Alzheimer's is a degenerative disease that slowly and progressively destroys the brain.

Alzheimer's disease is a degenerative disease that slowly and progressively destroys brain cells. It is neither infectious nor contagious, but is the single most common cause of dementia - a condition that affects about 10 per cent of those aged over 65 and about 20 per cent of those aged over 75.

In the UK, up to 750 000 people suffer from dementia, costing billions of pounds - mostly for institutional care - and causing incalculable distress and upset to the carers and relatives of patients.

What is dementia?

Dementia is the term used to describe a general decline in all areas of mental ability. The symptoms involve a deterioration in cognitive processes - memory, language, thinking and so on - with important repercussions on behaviour. About 50 per cent of people with dementia are suffering from Alzheimer's disease, about 20 per cent from vascular dementia (caused by blockages in the supply of blood to the brain), and about 20 per cent from Lewy body dementia (characterised by tiny spherical deposits in the brain).

Only about 10 per cent of dementia cases are treatable or potentially reversible, being caused by illnesses such as vitamin B12 deficiency, a brain tumour, syphilis, alcohol dependence or a subdural haematoma.

Traditionally, dementing illnesses were divided into 'presenile' (under 65 years of age at onset) and 'senile' (over 65 years). Although this is now seen as rather an arbitrary division, it has helped in the search for genes that might underlie early-onset Alzheimer's.

Who discovered it?

Alois Alzheimer (1864-1915), who first described the disease, was a German psychiatrist and pathologist. In 1906, he carried out an autopsy on the brain of a 56-year-old woman, Auguste D of Frankfurt. Auguste had died after several years of progressive mental deterioration characterised by confusion and memory loss. In her cerebral cortex, the part of the brain responsible for reasoning and memory, he found strange bundles of nerves, which he termed neurofibrillary tangles, and accumulations of cellular debris around the nerves, which he termed senile plaques.

He presented his findings at a meeting in late 1906, and published them in 1907, speculating that the nerve tangles and plaques were responsible for Auguste's dementia.

Who is likely to get Alzheimer's disease?

A number of risk factors for the disease have been identified, so some people are more likely to be affected than others, but it is unlikely that the disease could be traced to a single cause. It is more likely that a combination of factors lead to its development, with the importance of particular factors differing from one person to another. Although age obviously plays a key role - less than one person in a thousand under the age of 65 suffers from Alzheimer's, rising to about one person in 20 over 65 - most people over 80 stay mentally alert. So while the likelihood of suffering from the disease increases with age, old age itself does not cause the disease.

A great amount of research has gone towards the identification of genetic factors that might lead to Alzheimer's disease. Four genes have been identified so far that play a role in a proportion of cases.

Race, culture, environment and lifestyle appear to have few, if any, effects on the risk of developing Alzheimer's. Some evidence suggests that people with a higher level of education are at less risk than those with lower levels of education, but these findings remain controversial. Other evidence suggests that a severe blow to the head can induce the disease. The risk appears to be greater if the person is over 50 at the time of the injury, has a specific gene (ApoE e4) and lost consciousness just after the injury. Indeed, boxers often develop a form of dementia called 'punch-drunk syndrome'. Their brains are extensively damaged, and some of the changes resemble those of Alzheimer's disease.

Is Alzheimer's inherited?

Alzheimer's disease is not usually inherited, but genes do play a role in a proportion of cases. In a very small number of families, the disease is a dominant genetic disorder – three genes have been identified that, if defective, cause the disease. The age of onset in such families tends to be both relatively low, usually between 35 and 60, and similar within the family. In the majority of cases, however, there is no discernible family history of the disease.

Another gene has been identified that can influence the risk of developing the disease. This gene is responsible for the production of a protein called apolipoprotein E (ApoE). The e4 variant of this gene, although uncommon, increases the risk but does not by itself cause the disease - only half the people with Alzheimer's have ApoE e4, and not everyone with ApoE e4 develops the disease.

What are the symptoms of Alzheimer's disease?

Although the symptoms vary among individuals, there are three broad stages to the disease. At first, the patient becomes increasingly forgetful - also a feature of normal ageing, and not, by itself, evidence of dementia. This forgetfulness shades into severe memory loss, however, often for recent events and for events of personal significance.

Concentration and numerical ability decline, and dysphasia (inability to find the right word) becomes noticeable. Anxiety increases, mood changes are unpredictable, and personality changes soon occur. In the third stage, patients become severely disoriented and confused. They may also suffer from the symptoms of psychosis, such as hallucinations and delusions. Some patients become very demanding and aggressive, while others become docile and helpless.

How is it diagnosed?

There is no single test that can definitively determine whether someone has Alzheimer's disease - as with many other medical conditions, diagnosis relies on a process of elimination. The most important step in diagnosis is usually a discussion with a carer or relative of the patient's symptoms and signs. Tests of mental state can indicate decreased intellectual ability, and brain imaging - using computer tomography scanning or magnetic resonance imaging - can indicate a reduction in the size of the brain. Yet the only way to diagnose Alzheimer's disease definitely is after death, as the hallmarks of the disease - plaques and tangles in the brain - can only be identified by autopsy.

How does it affect the brain?

We now know a lot more about the plaques and tangles first identified by Alois Alzheimer. The neurofibrillary tangles - which look like knotted string - are mostly made up of a protein called tau. The build-up of these tangles kills the nerve cells, although the tangles remain after the cells have died. The core of the plaques is a protein called b-amyloid, surrounded by the debris of dead and dying nerves. As the neurons die, the brain shrinks. Other changes in the brains of people with the disease are more subtle. For example, the levels of the neurotransmitter acetylcholine are much lower in the brains of patients with Alzheimer's, and the lower the levels, the more plaques and tangles there are and the greater the dementia.

Can Alzheimer's be treated?

As yet, there are no effective treatments that can reverse the memory loss associated with the disease. The drugs that have come to the market are designed to preserve the levels of acetylcholine in the brain by inhibiting the enzyme acetylcholinesterase. In some patients, these drugs help to improve memory and concentration, but they do not appear to slow the process of damage to the neurons in the brain and the benefits are only evident for a few months.