amyloid angiopathy (CAA): Aβ accumulates not only in plaques but within vessel walls of cerebral arterioles and capillaries, comprising cerebral amyloid angiopathy. CAA contributes to vascular dysfunction, impaired cerebral blood flow, increased vascular permeability, and risk of microhemorrhages (particularly lobar microhemorrhages) and macrohemorrhages. The interaction between amyloid pathology and cerebrovascular dysfunction is increasingly recognized as contributing to cognitive decline independent of neurodegeneration alone.

• Age: Age is the single strongest risk factor for AD. Incidence increases exponentially with age, doubling approximately every 5 years after age 65. The prevalence of AD increases from ~3% at age 65-74 to ~15% at age 75-84 to >40% at age 85+. The mechanism underlying age as a risk factor involves accumulated cellular damage, reduced DNA repair capacity, impaired protein clearance systems, and age-related neuroinflammation.

• Apolipoprotein E (APOE) genotype: The APOE ε4 allele is the most significant genetic risk factor for late-onset AD. APOE exists as three allelic variants (ε2, ε3, ε4) producing six possible genotypes. APOE ε4 carriers have increased AD risk in a gene-dose dependent manner: one APOE ε4 allele increases risk ~3-fold; two alleles increase risk ~8-12-fold. Conversely, APOE ε2 is protective. APOE ε4 effects begin in middle age; APOE ε4 homozygotes can develop AD in the 50s-60s. The mechanism involves effects on Aβ clearance, aggregation, tau phosphorylation, and neuroinflammation.

• Autosomal dominant mutations: PSEN1, PSEN2, and APP mutations cause autosomal dominant early-onset AD (EOAD) with nearly 100% penetrance. PSEN1 mutations account for ~50% of familial EOAD, with mean onset age of 43 years (range 24-60 years); PSEN2 mutations cause later-onset disease (mean ~55 years); APP mutations are least common but often present with additional features (amyloid angiopathy, myoclonus, seizures, migraine). These mutations are sufficient to cause disease through mechanisms that increase Aβ42 production or alter APP processing.

• Family history: A family history of dementia increases AD risk, reflecting both genetic predisposition and potentially shared environmental exposures. First-degree relatives of AD patients have increased lifetime risk. However, most AD cases are sporadic without clear family history.

• Cardiovascular risk factors: Hypertension, diabetes mellitus, hyperlipidemia, and obesity in midlife are associated with increased AD risk. These risk factors may act through vascular mechanisms (atherosclerosis, reduced cerebral perfusion, blood-brain barrier dysfunction) or through metabolic effects (insulin resistance, glycemic dysfunction affecting neuronal insulin signaling). The relationship between midlife hypertension and late-life dementia risk is particularly strong.

• Traumatic brain injury (TBI): Moderate-to-severe TBI is associated with increased AD risk, even decades after injury. The mechanism may involve direct axonal injury, neuroinflammation, or direct impact on amyloid and tau metabolism. Single TBI may increase risk modestly; repetitive TBI (as in chronic traumatic encephalopathy) markedly increases risk. This association has implications for athletes in contact sports.

• Cognitive reserve and education: Higher educational attainment and cognitive engagement are associated with reduced AD risk or delayed symptom onset. However, when AD pathology eventually develops, cognitive reserve does not prevent pathological progression—it merely delays symptom manifestation. This explains why highly educated individuals may have substantial AD pathology at autopsy despite minimal antemortem symptoms.

• Lifestyle and modifiable factors: Physical inactivity, cognitive inactivity, poor sleep quality/duration, depression, hearing loss, social isolation, and smoking increase AD risk. Conversely, Mediterranean diet, physical exercise, cognitive engagement, Mediterranean diet adherence, and active social engagement are protective. These associations have been most clearly demonstrated in observational studies; causality remains debated. Hearing loss may increase AD risk through several mechanisms: auditory system effects on cognition, social isolation secondary to hearing impairment, or common pathophysiology affecting both auditory and cognitive systems.

• APOE ε2/ε3 carriers: Although rare in practice, APOE ε2 carriers show reduced AD risk independent of other factors. ε2/ε3 carriers have lower risk than ε3/ε3 genotype.

Early-stage (Mild Cognitive Impairment [MCI] stage)

• Memory loss (episodic/declarative memory most affected): Patients experience difficulty recalling recent conversations, appointments, or events; the loss is noticeable to the patient and informants. Characteristically, patients may repeat questions or conversations multiple times. Remote/semantic memory remains relatively preserved early. The physiological basis reflects hippocampal pathology disrupting encoding and consolidation of new memories.
• Mild cognitive impairment without dementia: Some patients progress through an intermediate stage of MCI (cognitive impairment exceeding normal aging but insufficient to impair functional independence) before evolving to dementia. Not all MCI patients progress; roughly 10-15% progress to dementia annually.
• Subjective cognitive complaints: Patients or families may report "forgetfulness" or difficulty with complex tasks; objective cognitive testing may initially be normal or show only mild abnormalities.
• Preserved functional independence: Early-stage patients maintain independence in activities of daily living (ADLs) and instrumental ADLs (IADLs).

Middle-stage (Moderate Cognitive Impairment)

• Progressive memory loss with additional cognitive domains affected: Beyond memory, attention/executive function, language, and visuospatial abilities decline. Dysexecutive features (difficulty planning, organizing, problem-solving) become apparent. Language impairment (word-finding difficulties [anomia], reduced verbal fluency) emerges, reflecting temporal and inferior parietal lobe pathology.
• Functional decline: Patients require assistance with IADLs (managing finances, medications, complex household tasks). This stage typically lasts 2-10 years.
• Behavioral and neuropsychiatric changes: Personality changes, apathy, depression, anxiety, irritability, and agitation emerge in 50-80% of patients. Behavioral variant dementia may predominate with personality change and behavioral disinhibition as core features. Wandering and getting lost in familiar places occur due to visuospatial disorientation.
• Sleep disturbance: Fragmented nighttime sleep with daytime somnolence is common; "sundowning" (confusion and agitation in late afternoon/evening) occurs in 25-50% of patients, possibly related to circadian rhythm disruption and reduced environmental cues in dim lighting.
• Parkinsonism: Rigidity, bradykinesia, and gait slowing may develop late in the disease course, reflecting involvement of extrapyramidal circuits; resting tremor is uncommon.

Late-stage (Severe Dementia)

• Global cognitive decline: Severe impairment across all cognitive domains; patients may not recognize family members.
• Functional dependence: Complete dependence on caregivers for ADLs including grooming, dressing, toileting, eating. Patients eventually lose ability to communicate meaningfully.
• Motor features: Generalized rigidity, flexed posture, myoclonus, seizures (occur in 5-10% of AD patients, more common in severe disease).
• Incontinence: Urinary and fecal incontinence develop due to loss of cortical control.
• Swallowing difficulties (dysphagia): Difficulty swallowing leads to aspiration risk; nutritional decline.

Atypical presentations

• Primary Progressive Aphasia (PPA): Language/speech disturbance predominates early, with relative sparing of memory; may eventually show AD pathology on imaging/autopsy. Two variants exist: non-fluent/agrammatic variant (effortful, agrammatic speech with preserved comprehension) and semantic variant (fluent but meaningless speech with severe comprehension loss, often with prominent behavioral features).
• Posterior Cortical Atrophy (PCA/visual variant): Visuospatial disturbance, visual field defects, alexia, acalculia, and object agnosia predominate early; presents with AD pathology in parietal-occipital regions. These atypical presentations typically occur with earlier onset (50s-60s) and faster progression.
• Behavioral variant: Personality change, behavioral disinhibition, apathy, and impaired judgment predominate before memory loss develops; sometimes presents as primary progressive non-fluent aphasia-like syndrome.

Physical examination findings

• Cognitive testing: Demonstrates impairment on standardized instruments (Mini-Cog, Montreal Cognitive Assessment [MoCA], Mini-Mental State Exam [MMSE] in early stages; severe impairment in advanced disease). Specific patterns include impaired memory (free recall more impaired than recognition), reduced verbal fluency, naming difficulty (anomia), visuospatial dysfunction (clock-drawing test), and executive dysfunction.
• Neurological examination: Typically normal in early-to-middle stages. Late features include general slowing, rigidity, bradykinesia, gait disturbance, myoclonus, and increased deep tendon reflexes. Primitive reflex

Step 1 — Confirm the syndrome

• History with an informant: essential, since anosognosia makes patient self-report unreliable. Document insidious onset, gradual progression, and impairment of independence (dementia/*major neurocognitive disorder*) versus preserved independence (MCI/*mild neurocognitive disorder*), per DSM-5-TR.
• Objective cognitive testing: brief instruments include the Mini-Cog, MoCA, and MMSE. On the MoCA, scores below 26/30 are generally abnormal; on the MMSE, scores below roughly 24/30 suggest dementia, with education and language adjustments needed. Testing typically shows impaired delayed free recall that does not improve with cueing or recognition — the amnestic pattern of hippocampal/entorhinal disease.

Step 2 — Exclude reversible and alternative causes

• Laboratory screen: CBC, metabolic panel, calcium, TSH, and vitamin B12; add syphilis or HIV serology and depression screening when history suggests. The AAN dementia evaluation guidance endorses this targeted approach rather than shotgun testing.
• Structural neuroimaging: MRI brain (noncontrast CT if MRI unavailable) to exclude subdural hematoma, tumor, strategic infarcts, and normal pressure hydrocephalus. Supportive AD findings are disproportionate medial temporal/hippocampal atrophy with temporoparietal cortical volume loss.

• CSF: low Aβ42 (or low Aβ42/Aβ40 ratio) with elevated phosphorylated tau and total tau.
• Amyloid PET (e.g., florbetapir) shows cortical tracer retention; tau PET maps neurofibrillary burden. FDG-PET classically shows temporoparietal and posterior cingulate hypometabolism, contrasting with frontotemporal hypometabolism in FTD.
• Plasma phosphorylated tau assays (p-tau217) are entering practice as triage tests.

Immediate/foundational measures

• Safety and capacity assessment: driving evaluation, firearm and stove access, wandering precautions, medication supervision, advance directives and surrogate decision-maker designation while capacity remains.
• Nonpharmacologic care: structured routines, exercise, sleep hygiene, hearing and vision correction, and caregiver education/respite. The AAN and Alzheimer's Association emphasize caregiver support as core therapy, not an adjunct.

First-line pharmacotherapy

• Cholinesterase inhibitors (donepezil, rivastigmine, galantamine): block acetylcholinesterase to offset degeneration of cholinergic projections from the nucleus basalis of Meynert. Indicated across mild to severe disease (rivastigmine also available transdermally). Benefits are modest and symptomatic — they do not alter pathology.
• NMDA receptor antagonist (memantine): blunts glutamatergic excitotoxicity; used in moderate-to-severe disease, often combined with a cholinesterase inhibitor. Requires dose reduction in significant renal impairment.

Disease-modifying escalation

• Anti-amyloid monoclonal antibodies (lecanemab, donanemab): clear amyloid-beta and modestly slow decline. Restricted to MCI or mild dementia due to AD with biomarker-confirmed amyloid; require baseline MRI, APOE genotyping for risk stratification, and serial MRI surveillance for ARIA. Not indicated in moderate-to-severe dementia.

Contraindicated / avoid

• Anticholinergics (diphenhydramine, oxybutynin, tertiary TCAs) — worsen cognition; listed on the AGS Beers Criteria.
• Benzodiazepines — delirium, falls, paradoxical agitation.
• Antipsychotics carry an FDA boxed warning for increased mortality in elderly patients with dementia-related psychosis.
• Cholinesterase inhibitors: caution with bradycardia, sick sinus syndrome, or AV block (syncope risk).

Complications of the disease

• Aspiration pneumonia: late dysphagia from loss of cortical swallowing control leads to silent aspiration; signals include coughing with meals, recurrent fevers, and new hypoxia with a right lower lobe infiltrate. This is the most common terminal event in AD and is an emergency when accompanied by sepsis physiology.
• Delirium superimposed on dementia: reduced cognitive reserve makes any infection, drug, or metabolic insult precipitate acute inattention and fluctuating consciousness. An abrupt change over hours to days is never AD progression — emergency workup for UTI, pneumonia, hyponatremia, hypoglycemia, subdural hematoma, or offending drugs.
• Falls, hip fracture, and subdural hematoma: gait apraxia, parkinsonism, and impaired judgment; a fall with delayed cognitive worsening or focal deficit demands urgent head imaging.
• Malnutrition and weight loss: apraxia of feeding and dysphagia. Feeding tubes do not improve survival or prevent aspiration in advanced dementia — careful hand-feeding is preferred.
• Wandering/elopement, incontinence, pressure injuries, and seizures (myoclonus and seizures appear in advanced disease).
• Caregiver burnout, depression, and elder abuse/neglect: screen the caregiver, not just the patient.

Complications of treatment

• Cholinesterase inhibitors: excess muscarinic tone → nausea, diarrhea, anorexia and weight loss, vivid dreams, urinary urgency, and bradycardia with syncope or AV block (may prompt inappropriate pacemaker placement).
• Memantine: dizziness, headache, confusion; accumulates in renal impairment.
• Anti-amyloid antibodies — ARIA: antibody-mediated removal of vascular amyloid destabilizes vessels affected by cerebral amyloid angiopathy, producing ARIA-E (vasogenic edema) and ARIA-H (microhemorrhage/siderosis). Most is asymptomatic and MRI-detected, but headache, confusion, visual change, or seizure signals symptomatic ARIA — an emergency requiring drug hold and urgent MRI. Risk is highest in APOE ε4 homozygotes and with concurrent anticoagulation, which raises the risk of fatal intracerebral hemorrhage.
• Antipsychotics: increased mortality and cerebrovascular events, parkinsonism, QT prolongation, and neuroleptic malignant syndrome (emergency).

• **Cholinergic deficit localizes to the *nucleus basalis of Meynert***: degeneration of these basal forebrain cholinergic neurons is the rationale for donepezil, rivastigmine, and galantamine. The reciprocal fact examiners love: anticholinergics such as diphenhydramine or oxybutynin acutely worsen cognition and appear on the AGS Beers Criteria.
• Amnestic pattern that does not improve with cueing distinguishes AD from the retrieval-deficit pattern of depression ("pseudodementia"), where recall improves with prompting and the patient emphasizes their deficits rather than minimizing them.
• Single best next step in a new dementia workup: TSH and vitamin B12 plus structural neuroimaging to exclude reversible and structural causes — not an amyloid PET, and not APOE genotyping. APOE ε4 is a risk allele, not a diagnostic test, and is checked mainly to stratify ARIA risk before anti-amyloid therapy.
• Trisomy 21: three copies of the APP gene on chromosome 21 mean nearly universal AD pathology by the fourth to fifth decade — the classic genetics association tested.
• Histology buzzwords: extracellular Aβ neuritic plaques and intracellular neurofibrillary tangles of hyperphosphorylated tau (paired helical filaments); tangle distribution (Braak staging) correlates with cognition better than plaque burden. Hirano bodies and granulovacuolar degeneration are supporting findings.
• Common distractors: fluctuating cognition with visual hallucinations, REM sleep behavior disorder, parkinsonism, and antipsychotic sensitivity → dementia with Lewy bodies; early disinhibition or apathy in a patient in their 50s–60s → frontotemporal dementia; gait apraxia, urinary incontinence, and dementia with ventriculomegaly out of proportion to atrophy → normal pressure hydrocephalus (treat with shunt, not donepezil); stepwise decline with focal deficits → vascular dementia.
• Rapid decline over weeks with myoclonus and periodic sharp waves on EEG points to Creutzfeldt-Jakob disease, not AD.
• Antipsychotics for agitation carry a boxed warning for increased mortality in dementia; the APA guideline requires trying nonpharmacologic measures and searching for pain, infection, or constipation first.