Young-onset dementia

2. What causes young-onset dementia?

The most common causes of young-onset dementia are the same progressive diseases that cause most cases of dementia in older people. Studies have produced a range of figures that show the causes of young-onset dementia. It is clear that the mix of causes of young-onset dementia is different from that in older people. The same disease in younger people can also have different symptoms. It is also sometimes thought that dementia progresses more rapidly in younger than in older people, but the evidence for this is not strong.

Early onset of common dementias

Alzheimer's disease

Alzheimer's disease develops when proteins build up in the brain to form structures called 'plaques' and 'tangles'. Alzheimer's is the most common type of dementia in younger people and may affect 30 to 35 per cent of younger people with dementia. However, this is a much smaller proportion than in older people with the condition, up to two thirds of whom have Alzheimer's disease.

Another difference is that younger people are much more likely to have an 'atypical' (ie unusual) form of Alzheimer's disease than older people. Atypical Alzheimer's disease is when the first symptoms are not memory loss, which is the most common symptom of late-onset Alzheimer's disease. Instead, the first symptoms are usually problems with vision (in posterior cortical atrophy), speech (in logopenic aphasia) or planning, decision-making and behaviour (in frontal variant Alzheimer's disease). These atypical forms of Alzheimer's disease account for up to one third of all Alzheimer's disease in younger people but only 5 per cent of all Alzheimer's disease in older people.

In some people with early-onset Alzheimer's disease there is a very clear inheritance of the disease from one generation to the next. This genetic form of the dementia - familial Alzheimer's disease - is caused by rare mutations (defects) in three genes. These mutations are found in between 7 and 12 per cent of all people with early-onset Alzheimer's.

Symptoms of familial Alzheimer's disease usually start in someone's 30s, 40s or 50s; the earlier the symptoms start, the more likely the disease is to be genetic. However, familial Alzheimer's disease is extremely rare. It affects only about 500 known families worldwide and probably accounts for much less than 1 per cent of Alzheimer's disease when all ages, young or old, are considered.

People with Down's syndrome and other learning disabilities can also develop dementia at an early age. Alzheimer's disease is the most common type of dementia in people with Down's syndrome. This increased risk is thought to be associated with the extra copy of chromosome 21 which most people with Down's syndrome have. Chromosome 21 carries the gene for amyloid which forms the plaques in Alzheimer's disease.

Vascular dementia

Vascular dementia occurs when there are problems in the blood supply to the brain. It is closely linked to diabetes and to cardiovascular diseases such as stroke and heart disease. It is probably the second most common type of dementia in younger people - around 15 per cent of dementia in this age group may be vascular dementia.

Symptoms of vascular dementia vary. Early memory loss is less common than in Alzheimer's disease, whereas problems with thinking things through and slower speed of thought are more common. When vascular dementia follows a stroke, physical symptoms (eg limb weakness) are common.

There is a genetic form of vascular dementia known as CADASIL (cerebral autosomal dominant arteriopathy with subcortical infarcts and leukoencephalopathy). CADASIL is rare and is most common in people aged 30 to 50. Symptoms include migraines, repeated strokes, fits, low mood and progressive loss of mental abilities. CADASIL is caused by defects in a gene called NOTCH3 and is inherited in a simple pattern similar to familial Alzheimer's disease.

Frontotemporal dementia

Frontotemporal dementia is caused by damage to the lobes at the front and/or sides of the brain. Around 10 to 15 per cent of younger people with dementia may have this form - much higher than in older people. Frontotemporal dementia is most often diagnosed between the ages of 45 and 65. This is in contrast to Alzheimer's disease, vascular dementia and dementia with Lewy bodies, all of which become more likely with age.

There are three different types of frontotemporal dementia - the behavioural variant, which causes changes in personality and behaviour first, and two types in which language is affected first.

In around 30 per cent of people with this type of dementia, there is good evidence that the disease is genetic. In this instance, genetic counselling and testing may be offered (see 'Genetic testing').

Dementia with Lewy bodies

This type of dementia is caused by the build-up of tiny protein deposits (Lewy bodies) in the brain. About 5 per cent of younger people with dementia may have dementia with Lewy bodies. Lewy bodies also cause Parkinson's disease and about one third of people with Parkinson's eventually develop dementia. Symptoms of dementia with Lewy bodies include varying levels of alertness, and people can also develop hallucinations and the features of Parkinson's disease (eg slower movement, stiffness, trembling of the limbs).

Alcohol-related brain damage

Alcohol-related brain damage includes Korsakoff's syndrome and alcoholic (or alcohol-related) dementia. It occurs in people, most often in their 50s, who regularly consumed excessive amounts of alcohol. Alcohol-related brain damage is caused by a lack of thiamine (vitamin B1), direct damage to nerve cells from alcohol, head injuries (eg falls, fights) and a poor diet.

At least 10 per cent of younger people with dementia may have alcohol-related brain damage. Symptoms overlap with those of Alzheimer's disease and vascular dementia. Alcohol-related brain damage differs from common dementias because it can be halted or even reversed in some people with treatment, abstinence and a good diet.

Rarer forms of dementia

Around 20 to 25 per cent of younger people with dementia are thought to have a rarer cause of the condition - a much higher proportion than in older people. These causes include degenerative neurological conditions (where there is progressive damage to the nervous system) such as Huntington's disease, progressive supranuclear palsy, corticobasal degeneration and Creutzfeldt-Jakob disease (CJD).

In most cases these diseases cause problems with movement as well as dementia. Some of these rarer causes of dementia, such as CJD, can progress very rapidly over just a few months.

Progression

Everyone's experience of dementia is different, but for all the dementias mentioned above most people will become more frail as the condition progresses. This means that they will gradually become dependent on others for their care.

This may be very difficult for all those affected to think about. However, knowing this can enable the person with dementia to think ahead about the kind of treatment and care they might want. Many people decide to set up a Lasting power of attorney or make an advance decision before they reach this stage. This gives the person input into how they will be cared for. It also allows carers and family to think about these aspects too.

Other causes

A wide range of other diseases can cause symptoms of dementia, often along with other symptoms (eg spleen or liver problems) which can be very different.

These diseases include inherited metabolic disorders such as Gaucher's disease, Tay Sach's disease and Niemann-Pick's disease. These develop more often in childhood or adolescence and so their development in someone's 30s or 40s is much later than usual.

Other causes of dementia symptoms in younger people include hormone disorders (eg thyroid problems, Addison's disease), vitamin (eg B12) deficiencies, inflammatory conditions (eg multiple sclerosis) and infections (eg HIV). Memory problems can also be caused by sleep apnoea, where breathing stops for a few seconds or minutes during sleep.

These conditions are not discussed further here but it is important that they are diagnosed because some (eg vitamin deficiency, thyroid problems, sleep apnoea) can be treated.