TYPES OF DEMENTIA
THERE ARE OVER 100 established types of dementia, but most of them are extremely rare. In this chapter I will describe the four main types of dementia seen in clinical practice—Alzheimer's disease, vascular dementia, frontotemporal dementia and dementia with Lewy bodies, which account for 90 to 95 per cent of all cases—and a number of the less common types, as listed in Table 5.1.
In the previous chapters I have outlined the common risk factors for dementia and described the clinical features, albeit with a particular focus on Alzheimer's disease. Here my focus is on what is understood about why these disorders occur, and the specific features that distinguish them from each other.
Apart from vascular dementia, most of the dementias are categorised as neurodegenerative disorders because, essentially, they involve the progressive degeneration and death of nerve cells. There is now mounting evidence for a common theme uniting these disorders. In a nutshell, neurodegenerative disorders are fundamentally caused by the abnormal accumulation of insoluble proteins in the brain. These proteins are toxic and exert a deleterious effect on selective nerve cells, impairing their function and eventually leading to cell death. The abnormal proteins also affect synapses (spaces) between nerve cells, hence the chemical information between cells might not be transmitted properly and nerve circuits might be interrupted. 1 This is a fast moving area with new discoveries happening almost every week. I concentrate on what seems to be reasonably well accepted by the scientific community rather than on what remains speculative.