At the University of Washington’s School of Medicine there is a computer database that states with certainty – albeit heavily encrypted – whether or not Sophie Leggett will develop a form of genetically inherited early onset Alzheimer’s disease. But she has chosen not to find out what it says.

A blood test is available to adult children and siblings of those who develop Alzheimer’s at a young age and have a family history of the disease. It identifies whether they carry one of the three faulty genes known to cause familial early onset Alzheimer’s, presenilin 1 (the mutation affecting Leggett’s family), presenilin 2 and amyloid precursor protein. All result in the overproduction of amyloid, a protein that builds up into the plaques on the brain which are the hallmark of Alzheimer’s.

Now 39, Leggett saw her mother and aunt develop familial early onset Alzheimer’s in their early 40s – as had their father. All died in their 50s. There is a one-in-two chance that Leggett has inherited the gene mutation.

“I focus on the 50% chance that I haven’t got it,” she says. “I know that my world would change totally if I came home one day and said to my family: ‘I know I’ve got it.’”

This particular form of Alzheimer’s is rare, and genetic testing is not available for the vast majority of people. In most cases of Alzheimer’s, the cause still remains unclear, a complex interplay of environmental and genetic factors not yet fully understood. Several gene variations that increase a predisposition to Alzheimer’s – as well as some which decrease it – have been identified and it is expected that more will be found. Researchers hope that developing better mapping of the pathways of the disease will in turn enable them to develop drugs that may prevent, delay or stop progression.

Leggett was in her early 20s when her mother, Gill, began to behave unusually, a few years after her sister – Sophie’s aunt – had been diagnosed with early onset dementia. “She became very anxious and had short-term memory problems,” recalls Leggett. Yet at this stage, a diagnosis of anxiety was given.

Within a few years, though, Gill was regularly wandering at night, often being picked up by the police in her nightie. She was eventually diagnosed with early onset dementia shortly before her 50th birthday. Soon afterwards, she was sectioned and, after a distressing four weeks, transferred to a specialist nursing home.

“I told her lies,” recalls Leggett. “I said it was a hotel where they would do her washing and cooking. I didn’t want to tell her the truth.”

But the real cause of her terror, Leggett says, is the knowledge that her own daughter, now 15, might be affected. “I can cope with it happening it to me, but not with the fact that, if it does, then she, too, has a 50% chance of it happening to her. That is horrendous.”

Coming to terms with this possibility is a major source of anxiety for potential sufferers, says Susie Henley, a clinical and research psychologist at the Dementia Research Centre, University College Hospital, London, who works with patients affected by this and other rare forms of dementia. “Often, partly because early onset dementia is so rare, and awareness even in the health profession remains low, there has been a very stressful and drawn-out process of diagnosis. Then, suddenly, there is the sledgehammer effect of all the possible consequences,” she says.

Knowing that the gene is or may be present can lead to terrifying cycles of anxiety. “You become very vigilant for signs that you have it – lost keys, forgotten words – but don’t notice signs that you don’t. The anxiety in itself has the effect of making you forget more things. Patients have also usually seen the disease played out before them, increasing anxiety further.”

Talking through these memories, involving family members and making sure the patient understands as much as possible about the disease Henley works with a model brain – and seeing how it progresses can be very helpful. “It gives some sense of control in a situation where they often feel they have none,” she says.

She encourages her patients to become more aware of positive signs in their behaviour and memory and to seek out activities and hobbies they enjoy. “Some people are able to develop a real ‘make the most of my life’ perspective, whether or not they know they have the gene.”

Leggett herself is now involved in the worldwide DIAN-TU trial, which is testing two potential disease-modifying drugs for individuals at risk of familial early onset dementia. By working with those who are not yet showing signs of disease, researchers hope to learn more about bio markers for Alzheimer’s, means for early detection and the benefits of early treatment.

“I wanted to do something for my family, but also for everyone,” Leggett says. “I think: ‘Well, Alzheimer’s, you might get me, but I will do everything to get you first.’”

Her family talk about Alzheimer’s openly and honestly. “We look at the positives – the amazing research, the opportunities to have genetic embryo screening before having children.” In due course, says Leggett, her daughter can make her own decision about genetic testing. But she remains confident of her own decision. “A couple of times a year, I lie awake at night, picturing myself being able to tell my daughter she can’t have it. That would be the most amazing thing in the world. Then I think of being told I do have it. I cannot cope with it being an absolute certainty.”

For more information visit alzheimersresearchuk.org