Siblings of people with dementia face an increased risk of shortened lifespan, even when they are not diagnosed themselves with the condition, a new study has revealed.

Based on data from the Swedish Twin Registry, researchers from the University of Southern California (USC) Dornsife College of Letters, Arts and Sciences, in collaboration with researchers from Karolinska Institutet in Sweden, found that a combination of genetics and environmental factors reduce longevity in dementia patients. The same combination may also shorten the life of their siblings even without dementia, according to the study published in Alzheimer's & Dementia: The Journal of the Alzheimer's Association.

Dementia impairs one's ability to remember, think or make decisions in daily life. The researchers began the study by estimating the average life expectancy of a person after a dementia diagnosis.

"One of the most frequently asked questions when a family member receives a diagnosis of dementia is: How much time do we have?" said lead author Jung Yun Jang. "We believed that asking this question in twins would inform patients and families in making their financial and end-of-life decisions as they deal with this disease that causes losses over time."

From a large cohort of more than 45,000 Swedish twins, the team looked at 90 pairs of identical twins and 288 pairs of fraternal twins, in which one twin had dementia while the other did not.

They found that dementia diagnosis shortened the lifespan of a patient by about seven years on average after the diagnosis, a finding that has been confirmed in previous studies.

However, the study also brought out other interesting links.

"In identical twins, when one is diagnosed with dementia, both twins have a similarly shortened life expectancy. In fraternal twins, when one is diagnosed with dementia, the twin who has not developed dementia has a slightly shortened life expectancy compared to someone who has no sibling with dementia," the researchers said in a news release.

"We expected a different result. We expected that, in twins where one developed dementia and the other did not, the difference in lifespan would be just like we see in unrelated people," Jang said.

Researchers believe this is because dementia itself is not the sole cause of a shortened lifespan, but a combination of shared genes and environment.

"We assumed the reason a person who has developed dementia has a shortened life expectancy is because the dementia leads to other medical conditions that affect mortality. What we're seeing instead is the increased risk of mortality is not due to just the dementia itself, but also a whole package of other influences that the person brings to their disease," Jang added.

"What happens early in the life course is really important. You may not be able to change that for yourself, but it does seem like the message to parents is, make sure your kid eats healthy, make sure your kid gets exercise, make sure your kid gets an education. You're actually contributing to giving that kid a lower chance of developing dementia 75 years later," said Margaret Gatz, a study author.