Dementia may run in the wider family, suggests a new study.
Researchers found that having great-grandparents or cousins with Alzheimer’s disease is linked to a higher risk of the degenerative illness.
And they say that an extended family history may give bigger clues to a person’s chances of being struck down.
Having a parent with Alzheimer’s disease has been known to raise a person’s risk of developing the disease.
But the new study, published in the online issue of the journal Neurology, suggests that having second- and third-degree relatives who have had Alzheimer’s may also increase risk.
First-degree relatives include parents and siblings who share both parents.
Second-degree relatives include grandparents, blood-related aunts and uncles, and siblings who share one parent.
Third-degree relatives include great-grandparents, great uncles, great aunts and first cousins.
Study author Professor Lisa Cannon-Albright, of the University of Utah School of Medicine in the United States, said: “Family history is an important indicator of risk for Alzheimer’s disease, but most research focuses on dementia in immediate family members, so our study sought to look at the bigger family picture.
“We found that having a broader view of family history may help better predict risk.
“These results potentially could lead to better diagnoses and help patients and their families in making health-related decisions.”
For the study, researchers looked at the Utah Population Database, which includes the genealogy of Utah pioneers from the 1800s and their descendants up until the present day.
The database is linked to Utah death certificates, which show causes of death, and in a majority of cases, contributing causes of death.
In that database, researchers analysed data from more than 270,000 people who had at least three generations of genealogy connected to the original Utah pioneers including genealogy data for both parents, all four grandparents and at least six of eight great-grandparents.
Of those, 4,436 have a death certificate that indicates Alzheimer’s disease as a cause of death.
Researchers found that people with one first-degree relative with Alzheimer’s had a 73 per cent increased risk of developing the disease.
People with two first-degree relatives were four times more likely to develop the disease; those with three were two-and-half more times likely; and those with four were nearly 15 times more likely to develop Alzheimer’s.
Of the 21 people in the study with four first-degree relatives with Alzheimer’s, six had the disease; researchers would have expected only 0.4 people to develop the disease.
Those with one first-degree relative and one second-degree relative had a 21 times greater risk. Examples of this would be a parent and one grandparent with the disease, or a parent and one aunt or uncle.
Those who had only third-degree relatives, and three such relatives, with Alzheimer’s disease had a 43 per cent greater risk of developing the disease. An example would be two great-grandparents with the disease, along with one great uncle, but no parents or grandparents with the disease.
Prof Cannon-Albright said: “More and more, people are increasingly seeking an estimate of their own genetic risk for Alzheimer’s disease.
“Our findings indicate the importance of clinicians taking a person’s full family history that extends beyond their immediate family members.”
She noted that among all of the study participants, three per cent had a family history that doubled their risk of Alzheimer’s disease, and a little over 0.5 per cent had a family history that increased their risk by three or more times that of a person without a family history of the disease.
Prof Cannon-Albright, who said death certificates are known to underestimate the prevalence of Alzheimer’s, added: “There are still many unknowns about why a person develops Alzheimer’s disease.
“A family history of the disease is not the only possible cause. There may be environmental causes, or both.
“There is still much more research needed before we can give people a more accurate prediction of their risk of the disease.”