Dementia is not a disease in itself. It is a word used to describe a group of symptoms that occur when brain cells stop working properly.

This happens inside specific areas of the brain and can affect how you think, remember, move and communicate.

The symptoms of dementia are caused by diseases that affect the brain.

There are several of these diseases, and the most common are:

Alzheimer’s disease is the most common, and there are around 670,000 people affected by Alzheimer’s disease in the UK.

There are also several rarer diseases and conditions that can lead to dementia or dementia-like problems. A rare disease is defined by the UK Government as one that affects less than five in 10,000 people in the general population, or one in every 2,000 people.

Less than 1% - 35,000 people in total – of the UK population are thought to be affected by these rarer causes of dementia combined.

If you’re worried you or a loved one may be experiencing symptoms of the conditions listed on this page, a doctor can test for these conditions separately to ensure that a correct diagnosis is given.

This page provides information about these rare conditions. The conditions have been divided into two categories:

  • those that directly cause dementia.
  • those that can cause memory and cognitive problems similar to symptoms seen in dementia.

We also provide information about organisations that offer specific help and support for people affected by these conditions.

Rare forms of dementia

Chronic traumatic encephalopathy (CTE)

Chronic traumatic encephalopathy is a brain condition associated with repeated blows to the head.

Early symptoms of CTE usually appear around 10 years after brain injuries occur, but changes can be gradual and not always noticeable at first.

Research suggests that people with a traumatic brain injury could be 1.5 times more likely to develop dementia. But head injuries can vary enormously, and so can the diseases that cause dementia, so the relationship is hard to unravel.

We do know, however, that CTE is specifically linked to repeated head injury. In CTE, a protein called tau forms clumps that spread throughout the brain, damaging brain cells.

Corticobasal syndrome (CBS)

Corticobasal syndrome usually affects people aged between 50 and 70. The condition is estimated to affect five people in every 100,000 in the general population. You may also hear it called corticobasal degeneration (CBD).

CBS is caused by damage to cells on the surface of the brain, called the cortex, and a deep part of the brain called the basal ganglia.

Labelled brain showing the cerebrum, which is the outer layer of the brain, and the basal ganglia which sits in the middle of our brain

We don’t know what damages the cells and causes them to die. However, researchers have found a number of different toxic proteins in the brains of people with CBS. Ongoing research is trying to find out why these build up, and how they affect the brain.

A protein called tau is most frequently involved. This protein occurs naturally in the brain, but it is usually broken down before it reaches high levels.

In CBS, it isn’t broken down properly and forms harmful clumps in brain cells. This can cause the symptoms a person experiences.

Progressive supranuclear palsy (PSP)

PSP is thought to affect over 4,000 people in the UK. This is roughly five in every 100,000 people.

It is caused by damage to areas of the brain that control thinking and body movement, including the brainstem, cerebellum and basal ganglia.

PSP sections

PSP has been linked to an abnormal build-up of tau protein, which is also linked to Alzheimer’s disease.

Creutzfeldt-Jakob disease (CJD)

CJD is a very rare disease affecting less than two in every million people in the UK per year.

It is caused by a type of abnormal protein, called a prion. Prion proteins clump together and cause brain cells to die, resulting in sponge-like brain tissue.

There are different types of CJD:

Sporadic CJD

Sporadic CJD is the most common and develops spontaneously, for no known reason. In the UK, sporadic CJD usually starts in a person’s 60s.

Familial CJD

Familial CJD develops because a person has inherited certain faulty genes from one of their parents. Typically, the disease starts between 30 and 55 years old.

Acquired CJD

Acquired CJD, also known as variant CJD, can happen when someone comes into contact with abnormal prion protein. For example, by eating meat from a cow that had bovine spongiform encephalopathy (BSE or ‘mad cow disease’).

The average age of symptoms of acquired CJD is 26 years and there have been only 178 confirmed cases in the UK between 1970 and 2017.

These cases are extremely rare since measures to remove infected animals from the food chain came into force.

Huntington's disease

Huntington’s disease may appear at any age, but usually develops between the ages of 30 and 55.

It is thought that around 12 people in every 100,000 are living with the condition in the UK.

Our research

Alzheimer’s Research UK are committed to funding research into all types of dementia, including those listed on this page.

Bringing together expertise from across the UK is important to achieve this. We are a founding funder of the UK Dementia Research Institute, who have invested £1.5 million into Huntington’s disease – from basic science all the way to clinical trials testing potential new treatments.

We’re also funding research at UCL aiming to unravel the role of genes in diseases like CBS, and carried out a comprehensive review into the links between sport and dementia risk. As well as looking at the current evidence base for the benefits and risks of sports, the review involved consultation with clinical, research and sporting stakeholders to understand any existing gaps in knowledge in order to shape future research and policy.

We have also helped to set up a database, bringing together all the available research on traumatic brain injury (TBI) and making it accessible to scientists across the world. By doing so, we hope to aid the development of new diagnostic techniques, therapeutic interventions, and personalized treatments for TBI.

By understanding more about the links between sport and dementia, particularly CTE, people will be able to make informed decisions about their health.

Conditions that can cause dementia-like symptoms

HIV-related cognitive impairment

HIV (human immunodeficiency virus) is an infection that weakens the body’s immune system. Around 105,000 people in the UK are thought to be living with HIV.

This is not a type of dementia, but it can cause similar symptoms, and this is known as HIV-related cognitive impairment.

Antiretroviral drugs for the treatment of HIV have improved significantly in recent years. This means that the number of people with HIV who go on to develop AIDS (acquired immunodeficiency syndrome) has fallen dramatically.

Because of this, there has also been a fall in the rate of HIV-related cognitive impairment.


Your thyroid gland produces hormones that regulate the energy demands of every cell in your body. These hormones affect your metabolism, which is the process by which your body turns the food you eat into energy.

Hypothyroidism, otherwise known as an underactive thyroid, is a disorder in which your thyroid gland stops producing enough hormones.

This is not a type of dementia, but its symptoms can be similar.

The condition is thought to affect two in every 100 people in the UK and is 10 times more common in women than men.

Multiple sclerosis (MS)

Multiple sclerosis is a neurological condition affecting the brain and spinal cord.

It is not a type of dementia, but its symptoms can be similar.

The condition is thought to affect over 100,000 people in the UK.

It causes damage to the white matter in the brain,, which is made up of nerve fibres covered by a protective layer of fat called myelin.

In MS, the immune system attacks the myelin, exposing the nerve fibres which affects the way they work.

Niemann-Pick disease type C

Niemann-Pick disease type C (NPC) is a rare, inherited condition that prevents cholesterol and other fats from moving around the body properly. This leads to a build-up of fat inside cells in the liver, spleen, and brain.

It is thought to affect one in 120,000 people in the UK.

It is not a type of dementia, but symptoms can be similar.

Normal pressure hydrocephalus (NPH)

Spinal fluid usually circulates around the brain and spinal cord. Normal pressure hydrocephalus occurs when spinal fluid builds up inside hollow, fluid-filled spaces in the brain called ventricles. As ventricles enlarge, they can damage the surrounding brain tissue.

In most cases there is no obvious cause. This is known as idiopathic NPH.

However, the condition can arise as a result of bleeding on the brain, head injury or meningitis.

Vitamin B12 deficiency

Vitamin B12 deficiency is thought to affect around six in 100 people under 60 years old and around 20 in 100 older people in the UK.

Some people are unable to absorb vitamin B12 due to:

  • certain genetic disorders
  • medical conditions such as pernicious anaemia
  • as a result of taking prescription drugs like metformin.

Common sources of B12 include meat, eggs, and dairy. People who limit consumption of meat and dairy, for example vegetarians and vegans, may not get enough B12 through their diet. Alternative sources include some soy products and fortified breakfast cereals.

Alcohol-related cognitive impairment

Long-term, moderate to heavy alcohol consumption can have a negative effect on brain function, memory, and our ability to process information. Research has found that excessive alcohol consumption may cause the brain to shrink and become damaged.

People who drink heavily long-term are at risk of developing irreversible alcohol-related brain syndromes, such as Wernicke-Korsakoff syndrome.

Wernicke-Korsakoff syndrome

Korsakoff syndrome is a form of dementia most commonly brought on by long-term alcohol misuse.

It often follows an episode of Wernicke’s encephalopathy, a medical emergency caused by a sudden lack of thiamine (vitamin B1).

This information was updated in February 2024 and is due for review in February 2026. It was written by Alzheimer’s Research UK’s Information Services team with input from lay and expert reviewers. It does not replace any advice that doctors, pharmacists, or nurses may give you. Please contact us if you would like a version with references.

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