I’m having a busy week writing information booklets for the friends and family of people with schizophrenia. I’ve not had a lot of time to write a blog post, so the following information comes with many thanks to Alzheimer’s Australia.
Why do some people develop dementia, while others live to a ripe old age with their mind as sharp as a 20 year old? We don’t yet know the answer to this question. What we do know is that several things affect your risk of developing dementia – your age, your genes, certain health factors and your lifestyle.
Old age is the largest risk factor for dementia
Dementia mostly affects older people, and the risk of dementia increases with increasing age. The older you are, the more likely you are to be affected by dementia. Approximately 1 in 70 people aged 65-69 have dementia. Nearly 1 in 4 people aged 85-89 have dementia.
It is rare for someone under 65 to have dementia, but it does occur at younger ages and we call this ‘younger onset dementia’.
Your genes may affect your dementia risk
People often wonder whether dementia is inherited. The answer for most of us is, no. The common forms of dementia are likely caused by a combination of genetic and environmental factors.
If you have a family history of dementia, you have a higher risk of developing it yourself compared to people with no history of dementia in their family. That doesn’t mean that you will definitely get it, just that you have a slightly increased risk.
Where does this increased risk come from? There are ‘susceptibility genes’ that we might inherit from our parents that increase the risk of developing dementia. Several susceptibility genes have been found, and there are likely to be others.
Some people with susceptibility genes will develop dementia but others won’t. And some people will develop dementia even though they don’t carry these genes. So they are just a risk factor, not a cause of dementia.
There are a few very rare forms of inherited dementia. In these families, a particular gene is passed down that directly causes dementia.
Familial Alzheimer’s disease
One rare form of Alzheimer’s disease is passed from generation to generation. This is called Familial Alzheimer’s disease (FAD). If a parent has a mutated gene that causes FAD, each child has a 50% chance of inheriting it. The presence of the gene means that the person will eventually develop Alzheimer’s disease, usually in their 40s or 50s. This form of Alzheimer’s disease affects an extremely small number of people – probably no more than 100 at any given time among the whole population of Australia.
Three genes have been identified which, if mutated in certain ways, will cause FAD. These are called presenilin 1 (chromosome 14), presenilin 2 (chromosome 1) and the amyloid precursor protein gene (APP) on chromosome 21.
If familial Alzheimer’s disease is suspected
Genetic testing can identify specific changes in a person’s genes. This test can tell if a person has FAD and if a child has inherited the changed gene from a parent and will develop the disease in the future. It cannot determine when the symptoms will begin.
It is essential to ensure that suspected cases in the family have, or have had, Alzheimer’s disease and not some other form of dementia. This can only be done through a medical examination, or a careful analysis of past medical records if the person is no longer alive.