Alzheimer's Disease: Causes, Stages, and Treatment Options

6.9 Million Americans, One Disease, No Cure

Alzheimer's disease affects approximately 6.9 million Americans aged 65 and older as of 2024, according to the Alzheimer's Association — a figure projected to reach 13 million by 2050 as the population ages. It is the sixth leading cause of death in the United States and the only top-ten cause with no treatment capable of stopping its progression. The total cost of Alzheimer's and related dementias to the US economy, including unpaid caregiving, exceeds $360 billion annually. Behind these numbers are 11 million family caregivers providing an estimated 18.4 billion hours of unpaid care each year.

Alzheimer's is not a normal part of aging, though age is its strongest risk factor. The disease is a specific neurodegenerative condition defined by the abnormal accumulation of two proteins in the brain: amyloid-beta plaques that form between neurons and tau tangles that form within neurons. These accumulations are present years — often more than a decade — before any cognitive symptoms appear.

The Biological Mechanism: Plaques, Tangles, and Synaptic Collapse

Amyloid-beta is a fragment cleaved from amyloid precursor protein (APP) by the enzymes beta- and gamma-secretase. Under normal conditions, amyloid-beta is produced and cleared continuously. In Alzheimer's, either overproduction or impaired clearance causes the protein to misfold and aggregate into oligomers — small, soluble clusters that are particularly toxic to synapses — and ultimately into insoluble plaques. The amyloid cascade hypothesis, dominant since the early 1990s, proposes that amyloid accumulation initiates the disease process.

Tau protein normally stabilizes microtubules, the structural scaffold of neurons. In Alzheimer's, tau becomes hyperphosphorylated, detaches from microtubules, and aggregates into neurofibrillary tangles within neurons. The spread of tau tangles through the brain correlates more closely with cognitive decline than amyloid burden — tau pathology follows a predictable anatomical pattern from the hippocampus outward that can be tracked by PET imaging and, increasingly, by blood biomarkers.

Genetic Risk: APOE ε4 and Early-Onset Mutations

The apolipoprotein E gene (APOE) has three common alleles: ε2, ε3, and ε4. APOE ε4 is the most significant genetic risk factor for late-onset Alzheimer's disease (onset after age 65). Carrying one copy of APOE ε4 increases risk approximately three-fold; carrying two copies (homozygous ε4) increases risk approximately 8- to 12-fold. Approximately 25% of the US population carries at least one ε4 allele. APOE ε2 appears to be mildly protective.

Early-onset Alzheimer's (before age 65) accounts for approximately 5–10% of cases and is more strongly genetic. Rare autosomal dominant mutations in three genes — APP, PSEN1 (presenilin 1), and PSEN2 (presenilin 2) — cause familial Alzheimer's with near-complete penetrance. These mutations are rare but devastating, sometimes causing symptoms in people in their 30s and 40s.

The Seven Stages of Alzheimer's Progression

Diagnostic Tests: MMSE, MoCA, and Blood Biomarkers

The Mini-Mental State Examination (MMSE) has been the standard cognitive screening tool since the 1970s, scoring performance on orientation, registration, attention, recall, and language on a 30-point scale. A score of 24 or below suggests cognitive impairment. The Montreal Cognitive Assessment (MoCA) is more sensitive for mild cognitive impairment, covering a broader range of cognitive domains including visuospatial ability and executive function.

The 2024 diagnostic landscape has been transformed by plasma biomarker tests. The p-tau217 blood test — which detects phosphorylated tau protein at the threonine-217 position — can identify Alzheimer's pathology with accuracy approaching cerebrospinal fluid (CSF) testing and amyloid PET scans, at a fraction of the cost. The Precivity AD2 test and similar assays can now distinguish Alzheimer's from other causes of cognitive decline with approximately 90% accuracy in primary care settings. This diagnostic revolution enables earlier intervention and more precise patient selection for clinical trials and disease-modifying therapy.

FDA-Approved Disease-Modifying Therapies

The approval of lecanemab (brand name Leqembi, developed by Biogen and Eisai) in July 2023 marked a watershed moment: the first drug to receive traditional FDA approval for slowing Alzheimer's progression. In the CLARITY AD trial of 1,795 patients with early Alzheimer's, lecanemab slowed clinical decline by 27% over 18 months compared to placebo on the CDR-SB scale. Donanemab (Kisunla, developed by Eli Lilly) received full FDA approval in July 2024; in the TRAILBLAZER-ALZ 2 trial, it slowed progression by 35% in patients with low-to-medium tau burden.

Both drugs are anti-amyloid monoclonal antibodies administered by IV infusion and carry the risk of amyloid-related imaging abnormalities (ARIA) — brain swelling or microbleeds visible on MRI. ARIA is typically asymptomatic but can be severe; the risk is higher in APOE ε4 carriers. Both drugs are indicated only for early-stage disease (mild cognitive impairment or mild Alzheimer's dementia) confirmed by amyloid biomarker testing. Annual costs exceed $26,000 per patient. The effect size is real but modest.

Symptomatic Treatments: Cholinesterase Inhibitors and Memantine

Cholinesterase inhibitors — donepezil (Aricept), rivastigmine (Exelon), and galantamine (Razadyne) — inhibit the enzyme that breaks down acetylcholine, a neurotransmitter depleted in Alzheimer's. They modestly improve or stabilize cognitive symptoms in mild-to-moderate disease for 6–12 months in approximately 40–50% of patients. They do not slow the underlying disease. Memantine (Namenda), an NMDA receptor antagonist, is used in moderate-to-severe disease and is sometimes combined with donepezil. These drugs treat symptoms; the disease continues regardless.

Caregiver Burden and the Infrastructure Challenge

The human cost of Alzheimer's extends far beyond the diagnosed individual. Approximately 59% of caregivers are women, and more than one-third of family caregivers themselves have diagnosable depression. Caregivers of people with Alzheimer's spend an average of 47 hours per week providing care — a figure that increases as the disease progresses. The disease's long arc — average time from diagnosis to death is 8–10 years, though some live 20 years — creates chronic stress unlike the acute demands of many other serious illnesses.

The Alzheimer's Association (alz.org) operates a 24-hour helpline at 1-800-272-3900 and provides caregiver education, local support groups, and care navigation services. The care navigation challenge — figuring out what services exist, how to pay for them, and how to coordinate them — is as significant as any clinical challenge in Alzheimer's management. The system was not built for this disease.