Understanding dementia
Last verified 2026-09-13
The word is familiar; what is actually happening is less so. This page sets out which illnesses lead to the state called dementia, what goes on in the brain, and what an appointment involves — from public sources. It is background reading, not a diagnosis.
Sources are the Ministry of Health, Labour and Welfare and the National Center for Geriatrics and Gerontology. Test thresholds are deliberately left out: those belong with a doctor, not on a web page.
Ageing forgetfulness, and something else
Everyone forgets. What people describe as different is forgetting the experience itself, not part of it — not "I forget what I ate", but "I do not remember eating". Another difference often mentioned is whether the person notices the forgetting.
But this is not something to judge at home. It is one of the things to bring to a doctor.
Dementia is not one illness
Dementia is a state — brain cells are lost or stop working, and daily life becomes difficult. Many different illnesses lead to it. The Ministry divides them broadly into degenerative diseases, where nerve cells die off slowly, and vascular causes, where blood stops reaching them.
Degenerative The most common
Alzheimer's disease
One of the degenerative diseases, in which brain cells die off slowly.
Shrinkage is known to begin around the hippocampus, the seat of memory.
It starts with forgetfulness and progresses slowly, over months and years.
Vascular The next most common
Vascular dementia
Caused by stroke, haemorrhage or arteriosclerosis cutting off oxygen and nutrients to brain cells.
Which abilities are affected depends on where the damage is.
It can worsen in steps, with each event. What the person can and cannot do may differ sharply.
Degenerative A degenerative disease
Dementia with Lewy bodies
Caused by a protein called Lewy bodies building up in nerve cells.
Closely related to Parkinson’s disease.
Known for visual hallucinations, tremor and stiffness, and marked day-to-day variation.
Degenerative A degenerative disease
Frontotemporal dementia
Degeneration of nerve cells in the frontal and temporal lobes.
The frontal lobe is where impulses are held in check and plans are made.
Changes in character or behaviour can appear before forgetfulness. It can begin at a relatively young age.
These are introductions, not a way to tell them apart. Which illness it is — and whether it is dementia at all — is for a doctor to determine. Types can also occur together.
Why recent things go first
New experiences are held briefly, then filed into longer-term storage. The hippocampus does that filing. In Alzheimer's disease, shrinkage is known to begin around there.
That is why long-ago memories stay vivid while this morning's conversation does not — it was never filed. Being asked the same question repeatedly is not stubbornness; for the person, it is the first time.
What happens at the appointment
- The interview When it began and what has been happening, from both the person and the family. What the family reports is part of the diagnosis, so go along if you can.
- Cognitive testing A question-based test of memory and orientation. A score alone does not decide a diagnosis.
- Imaging (CT / MRI) To look at shrinkage and at any trace of stroke or bleeding — and to check that nothing surgically treatable is hiding underneath.
- Blood tests To look for other causes that imitate dementia, such as thyroid problems or vitamin deficiency.
Some causes can be treated
This is the part worth knowing before deciding whether to go. Conditions such as normal pressure hydrocephalus and chronic subdural haematoma cause symptoms that look like dementia, and surgery can improve them. Thyroid problems and vitamin deficiency can do the same.
CT and MRI are considered essential precisely to rule these out. "It will not get better anyway" is the reasoning that misses them.
For the family
You do not have to correct them
Correcting rarely lands, and what remains is the feeling of being contradicted. Going along with it causes less trouble than people expect.
Do not change the surroundings too much
A familiar home and a familiar routine are the handholds. A well-meant reorganisation or move can trigger confusion.
Do not take over what they can still do
Leave what the person can do, even slowly. Doing everything for them costs those abilities faster.
Do not carry it alone
If the carer goes down, care at home stops. Asking a community support centre is free, and does not require a certification.
Where to ask
Asking is free, and does not require a diagnosis or a certification. "Something seems different lately" is enough to start with.
Sources
- Ministry of Health, Labour and Welfare — Understanding dementia
- National Center for Geriatrics and Gerontology — Diagnosis and treatment
- National Center for Geriatrics and Gerontology — Imaging in dementia
- National Center for Geriatrics and Gerontology — Normal pressure hydrocephalus
This page explains things in general terms and is not a medical diagnosis. Symptoms and progression differ from person to person. Please take any decision with a doctor.