Alzheimer's disease (AD) is the most common cause of dementia worldwide and the prototypical neurodegenerative disorder of late life, defined neuropathologically by extracellular amyloid-beta plaques and intraneuronal neurofibrillary tangles of hyperphosphorylated tau. DSM-5-TR classifies the clinical syndrome under major or mild neurocognitive disorder due to Alzheimer's disease, while the 2018 NIA-AA research framework re-conceptualizes AD as a biological entity defined by amyloid, tau, and neurodegeneration biomarkers (the A/T/N scheme) rather than by symptoms alone. Most patients present with an insidious, progressive amnestic syndrome that broadens to involve language, visuospatial function, and executive control, with neuropsychiatric symptoms (apathy, depression, agitation, psychosis) emerging across the course and driving much of the caregiver burden. Symptomatic pharmacotherapy with cholinesterase inhibitors and memantine produces modest cognitive and functional benefits; anti-amyloid monoclonal antibodies (aducanumab, lecanemab, donanemab) now offer the first disease-modifying option for early symptomatic disease but carry meaningful risk of amyloid-related imaging abnormalities (ARIA). The bottom line for the clinician: diagnose early using a structured cognitive, functional, and biomarker-informed workup, treat the syndrome and the patient — not the scan — and weigh disease-modifying therapy against ARIA risk and apolipoprotein E genotype before prescribing.

Alzheimer's disease accounts for roughly 60-80% of all dementia cases and is the dominant driver of the global dementia epidemic projected to triple by 2050.^[1-2] Incidence rises sharply with age, doubling roughly every five years after age 65.^[1]

Prevalence and age

• Estimated 6.7 million Americans aged 65 and older live with clinical AD dementia in 2024, projected to reach 13.8 million by 2060.^[1]
• Age-specific prevalence: approximately 5% at age 65-74, 13% at 75-84, and 33% at 85 and older.^[1]
• Global prevalence of all-cause dementia exceeds 55 million, with AD as the leading subtype; low- and middle-income countries bear approximately two-thirds of cases.^[2]

Sex distribution

• Women carry approximately two-thirds of the AD burden in the United States, attributable largely to greater longevity though sex-specific biological contributions remain under investigation.^[1]
• After adjusting for age, sex differences in incidence are smaller than prevalence figures suggest.^[1]

Risk factors

• Non-modifiable: advancing age, female sex, family history, and the apolipoprotein E (APOE) ε4 allele, which confers roughly 3-fold risk in heterozygotes and 8-12-fold risk in homozygotes.^[4,38]
• Modifiable: the 2024 Lancet Commission identified 14 potentially modifiable risk factors accounting for up to 45% of dementia risk, including less education, hearing loss, hypertension, smoking, obesity, depression, physical inactivity, diabetes, excessive alcohol, traumatic brain injury, air pollution, social isolation, untreated vision loss, and elevated LDL cholesterol.^[5]
• Cardiovascular risk in midlife (hypertension, diabetes, dyslipidemia) is consistently associated with late-life AD risk.^[5]

Comorbidity

• Cerebrovascular disease coexists in 30-50% of AD brains at autopsy, often producing a mixed dementia phenotype.^[6]
• Major depressive disorder occurs in approximately 20-30% over the course of illness; apathy is even more common.^[21]
• Late-life depression may be both a risk factor and an early manifestation of underlying neurodegeneration.^[5,21]

The dominant pathophysiologic model — the amyloid cascade hypothesis — proposes that abnormal accumulation of amyloid-beta (Aβ) peptides initiates a sequence of downstream events including tau hyperphosphorylation, synaptic dysfunction, neuroinflammation, and neuronal loss.^[8] This model has been refined but not replaced by the 2018 NIA-AA biological framework, which defines AD as the joint presence of amyloid (A+) and tau (T+) pathology demonstrable in vivo.^[15]

Molecular pathology

• Extracellular plaques of aggregated Aβ42, generated from amyloidogenic cleavage of amyloid precursor protein (APP) by β- and γ-secretases.^[8]
• Intraneuronal neurofibrillary tangles of hyperphosphorylated tau, whose regional distribution correlates more tightly with cognitive decline than amyloid burden.^[8,15]
• Reactive astrocytosis, microglial activation, and synaptic loss accompany the proteinopathies.^[8]

Neurotransmitter systems

• Marked degeneration of basal forebrain cholinergic neurons (nucleus basalis of Meynert) underlies the cholinergic hypothesis and the rationale for cholinesterase inhibitor therapy.^[11]
• Glutamatergic dysregulation with excessive NMDA receptor activation contributes to excitotoxicity, providing the rationale for memantine.^[12]
• Monoaminergic systems (locus coeruleus noradrenergic, raphe serotonergic) also degenerate, contributing to neuropsychiatric symptoms.^[13]

Circuit and structural findings

• Earliest tau pathology localizes to entorhinal cortex and transentorhinal region (Braak stages I-II), spreading to hippocampus and limbic structures (III-IV), then to neocortex (V-VI).^[8,15]
• Structural MRI shows medial temporal atrophy, particularly hippocampal and entorhinal volume loss, as an early imaging signature.^[14]
• FDG-PET demonstrates temporoparietal and posterior cingulate hypometabolism; the default mode network shows early functional disruption.^[15]

Genetics

• Heritability of late-onset AD is estimated at 60-80%.^[4]
• APOE ε4 is the strongest common genetic risk factor; the ε2 allele appears protective.^[4]
• Autosomal dominant early-onset AD (<1% of cases) results from mutations in APP, PSEN1, or PSEN2, all converging on increased Aβ42 production.^[4]
• GWAS have identified more than 75 risk loci, implicating lipid metabolism (CLU, ABCA7), endocytosis (BIN1, PICALM), and innate immunity (TREM2, CR1).^[4]

DSM-5-TR classifies the clinical syndrome as either major or mild neurocognitive disorder due to Alzheimer's disease, layered onto the generic NCD criteria.^[9] The 2018 NIA-AA research framework, increasingly adopted in clinical practice as biomarkers become available, recasts AD as a biological diagnosis independent of symptom severity.^[15]

DSM-5-TR essentials

• Evidence of significant (major NCD) or modest (mild NCD) cognitive decline from a prior level in one or more cognitive domains, based on clinical concern and standardized or quantified assessment.^[9]
• For major NCD: deficits interfere with independence in instrumental activities of daily living (e.g., paying bills, managing medications).^[9]
• For mild NCD: deficits do not interfere with independence, though greater effort or compensatory strategies may be required.^[9]
• Insidious onset and gradual progression of impairment in one or more cognitive domains (for major NCD, at least two; for mild NCD, at least one).^[9]
• No evidence of mixed etiology (cerebrovascular, other neurodegenerative, systemic, or substance-related).^[9]

Probable vs possible AD (DSM-5-TR)

• Probable AD requires either evidence of a causative genetic mutation (from genetic testing or family history) or all three of: clear evidence of decline in memory and learning plus one other domain, steadily progressive gradual cognitive decline without plateaus, and no evidence of mixed etiology.^[9]
• Possible AD applies when the genetic criterion is absent and one of the three clinical features is not met.^[9]

Specifiers

• With or without behavioral disturbance (e.g., psychotic symptoms, mood disturbance, agitation, apathy).^[9]
• Severity (for major NCD): mild (difficulty with instrumental ADLs), moderate (difficulty with basic ADLs), severe (fully dependent).^[9]

NIA-AA biological framework (2018)

• AD is defined by amyloid (A) and tau (T) biomarker positivity; neurodegeneration (N) is a non-specific staging marker.^[15]
• A+T+ defines AD regardless of symptoms; A+T- is termed Alzheimer's pathologic change.^[15]
• Blood-based biomarkers (plasma p-tau217, Aβ42/40 ratio) are increasingly accurate and may replace CSF or PET in some settings.^[41]

ICD-11 differences

• ICD-11 retains "dementia due to Alzheimer disease" within neurocognitive disorders (chapter 06), with onset specifiers (early vs late) and severity codes broadly parallel to DSM-5-TR.^[17]
• ICD-11 explicitly accommodates mild neurocognitive disorder as a separate entity, similar to DSM-5-TR.^[17]

The prototypical presentation is an insidiously progressive amnestic syndrome with episodic memory loss as the earliest and most prominent deficit, followed by progressive involvement of language, visuospatial function, executive function, and ultimately basic activities of daily living.^[19] Atypical presentations — including posterior cortical atrophy, logopenic variant primary progressive aphasia, and a frontal/behavioral variant — account for roughly 5-15% of cases and disproportionately affect younger patients.^[20]

Cognitive stages

• Preclinical AD: biomarker-positive, cognitively normal; may last 10-20 years.^[15]
• Mild cognitive impairment (MCI) due to AD: measurable cognitive decline without functional impairment; approximately 10-15% per year progress to dementia.^[16]
• Mild dementia: impaired instrumental ADLs (finances, driving, medications), prominent short-term memory loss, word-finding difficulty.^[19]
• Moderate dementia: basic ADL impairment, disorientation, language deterioration, behavioral symptoms peak.^[19]
• Severe dementia: total dependence, mutism, gait apraxia, incontinence, dysphagia, ultimately death from intercurrent illness.^[19]

Cognitive domains affected

• Episodic memory loss with rapid forgetting and impaired delayed recall, classically not improved by cueing or recognition.^[19]
• Anomic aphasia progressing to semantic and ultimately global language deterioration.^[19]
• Visuospatial impairment with topographical disorientation and constructional apraxia.^[19]
• Executive dysfunction with impaired planning, set-shifting, and judgment.^[19]

Neuropsychiatric symptoms

• Apathy is the most common neuropsychiatric symptom, present in up to 70% across the course.^[21]
• Depression occurs in 20-30%; can be difficult to distinguish from apathy and from cognitive symptoms.^[7]
• Agitation, aggression, and psychosis (typically paranoid delusions of theft or misidentification, less often hallucinations) emerge in moderate stages.^[21]
• Sleep-wake disturbance, including sundowning, is common.^[21]

Atypical presentations

• Posterior cortical atrophy: progressive visuospatial and visuoperceptual deficits (Balint syndrome, alexia, simultanagnosia) with relatively preserved memory early on.^[20]
• Logopenic variant primary progressive aphasia: impaired single-word retrieval and sentence repetition; most cases have AD pathology.^[20]
• Frontal/behavioral variant: dysexecutive and behavioral features that can mimic frontotemporal dementia.^[20]

The differential for an older adult with progressive cognitive decline is broad, and a confident clinical diagnosis of AD rests on excluding reversible mimics and recognizing alternative neurodegenerative patterns. Biomarkers now permit positive identification rather than diagnosis by exclusion alone.^[15-16]

Other neurodegenerative dementias

• Dementia with Lewy bodies (Dementia with Lewy bodies): fluctuating cognition, recurrent well-formed visual hallucinations, REM sleep behavior disorder, and parkinsonism within one year of cognitive onset; severe neuroleptic sensitivity is a red flag.^[23]
• Frontotemporal dementia: early personality change, disinhibition, apathy, or progressive aphasia with relative sparing of episodic memory; typically younger onset (45-65).^[24]
• Vascular cognitive impairment: stepwise decline, focal neurologic signs, prominent executive dysfunction, and strategic infarcts or extensive small-vessel disease on imaging.^[25]
• Parkinson disease dementia: motor symptoms precede cognitive decline by more than one year, distinguishing it from DLB.^[23]
• Creutzfeldt-Jakob disease: rapidly progressive dementia over weeks to months with myoclonus and cortical ribboning on diffusion-weighted MRI.^[26]

Psychiatric mimics

• Major depressive disorder with cognitive features (pseudodementia): subjective complaints often exceed objective deficits, with prominent anhedonia, psychomotor slowing, and a 'don't know' response style on testing.^[27]
• Delirium: acute onset, fluctuating attention, and altered level of consciousness — always evaluated before attributing new cognitive decline to a dementia.^[28]
• Late-onset psychotic disorder or severe anxiety can secondarily impair attention and memory.^[27]

Reversible and medical mimics

• B12 deficiency, hypothyroidism, and hyponatremia are the classic reversible workup targets.^[29]
• Normal pressure hydrocephalus: gait apraxia, urinary incontinence, and cognitive slowing (Hakim triad) with ventriculomegaly out of proportion to atrophy.^[30]
• Neurosyphilis and HIV-associated neurocognitive disorder should be considered in appropriate epidemiologic contexts.^[29]
• Chronic subdural hematoma, particularly in patients on anticoagulation or with falls.^[29]
• Medication-induced cognitive impairment: anticholinergics, benzodiazepines, opioids, and sedating antihistamines are frequent culprits in older adults.^[31]

Evaluation integrates a structured history, informant interview, bedside cognitive testing, targeted laboratories, structural imaging, and — when diagnostic uncertainty remains or disease-modifying therapy is contemplated — fluid or imaging biomarkers of amyloid and tau.^[15-16]

History and interview

• Obtain a detailed cognitive timeline from the patient and a knowledgeable informant; informant report is more sensitive than self-report in established AD.^[34]
• Document functional decline using activities of daily living and instrumental activities of daily living; functional change distinguishes major from mild neurocognitive disorder.^[9]
• Screen for vascular risk factors, head trauma, sleep apnea, alcohol use, and exposure to anticholinergic or sedating medications.^[31]
• Elicit neuropsychiatric symptoms across the apathy-agitation-mood-psychosis-sleep axis; the Neuropsychiatric Inventory (NPI) is the standard structured instrument.^[22]

Bedside cognitive screening

• The Montreal Cognitive Assessment (MoCA) is more sensitive than the Mini-Mental State Examination (MMSE) for mild neurocognitive disorder; a score below 26 of 30 prompts further evaluation, with education-adjusted norms.^[35]
• The Mini-Cog (three-item recall plus clock drawing) is useful for very brief screening in primary care.^[35]
• Neuropsychological testing is indicated when screening is equivocal, when the cognitive profile is atypical, or when baseline characterization will inform follow-up.^[36]

Physical and neurologic examination

• A focused neurologic exam looks for parkinsonism, frontal release signs, focal deficits, gait abnormalities, and signs suggestive of an alternative diagnosis.^[16]
• Screen vision and hearing; uncorrected sensory loss inflates cognitive deficits and worsens trajectory.^[37]

Laboratory workup

• Standard reversible workup: TSH, vitamin B12, comprehensive metabolic panel, complete blood count.^[29]
• Consider HIV, RPR/treponemal testing, and heavy metal screening when clinically indicated.^[29]
• ApoE genotyping is not required for diagnosis but is now clinically relevant when anti-amyloid mAb therapy is considered because APOE ε4 homozygotes have substantially higher ARIA risk.^[38,49]

Neuroimaging

• Structural MRI (or CT if MRI contraindicated) is recommended in every new dementia workup to exclude mass lesions, stroke, hydrocephalus, and to characterize atrophy patterns; medial temporal atrophy supports AD.^[14,16]
• FDG-PET shows characteristic temporoparietal and posterior cingulate hypometabolism in AD and helps distinguish AD from FTD.^[16]
• Amyloid PET (florbetapir, florbetaben, flutemetamol) and tau PET (flortaucipir) provide in vivo biomarker confirmation; appropriate-use criteria restrict amyloid PET to cases with diagnostic uncertainty or candidacy for disease-modifying therapy.^[16,39]

Fluid biomarkers

• CSF Aβ42 (low), Aβ42/Aβ40 ratio (low), total tau (high), and phosphorylated tau (p-tau181, p-tau217; high) reliably identify AD pathology.^[16,40]
• Plasma p-tau217 has emerged as the most accurate blood-based biomarker, with diagnostic performance approaching CSF in specialty settings, although standardization across assays remains incomplete.^[40-41]

What NOT to order routinely

• Routine EEG, genetic testing for autosomal dominant AD in late-onset cases, and untargeted antibody panels are not indicated outside specific clinical scenarios.^[16]

Management of AD combines symptomatic pharmacotherapy, disease-modifying anti-amyloid antibodies in eligible patients, psychosocial and caregiver-directed interventions, and targeted treatment of neuropsychiatric symptoms. No intervention to date halts progression, and goals of care should be revisited at every transition in disease severity.^[42,49]

Pharmacotherapy

Cholinesterase inhibitors (ChEIs) and the NMDA receptor antagonist memantine remain the symptomatic backbone, producing small but reproducible improvements in cognition, global function, and behavior versus placebo in randomized trials.^[42,45-46]

• donepezil 5-10 mg PO QHS is approved for mild, moderate, and severe AD; the 23 mg formulation offers marginal benefit in moderate-to-severe disease at the cost of more gastrointestinal effects.^[42,44]
• rivastigmine 1.5-6 mg PO BID or transdermal 4.6-13.3 mg/24 h patch — the patch reduces GI adverse effects relative to oral dosing.^[42,44]
• galantamine 8-12 mg PO BID (or 16-24 mg extended-release once daily); avoid in severe hepatic or renal impairment.^[42,44]
• memantine 10 mg PO BID (or 28 mg extended-release once daily) for moderate-to-severe AD, alone or combined with a ChEI; the donepezil-memantine fixed combination simplifies dosing.^[45-46]
  • Combination ChEI + memantine has small additive benefits on cognition and behavior versus ChEI monotherapy in moderate-to-severe disease.^[45-46]

Disease-modifying anti-amyloid therapy

Three monoclonal antibodies targeting aggregated amyloid-beta have received FDA approval for early symptomatic AD (mild cognitive impairment or mild dementia due to AD) with confirmed amyloid pathology.^{[32-33,50-51]}

• Lecanemab and donanemab slow clinical decline by roughly 25-35% over 18 months in patients with biomarker-confirmed early AD.^[32-33]
• lecanemab 10 mg/kg IV every two weeks; CLARITY-AD demonstrated a 27% slowing of CDR-SB progression at 18 months versus placebo.^[32]
• donanemab 700-1400 mg IV every four weeks until amyloid clearance on PET; TRAILBLAZER-ALZ 2 showed 35% slowing of iADRS decline in low-to-medium tau participants.^[33]
• aducanumab 10 mg/kg IV every four weeks received accelerated approval based on amyloid reduction; clinical-benefit data were conflicting and the manufacturer discontinued commercialization in 2024.^[50]

• ARIA-E occurs in 12-13% of lecanemab-treated and 24% of donanemab-treated patients; symptomatic ARIA is less common but can include headache, confusion, seizure, and rarely fatal cerebral edema or hemorrhage.^[32-33,52]
• APOE ε4 homozygotes show approximately 30-40% incidence of ARIA-E; the FDA label and Appropriate Use Recommendations advise caution and shared decision-making in this group.^[49,52]
• Concurrent anticoagulation increases hemorrhagic ARIA risk; tissue plasminogen activator administration during anti-amyloid therapy has been associated with fatal intracerebral hemorrhage.^[49,52]

Psychotherapy

Non-pharmacologic interventions are first-line for behavioral and psychological symptoms of dementia (BPSD) and supplement pharmacotherapy across the disease course.^[54-55]

• Cognitive stimulation therapy (group-based, manualized) improves cognition and quality of life in mild-to-moderate dementia with effect sizes comparable to ChEIs.^[54]
• Structured caregiver psychoeducation and skill-building (e.g., the REACH II / Savvy Caregiver programs) reduces caregiver depression and delays institutionalization.^[55-56]
• Reminiscence therapy and music-based interventions improve mood and agitation in moderate-to-severe stages; evidence is moderate-quality but harms are negligible.^[54,57]
• Tailored activity programs and environmental modification reduce agitation and rejection of care; the DICE approach (Describe, Investigate, Create, Evaluate) operationalizes assessment of BPSD triggers.^[55-56]

Neuromodulation

Neuromodulation in AD remains investigational; no device-based therapy is approved as a primary treatment.^[58]

• Repetitive transcranial magnetic stimulation (rTMS) and combined rTMS-cognitive training show small, short-term cognitive benefits in pilot trials but lack durable disease-modifying evidence.^[58]
• Deep brain stimulation of the fornix has been studied in phase II trials with mixed results; subgroup signals in older patients are hypothesis-generating only.^[58]
• Electroconvulsive therapy (ECT) is reserved for severe, treatment-refractory depression or agitation in advanced dementia and can produce rapid behavioral improvement, with transient post-ictal confusion expected.^[59]

Adjunctive

Treat neuropsychiatric symptoms, cardiovascular risk factors, and reversible contributors aggressively — they drive function as much as the underlying pathology.^[3,56]

• Depression: sertraline 25-100 mg PO QD or citalopram 10-20 mg PO QD (cap citalopram at 20 mg in patients over 60 due to QT prolongation risk) are first-line; tricyclics are avoided for anticholinergic burden.^[57,62]
• Agitation and psychosis: Antipsychotics in dementia carry a black-box warning for increased mortality; reserve for severe, dangerous symptoms after non-pharmacologic measures fail.^[56,65]
  • brexpiprazole 2 mg PO QD is FDA-approved (2023) specifically for agitation associated with AD dementia, with modest reduction in CMAI scores versus placebo.^[60-61]
  • Low-dose risperidone 0.25-1 mg PO QD or quetiapine 25-100 mg PO QD are used off-label; reassess every 3 months for ongoing need and tapering.^[56]
• Sleep: avoid benzodiazepines and Z-drugs due to fall and delirium risk; trazodone 25-100 mg at bedtime is commonly used with limited high-quality evidence.^[56,66]
• Manage vascular risk factors (hypertension, diabetes, dyslipidemia, atrial fibrillation); intensive blood pressure control reduces incidence of mild cognitive impairment in trial data.^[3]
• Address sensory impairment — hearing aids in those with hearing loss reduce 3-year cognitive decline in at-risk older adults (ACHIEVE trial).^[64]

The table below summarizes the central interventions in AD, weighted toward symptomatic cholinesterase therapy and anti-amyloid monoclonal antibodies as the dominant evidence streams.

The harms picture in AD pharmacotherapy is dominated by tolerability ceilings on symptomatic agents and by ARIA risk in disease-modifying therapy. Evidence base limitations include trial populations that under-represent older, frailer, and ethnically diverse patients most affected by AD in practice.

Common adverse effects

• Cholinesterase inhibitors cause dose-related nausea, diarrhea, anorexia, weight loss, vivid dreams, urinary urgency, and muscle cramps; bradycardia and syncope are clinically important in older patients on rate-controlling medications ^[42,44].
• Memantine is generally well tolerated; dizziness, headache, constipation, and rare confusion or hallucinations are reported ^[45].
• Anti-amyloid monoclonal antibodies produce infusion reactions, headache, and ARIA detected on protocol MRI; most ARIA episodes are radiographic and asymptomatic but can cause headache, confusion, focal deficits, or seizures ^[47,49-51].
• Antipsychotics used for agitation cause sedation, parkinsonism, metabolic effects, orthostasis, and falls; brexpiprazole's adverse-effect profile in AD trials was milder than older agents but the class signal persists ^[60-61,65].

Serious or rare adverse effects

• Symptomatic intracerebral hemorrhage on anti-amyloid antibodies, particularly in APOE ε4 homozygotes, with reported fatalities in real-world and trial settings ^[49-51].
• All atypical antipsychotics carry an FDA black-box warning for increased mortality in dementia-related psychosis driven by cardiovascular and infectious deaths ^[65].
• Citalopram and escitalopram at higher doses prolong the QTc interval, with the FDA capping citalopram at 20 mg/day in adults over 60 years ^[62].
• Anticholinergic burden from any source (antihistamines, urinary antispasmodics, tricyclics) worsens cognition and can precipitate delirium superimposed on dementia ^[66].

Monitoring, withdrawal, and discontinuation

• Cholinesterase inhibitor discontinuation can produce acute cognitive and behavioral worsening over days to weeks; taper when stopping in moderate–severe disease ^[42].
• Anti-amyloid antibody protocols require MRI at baseline and before scheduled doses (with additional scans for symptomatic ARIA), restricting use to centers with neuroradiology capacity ^[33,47,49].
• Long-term safety beyond 18–24 months for lecanemab and donanemab remains under post-marketing surveillance ^[33,47].

Limitations of the evidence base

• Most disease-modifying trials enrolled predominantly non-Hispanic white participants with high education, limiting generalizability ^[33,47].
• Cognitive scales used in trials (CDR-SB, ADAS-cog, iADRS) capture statistically significant change that may not translate to patient-perceived benefit at the individual level ^[48,51,67].
• The clinical meaningfulness of 25–35% slowing of decline is debated; the minimal clinically important difference for CDR-SB in early AD is not firmly established ^[48,67].

Tailoring AD care across populations addresses the dominant comorbid and contextual factors that drive outcome in practice. Early-onset (presenilin / amyloid precursor protein mutations):

• Autosomal dominant familial AD presents in the 30s–50s and accounts for under 1% of cases; key genes are PSEN1, PSEN2, and APP ^[9-10].
• The Dominantly Inherited Alzheimer Network (DIAN) cohort and trials provide the principal evidence base for this population; gantenerumab and solanezumab in DIAN-TU did not meet primary cognitive endpoints in symptomatic carriers but reduced biomarker burden ^[68].
• Genetic counseling is required before testing; psychiatric sequelae of presymptomatic testing are well documented ^[10].

Down syndrome

• Trisomy 21 triples APP gene dosage; by age 40 nearly all individuals with Down syndrome have AD neuropathology, with clinical dementia in 50–70% by age 60 ^[69].
• Baseline intellectual disability complicates cognitive assessment; specialized instruments such as the Cambridge Cognitive Examination for adults with Down syndrome (CAMCOG-DS) are recommended ^[69].
• Anti-amyloid therapy in Down syndrome is under active investigation; trial enrolment requires careful capacity assessment ^[69].

Geriatric and frail patients

• Polypharmacy and anticholinergic burden are major modifiable contributors to cognitive impairment; the Beers Criteria flag agents to avoid in older adults ^[66].
• Frailty modifies the risk–benefit calculus for both cholinesterase inhibitors and anti-amyloid therapy; expert guidance recommends individualized goals-of-care discussion ^[42,49].

Comorbid depression and late-life depression

• Depression both increases AD risk and complicates its presentation; treat with SSRIs while avoiding paroxetine and tricyclics due to anticholinergic effects ^[56-57,66].
• Pseudodementia of depression typically improves with antidepressant treatment but is associated with elevated risk of later dementia, requiring follow-up ^[57].

Cultural and ethnic considerations

• Black and Hispanic populations have higher AD prevalence but are under-represented in disease-modifying trials, and biomarker reference ranges have been derived predominantly in non-Hispanic white cohorts ^[1,3,33,47].
• Culturally adapted cognitive instruments and trained interpreters reduce misclassification in non-English-speaking patients ^[9].

AD is a progressive, ultimately fatal disorder; median survival from diagnosis ranges from approximately 4 to 8 years and is influenced by age at onset, comorbidity, and care environment ^[1,7].

Trajectory

• Preclinical AD (biomarker-positive, cognitively unimpaired) can persist for 10–20 years before symptom onset ^[15].
• Mild cognitive impairment due to AD progresses to dementia at roughly 10–15% per year, though some patients remain stable or revert ^[9,16].
• The dementia stage typically lasts 4–8 years from diagnosis, with progressive loss of independence in instrumental and then basic activities of daily living ^[1,7,9].

Predictors of faster decline

• Younger age at onset, APOE ε4 homozygosity, early extrapyramidal or psychotic features, and high baseline tau burden predict more rapid progression ^[15,19].
• Vascular comorbidity, uncontrolled diabetes, and recurrent delirium accelerate functional decline ^[3,7].

Mortality and cause of death

• Common terminal events are aspiration pneumonia, urinary tract infection with sepsis, and complications of immobility ^[7].
• Hospice eligibility under US Medicare uses Functional Assessment Staging (FAST) stage 7 plus a medical complication as a practical threshold ^[70].

Acute presentations in AD typically arise from superimposed delirium, behavioral crises, falls, wandering, or anti-amyloid therapy complications, and a structured response prevents both undertreatment and iatrogenic harm.

Delirium superimposed on dementia

• New or rapidly worsening confusion in an AD patient is delirium until proven otherwise; pursue infection, dehydration, urinary retention, constipation, pain, medication change, and metabolic derangement ^[71].
• Manage with environmental measures, treat the underlying cause, and use antipsychotics only briefly for safety-threatening agitation while accepting the class mortality signal ^[65,71].

Agitation and aggression

• DICE-guided non-pharmacologic approaches are first-line; reserve pharmacotherapy for behaviors that threaten safety or prevent essential care ^[55-56].
• Brexpiprazole is the only FDA-approved agent for AD agitation; risperidone, quetiapine, and olanzapine remain widely used off-label with informed consent about the black-box warning ^[60-61,65].

Suicide risk

• Suicide risk is elevated around the time of diagnosis and in early-stage disease when insight is preserved; screen explicitly and address depression aggressively ^[72].
• Means restriction (firearms, large medication supplies) and driving safety planning are routine in early AD ^[72].

Wandering, driving, and capacity

• Wandering occurs in up to 60% of community-dwelling patients; document elopement plans and consider GPS tracking with consent ^[73].
• Driving cessation is a clinical and legal threshold; on-road testing through occupational therapy is the most accurate predictor of safety ^[74].
• Capacity assessment is decision-specific; document the standard applied (understanding, appreciation, reasoning, expressing a choice) ^[75].

Anti-amyloid therapy emergencies

• Symptomatic ARIA-E or ARIA-H requires immediate MRI, infusion suspension, and neurology consultation; symptomatic intracerebral hemorrhage is a medical emergency ^[33,47,49].
• Patients on lecanemab or donanemab who develop stroke symptoms should not receive thrombolytics outside expert consultation given case reports of fatal hemorrhage ^[33,49].

AD sits at the intersection of an aging neurobiological framework, a contested set of disease-modifying approvals, and a public-health risk-reduction agenda; the live debates are clinically consequential.

Amyloid hypothesis and biological definition

• The amyloid cascade hypothesis has guided drug development for three decades, but anti-amyloid trials have shown that substantial plaque clearance produces only modest clinical benefit, prompting renewed interest in tau and other targets ^[8,33,47].
• The 2018 NIA-AA biological definition (A/T/N) is research-oriented; its translation into clinical care remains contested, particularly the question of when biomarker-positive cognitively unimpaired individuals should be told they have AD ^[15].

Clinical meaningfulness of disease-modifying therapy

• Lecanemab and donanemab slow CDR-SB decline by ~25–35% over 18 months, but whether this constitutes a clinically meaningful benefit for an individual patient is debated ^[48,51,67].
• Cost (US list prices of $26,500–$32,000 per year), MRI surveillance burden, and infusion logistics constrain real-world uptake even among eligible patients ^[49].

Aducanumab and accelerated approval

• The 2021 accelerated approval of aducanumab over the FDA advisory committee's near-unanimous negative vote remains a touchstone case in regulatory science ^[52-53].
• Multiple national insurers restricted coverage, and the manufacturer discontinued the product in 2024 ^[53].

APOE genotyping and equity

• APOE ε4 homozygotes have markedly higher ARIA risk on anti-amyloid therapy, raising the question of routine pre-treatment genotyping; counseling, insurance, and equity implications are unresolved ^[33,47,49].
• Recent observations suggest APOE ε4 homozygosity itself may represent a distinct genetic form of AD, further complicating risk communication ^[76].

Modifiable risk factor framework

• The Lancet Commission identifies 14 potentially modifiable risk factors accounting for around 45% of dementia risk worldwide; the population-attributable fractions are estimates, not individual-patient predictions ^[3].
• Public-health translation of multidomain interventions (FINGER, US POINTER) is ongoing; effect sizes remain modest ^[63].

Antipsychotic use for agitation

• The black-box mortality warning is well established, but residual clinical need for off-label antipsychotics persists when behaviors threaten safety and non-pharmacologic measures fail ^{[54-55,60,65]}.
• Brexpiprazole's 2023 approval narrows the off-label space but does not eliminate the trade-off ^[60-61].

Blood-based biomarkers

• Plasma p-tau217 and related assays approach the performance of CSF and PET for amyloid status and are entering clinical use, with implications for primary-care triage ^[77].
• Standardization across platforms and performance in diverse populations remain works in progress ^[77].

• AD is the most common cause of dementia, accounting for 60–80% of cases in older adults ^[1-2].
• DSM-5-TR places AD under major or mild neurocognitive disorder due to Alzheimer's disease, classified as probable or possible based on genetic evidence and clinical course ^[9].
• Hippocampal and medial temporal atrophy on MRI, hypometabolism in temporoparietal and posterior cingulate regions on FDG-PET, and amyloid-positive PET or CSF Aβ42 reduction support the diagnosis ^{[14-15,39-40]}.
• APOE ε4 is the strongest common genetic risk factor; one copy roughly triples and two copies roughly increase risk 8–12 fold for late-onset AD ^[4,38].
• Autosomal dominant familial AD (under 1% of cases) is caused by mutations in PSEN1, PSEN2, or APP ^[9-10].
• Cholinesterase inhibitors (donepezil, rivastigmine, galantamine) are first-line for mild–moderate AD; bradycardia and syncope are key adverse effects ^[42,44].
• Memantine is approved for moderate–severe AD and is commonly combined with a cholinesterase inhibitor ^[45-46].
• Lecanemab and donanemab are FDA-approved for early symptomatic AD and slow CDR-SB decline by 25–35% over 18 months at the cost of meaningful ARIA risk ^[33,47].
• ARIA-E (edema/effusion) and ARIA-H (hemorrhage/hemosiderosis) are the defining adverse events of anti-amyloid monoclonal antibodies; APOE ε4 homozygotes are at highest risk ^[33,47,49].
• Aducanumab received accelerated approval in 2021 and was discontinued by its manufacturer in 2024 ^[53].
• Brexpiprazole is the only FDA-approved pharmacotherapy specifically for agitation in AD; all atypical antipsychotics carry a black-box warning for increased mortality in dementia-related psychosis ^[60-61,65].
• New or rapidly worsening cognition in an AD patient is delirium until proven otherwise ^[71].
• The Lancet Commission identifies hearing loss, hypertension, depression, smoking, physical inactivity, social isolation, and low education among modifiable risk factors accounting for around 45% of dementia risk ^[3].
• The FINGER trial supports a multidomain lifestyle bundle for at-risk older adults ^[63].
• AD has a typical disease duration of 4–8 years from diagnosis; aspiration pneumonia is the most common terminal event ^[1,7].

No external funding. No conflicts of interest declared. Peer-review status: pending.