How Alzheimer's Disease Works: Brain Changes, Stages, and Research

What Is Alzheimer's Disease?

Alzheimer's disease is a progressive, irreversible neurodegenerative disorder that gradually destroys memory, thinking, and the ability to carry out simple tasks. It is the most common cause of dementia — a broad term for a decline in cognitive function severe enough to interfere with daily life — accounting for 60 to 80 percent of dementia cases.

More than 55 million people worldwide live with Alzheimer's or other dementias. In the United States alone, about 7 million people have Alzheimer's — a number projected to nearly double by 2050 as the population ages.

What Happens in the Brain

Alzheimer's disease is characterized by the abnormal buildup of two proteins in the brain:

Amyloid Plaques

Beta-amyloid is a protein fragment that normally occurs in the brain and is broken down and eliminated. In Alzheimer's, these fragments accumulate between neurons, clumping together into sticky deposits called amyloid plaques. Plaques disrupt communication between neurons and trigger inflammatory responses that kill brain cells.

Amyloid accumulation begins in the brain 15 to 20 years before symptoms appear, making early detection possible with PET scans or spinal fluid analysis.

Tau Tangles

Tau is a protein that stabilizes structures inside neurons called microtubules — essentially the internal scaffolding and transport system of nerve cells. In Alzheimer's, tau becomes abnormally twisted and forms neurofibrillary tangles inside neurons, disrupting the neuron's ability to transport nutrients and communicate. Neurons eventually die.

Tau pathology typically begins in the hippocampus — the brain region critical for forming new memories — before spreading to other areas.

Neuroinflammation and Synaptic Loss

Beyond plaques and tangles, Alzheimer's involves chronic neuroinflammation as the brain's immune cells (microglia) attempt to clear the damage. Over time, synapses — the connections between neurons — are lost before neurons themselves die. Synapse loss correlates most closely with cognitive decline.

Stages of Alzheimer's

Preclinical Stage

Brain changes (amyloid and tau accumulation) occur for years or decades before any symptoms appear. This stage can only be detected through biomarker testing, not clinical observation.

Mild Cognitive Impairment (MCI) due to Alzheimer's

Subtle but measurable changes in memory and thinking that are noticeable to the person and close family but do not significantly impair daily life. Not everyone with MCI develops Alzheimer's.

Mild Alzheimer's (Early Stage)

Memory lapses become more obvious — forgetting recent conversations, names of people just met, misplacing items. Getting lost in familiar places. Difficulty with complex tasks like managing finances. Personality and mood changes may begin.

Moderate Alzheimer's (Middle Stage)

Memory loss and confusion intensify. Difficulty recognizing family members. Significant problems with language. Inability to learn new things or recall recent events. May begin wandering. Needs help with daily activities like dressing and bathing.

Severe Alzheimer's (Late Stage)

Loss of ability to communicate verbally. Requires full-time care for all activities. Difficulty swallowing and walking. Highly vulnerable to infections, particularly pneumonia. This stage can last from weeks to years.

Risk Factors

Non-Modifiable

Age: The biggest risk factor. Risk doubles every 5 years after age 65; nearly 1 in 3 people over 85 has Alzheimer's.
APOE ε4 gene: One copy increases risk 3-fold; two copies increase it 8–12-fold. However, many people with APOE ε4 never develop Alzheimer's.
Family history

Modifiable Risk Factors

Research increasingly shows that the same cardiovascular risk factors that damage blood vessels also increase Alzheimer's risk:

High blood pressure
Type 2 diabetes
Obesity
Physical inactivity
Smoking
Social isolation
Hearing loss (untreated)
Low educational attainment (lower cognitive reserve)

Diagnosis

Alzheimer's is diagnosed through clinical evaluation (detailed medical history, cognitive testing, neurological exam), brain imaging (MRI or CT to exclude other causes, PET to detect amyloid), and increasingly through biomarker testing in cerebrospinal fluid or blood tests measuring amyloid and tau levels. A definitive diagnosis has traditionally required autopsy to confirm pathology.

Treatment and Research

For decades, available medications (cholinesterase inhibitors like donepezil, and memantine) only modestly managed symptoms without altering disease progression.

In 2023, a new class of drugs — amyloid-clearing immunotherapies — achieved a landmark: the FDA approved lecanemab (Leqembi) as the first drug to demonstrably slow clinical decline in early Alzheimer's by clearing amyloid from the brain. These drugs are not cures and carry significant side effects (brain swelling and bleeding), but represent a major conceptual shift in treatment.

Research frontiers include tau-targeting drugs, neuroinflammation modulation, prevention strategies in preclinical populations, and precision medicine approaches based on genetic and biomarker profiles.