Alzheimer’s disease slowly destroys brain cells, starting with the areas responsible for memory and gradually spreading to regions that control language, reasoning, and eventually basic body functions like swallowing and movement. An estimated 7.2 million Americans age 65 and older are living with Alzheimer’s dementia in 2025, and that number could reach 13.8 million by 2060.
How It Damages Brain Cells
Two types of abnormal protein buildup drive the destruction. The first involves beta-amyloid, a naturally occurring protein that begins clumping together into sticky plaques between brain cells. These plaques disrupt how cells function and communicate with each other. The second involves tau, a protein that normally helps stabilize the internal transport system within neurons. In Alzheimer’s, tau molecules undergo abnormal chemical changes, detach from their normal structure, and clump into tangles inside the neurons themselves. These tangles block the neuron’s internal transport system, cutting off the supplies cells need to survive and communicate.
The two processes feed each other. Once beta-amyloid levels reach a tipping point, tau spreads rapidly throughout the brain. The early loss of connections between neurons, called synapses, is one of the primary drivers of cognitive decline. Over time, neurons stop functioning, lose their connections to neighboring cells, and die. The damage is not contained to one spot. It spreads progressively across the brain over years.
The Brain’s Immune System Backfires
The brain has its own immune cells, and they initially try to help. When amyloid plaques first appear, these immune cells activate and attempt to engulf and clear the protein buildup. In the short term, this response is protective. But as the disease progresses and plaques keep accumulating, the immune cells become chronically overactivated. They lose their ability to clear the plaques effectively and instead start releasing toxic inflammatory molecules that damage healthy neurons, destroy the connections between cells, and actually worsen both the amyloid and tau pathology. What begins as a defense mechanism becomes a third front of brain destruction.
Where the Damage Starts
Alzheimer’s does not hit the whole brain at once. The plaques and tangles first appear in the entorhinal cortex, a small region that acts as a gateway to the hippocampus, the brain’s memory center. From there, tau pathology spreads into the hippocampus itself, which begins to shrink rapidly in the early stages. The hippocampus is responsible for forming new episodic memories, the kind that let you recall what happened yesterday or where you parked your car. Its advanced shrinkage is directly responsible for the short-term memory loss that defines early Alzheimer’s.
As the disease progresses, neurodegeneration extends outward from the hippocampus into the temporal cortex, then the frontoparietal cortex, and eventually deeper brain structures. Each region’s decline brings a new wave of lost abilities.
The Chemical Shortage
Alzheimer’s also depletes the brain’s supply of acetylcholine, a chemical messenger critical for memory and learning. The neurons that produce acetylcholine degenerate severely as the disease advances, and the enzyme needed to manufacture it becomes significantly less active. Meanwhile, the enzyme that breaks acetylcholine down becomes more active, compounding the shortage. The amyloid plaques themselves accelerate this process, particularly in the hippocampus and the prefrontal cortex, the region involved in working memory and decision-making. This chemical deficit is one reason memory and the ability to learn new information deteriorate so early in the disease.
What Changes in the Early Stage
Memory problems are typically the first noticeable sign, but they are not always the first sign. For some people, the earliest changes involve difficulty finding the right words, trouble judging distances or spatial relationships, or impaired reasoning and judgment. In practical terms, the early stage looks like repeating the same questions, taking longer to complete familiar tasks, mishandling money or bills, getting lost in familiar places, and subtle personality shifts that family members may initially dismiss as normal aging.
What Changes in the Middle Stage
The middle stage is typically the longest and brings the most dramatic behavioral changes. Sleep patterns often reverse, with the person sleeping during the day and becoming agitated or restless at night. Wandering becomes more frequent and more dangerous, as the person may leave home and be unable to find their way back. Personality changes deepen into suspiciousness, delusions, and compulsive repetitive behaviors like hand-wringing or shredding tissues. The person may no longer recognize family members or remember significant life events. They increasingly need help with daily activities like dressing, bathing, and using the bathroom.
What Happens in the Late Stage
In the final stage, Alzheimer’s moves beyond cognition and begins dismantling basic physical abilities. The person loses the ability to move independently, and without regular range-of-motion exercises, their arms and legs can become stiff and immobile. Some people develop sudden involuntary muscle spasms or jerks in their limbs or entire body.
One of the most serious late-stage complications involves swallowing. The person may no longer be able to chew and swallow safely, which creates a significant risk of choking. If food enters the lungs instead of the stomach, it can cause a type of pneumonia that is one of the most common causes of death in people with advanced Alzheimer’s. Communication is typically reduced to a few words or none at all, and the person becomes fully dependent on caregivers for every aspect of daily life.
How It Is Diagnosed
Diagnosis has historically relied on cognitive testing, brain imaging with MRI to measure hippocampal shrinkage, and PET scans to detect amyloid plaques. But PET scans require specialized facilities and are not widely available, which has limited access to definitive diagnosis. Blood-based biomarker tests are now emerging as a practical alternative. These tests can measure the ratio of specific amyloid proteins, levels of phosphorylated tau, and markers of nerve cell damage and brain inflammation. They are not yet standard in every clinic, but they represent a major shift toward earlier and more accessible detection. Confirming the presence of amyloid in the brain is now required before a person can be prescribed the newest class of treatments.
Current Treatments
For decades, the only medications available for Alzheimer’s worked by boosting acetylcholine levels in the brain, addressing the chemical shortage but doing nothing to slow the underlying disease. That changed with the FDA’s approval of drugs that directly target amyloid plaques. The most recent, approved in 2024, is an infusion therapy that clears amyloid from the brain. In clinical trials, treatment was stopped once amyloid levels dropped below a certain threshold on PET scans, suggesting the plaques had been substantially reduced. These treatments do not reverse damage already done, and they carry risks including brain swelling and small bleeds. But they represent the first therapies that alter the biological process driving the disease rather than just managing symptoms.