PET IMAGING · AMYLOID SCAN ACTIVE · TOPIC: ALZHEIMER'S DISEASE · REF: AA / NIA / FDA
The Amyloid Ledger Neurology reference · U.S. edition
Neurology · Neurodegenerative

Alzheimer's begins in the brain decades before the first symptom.

About 7 million Americans live with Alzheimer's disease, and the number is projected to nearly double by 2050. For most of its history, the diagnosis came late and the treatment was limited. What has changed in the past five years is not the cure — there still isn't one — but the diagnostic tooling and the arrival of the first therapies that target the underlying biology rather than just the symptoms.

17 min read
AA & NIA aligned
Biomarkers · 4 drug classes
7M+
U.S. adults live with Alzheimer's disease — projected to reach 13 million by 2050.
6th
Leading cause of death in the U.S. — and the only one in the top 10 without a way to prevent or cure it.
20 yrs
Preclinical period — amyloid accumulates for up to two decades before symptoms appear.
11M+
Family caregivers provide unpaid care worth hundreds of billions of dollars annually.
Section 01 · The mechanism

Two proteins, one progressive cascade — and a long silent phase.

Alzheimer's disease is defined neuropathologically by two proteins that accumulate abnormally in the brain. Amyloid-β (Aβ) forms extracellular plaques — dense aggregates that trigger inflammation and disrupt synaptic signaling. Tau, a protein that normally stabilizes microtubules inside neurons, becomes hyperphosphorylated and forms intraneuronal neurofibrillary tangles. Both are required for the pathological diagnosis — but they follow different timelines and originate in different places.

Amyloid-β is produced continuously throughout life as a byproduct of normal membrane metabolism and is normally cleared by microglia, astrocytes, and the cerebrospinal fluid circulation. In Alzheimer's disease, this clearance fails. Amyloid begins to accumulate in the neocortex up to 20 years before the first symptom — a period called preclinical Alzheimer's — then spreads through the brain following a predictable spatial pattern described by the Thal phases.

Tau pathology begins later, appears first in the entorhinal cortex and hippocampus (regions critical for memory), and spreads through six stages described by Braak staging. The spread of tau correlates far more closely with symptom onset and severity than amyloid burden does. This is why a patient can have extensive amyloid plaques on PET but remain cognitively normal for years — the tau is what drives the clinical decline.

Neuroinflammation amplifies the damage. Microglia and astrocytes that normally support neurons become chronically activated, releasing inflammatory cytokines that damage bystander neurons and impair synaptic function. This is now understood to be an active part of the disease, not just a response to it, and it is one of the reasons the search for treatment has expanded beyond amyloid alone.

Cholinergic degeneration in the basal forebrain nucleus basalis of Meynert is another characteristic feature and the reason cholinesterase inhibitors were the first approved drugs for Alzheimer's. Loss of cholinergic neurons produces the memory and attention deficits most associated with the disease. Glutamatergic overactivity, particularly at NMDA receptors, contributes to excitotoxicity and is the target of memantine.

The amyloid cascade — five stages over 20 years
Preclinical → prodromal → clinical

Overproduction

Aβ is generated from APP. In most cases production is normal, but clearance is impaired.

→

Oligomer formation

Soluble Aβ oligomers — the most synaptotoxic form — form before visible plaques appear.

→

Plaque deposition

Insoluble amyloid plaques form in neocortex (Thal phase 1) and spread predictably.

→

Tau spread

Tau pathology begins in entorhinal cortex (Braak I–II) and spreads through six stages.

→

Neuronal loss & symptoms

Synaptic loss and neuronal death produce the memory and cognitive decline that brings patients in.

Why the long silent phase matters. By the time the first symptom appears, amyloid has been accumulating for 15 to 20 years and tau pathology is usually already established in the medial temporal lobe. This is why anti-amyloid therapies work best in early symptomatic disease — they can slow further accumulation but cannot undo existing neuronal loss. The future of Alzheimer's treatment likely involves identifying and treating patients before symptoms begin.
Section 02 · Clinical stages

Four clinical stages — from silent biomarker change to severe dementia.

Alzheimer's is classified by clinical symptoms and, increasingly, by biomarkers. The two do not always move together — a patient can be biomarker-positive but cognitively normal for years.

Stage 1
Preclinical · 10–20 years

Biomarker-positive, no symptoms

Amyloid PET is positive or CSF biomarkers are abnormal, but cognitive testing is completely normal. Not diagnosed clinically. Currently only relevant for research and clinical trials. Some prevention trials now enroll at this stage.

Stage 2
MCI · 2–7 years

Mild cognitive impairment

Measurable cognitive decline that is greater than expected for age but does not impair daily function. Patients notice it; family may or may not. About 10 to 15 percent progress to dementia per year. Memory-predominant MCI with positive biomarkers is highly likely to be Alzheimer's.

Stage 3
Mild–moderate · 2–10 years

Mild-to-moderate dementia

Cognitive decline now interferes with instrumental activities of daily living — managing finances, driving, cooking, medication. Memory, language, and executive function are all affected. Behavioral symptoms (apathy, anxiety, agitation) may appear. This is the stage at which most patients receive a diagnosis.

Stage 4
Severe · 1–3 years

Severe dementia

Requires full assistance with basic activities of daily living — eating, dressing, bathing, toileting. Language and mobility are severely impaired. Complications include dysphagia (leading to aspiration), immobility (leading to pressure ulcers, DVT), and infection. Hospice care is often appropriate.

Section 03 · Cognitive testing

How cognition is measured — two tests most patients will encounter.

Cognitive testing is the entry point for evaluation. Two screening instruments dominate in U.S. clinical practice, with more detailed neuropsychological batteries used when clarification is needed.

Screening instruments · comparison
Both are brief screens, not diagnoses
Instrument Range Time Notes
MMSE 0–30 5–10 min Mini-Mental State Examination. The classic instrument. < 24 suggests cognitive impairment, adjusted for education. Heavy emphasis on orientation and memory. Less sensitive to mild MCI than MoCA.
MoCA 0–30 10–15 min Montreal Cognitive Assessment. < 26 suggests impairment. More sensitive for MCI and early Alzheimer's. Includes clock drawing, trail-making, abstraction, and delayed recall. Now the preferred screen in most memory clinics.
CDR-SB 0–18 30–45 min Clinical Dementia Rating — Sum of Boxes. Combines cognitive and functional assessment across six domains. Used as the primary endpoint in most anti-amyloid trials. 0 = normal; 0.5 = MCI; 1+ = dementia.
ADAS-Cog 0–70 45 min Alzheimer's Disease Assessment Scale — cognitive subscale. Traditional primary endpoint in older AD trials. More sensitive to change over time than MoCA but longer to administer.
Neuropsych battery Varies 2–4 hrs Full battery covering memory, executive function, language, visuospatial ability, and processing speed. Used when screening results are ambiguous or when a more precise cognitive profile is needed.

What matters beyond the score. Cognitive testing is one input among several. A diagnosis also requires a functional history (what can the patient still do?), an informant's perspective (what has changed?), a neurological exam, bloodwork to rule out reversible causes (B12, thyroid, syphilis, HIV), and imaging. MRI of the brain is standard — both to rule out strokes, tumors, and hydrocephalus, and because medial temporal atrophy is characteristic of Alzheimer's. Biomarker testing (amyloid PET, CSF Aβ42/tau, or plasma p-tau217) confirms the underlying pathology and is required before starting anti-amyloid therapy.

Reversible mimics of dementia that must be ruled out: depression (pseudodementia), B12 deficiency, hypothyroidism, subdural hematoma, normal pressure hydrocephalus, medication side effects (especially anticholinergics, benzodiazepines, and opioids), obstructive sleep apnea, and chronic alcohol use. Each is treatable, and each can produce a cognitive picture that resembles early Alzheimer's.

Section 04 · Medications

Four classes — from symptomatic treatment to disease-modifying therapy.

For decades, treatment meant managing symptoms. The approval of anti-amyloid monoclonal antibodies in 2021 and 2023 introduced the first therapies that target the underlying pathology — modest in effect, but a genuine change in kind.

Cholinesterase inhibitors — symptomatic
Mild–severe AD · once daily

Block acetylcholinesterase, the enzyme that breaks down acetylcholine, increasing its availability at synapses. Modest but measurable improvement in cognition, behavior, and daily function in mild-to-moderate Alzheimer's. Effects are typically small and often temporary — six to twelve months of benefit on average — but many patients continue on them indefinitely.

Donepezil is once-daily, available as a tablet and as a transdermal patch (rivastigmine). Common side effects: nausea, diarrhea, vivid dreams, bradycardia. Caution with cardiac conduction abnormalities. Donepezil is the only one approved for severe disease.

Examplesdonepezil (Aricept) · rivastigmine (Exelon, patch) · galantamine (Razadyne)
Memantine — NMDA antagonist
Moderate–severe AD

Blocks NMDA receptors, reducing glutamate-mediated excitotoxicity. Approved for moderate-to-severe Alzheimer's and often combined with a cholinesterase inhibitor — the combination has modest additional benefit over either alone in some studies. Also available as an extended-release formulation (Namenda XR).

Side effects are usually mild: dizziness, headache, constipation, confusion. Requires dose reduction in severe kidney impairment. Well tolerated in most older adults, which is one reason it remains widely used.

Examplesmemantine IR · memantine ER (Namenda XR) · combination capsule with donepezil (Namzaric)
Anti-amyloid antibodies — disease-modifying
IV infusion · early symptomatic AD

Monoclonal antibodies that bind and clear amyloid-β plaques. Lecanemab (Leqembi) and donanemab (Kisunla) are FDA-approved for early symptomatic Alzheimer's — mild cognitive impairment or mild dementia with confirmed amyloid pathology. Both slow cognitive decline by roughly 25 to 35 percent over 18 months on the CDR-SB scale.

Given as IV infusions every 2 to 4 weeks in an infusion center. Require confirmation of amyloid by PET or CSF before starting. Carry a boxed warning for ARIA (amyloid-related imaging abnormalities). APOE ε4 carriers are at higher risk. Require scheduled MRI monitoring.

Exampleslecanemab (Leqembi) · donanemab (Kisunla) · aducanumab (Aduhelm — largely discontinued)
Symptomatic treatment — behavioral and psychiatric
Used judiciously

Up to 90 percent of patients develop behavioral symptoms — apathy, agitation, depression, anxiety, sleep disturbance, delusions. Non-drug approaches are first-line: routine, environmental modification, caregiver education. When medication is needed, SSRIs are preferred for depression and anxiety.

Brexpiprazole is FDA-approved for agitation in Alzheimer's. Antipsychotics (risperidone, quetiapine, olanzapine) are used off-label but carry a boxed warning for increased mortality in older adults with dementia and should be reserved for severe symptoms that are dangerous or refractory. Trazodone is sometimes used for sleep.

Examplessertraline · citalopram · brexpiprazole · trazodone · risperidone (short-term, cautious)
Section 05 · Anti-amyloid safety

ARIA — the risk every patient on anti-amyloid therapy must understand.

ARIA (amyloid-related imaging abnormalities) are the primary adverse effect of anti-amyloid monoclonal antibodies. They occur because clearing amyloid from blood vessel walls transiently weakens the vessel, leading to fluid leakage or small hemorrhages. They are usually asymptomatic but can be serious.

BOXED WARNING · MRI MONITORING REQUIRED

ARIA occurs in about 20 to 35 percent of patients on anti-amyloid therapy — most commonly in APOE ε4 carriers.

ARIA is divided into two types, which can occur together. ARIA-E is edema — fluid leakage into the brain parenchyma or sulci, visible as T2/FLAIR hyperintensity on MRI. ARIA-H is hemorrhage — small microhemorrhages or superficial siderosis, visible as susceptibility artifact on gradient-echo or SWI sequences.

Most ARIA is asymptomatic and detected only on scheduled MRI. When symptomatic, it can produce headache, confusion, visual disturbance, dizziness, nausea, or seizures. In rare cases, ARIA can be severe or fatal — particularly when anticoagulation is involved.

MRI monitoring schedule

Baseline MRI before starting. Follow-up MRI at defined intervals (typically before the 5th, 7th, and 14th infusions, plus any time symptoms appear). Do not skip these scans.

APOE ε4 genotype

Testing is recommended before starting. Homozygous ε4 carriers (two copies) have the highest risk of ARIA — approaching 30 to 40 percent for ARIA-E with lecanemab in trials. Some clinicians do not offer therapy to this group.

Anticoagulation

Concurrent anticoagulation substantially increases the risk of severe ARIA-H. Most clinicians avoid anti-amyloid therapy in patients on warfarin or DOACs. Antiplatelet therapy requires careful consideration.

When ARIA is detected

Asymptomatic ARIA-E usually requires holding the infusion until resolution on MRI. Symptomatic ARIA requires stopping the drug and may require corticosteroids. Severe ARIA requires hospitalization.

Section 06 · Caregiving

The unpaid workforce — and what caregivers need to know.

More than 11 million Americans provide unpaid care to someone with dementia. Caregiver health, burnout, and financial strain are as much a part of Alzheimer's care as the medications themselves.

Four domains of caregiver support
Practical · emotional · financial · legal

Safety at home

Remove trip hazards, install grab bars, lock away medications and car keys, disable the stove, and consider a medical alert system. Wandering is common — ID bracelets and door alarms help.

Daily routine & communication

Consistent schedule, short sentences, one question at a time, and validation rather than correction. Do not argue with delusions — redirect instead. Music, photos, and familiar objects can calm agitation.

Legal & financial planning

Early on, while the person can still participate: durable power of attorney, healthcare proxy, advance directives, and possibly a trust. Medicare does not cover long-term custodial care — plan ahead for Medicaid or private funds.

Caregiver self-care

Respite care, support groups (in person or online), and attention to the caregiver's own health. Caregiver depression and burnout are common and undertreated. The Alzheimer's Association 24/7 Helpline is 800-272-3900.

Section 07 · What to watch for

Changes that require prompt medical evaluation.

Evaluate promptly

If any of these appear in someone with memory concerns or known dementia, seek medical evaluation.

Sudden cognitive change over hours to days. Suggests delirium — often triggered by infection (UTI, pneumonia), medication side effects, dehydration, or metabolic disturbance. Delirium is a medical emergency and is often reversible.
New focal weakness, facial droop, or difficulty speaking. Could indicate stroke. Time-critical — call emergency services.
New headaches, vision changes, or gait instability. Requires imaging — could indicate subdural hematoma, tumor, or normal pressure hydrocephalus, all potentially treatable.
New or worsening agitation, hallucinations, or delusions. Requires evaluation for pain, infection, medication effects, and other causes before assuming it is "just the disease."
Headache, confusion, or visual changes in a patient on anti-amyloid therapy. Possible ARIA. Contact the treating clinician — do not wait for the next scheduled MRI.
Weight loss, difficulty swallowing, or recurrent choking. Suggests dysphagia — increases aspiration risk. Requires swallow evaluation, often with speech therapy.
Section 08 · Access & affordability

Alzheimer's medication is expensive. Coverage exists — with planning.

Donepezil, rivastigmine, galantamine, and memantine are all generic and inexpensive — typically less than $30 per month. This makes symptomatic therapy accessible to nearly everyone with Medicare Part D or commercial insurance. For generics, the same tablet can still cost several times more at one licensed pharmacy than another, and a live prescription price comparison shows the current range in about two minutes. If you are filling an online refill for the first time, the process is documented step by step in this how to order prescriptions online guide — prescription first, licensed seller second, small first order third.

Anti-amyloid antibodies are a different picture. Lecanemab costs about $26,500 per year for the drug alone — plus the cost of PET scans, MRI monitoring, infusion center visits, and specialist care. Medicare Part B covers the drug for patients with confirmed amyloid and a qualifying diagnosis, but with a 20 percent coinsurance after deductible, plus additional costs for the amyloid PET and monitoring MRIs. Manufacturer assistance programs and patient assistance foundations help with the coinsurance. Many patients also face substantial non-medical costs — imaging, neurology visits, caregiver support. The clinical benefit is real but modest, and the practical barriers remain significant.

Before filling from any seller — online or local — verify them. A working VIPPS seal that resolves to a live NABP record, a real U.S. street address, and a prescription requirement that cannot be waived. A transparency page like this one about Metformin4You is a reasonable example of the standard to expect.

Section 09 · Questions

Five asked most often at the memory clinic.

Q·01 What is the difference between Alzheimer's disease and dementia? +
Dementia is a syndrome — a decline in memory and other cognitive functions severe enough to interfere with daily life. Alzheimer's disease is the most common cause of dementia, accounting for 60 to 80 percent of cases. Other causes include vascular dementia, Lewy body dementia, and frontotemporal dementia, and mixed pathology is common in older adults.
Q·02 Is Alzheimer's disease hereditary? +
The most common form — late-onset Alzheimer's, beginning after age 65 — is not inherited in a simple pattern. The APOE ε4 allele is the strongest common genetic risk factor: one copy roughly triples risk and two copies increase it more substantially, but many carriers never develop the disease. Rare early-onset familial forms, caused by mutations in APP, PSEN1, or PSEN2, are inherited in an autosomal dominant pattern and typically present before age 60.
Q·03 What do cholinesterase inhibitors actually do? +
They increase acetylcholine in the brain by blocking the enzyme that breaks it down. Donepezil, rivastigmine, and galantamine produce modest but measurable improvements in cognition and daily function in mild-to-moderate Alzheimer's disease, and are usually continued into severe disease. They do not change the underlying course of the disease, and benefits are typically modest and temporary.
Q·04 Are the new anti-amyloid antibodies a cure? +
No. Lecanemab and donanemab slow cognitive decline by roughly 25 to 35 percent over 18 months in patients with early symptomatic Alzheimer's disease — a real but modest effect. They are not a cure, they do not reverse existing damage, and they carry meaningful risks including ARIA (amyloid-related imaging abnormalities) that require MRI monitoring. They require confirmation of amyloid pathology by PET or CSF before starting.
Q·05 When should someone with memory problems see a doctor? +
Any new, persistent, or progressive memory change that interferes with daily function warrants evaluation — especially if it comes on suddenly, is accompanied by confusion, personality change, or difficulty with language or spatial tasks. Sudden cognitive change may indicate delirium, stroke, medication side effects, or infection, all of which are treatable. Early evaluation rules out reversible causes and opens access to symptomatic and disease-modifying options.
◆ Sources & Verification
This reference is aligned with public clinical guidance from the Alzheimer's Association, the National Institute on Aging (NIH), the National Institute of Neurological Disorders and Stroke (NIH), and the U.S. Food and Drug Administration's BeSafeRx program. Drug-label information draws on the National Library of Medicine's DailyMed database and MedlinePlus. Pharmacy verification follows the NABP VIPPS program. Caregiver support is available 24/7 through the Alzheimer's Association Helpline at 800-272-3900.
◆ Before the next refill

Symptomatic therapy is affordable. It should be priced like the generic it is.

Donepezil, memantine, and the other symptomatic Alzheimer's medications are all available as low-cost generics, and the same tablet can cost several times more at one licensed pharmacy than another. A two-minute comparison across licensed U.S. sellers is a useful habit to build. Begin with the current Metformin price comparison as a live demonstration of how wide the spread runs.

Open the price comparison