What is dementia?

Find out more about what dementia is and what causes it.

Causes of dementia

Many different illnesses can damage a person’s brain and cause dementia, but some are much more common than others. The chart below shows you the four main types of dementia and which are the most common.

Different types of dementia can affect different parts of the brain at first, and have their own pattern of signs and symptoms. For some people, the first signs of dementia may be forgetfulness or confusion. For others, it may be changes in behaviour and mood, or problems with speaking or seeing what and where things are.

As time goes on, the illnesses that cause dementia affect more of a person’s brain and cause new problems. It is also possible to have more than one type of dementia at the same time. This is called ‘mixed dementia.’

By understanding the illnesses that cause dementia, scientists hope to find ways to prevent, treat and even cure dementia in the future. You can find out more about dementia research by exploring our dementia research section.


Alzheimer’s disease

Alzheimer’s disease causes more than half of all cases of dementia. When people have this disease, the structure and chemistry of their brain begins to change. Sticky lumps of protein build up inside and around brain cells. This harms brain cells, and causes them to die. We don’t yet fully understand what causes a person’s brain to start changing in this way.

Common symptoms can include:

Memory Memory

Regularly forgetting recent events, names and faces.

Confusion Confusion

Uncertainty about the time of day.

Disorientation Disorientation

Disorientation, especially away from normal surroundings. Getting lost.

Language Language

Problems finding the right words.

Mood and behaviour Mood and behaviour

Some people become disinterested in what’s happening around them, become irritable, or lose confidence.

Read more detailed information about Alzheimer’s disease.

Vascular dementia

Vascular dementia is the second most common type of dementia. It is caused when blood vessels in the brain are damaged, meaning that less blood reaches the brain than normal. The damage may happen gradually over time, or suddenly, when a person has a stroke. In both cases, brain cells are damaged because they don’t get the oxygen and nutrients they need.

Common symptoms can include:

Thinking skills Thinking skills

Becoming slower in thinking.

Personality changes Personality changes

These may include depression and apathy (becoming disinterested in things). People may also become more emotional.

Movement problems Movement problems

Difficulty walking or changes in the way a person walks.

Bladder problems Bladder problems

Frequent urge to urinate or other bladder symptoms. This can be common in older age, but can be a feature of vascular dementia when seen with other symptoms.

You can read more detailed information about vascular dementia here.

Dementia with Lewy bodies

Dementia with Lewy bodies (DLB) is the third most common cause of dementia. Lewy bodies are tiny, round clumps of protein that build up inside brain cells. They stop brain cells from working properly, causing dementia symptoms such as memory loss. They can cause other problems too, such as hallucinations (seeing things that aren’t really there), slowed stiff movements and changes with sleep, causing people to talk or move about in their sleep. People with DLB often have big differences between their good days and bad days. On bad days, they may be much more confused and less alert.

Some of the ways people are affected are:

Alertness Alertness

Changes in alertness, attention and confusion, which may be unpredictable and change from hour to hour or day to day.

Movement problems Movement problems

Parkinson’s disease-type symptoms such as slowed movements, muscle stiffness and tremors.

Visual hallucinations Visual hallucinations

These can involve seeing people or animals that aren’t really there.

Restlessness Restlessness

Sleep disturbances which can cause people to move or talk in their sleep.

Stability Stability

Fainting, unsteadiness and falls.

Find out more about dementia with Lewy bodies.

Frontotemporal dementia

Frontotemporal dementia (FTD) is caused by damage to the frontal and temporal lobes of the brain. These are the parts of the brain that control personality, emotions, behaviour, thinking and language. The symptoms of FTD can vary, depending on which parts of the frontal and temporal lobes are most affected. Although frontotemporal dementia is rare compared to Alzheimer’s disease and vascular dementia, it is the second most common cause of early-onset dementia, which is the name for dementia in people younger than 65 years old.

Below are some of the common symptoms of FTD. Visit our main website for more detailed information about frontotemporal dementia.


Personality changes Personality changes

This may include a change in how people express their feelings towards others or a lack of understanding of other people’s feelings. They may also show a lack of interest or concern, become disinhibited or behave inappropriately.

Lack of personal awareness Lack of personal awareness

People may fail to maintain their normal level of personal hygiene and grooming.

Social Awareness Lack of social awareness

This might include making inappropriate jokes, or showing a lack of tact.

Diet Diet

Changes in food preference, over-eating or over-drinking.

Behaviour changes Behaviour changes

People may change their humour or sexual behaviour, become violent or develop unusual beliefs, interests or obsessions. Some people become impulsive or easily distracted.

This information was updated in December 2017 and is due for review in December 2019. Please contact us if you would like a version with references.