Concussion does not have to result in loss of consciousness for it to be followed by a higher risk of developing dementia.

This was the conclusion of a large study of veterans in the United States that found that the risk of developing dementia more than doubled following mild traumatic brain injury (TBI) without loss of consciousness.

The study, by researchers at the University of California, San Francisco (UCSF) and now published in JAMA Neurology, addresses a gap in the evidence linking TBI with dementia.

Other studies have shown that moderate and severe TBI are followed by a higher likelihood of developing dementia, but evidence of a link with mild TBI—particularly where no loss of consciousness occurs—was somewhat patchy.

"There are several mechanisms," says senior study author Kristine Yaffe, who is a UCSF professor in neurology, psychiatry, and epidemiology and biostatistics, "that may explain the association between TBI and dementia."

Some have suggested that TBI triggers or speeds up the "accumulation of abnormal proteins" that kills brain cells, such as that which occurs in Alzheimer's disease.

Others are of the view that TBI increases vulnerability to damage from further injuries or aging. But Professor Yaffe reminds us that these are just theories and that "we need more work in this area."

Mild TBI accounts for most TBIs

A TBI is a blow, jolt, or bump to the head that causes disruption to normal brain function. Its severity depends on whether there is loss of consciousness, and the extent of any memory impairment, or altered mental status, and how long these changes last.

Mild TBI, also known as concussion, accounts for most cases of TBI in the US.

The researchers note that of the 2.8 million TBIs that occurred during 2013 in the US, around 80% were mild TBIs.

They refer to a survey of men and women who had served in Iraq and Afghanistan that found that as many as 17% of personnel said that they had experienced mild TBIs in the field, of whom more than half reported having had more than one.

The majority of these cases, note the authors, are caused by "shockwaves from blasts, rather than blunt trauma," and they are not necessarily accompanied by loss of consciousness.

Study tracked more than 350,000 veterans

For their study, the researchers used two sources of health information on US veterans. One source was a database of "all-era" veterans whose TBIs could have occurred either during service or during their lives as civilians.

The other database only contained records of veterans who had served in Iraq and Afghanistan, and most of the TBIs they detailed had occurred during military service.

Altogether, they included 178,779 individuals diagnosed with TBI, with 54% having experienced mild TBI, or concussion.

The researchers compared them with the same number of individuals with no history of TBI, bringing the total tracked by the study to more than 350,000 veterans.

The average age of the veterans was 49, and the study followed them for an average of 4.2 years, noting any cases of diagnosed dementia. The vast majority of the vets were males, and nearly three-quarters were white.

More than double dementia risk

The researchers found that, even after they had taken into account gender, race, age, and medical conditions that might affect the results, TBI was linked to an increased risk of dementia, as follows:

• Mild TBI without loss of consciousness was tied to a 2.36 times higher risk.

• Mild TBI with loss of consciousness was tied to a 2.51 times higher risk.

• Moderate to severe TBI was linked to a 3.77 times higher risk.

The results were largely similar for the two sources of data, leading the team to conclude that mild TBIs that occur during military life are just as likely to have a link to dementia as those that occur in the population at large.

The researchers believe that their findings show that more should be done to reduce the risks of brain injury.

"In older adults," says first study author Deborah Barnes, who is a UCSF professor of psychiatry, epidemiology, and biostatistics, "exercise and multifactorial interventions may limit the risks of falls, which are a leading cause of head injury."

In 2013, falls accounted for nearly half of all TBI visits to emergency departments, hospital admissions, and deaths in the US.

In an accompanying editorial, Professor Kimbra Kenney, from the Uniformed Services University of the Health Services in Bethesda, MD, and Professor Ramon Diaz-Arrastia, of the University of Pennsylvania in Philadelphia, note that the large study is "the best information to date" to show that US veterans face an increased risk of dementia as a result of being injured while serving their country.

In the meantime, Professor Barnes advises that you seek medical attention if you have a concussion and allow it "time to heal and try to avoid repeat concussions."

"Although our study did not directly examine this issue, there is growing evidence that repeated concussions appear to have a cumulative effect."

–Professor Deborah Barnes