Dementia in One Twin Tied to Shorter Lifespan for Both Twins

A new study published  in the journal, Alzheimer’s & Dementia: The Journal of the Alzheimer’s Association, suggests that the same combination of “nature plus nurture” that shortens longevity in someone with dementia may also shorten the life of their siblings without the disease.

Uniquely Designed Study

The study used data from the Swedish Twin Registry, focusing on 2,147 pairs of twins – both identical and fraternal – where at least one twin had dementia. University of Southern California and Swedish researchers employed a case-control design and a co-twin control model to analyze the twin pairs. 

A case-control design is a research method that compares two groups in order to understand the differences between them. In this instance, one group had dementia and the other group didn’t. Adding the co-twin control model took the analysis one step further. It compared twins who were genetically identical, or at least very similar, who both had cognitive decline, to see how each was uniquely affected by the disease. This is the first study to use this methodology to investigate whether the link between dementia and higher death rates is a result of genetics, shared environmental factors, or if it is the condition itself that primarily reduces lifespan.

Surprising Findings

The researchers observed that twins with dementia had lower education levels than those with no disease. However, there was no significant difference in self-rated health between the two groups. Notably, a higher percentage of twins with dementia had passed away compared to their healthy counterparts.

A survival analysis revealed that twins with dementia lived, on average, 7.06 years after diagnosis, whereas those without lived about 10.25 years. According to the researchers, this indicated that dementia doubles the risk of mortality. In comparisons within twin pairs, where one twin had dementia and the other didn’t, the mortality risk for the affected twin was still higher. In fraternal (DZ) twins, the risk was 1.67 times greater, and in identical (MZ) twins, it was 1.48 times higher. The results held even after adjusting for factors like age at diagnosis, gender, education, and self-rated health.

In monozygotic (MZ) twin pairs, a dementia diagnosis in one twin lead to both twins experiencing similarly reduced lifespans. For dizygotic (DZ) twins, if one developed dementia, the other, even without the condition, tended to have a slightly shorter lifespan compared to individuals with no siblings diagnosed with the condition.

The study also explored differences in causes of death. It found no clear pattern distinguishing the mortality of MZ twins who died before or after their twin with dementia. 

Nature and Nurture

These results suggest that dementia itself is not the sole cause of a shortened lifespan. Instead, it appears to be a multifaceted interaction of shared genes and environment. Both twins may develop cardiovascular disease, for example, which can contribute to dementia in one and shorten the lifespan in both.

“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,” said lead author Jung Yun Jang, who conducted the research as part of her doctoral studies in clinical science in the Department of Psychology at the USC Dornsife College of Letters, Arts and Sciences.

Jang added: “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.”

The authors emphasized the lifelong impact of early life choices, suggesting that healthy habits and education in childhood can significantly reduce the risk of developing Alzheimer’s disease and dementia later in life. They said that they conducted their research to address common questions about life expectancy after a dementia diagnosis, aiming to help families plan for the future by better understanding how the disease progresses.

Further Reading:

How the ‘Big 5’ Personality Traits Influence Dementia Risk

Distinguishing Frontotemporal Dementia From Behavioral Variant Alzheimer’s Disease

Study on Strawberries and Brain Health Bears Fruit