Traumatic brain injury (TBI) that occurs in early adulthood is associated with cognitive decline in later life, results from a study of identical twins who served in World War II show.
The research, which included almost 9000 individuals, showed that twins who had experienced a TBI were more likely to have lower cognitive function at age 70 versus their twin who did not experience a TBI, especially if they had lost consciousness or were older than age 24 at the time of injury. In addition, their cognitive decline occurred at a more rapid rate.
"We know that TBI increases the risk of developing Alzheimer's disease and other dementias in later life, but we haven't known about TBI's effect on cognitive decline that does not quite meet the threshold for dementia," study investigator Marianne Chanti-Ketterl, PhD, assistant professor, Duke University, Durham, North Carolina, told Medscape Medical News.
"We know that TBI increases the risk of dementia in later life, but we haven't known if TBI affects cognitive function, causes cognitive decline that has not progressed to the point of severity with Alzheimer's or dementia," she added.
Being able to study the impact of TBI in monozygotic twins gives this study a unique strength, she noted.
"The important thing about this is that they are monozygotic twins, and we know they shared a lot of early life exposure, and almost 100% genetics," Chanti-Ketterl said.
The study was published online September 6 in Neurology.
For the study, the investigators assessed 8662 participants born between 1917 and 1927 who were part of the National Academy of Sciences National Research Council's Twin Registry. The registry is comprised of male veterans of World War II with a history of TBI, as reported by themselves or a caregiver.
The men were followed up for many years as part of the registry, but cognitive assessment only began in the 1990s. They were followed up at four different time points, at which time the Telephone Interview for Cognitive Status (TICS-m), an alternative to the Mini-Mental State Examination which must be given in person, was administered.
A total of 25% of participants had experienced concussion in their lifetime. Of this cohort, there were 589 pairs of monozygotic twins who were discordant (one twin had TBI and the other had not).
Among the monozygotic twin cohort, a history of any TBI and being older than age 24 at the time of TBI, was associated with lower TICS-m scores.
A twin who experienced TBI after age 24 scored 0.59 points lower on the TICS-m at age 70 than his twin with no TBI, and cognitive function declined faster, by 0.05 points per year.
First Study of its Kind
Holly Elser, MD, PhD, MPH, an epidemiologist and resident physician in neurology at the University of Pennsylvania, Philadelphia, and co-author of an accompanying editorial, told Medscape Medical News that the study's twin design was a definite strength.
"There are lots of papers that have remarked on the apparent association between head injury and subsequent dementia or cognitive decline, but to my knowledge, this is one of the first, if not the first, to use a twin study design, which has the unique advantage of having better control over early life and genetic factors than would ever typically be possible in a dataset of unrelated adults," said Elser.
She added that the study findings "strengthen our understanding of the relationship between TBI and later cognitive decline, so I think there is an etiologic value to the study."
However, Elser noted that the composition of the study population may limit the extent to which the results apply to contemporary populations.
"This was a population of White male twins born between 1917 and 1927," she noted. "However, does the experience of people who were in the military generalize to civilian populations? Are twins representative of the general population or are they unique in terms of their risk factors?"
It is always important to emphasize inclusivity in clinical research, and in dementia research in particular, Elser added.
"There are many examples of instances where racialized and otherwise economically marginalized groups have been excluded from analysis, which is problematic because there are already economically and socially marginalized groups who disproportionately bear the brunt of dementia.
"This is not a criticism of the authors' work, that their data didn't include a more diverse patient base, but I think it is an important reminder that we should always interpret study findings within the limitations of the data. It's a reminder to be thoughtful about taking explicit steps to include more diverse groups in future research," she said.
The study was funded by the National Institute on Aging/National Institutes of Health and the Department of Defense. Chanti-Ketterl and Elser have reported no relevant financial relationships.
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Cite this: Unique Twin Study Sheds New Light on TBI and Risk of Cognitive Decline - Medscape - Sep 25, 2023.