What causes dementia?

Dementia isn't a single disease. Dementia is a term used to describe the symptoms that occur when there's a decline in brain function.

Several different diseases can cause dementia. Many of these diseases are associated with an abnormal build-up of proteins in the brain.

This build-up causes nerve cells to function less well and ultimately die. As the nerve cells die, different areas of the brain shrink.

Causes of Alzheimer's disease

Alzheimer's disease is the most common type of dementia.

In the brain of someone with Alzheimer's disease, there are two different proteins called amyloid and tau.

Deposits of amyloid, called plaques, build up around brain cells. Deposits of tau form "tangles" within brain cells.

Researchers don't yet fully understand how amyloid and tau are involved in the loss of brain cells, but this is an area of active research.

As brain cells become affected in Alzheimer's, there's also a decrease in chemical messengers (called neurotransmitters) involved in sending messages, or signals, between brain cells.

Levels of one neurotransmitter, acetylcholine, are particularly low in the brains of people with Alzheimer's disease.

Medicines like donepezil increase levels of acetylcholine, and improve brain function and symptoms.

Read more about treatments for dementia.

The symptoms that people develop depend on the areas of the brain that have been damaged by the disease.

The hippocampus is often affected early on in Alzheimer's disease. This area of the brain is responsible for laying down new memories. That's why memory problems are one of the earliest symptoms in Alzheimer's.

Unusual forms of Alzheimer's disease can start with problems with vision or with language.

Read more about Alzheimer's disease.

Causes of vascular dementia

Vascular dementia is caused by reduced blood flow to the brain.

Nerve cells in the brain need oxygen and nutrients from blood to survive. When the blood supply to the brain is reduced, the nerve cells function less well and eventually die.

Reduced blood flow can be caused by:

• narrowing of the small blood vessels deep inside the brain - known as small vessel disease (subcortical vascular dementia); this is the main cause of vascular dementia and is more common in people who smoke, or have high blood pressure or diabetes
• a stroke (where the blood supply to part of the brain is suddenly cut off, usually as a result of a blood clot) - called post-stroke dementia
• lots of "mini strokes" that cause widespread damage to the brain - known as multi-infarct dementia

Not everyone who's had a stroke will go on to develop vascular dementia.

Read more about vascular dementia.

Mixed dementia

Because both vascular dementia and Alzheimer's disease are common - especially in older people - they may be present together.

This is often called mixed dementia because a mix of these two conditions is thought to be the cause of the dementia.

It can be difficult to be sure how much each cause is contributing to a person's problems.

Causes of dementia with Lewy bodies

Lewy bodies are tiny clumps of a protein called alpha-synuclein that can develop inside brain cells.

These clumps damage the way the cells work and communicate with each other, and the cells eventually die.

Dementia with Lewy bodies is closely related to Parkinson's disease and often has some of the same symptoms, including difficulty with movement and a higher risk of falls.

Read more about dementia with Lewy bodies.

Causes of frontotemporal dementia

This is an important cause of dementia in younger people. It's most often diagnosed between the ages of 45 and 65.

It's caused by an abnormal clumping of proteins, including tau, in the frontal and temporal lobes at the front and sides of the brain.

The clumping of these proteins damages nerve cells in the frontal and temporal lobes, causing brain cells to die. This leads to shrinking of these areas of the brain.

Frontotemporal dementia is more likely to run in families and have a genetic link than other, more common causes of dementia.

Read more about frontotemporal dementia.

Rarer causes of dementia

There are many rarer diseases and conditions that can lead to dementia, or dementia-like symptoms.

These conditions account for only 5% of dementia cases in the UK.

They include:

• Huntington's disease
• corticobasal degeneration
• progressive supranuclear palsy
• normal pressure hydrocephalus

Mild cognitive impairment

Mild cognitive impairment (MCI) isn't a cause of dementia.

It refers to a condition in which someone has minor problems with cognition, or their memory and thinking, such as:

• memory loss
• difficulty concentrating
• problems with planning and reasoning

These symptoms aren't severe enough to cause problems in everyday life, so aren't defined as dementia.

MCI can be caused by an underlying illness, such as depression, anxiety or thyroid problems.

If the underlying illness is treated or managed, symptoms of MCI often disappear and cause no further problems.

But in some cases, people with MCI are at increased risk of going on to develop dementia, which is usually caused by Alzheimer's disease.

Join dementia research

There are dozens of dementia research projects and clinical trials going on around the world, many of which are based in the UK.

If you have a dementia diagnosis or are worried about memory problems, you can help scientists understand more about the disease, and develop possible treatments, by taking part in research.

If you're a carer, you can also take part as there are studies into the best ways to care for someone with a dementia diagnosis.

You can sign up to take part in trials on the NHS Join Dementia Research website.

What are the treatments for dementia?