Dementia is caused by damage to brain cells that disrupts their ability to communicate with each other. The specific source of that damage varies. Alzheimer’s disease accounts for the majority of cases, but vascular problems, abnormal protein deposits, and a surprisingly long list of lifestyle and environmental factors all play a role. About 1.7% of adults ages 65 to 74 have a dementia diagnosis, rising to 5.7% for ages 75 to 84 and 13.1% for those 85 and older.
How Alzheimer’s Disease Damages the Brain
Alzheimer’s is the most common cause of dementia, and it starts with two proteins behaving badly. The first, beta-amyloid, is a sticky fragment that builds up between brain cells and clumps into plaques. The second, tau, normally helps stabilize the internal scaffolding that neurons use to transport nutrients. In Alzheimer’s, tau becomes chemically altered, detaches from that scaffolding, and tangles together inside the cell. Those tangles disrupt the neuron’s transport system and eventually kill it.
The relationship between these two proteins matters. Amyloid buildup appears to come first and actively drives the spread of tau tangles into new brain regions. But the picture is more nuanced than “plaques cause dementia.” Healthy older adults can accumulate plaques without cognitive decline, and the number of plaques often doesn’t correlate with the severity of symptoms. The real damage in early stages may come from smaller, soluble clusters of amyloid that poison synapses, the junctions where neurons pass signals to each other. These toxic clusters appear to trigger a cascade that accelerates tau accumulation and its spread from cell to cell. As more neurons die, brain tissue shrinks and cognitive abilities decline.
Vascular Dementia: When Blood Flow Falls Short
The brain consumes roughly 20% of your body’s oxygen supply, so anything that restricts blood flow can cause lasting damage. Vascular dementia develops when small blood vessels deep in the brain narrow, become blocked, or rupture. This can happen gradually through chronic conditions like high blood pressure, or suddenly through strokes.
The most common pattern involves disease of the brain’s tiny blood vessels. As these vessels stiffen and narrow over years, they progressively starve the surrounding tissue of oxygen. This chronic shortage damages the white matter, the wiring that connects different brain regions. Small vessel disease also causes lacunar infarcts, tiny areas of dead tissue concentrated in the deep brain structures that handle processing speed, attention, and executive function. That’s why vascular dementia often shows up first as slowed thinking and difficulty planning rather than the memory loss typical of Alzheimer’s. Many people have both vascular damage and Alzheimer’s pathology simultaneously, a combination called mixed dementia.
Lewy Body Dementia and Protein Clumps
Lewy body dementia is caused by deposits of a misfolded protein called alpha-synuclein that accumulate inside neurons. These deposits, known as Lewy bodies, form in the cell body and along nerve fibers, where they block the internal transport system that neurons depend on. Over time, affected neurons essentially disconnect from each other. When this happens in the brain’s outer cortex, one region loses its ability to communicate with another, producing the hallmark symptoms: visual hallucinations, fluctuating alertness, and cognitive decline. When it happens in the movement-control areas, it produces Parkinson’s-like stiffness and slowness.
Alpha-synuclein is normally a soluble protein concentrated at synapses, but under certain conditions it folds incorrectly and begins clumping with other proteins. This can happen even without a genetic mutation, which is why most Lewy body dementia cases are sporadic rather than inherited.
Frontotemporal Dementia: Personality and Language Changes
Frontotemporal dementia (FTD) strikes the front and side regions of the brain, areas that govern personality, behavior, and language. It tends to appear earlier than other dementias, often between ages 45 and 65. Two different protein problems each account for roughly half of all FTD cases.
In one form, a protein called TDP-43 gets displaced from the cell’s nucleus, where it belongs, into the surrounding cytoplasm, where it clumps into aggregates. These aggregates damage neurons in the frontal and temporal cortex, the limbic system, and the hippocampus. In the other major form, abnormal tau protein accumulates. Pick’s disease, for example, involves spherical clumps of tau that appear inside neurons in the prefrontal area and the undersides of the temporal lobes, causing dramatic personality changes and loss of social awareness. Another tau-related variant, corticobasal degeneration, produces dense tangles in the deeper layers of the cortex, often affecting one side of the brain more than the other.
Genetics and the APOE4 Gene
Your genes don’t guarantee dementia, but they can significantly shift the odds. The strongest known genetic risk factor for late-onset Alzheimer’s is a variant of the APOE gene called APOE4. Everyone inherits two copies of APOE, one from each parent. Carrying one APOE4 copy raises your risk moderately. Carrying two copies changes the picture dramatically: people with two APOE4 copies have an estimated 60% chance of developing Alzheimer’s dementia by age 85, and they begin experiencing symptoms around age 65 on average, roughly 7 to 10 years earlier than people without the variant.
Although APOE4 homozygotes make up only about 2% of the general population, they account for an estimated 15% of Alzheimer’s cases. Nearly all of them show Alzheimer’s brain pathology from age 55 onward, and by 65, almost all have abnormal levels of amyloid in their spinal fluid. A separate, much rarer category involves mutations that directly cause early-onset Alzheimer’s, typically before age 65. These autosomal dominant forms are essentially genetically determined, but they account for a small fraction of total cases. Down syndrome is also associated with genetically driven Alzheimer’s pathology.
The 14 Modifiable Risk Factors
Perhaps the most empowering finding in dementia research is that a large share of cases are tied to factors people can influence. The 2024 Lancet Commission report identified 14 modifiable risk factors that, if fully addressed, could potentially prevent about 45% of dementia cases worldwide. That number is striking because it means nearly half of all dementia is not an inevitable consequence of aging or genetics.
The risk factors span different life stages:
- Early life: lack of education
- Midlife: hearing loss, traumatic brain injury, high blood pressure, excessive alcohol use, and obesity
- Late life: smoking, depression, social isolation, physical inactivity, diabetes, and air pollution exposure
- Added in 2024: vision loss and high LDL cholesterol
Hearing loss is one of the largest individual contributors. The prevailing theory is that when the brain receives degraded auditory input for years, the regions responsible for higher-level processing have to work harder just to decode sound, leaving fewer resources for memory and thinking. Social isolation likely compounds this, since hearing loss often leads people to withdraw from conversation. Air pollution, specifically fine particulate matter (PM2.5), can enter the bloodstream through the lungs, cross into the brain, and cause direct damage to neurons. High blood pressure and diabetes injure blood vessels over decades, contributing to the vascular damage described earlier.
Conditions That Mimic Dementia
Not every case of memory loss and confusion is dementia. Several treatable medical conditions can produce symptoms that look almost identical. Vitamin B12 deficiency, thyroid disorders, diabetes, obstructive sleep apnea, depression, and normal-pressure hydrocephalus (a buildup of fluid in the brain) can all cause significant cognitive impairment. Some medications, particularly in older adults taking multiple prescriptions, can fog thinking and mimic a dementia-like picture.
The distinction matters enormously because these conditions are often reversible. Correcting a thyroid imbalance or a vitamin deficiency can restore cognitive function that seemed permanently lost. Mixed presentations are common too, where someone has both an early neurodegenerative process and a treatable condition making it worse. Identifying and addressing the treatable layer can meaningfully improve function even when an underlying dementia is also present.
Why Age Is the Biggest Risk Factor
Age is not a cause of dementia in the way that amyloid plaques or vascular disease are, but it is the single strongest predictor. The jump from 1.7% prevalence at ages 65 to 74, to 13.1% at 85 and older reflects the reality that the biological processes behind dementia, protein misfolding, vascular wear, chronic inflammation, and declining cellular repair, all accelerate with time. Decades of exposure to risk factors compound. Blood vessels that tolerated high blood pressure at 50 may begin failing at 75. Protein clearance systems that kept amyloid in check for years gradually lose efficiency.
But age alone doesn’t make dementia inevitable. Plenty of people reach their 90s with intact cognition. The interaction between genetic predisposition, accumulated risk factors, and the brain’s individual resilience determines who crosses the threshold into clinical dementia and who doesn’t.