When a loved one is diagnosed with dementia, families often grapple with a challenging question: “How much time do we have?” Predicting the trajectory of their decline, as memories fade and confusion sets in, is complex. Life expectancy post-dementia diagnosis varies, typically ranging from 3 to 7 years, influenced by factors such as age and the specific type of dementia.

Notably, individuals with dementia generally experience shorter lifespans than their healthy counterparts. Surprisingly, a recent study suggests that this correlation may extend to their seemingly unaffected identical twins.

This peculiar association emerged from an extensive study involving hundreds of Swedish twins, where one twin received a dementia diagnosis while the other did not.

The study encompassed a broader analysis, involving nearly 1,000 individuals with dementia who were twins and almost 3,000 healthy twins, not necessarily siblings of the affected individuals.

As expected, individuals without dementia surpassed the life expectancy of those with the condition. Notably, the research revealed that individuals diagnosed with dementia lived an average of 7 years post-diagnosis, aligning with previous estimations.

Surprisingly, identical twin pairs exhibited similar life expectancies after one sibling received a dementia diagnosis.

The findings raise questions about the increased risk of a shorter lifespan associated with dementia, extending to the cognitively unaffected twin. This prompts inquiries into the role of genetics versus the disease itself or environmental factors in premature mortality.

Jung Yun Jang, a medical researcher at the University of California Irvine and the lead author of the study clarifies, “We assumed that the shortened life expectancy in a person with dementia is due to the condition leading to other medical issues affecting mortality,” she explains.

“Instead, what we observe is that the heightened risk of mortality is not solely attributed to dementia itself but encompasses a range of other influences that the individual brings to their disease.”

The study leveraged data from the Swedish Twin Registry, a comprehensive cohort study involving over 45,000 Swedish twins.

Jang and her colleagues conducted a primary analysis involving 90 pairs of identical twins, 288 pairs of fraternal twins, and 5 twin pairs of unknown zygosity, where one twin had dementia and the other did not.

Twin studies are extremely valuable natural experiments, enabling researchers to explore questions that would otherwise be unfeasible, shedding light on the impact of genetics and lifestyle factors on an individual’s health.

While born simultaneously, twins share largely the same genetic makeup. However, their exposure to environmental influences and lifestyles may diverge after childhood, leading to variations in their health.

Upon comparing the years lived after one member of fraternal and identical twin pairs received a dementia diagnosis, the researchers discovered a more significant difference in life expectancy among fraternal twins compared to identical twins.

This implies that it is the shared genetic factors among identical twins that contribute to their similarly shortened lifespans, although environmental factors unique to each twin can still exert an influence.

In a subsequent analysis, an individual in good health, whose twin had dementia, faced a slightly increased risk of a shorter lifespan compared to someone from a pair in which neither twin was diagnosed with the condition.

This suggests that the risk of a shortened lifespan associated with dementia is partly familial, exhibiting similarities between identical twin siblings, and not solely linked to the disease itself.

“Even though identical twins with co-twins affected by dementia may never suffer from dementia themselves, they, too, may have a shortened lifespan,” the researchers state in their paper.

While the number of twins in this study is substantial, the findings could be bolstered if replicated in another twin registry from outside of Sweden. The majority of dementia cases included in the analysis were of Alzheimer’s disease, so life expectancy patterns may vary for other forms of the disorder.

The study has been published in Alzheimer’s & Dementia: The Journal of the Alzheimer’s Association.