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Traumatic brain injuries linked to cognitive decline later in life

A study of twins who served in World War II showed that traumatic brain injuries are associated with faster rates of cognitive decline as we age

By Teddy Amenabar

Traumatic brain injuries are associated with cognitive decline later in life, and a sharper drop in cognition as we age, a study of twins who served in World War II shows.

There is robust research demonstrating a relationship between head injuries and cognitive impairment or dementia later in life, “but I do not know of any others that use a twin-study design,” said Holly Elser, an epidemiologist and resident physician in neurology at the University of Pennsylvania who peer-reviewed the study.

The study published in Neurology on Wednesday found that individuals who had a traumatic brain injury were more likely to have lower scores on cognitive tests when they were about 70 years old.

They were also more likely to have rapidly declining scores after their first test if they had multiple traumatic brain injuries, lost consciousness because of a head injury or were 25 or older when the injury happened.

“Even if it’s just a single traumatic brain injury, we now know that it led to worse cognitive outcomes later in life,” said Marianne Chanti-Ketterl, the lead author of the study and an assistant professor in psychiatry and behavioral sciences at the Duke University School of Medicine.

Advantages of twin studies

The study of identical and fraternal twins allows researchers to compare participants to each other while controlling for some, if not all, of the underlying genetic factors and some of the twins’ early life conditions. Identical twins share 100 percent of their genes, while fraternal twins share about half.

Primarily White and male twins who were World War II veterans participated in the study, which began in the 1990s and was conducted by Duke University. The twins, born between 1917 and 1927, were part of a national registry started in the 1950s.

The researchers spoke with 8,662 people; 7,188 participated as twin pairs, and 1,474 participated without their twin. A quarter of the twins in the study reported having at least one traumatic brain injury.

There were 589 pairs of identical twins in the study. Among that group, a history of at least one traumatic brain injury and having a concussion at age 25 or older were associated with lower scores on cognitive tests later in life.

“Those two kids shared the same household, the same environmental factors at that time. They were both in the military,” Chanti-Ketterl said. “Very, very similar exposures of everything.”

Faster cognitive decline with age

The researchers asked participants if they had suffered a traumatic brain injury; if it had been more than once; whether they had lost consciousness because of a traumatic brain injury; and what had led to the injuries.

The study did not define what constitutes a traumatic brain injury (TBI) and whether the participants had a mild or severe injury. The researchers asked if the participants had sought medical attention or had lost consciousness. The “International Classification of Disease (ICD) defines mild TBI as concussion and severe TBI as skull fracture, oedema, brain injury or bleed,” a Lancet report noted.

The researchers conducted cognitive tests with participants over the phone every three or four years for more than 12 years. About half of the participants completed all four of the tests, Chanti-Ketterl said.

To test memory and thinking, the researchers asked questions such as “Who’s the president of the United States?” and “Can you start subtracting 7 from 100 until I tell you to stop?” and the tests scored the responses from 0 to 50, Chanti-Ketterl said.

Among all participants, anyone with at least one traumatic brain injury, a head injury that led to a loss of consciousness or an injury that occurred at age 25 or older were more likely to have lower scores on the cognitive tests.

A loss of consciousness, more than one traumatic brain injury and one that occurred at age 25 or older were also associated with a faster rate of decline in cognitive test scores in subsequent tests.

The researchers controlled for alcohol overuse, smoking, and medical conditions such as a cardiovascular disease, cardiovascular risk, and neurological conditions such as Parkinson’s disease or depression. Most of the associations between traumatic brain injuries and worse cognition “remained statistically significant.”

The study did not control for a person’s physical activity or hearing loss, which have been found to be associated with cognitive decline later in life.

The study, written by researchers at Duke and the University of California at San Francisco, is the first to repeatedly measure cognition in association with traumatic brain injuries for more than a decade of someone’s “later life.”

It was funded by the National Institute on Aging and the Defense Department.

Protecting the brain

One limitation of the study is that the participants — or a proxy — self-reported whether they had had a traumatic brain injury. It’s possible someone forgot to report an injury, the researchers said.

The participants are also homogenous — male, primarily White World War II veterans born within a decade of one another in the United States. The researchers say the results may not be generalizable to women and people from other races, ethnicities and backgrounds.

The researchers did not analyze the cause of the injuries, such as while playing a sport or serving in the military, though those details are in the reports, Chanti-Ketterl said.

More time is needed to analyze that data, which include reports of some participants falling while on a boat, for instance, Chanti-Ketterl said.

The study results emphasize the importance of taking reasonable steps to avoid a head injury — by wearing a helmet and using a seat belt — said Elser, who also wrote an editorial about the research, which was published alongside the study Wednesday.

People who have suffered a traumatic brain injury should not feel “destined to have cognitive difficulties or develop dementia as an older adult,” she said.

“We know that there are other risk factors for dementia that include things like physical inactivity and poor diet that maybe you can have a little bit of control over,” Elser said. “So, talking to your doctor about other steps you can take to protect your brain as you are aging is a very reasonable thing to do, if you’ve had a head injury.”