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A test could indicate whether people in their 40s are more likely to develop dementia later in life, scientists say. But wouldn't many of us rather not know?

It is a devastating, heart-breaking, life-shattering condition - and one with an enormous impact not just on those who live with it, but on their loved ones too.

Dementia robs from us our memories and our ability to think - the very brain functions that shape our sense of self.

At the same time, it imposes a heavy cost on families, who can only watch as the person they once knew slips away from them.

So we could easily be forgiven for blocking out the possibility that it could happen to us.

This, however, is a decision that middle-aged people could find themselves taking.

Scientists at Brunel University have developed a test which they believe could eventually indicate whether people in their 40s have a higher chance of going on to develop the condition.

Although it does not offer a cast-iron prediction that the patient will develop or be free from dementia, it does allow those displaying indicators - tiny areas of damage in the brain - to receive early treatment and adapt their lifestyles as a preventative measure.

Alzheimer's disease is the most common form of dementia - or loss of cognitive ability - in the UK, affecting an estimated 465,000 people.

During the course of the disease the structure of the brain is altered, causing brain cells to die, and those with it tend to become confused and experience mood swings as their memory deteriorates.

There is, so far, no cure - although some drugs can lessen the symptoms and slow down the disease.

Doctors say early diagnosis can allow these treatments to have a greater impact.

In this respect, the Brunel team's test has the potential to improve the long-term condition of thousands of people.

But it raises an existential dilemma for otherwise healthy individuals: who would really want to live with the knowledge that they had a higher chance of developing such a devastating illness?

It is a conundrum researchers acknowledge can affect diagnosis rates. A (PDF) by a team of Israeli academics outlined the symptoms of Huntington's disease, a form of dementia, and asked respondents whether they would want to know if they or their partner would develop the condition. It concluded that "around half avoided finding out".

Little wonder, then, that according to the Alzheimer's Society, some two-thirds of people who die with dementia do so without a diagnosis.

One person who understands the dilemma is the former newsreader Angela Rippon, who watched her mother, Edna, succumb to dementia.

Ms Rippon acknowledges that witnessing the heart-breaking deterioration of Edna, once an independent-minded career woman, would be enough to put many people off confronting the possibility of the same thing happening to them.

"I can totally appreciate why some people would rather not know because I know how distressing it is to watch someone you love disappear from you," she says.

"But personally, I'd want to take the test. At least that way you can prepare. Otherwise, it's left to the families and loved ones to try and keep up with it."

According to Alzheimer's Society interim chief executive Ruth Sutherland, the low diagnosis rates for dementia can be attributed not just to patients not wishing to confront their condition, but also to lack of awareness and specialist training for GPs.

As such, she welcomes any test that, through increasing vigilance, might lead patients to seek diagnosis earlier - although she warns of the dangers of allowing such data to be misused by insurers and other organisations.

"What we know is that when people get a diagnosis, it's usually a relief to them," she says. "We also know what a big difference early diagnosis makes.

"But we need to be very careful that these tests were not used to discriminate against people. What we would also want to see is the right level of support and counselling available."

Indeed, the manner in which patients receive such information is likely to have a major impact on whether they judge retrospectively that taking the test was the right decision.

Dr Daniel Sokol, a medical ethicist at Imperial College, London, says what is most important is that patients are given their results honestly and with compassion.

"Before taking the test, they should ask themselves why they would like to know and how a diagnosis of Alzheimer's would affect the way they go about their lives," he says.

"Ultimately, it is a personal choice. Some would like to know as soon as possible so they can modify their lives accordingly; others would rather not know. There is no single right answer that applies to all."

It is, after all, the loss of control that sufferers and their families find so painful. Offering the choice of whether or not to look into the future at least offers more of that precious autonomy.