Older war veterans who sustained head injuries on the battlefield and the gridiron of American football likely to boost dementia risk in later life, show studies.
Older war veterans who experienced traumatic brain injury face a doubled risk of developing dementia, according to a study led by Kristine Yaffe, head of the Memory Disorders Program at the San Francisco Veterans Association medical centre.
Yaffe led a team that reviewed the medical records of 281,540 US veterans aged 55 and older who had had at least one medical visit from 1997 to 2000, and at least one follow-up visit over the following seven years.
Checking medical records for a history of traumatic brain injury (TBI), they found the risk of dementia was 15.3 percent in those who had such injuries, compared to 6.8 percent for those ex-soldiers who did not.
"This issue is important, because TBI is very common," Yaffe said in a statement.
"About 1.7 million people experience a TBI each year in the United States, primarily due to falls and car crashes."
Such injuries are also known as the "signature wound" of the conflicts in Afghanistan and Iraq, accounting for 22 percent of casualties overall and 59 percent of blast-related injuries.
The research suggests that the death and damage of axons -- long cell extensions that form connections among nerve cells in the brain -- may be to blame for the higher risk of dementia.
The swelling of the traumatised axons accompanies the accumulation of proteins called beta-amyloid, a hallmark of Alzheimer's.
Amyloid plaques similar to those found in the brains of people with Alzheimer's are present in up to 30 percent of TBI patients who do not survive their injuries, regardless of age.
In the second study, scientists led by Christopher Randolph of Loyola University Medical Center in Chicago compared the likelihood of decline in basic cognitive functions among retired footballs players and in older adults who had not played professional sports.
A handful of studies in recent years have suggested that the repeated head-on clashes typical of American football may -- despite protective gear -- boost the chances of long-term brain damage.
Of more than 500 ex-football players, mean age 61, who responded a health survey in 2008, just of over 35 percent gave answers suggesting possible dementia.
By comparison, some 13 percent of Americans over the age of 65 have some form of Alzheimer's, according the the Alzheimer's Association.
Researchers followed up on this data to identify players with Mild Cognitive Disorder (MCI), often a precursor to full-blown dementia or Alzheimer's.
The study compared neurological and psychological test results from this group with two other groups, neither of which had played pro sports: demographically similar adults who showed no cognitive decline, and adults diagnosed with MCI.
The former athletes were clearly impaired compared to the normal adults, suggesting that football may have had a role in the ex-players' impairment.
The athletes were slightly less impaired that the non-athlete group diagnosed with MCI, but were also considerably younger.
"It appears that there may be a very high rate of cognitive impairment in these retired football players compared to the general population," Randolph said, pointing to "repetitive head trauma" as the likely culprit.