Family history of dementia and Alzheimer's disease: what it means

Dementia is not a single disease. The NHS explains it is a term for the symptoms that occur when brain function declines, and that different diseases can cause it. Alzheimer's disease is the most common type, and other causes include vascular dementia, dementia with Lewy bodies and frontotemporal dementia.

Because dementia is common, many people have a relative who developed it. Alzheimer's Society says that around 1 in 4 people aged 55 and over has a close birth relative with dementia. In most of these families the pattern reflects shared risk factors, such as age, lifestyle and common gene variants, rather than a single inherited fault.

The NHS states that age is the single most significant factor in Alzheimer's disease, with the likelihood doubling every 5 years after 65. Family history is then a modest additional factor for most people.

Alzheimer's Society explains that nearly all cases of dementia are the result of a complex disease, where several factors act together, including environment and lifestyle and multiple gene variants known as risk variants. It is not possible to directly inherit a complex disease in the way you inherit a single-gene condition. A person with a family history will often have an increased chance, because they share risk factors with relatives, but they may never develop the disease.

The society gives an example: a person may have a parent who developed dementia in their 90s and a brother who developed it in his 70s. This is common, and it is not the same as familial dementia.

The best-known risk variants are in the APOE gene. Of its three common forms, APOE4 increases risk most, and the more copies a person has, the higher the risk of Alzheimer's disease. However, Alzheimer's Society stresses that APOE variants do not directly cause dementia, and says this is why testing for them is not helpful in telling whether someone will develop it.

In some rare cases, dementia is caused directly by a change in a single gene. The NHS says that in a few families Alzheimer's disease is caused by inheriting a single gene, and that the risks of it being passed on are much higher. Alzheimer's Society says that of the four most common types, frontotemporal dementia is the most likely to be caused by a single-gene change.

These forms tend to develop earlier, sometimes in the 40s or 50s, and run strongly in families, with several close relatives affected over generations. If a parent carries the changed gene, each child has a 1 in 2 chance of inheriting it, according to Alzheimer's Society.

The NHS suggests that if several family members have developed dementia over the generations, particularly at a young age, you may want to seek genetic counselling for information and advice about your chances.

Whatever your family history, some risk factors can be modified. The NHS says that several lifestyle factors and conditions associated with cardiovascular disease can increase the risk of Alzheimer's disease: smoking, obesity, diabetes, high blood pressure and high cholesterol. It advises stopping smoking, eating a healthy balanced diet, leading an active life both physically and mentally, losing weight if needed, drinking less alcohol and having regular health checks as you get older.

It adds that research points to hearing loss, untreated depression, loneliness or social isolation and a sedentary lifestyle as other important factors, although this does not mean they directly cause dementia. Alzheimer's Society confirms that people with risk variants who eat well, are physically active, do not smoke and do not drink too much alcohol reduce their overall risk.

For most families, genetic testing for dementia risk is not useful. APOE testing cannot tell you whether you will develop dementia, and Alzheimer's Society says this is the reason it is not helpful. Testing is considered in the rare situation where a single-gene dementia is suspected, usually through a specialist clinic with counselling before and after.

If you are worried, a GP is the first step. Tell them which relatives had dementia, what type if known, and at what age symptoms began. A very young onset or several affected relatives across generations are the features that may justify referral to a genetics service.

If you are noticing memory or thinking changes yourself, do not wait. The NHS describes mild cognitive impairment as minor memory and thinking problems that are not severe enough to affect everyday life, which can be caused by underlying illness such as depression, anxiety or thyroid problems, and which is worth having assessed.

Worrying about developing dementia is common if you have watched a parent decline. Focusing on the factors you can influence, keeping up health checks and staying socially and mentally active, can feel more constructive than dwelling on what cannot be changed.

Within the NHS, memory concerns are assessed through the GP and memory services, and genetic counselling is available through clinical genetics for suitable families. Direct-to-consumer genetic tests that report APOE status are sold privately. Given the points above, such results can alarm without being able to predict what will happen, so it is wise to discuss the idea with your GP or a genetic counsellor first.