Dementia by
Dr Jonathan Rohrer, Dementia Research Centre
"Dementia" is a term for progressive problems in thinking or behaviour - this may be:
- Memory, the main problem in Alzheimer's disease
- Behaviour, the main problem in frontotemporal dementia
- Language, the main problem in semantic dementia and progressive non-fluent aphasia
Or other things, such as arithmetic, planning and problem solving
Different problems are seen in different dementias - some dementias affect the whole brain, such as Alzheimer's disease, whereas others appear to affect only parts of the brain, for example the area for speech production in progressive non-fluent aphasia.
The most common form of dementia is Alzheimer's disease and this accounts for about 60-70% of all cases. Vascular dementia caused by damage to the blood vessels in the brain is probably the next most common cause. The 3rd and 4th most common causes are frontotemporal lobar degeneration or Pick's disease (which includes frontotemporal dementia, semantic dementia and progressive non-fluent aphasia) and dementia with Lewy bodies. In total these four causes account for about 95% of all cases of dementia. There are many rare causes of dementia and these account for the final 5% of cases.
The diagram above is a simplistic way of thinking about what is happening in people's brains as they develop dementia:
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For some people an abnormal gene triggers the disease (See the section "Is dementia inherited?" for more information). This is the case in up to 50% of people with frontotemporal lobar degeneration (Pick's disease) but only a very small amount of people with Alzheimer's disease.
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However, for the people who do not have an abnormal gene a mixture of things probably trigger the disease - a mix of something from the "environment" and other genes. We do not currently know what the triggers in the "environment" are. Suggestions in the past have included things such as head injury but it is really not clear whether this is the case.
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Once the disease is triggered, abnormal proteins become deposited in the brain. In frontotemporal lobar degeneration, these are proteins known as tau and TDP-43, in Alzheimer's disease it is tau and amyloid and in dementia with Lewy bodies it is something called alpha-synuclein.
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These abnormal proteins cause brain cells to die, and so brain cells from certain areas of the brain are slowly lost. In frontotemporal lobar degeneration the cells are lost from the frontal and temporal lobes of the brain while in Alzheimer's disease cells are lost from the whole brain.
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Once cells are lost, the amount of chemicals in the brain becomes lower. Brain chemicals or "transmitters" are needed for signals to pass from brain cell to another. For example, in Alzheimer's disease there is less of a chemical called acetylcholine. The aim of drugs such as donepezil (ARICEPT), rivastigmine (EXELON) and galantamine (REMINYL) is to try to increase the amount of this.
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Loss of cells and decreased amount of chemicals cause the symptoms and signs of the disease, which are different in different types of dementia.
Research in dementia takes place all over the world and more and more is becoming known about each of the different types of dementia. As our knowledge continues to increase, the hope is that we will be able to design treatments that will help combat the symptoms and signs of dementia, and eventually hopefully cure it.
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