Dementia is a progressive brain dysfunction which results in a restriction of daily activities and in most cases leads in the long term to the need for care. Many diseases can result in dementia, the most common one being Alzheimer's disease.
Alzheimer’s disease is a progressive neurodegenerative 
condition characterized clinically by a gradual decline in cognition, daily functioning and behaviour. Alzheimer's disease is the most common form of dementia: about 60% of all dementia patients suffer from Alzheimer's disease. In this case changes in the brain cells often occur, impairing them to 'function'. Those brain cells eventually die. This happens gradually to an increasing number of brain cells. Mainly cells in a certain part of the brain are affected, the cortex (the outside). That is the part we use to think with.
In 10-15 % of the cases of dementia we are dealing with multi-infarct dementia or vascular dementia. In that case the nerve cells suffer a lack of oxygen because of vascular hardening in the brain. Multi-infarct dementia is not always easy to distinguish from Alzheimer and sometimes both syndromes occur simultaneously. Multi-infarct dementia progresses less gradually; the worsening takes place suddenly, alternated with some improvement and then again a sudden worsening.
How common is dementia?
In our society of longer lifetime the probability of suffering from dementia increases with advancing age. Dementia predominantly occurs in the second half of our life, often after the age of 65 - some experts think that this is the 'price society has to pay' for our higher life expectancy and therefore the term 'dementia' activates similar fears and repression mechanisms as 'cancer' or 'AIDS'.
The frequency of dementia increases with rising age from less than 2 % for the 65-69-year-olds, to 5 % for the 75-79 year-olds and to more than 20 % for the 85-89 year-olds. Every third person over 90 years of age suffers from moderate or severe dementia (Bickel, Psycho 1996, 4-8). About half of those affected by dementia suffer from Alzheimer's disease
About 5 % of people above 65 years of age, about 20 % of those over 80 years and about 30 % of those over 90 suffer from Alzheimer's disease.
