Dementia is defined clinically: cognitive or behavioural decline from a previous level that interferes with work or usual activities. The decline must span at least two domains among memory, reasoning and judgment, visuospatial skill, language, and personality or behaviour, and it must not be explained by delirium or a major psychiatric disorder. Classification matters because the cause determines the treatment.
What dementia is not
The definition excludes two look-alikes. Delirium is an acute confusional state, and pseudodementia is cognitive impairment that follows depression. The table shows how each differs from dementia in onset, course, attention, memory, and mood.
| Feature | Dementia | Delirium | Pseudodementia |
|---|---|---|---|
| Onset | Insidious | Acute | Variable |
| Course | Progressive | Fluctuating | Follows mood |
| Attention | Preserved until late | Impaired early | Variable |
| Memory | Recent worse than remote | Fluctuating | Answers “I don’t know” |
| Mood | Normal, later apathy | Fearful or agitated | Depressed first |
Two further groups sit alongside true dementia: the stage before it, and treatable conditions that mimic it.
Mild cognitive impairment is measurable decline with daily activities largely preserved. Its staging, yearly progression risk, and how amyloid testing sharpens prognosis are set out under mild cognitive impairment.
Reversible causes are treatable mimics: depression, drug effects, hypothyroidism, and B12 and thiamine deficiency. They are compared in full under reversible dementias. Every new assessment screens for these before accepting a neurodegenerative diagnosis.
Major causes compared
Once these are set aside, the neurodegenerative and vascular causes can be compared. The table gives each cause’s approximate share of cases and the feature that most often identifies it.
| Cause | Share of cases | Signature |
|---|---|---|
| Alzheimer disease | Roughly 60% | Memory loss first, slow progression, temporal and parietal atrophy |
| Vascular dementia | Roughly 10–20% | Stepwise decline, focal signs, white matter lesions |
| Dementia with Lewy bodies | Roughly 5–10% | Early visual hallucinations, fluctuating cognition, Parkinsonism |
| Frontotemporal dementia | Under 5% | Personality or language change first, younger onset |
| Normal pressure hydrocephalus | Under 5% | Gait, bladder, and cognitive triad; full criteria under reversible dementias |
| Creutzfeldt-Jakob disease | Under 1% | Rapid progression over weeks to months, myoclonus (sudden muscle jerks), periodic EEG (electroencephalogram) changes |
The World Health Organization estimates Alzheimer disease may contribute to 60–70% of cases. Mixed forms often coexist, so single-cause shares never sum cleanly.
Alzheimer disease versus vascular dementia
This distinction carries a direct treatment consequence. Alzheimer disease is managed with cholinesterase inhibitors and memantine alongside non-pharmacological care, while vascular dementia is managed by controlling vascular risk factors such as hypertension, diabetes, and hyperlipidaemia. Mixed dementia is common, so comorbid Alzheimer pathology should be considered even when vascular features dominate; amyloid PET (positron emission tomography) can clarify this when the answer changes management.
Creutzfeldt-Jakob disease
Creutzfeldt-Jakob disease is a prion disease caused by accumulation of abnormal prion protein. It is the fastest dementia, progressing over weeks to months, with myoclonus, pyramidal or extrapyramidal signs, visual disturbance, and cerebellar signs. The EEG shows characteristic periodic sharp waves. Typical onset is between 57 and 62 years; most cases are sporadic, with smaller familial and iatrogenic shares, and average survival is only a few months. The variant form linked to bovine spongiform encephalopathy presents earlier with prominent psychiatric features.
Frontotemporal variants
Three clinical presentations cover most frontotemporal dementia: the behavioural variant with disinhibition, apathy, and loss of social norms; the nonfluent variant with effortful, halting speech and grammar errors; and the semantic variant with fluent speech that has lost word meaning. A right temporal presentation with progressive loss of face recognition is also recognised. Onset is typically between 50 and 65. Memory stays relatively preserved early, which is the bedside contrast with Alzheimer disease. About 15–20% of patients have an associated motor neuron disease, most often through shared TDP-43 pathology and C9orf72 genetics.
The Lewy body spectrum
Dementia with Lewy bodies and Parkinson disease dementia share the same α-synuclein pathology and differ in timing. When dementia appears within about a year of Parkinsonism, the convention is to call it dementia with Lewy bodies; when established Parkinson disease precedes dementia by more than a year, it is Parkinson disease dementia. This timing convention is a clinical rule of thumb rather than a sharp biological boundary.