Alzheimer disease

Overview

• Definition: Alzheimer disease (AD) is a progressive neurodegenerative disease and the most common memory-predominant dementia, causing insidious decline in memory, language, and visuospatial function that ultimately impairs activities of daily living.
• Core pathology: the disease is defined pathologically by extracellular fibrillar amyloid-beta (Abeta) plaques and intracellular neurofibrillary tau tangles, accompanied by progressive brain volume loss.
• Preclinical window: amyloid pathology begins accumulating 15 to 20 years before symptom onset and is diffusely distributed in the brain, while neurofibrillary tangles localize to the regions of greatest atrophy, explaining the delayed and regional onset of symptoms.
• Clinical signature: minimal motor or gait involvement until moderate-to-severe stages distinguishes AD from other neurodegenerative dementias that present with early motor or autonomic features.

Epidemiology

• Most common dementia: AD is the most common cause of dementia overall and the most common memory-predominant dementia syndrome.
• Age-driven prevalence: age is the single greatest risk factor; after age 65 years, prevalence doubles approximately every 5 years.
• Younger-onset pattern: younger-onset AD more often presents with nonmemory-predominant patterns (language, executive, or visuospatial predominant) rather than classic amnestic decline, and memory impairment can be minimal early on.
• Declining incidence, rising prevalence: as the aging population grows, prevalence of AD and mild cognitive impairment (MCI) is increasing, but age-adjusted incidence is declining as vascular risk factors (diabetes mellitus, hypertension, hyperlipidemia) are better controlled.

Risk factors

• Age: the dominant risk factor; risk rises sharply with each decade past 65.
• APOE e4 allele: one or two copies of the apolipoprotein-E e4 (APOE e4) allele increases susceptibility; it is not a causative gene and is not recommended for routine diagnostic testing.
• Family history: a family history of dementia raises risk.
• Female sex: an independent, though modest, risk factor.
• Vascular and cerebrovascular contributors: history of stroke and cardiovascular disease increase risk; to a lesser extent, head injury also contributes.
• Modifiable dementia risk factors (Lancet Commission): excessive alcohol use, head injury, air pollution, less education, hypertension, hearing loss, smoking, obesity, depression, physical inactivity, diabetes, and limited social contact all raise dementia risk broadly and are targets for prevention counseling.

Pathophysiology

• Amyloid cascade: extracellular Abeta plaques accumulate diffusely across the brain beginning years to decades before symptoms, reflecting disordered amyloid precursor protein processing and clearance.
• Tauopathy: intracellular neurofibrillary tangles composed of hyperphosphorylated tau develop in the regions of greatest neuronal loss, tracking more closely with the topography and severity of clinical disease than amyloid burden does.
• Regional vulnerability: the medial temporal lobe, particularly the hippocampus, is affected early, producing the characteristic amnestic presentation; parietal and temporal association cortices are affected as disease spreads, producing language and visuospatial deficits.
• Neurodegeneration and atrophy: progressive synaptic and neuronal loss produces measurable brain volume loss, most detectable as hippocampal atrophy on structural imaging.

Clinical features

• Insidious amnestic onset: the typical presentation is gradually worsening memory, language, and visuospatial ability, manifesting as forgetfulness (misplacing objects, missing appointments, repeating questions, missing bill payments).
• Language and navigation problems: word-finding difficulty, hesitant speech, and navigational problems (getting lost even in familiar places, difficulty following directions while driving) follow.
• Executive and calculation decline: problems with organization, multitasking, and managing finances emerge as the disease progresses.
• Behavioral and mood symptoms: apathy, depression, anxiety, irritability, and agitation appear; delusions become more common with progression, but visual hallucinations are rare (in contrast to dementia with Lewy bodies).
• Late motor involvement: motor symptoms and gait impairment typically do not appear until moderate stages when daily function is already significantly impaired; early hallucinations or motor signs suggest an alternative diagnosis.
• Severity staging: the Mini-Mental State Examination (MMSE) is used to stage severity: mild dementia corresponds to a score of roughly 20 to 23, moderate dementia 10 to 19, and severe dementia below 10.

Differential diagnosis

• Frontotemporal dementia (behavioral variant): distinguished by personality and behavioral change (disinhibition, apathy, loss of empathy, compulsive or dietary changes) preceding cognitive impairment, with relative sparing of memory and visuospatial function early; younger age at onset and shorter survival than AD.
• Dementia with Lewy bodies: early visual hallucinations, prominent fluctuations in cognition, REM sleep behavior disorder, and parkinsonism are absent early in AD, which rules out AD here.
• Normal pressure hydrocephalus: a triad of gait disorder (often the earliest and most pronounced feature) with urinary incontinence proximate to gait change and only mild cognitive impairment relative to the gait disturbance excludes AD, in which gait is spared until late.
• Vascular cognitive impairment: distinguished by stepwise decline temporally linked to vascular events, prominent emotional incontinence, pronounced apathy, cognitive slowing, and confirmatory vascular changes on MRI rather than isolated hippocampal atrophy.
• Mild cognitive impairment (MCI): distinguished by preserved activities of daily living despite measurable cognitive deficit; only 5% to 15% convert to dementia annually, and many revert to normal on repeat testing.
• Delirium/reversible causes: distinguished by acute or subacute onset, fluctuating attention, and an identifiable precipitant (infection, medication effect, metabolic derangement); reversible causes must be excluded before attributing decline to AD.
• Depression (pseudodementia): distinguished by prominent mood symptoms preceding cognitive complaints, "I don't know" answers versus confabulated wrong answers, and improvement in cognition with treatment of depression.
• Medication-induced cognitive impairment: cognition normalizes after the offending sedating, anticholinergic, or centrally acting drug is stopped, which is not seen when AD is the primary cause.
• Language-variant primary progressive aphasia: distinguished by isolated, progressive language deficits (word-finding, grammar, or comprehension) with relative preservation of other cognitive domains early in the course.

Diagnosis

• Clinical diagnosis: AD is diagnosed clinically based on a history of insidious, progressive amnestic decline affecting daily function, after reversible and alternative causes are excluded; there is no single confirmatory bedside test.
• First step, cognitive and functional assessment: validated bedside tools (e.g., MMSE) quantify severity and support staging (mild 20 to 23, moderate 10 to 19, severe less than 10).
• Then, exclude reversible and alternative causes: initial workup for cognitive impairment of more than 3 months' duration screens for reversible contributors (medication effects, sleep apnea, depression, metabolic derangement) and alternative diagnoses.
• Structural imaging supports the diagnosis: MRI of the brain, showing hippocampal volume loss disproportionate to the rest of the brain, supports a diagnosis of AD; in MCI, hippocampal atrophy predicts higher likelihood of progression to AD dementia.
• If MRI is normal or diagnosis remains uncertain: functional imaging with fluorodeoxyglucose (FDG) PET can be used, looking for the AD pattern of decreased bilateral parietal and temporal metabolism.
• If diagnosis remains uncertain or the patient is young: AD-specific biomarkers can be sought; cerebrospinal fluid (CSF) testing showing decreased Abeta42 with increased tau and phosphorylated tau is highly specific for AD, while a normal Abeta42 level has high negative predictive value. Amyloid PET scanning is FDA approved and, in dementia specialist practice, has been shown to change management in roughly 60% of patients tested.
• Reserve specialized testing for atypical cases: CSF biomarkers, dopamine transporter imaging, and PET scanning should be reserved for uncertain diagnoses or atypical presentations rather than used routinely.

Clinical pearl. In an older adult with a slowly progressive amnestic syndrome and preserved motor function, early hallucinations or early parkinsonism are red flags that argue against Alzheimer disease and should prompt evaluation for dementia with Lewy bodies instead.

Management

• First, address reversible contributors and optimize risk factors: treat sleep disorders (including obstructive sleep apnea) and cardiovascular risk factors, encourage an exercise program, and address any medication that may be worsening cognition.
• Start an acetylcholinesterase inhibitor for mild to moderate disease: donepezil 5 mg PO once daily, increased to 10 mg once daily after 4–6 weeks; rivastigmine 1.5 mg PO twice daily, increased by 1.5 mg twice daily at intervals of at least 2 weeks to 3–6 mg twice daily; or galantamine 4 mg PO twice daily for at least 4 weeks, then 8 mg twice daily, with an increase to 12 mg twice daily after at least 4 additional weeks, are approved for mild to moderate AD dementia and produce modest improvement in cognitive performance and stabilization of functional decline; donepezil and rivastigmine are also approved for moderate to severe disease. Use with caution or avoid in patients with bradycardia or other cardiac conduction abnormalities; the most common adverse effect is gastrointestinal upset, which leads to discontinuation in nearly 20% of patients, and donepezil should be used cautiously in patients with a seizure history.
• Add memantine 5 mg PO once daily, increasing by 5 mg/day at weekly intervals to 10 mg twice daily, once disease reaches the moderate to severe stage: memantine is added to an acetylcholinesterase inhibitor for moderate to severe dementia with significant functional impairment; it has no established benefit in mild disease and is not associated with adverse cardiovascular effects, with dizziness as its most common side effect.
• If behavioral symptoms (agitation, irritability, delusions) emerge: treat nonpharmacologically first (behavioral strategies, environmental modification, addressing pain or unmet needs); reserve pharmacologic behavioral management for symptoms that are dangerous or refractory, given the risks of centrally acting drugs in this population.
• Avoid unproven supplements: ginkgo biloba and high-dose vitamin E have not shown clinical benefit; high-dose vitamin E has been associated with increased all-cause mortality and heart failure in some studies, and neither should be recommended.
• Discuss prognosis and goals of care early: because AD is progressive and within a few years impairs decision-making capacity, advance care planning and goals-of-care discussions should begin while the patient can still participate.

Complications

• Loss of functional independence: progressive impairment of instrumental and then basic activities of daily living (finances, driving, dressing, feeding) is the central complication driving caregiver burden.
• Behavioral and psychiatric complications: agitation, delusions, and mood disturbance increase in frequency as disease advances and are a major source of caregiver distress and institutionalization.
• Falls and injury: although motor symptoms are late, gait and balance impairment in moderate-to-severe disease increases fall risk.
• Medication sensitivity: centrally acting and anticholinergic medications can disproportionately worsen cognition and precipitate delirium in AD.
• Aspiration and nutritional decline: advanced disease impairs swallowing and self-feeding, increasing risk of aspiration pneumonia and malnutrition.
• Caregiver burden: the chronic, progressive course places substantial physical, emotional, and financial strain on caregivers.

Prognosis

• Progressive and ultimately fatal course: AD is relentlessly progressive; average survival from diagnosis is measured in years, with eventual loss of independent function, mobility, and communication.
• Acetylcholinesterase inhibitors and memantine slow but do not reverse decline: treatment stabilizes symptoms and functional loss temporarily but does not alter the underlying neurodegenerative trajectory or restore lost function.
• Hippocampal atrophy predicts progression: in patients with MCI, decreased hippocampal volume on MRI predicts a higher likelihood of progression to AD dementia.
• End-stage disease: advanced AD leads to severe functional dependence, immobility, and vulnerability to intercurrent illness (aspiration pneumonia, infection), which are the typical proximate causes of death.

Prevention

• No proven disease-modifying prevention: no medications or dietary supplements have been shown to prevent progression from MCI to dementia once reversible causes have been excluded, and none are approved for prevention of AD itself.
• Modifiable risk factor control: aggressive control of vascular risk factors (diabetes mellitus, hypertension, hyperlipidemia) is associated with declining age-adjusted incidence of dementia and should be pursued starting well before age 65 years.
• Exercise as the leading modifiable factor: regular exercise is the single most important modifiable lifestyle factor shown to reduce risk of MCI onset.
• Cognitive and educational engagement: higher educational attainment and midlife cognitive stimulation are associated with decreased risk of MCI.
• Address broader Lancet Commission risk factors: limiting excessive alcohol use, treating hearing loss, avoiding head injury, treating depression, maintaining social engagement, and avoiding smoking and obesity each contribute incrementally to dementia risk reduction.

Key points

• AD is the most common cause of dementia, presenting as insidious amnestic decline with late motor involvement; age and the APOE e4 allele are the dominant risk factors.
• Core pathology is amyloid-beta plaques plus neurofibrillary tau tangles, with amyloid accumulation beginning 15 to 20 years before symptoms.
• MRI showing hippocampal atrophy supports the diagnosis; FDG-PET or CSF/amyloid PET biomarkers are reserved for uncertain or atypical (especially younger-onset) cases.
• Early visual hallucinations or early motor signs point away from AD and toward dementia with Lewy bodies or another alternative diagnosis.
• Donepezil, rivastigmine, or galantamine are first-line for mild to moderate disease; memantine is added for moderate to severe disease, not used alone in mild disease.
• No intervention currently prevents or reverses progression; exercise and vascular risk factor control are the best-supported modifiable levers.