There is no single definitive number, but medical sources generally recognize four major types of dementia along with numerous rarer forms, bringing the total to well over a dozen distinct conditions. The reason you won’t find one clean number is that dementia isn’t a single disease. It’s an umbrella term for a range of brain conditions that progressively impair memory, thinking, and the ability to handle daily life. Some of these conditions break down further into subtypes, and many people end up with more than one type at the same time.
The Four Major Types
The National Institute on Aging highlights four primary categories that account for the vast majority of dementia cases worldwide.
Alzheimer’s disease is by far the most common, responsible for 60 to 80 percent of all dementia cases. It involves the buildup of abnormal protein clumps in the brain that gradually destroy neurons, starting with areas that control memory.
Vascular dementia accounts for roughly 5 to 10 percent of cases. It results from reduced blood flow to the brain, often after strokes or damage to small blood vessels. Symptoms depend on which part of the brain lost blood supply, so the pattern of decline can look quite different from Alzheimer’s.
Lewy body dementia is caused by abnormal protein deposits (called Lewy bodies) that form inside nerve cells. It tends to cause visual hallucinations, fluctuating alertness, and movement problems that can resemble Parkinson’s disease. It is one of the more common dementias after Alzheimer’s and vascular dementia, though exact prevalence figures vary across studies.
Frontotemporal dementia targets the front and side regions of the brain, which control personality, behavior, and language. It tends to strike earlier than other dementias, often between ages 45 and 65. Unlike Alzheimer’s, memory may stay relatively intact in the early stages while behavior and speech deteriorate first.
Subtypes Within the Major Categories
Each of those four categories contains its own subtypes, which is part of why counting “how many dementias” gets complicated quickly. Frontotemporal dementia alone branches into several distinct disorders: behavioral variant FTD (where personality and social conduct change first), primary progressive aphasia (where language breaks down), and movement-related variants like corticobasal syndrome and progressive supranuclear palsy. Some people develop a combination of frontotemporal dementia and ALS, the motor neuron disease, which is classified as ALS-Frontotemporal Spectrum Disorder.
Vascular dementia also comes in different forms depending on how the brain’s blood supply was disrupted. A series of small strokes can cause multi-infarct dementia, while chronic damage to tiny blood vessels deep in the brain leads to subcortical vascular dementia. There is also a rare hereditary form called CADASIL, which causes migraines, small strokes, and progressive dementia starting in adulthood.
Other Conditions That Cause Dementia
Beyond the big four, a number of other diseases include dementia as a core feature. Parkinson’s disease dementia develops in many people with Parkinson’s, typically about 10 years after movement symptoms first appear. It shares a biological basis with Lewy body dementia. The key clinical distinction is timing: if cognitive problems come first or alongside movement issues, it’s usually classified as Lewy body dementia, but if someone has had Parkinson’s for years before thinking and memory decline, it’s classified as Parkinson’s disease dementia.
Creutzfeldt-Jakob disease is a rare, rapidly progressive dementia caused by misfolded proteins called prions. Huntington’s disease, an inherited condition, causes progressive cognitive decline alongside involuntary movements. Posterior cortical atrophy primarily affects visual processing rather than memory, making it harder to judge distances, read, or recognize objects. Korsakoff syndrome, linked to severe thiamine deficiency (often from chronic alcohol misuse), causes profound memory problems. Normal pressure hydrocephalus, where fluid builds up in the brain’s ventricles, causes a triad of walking difficulty, urinary problems, and cognitive decline.
When you tally the major types, their subtypes, and the less common conditions, the number of distinct dementia-causing disorders easily exceeds 20.
Mixed Dementia Is More Common Than Most People Realize
Many people don’t have just one type of dementia. Brain autopsy studies from the National Institute on Aging found that mixed pathologies, meaning signs of more than one dementia type, were present in 45 percent of people who had dementia. Among people who lived to age 90 and beyond, having a mix of pathologies was the norm rather than the exception. The most common combination is Alzheimer’s disease alongside vascular damage, but Lewy body changes can overlap with either one.
This overlap is one reason diagnosis can be so difficult during life. Symptoms of different dementias frequently look alike, and when two or more are present simultaneously, the clinical picture gets even murkier.
Reversible Conditions That Mimic Dementia
Not everything that looks like dementia is permanent. Several treatable medical problems can cause memory loss, confusion, and difficulty thinking clearly enough to be mistaken for early dementia. These include vitamin B12 or folate deficiency from poor nutrition, underactive thyroid, depression, heart or lung conditions that starve the brain of oxygen, and medication side effects (especially common in older adults taking multiple prescriptions). Normal pressure hydrocephalus, mentioned above, is also sometimes reversible with treatment.
These “reversible dementias” are important because, unlike Alzheimer’s or frontotemporal dementia, correct treatment can resolve or significantly improve the symptoms. This is one reason thorough medical evaluation matters when cognitive decline first appears.
How Dementia Is Formally Classified
The diagnostic manual used by psychiatrists and psychologists (the DSM-5) replaced the word “dementia” with the broader term “neurocognitive disorder,” though clinicians can still use traditional names like Alzheimer’s dementia or vascular dementia. The system divides cognitive decline into two levels. Mild neurocognitive disorder means noticeable cognitive slipping that doesn’t yet interfere with everyday independence. Major neurocognitive disorder, which corresponds to what most people mean by “dementia,” involves decline significant enough that a person needs help with daily tasks like managing medications or paying bills.
These two levels exist on a spectrum. The boundary between them is somewhat arbitrary, with standardized cognitive testing used to place someone on one side or the other. Major neurocognitive disorder is then further classified by severity (mild, moderate, or severe) and by the underlying cause, whether that’s Alzheimer’s, Lewy bodies, vascular disease, Huntington’s, or any of the other conditions on the list. This layered system is why there’s no single number for “how many dementias there are.” The answer depends on whether you’re counting broad categories, specific subtypes, or every distinct disease that can cause progressive cognitive decline.