Researchers believe that the findings also highlight the importance of documenting concussion events and their associated symptoms.
Researchers, from Boston University School of Medicine (BUSM) in Massachusetts, and colleagues report their findings - which show promise for detecting the effect of concussion on neurodegeneration - in the journal Brain.
They suggest that their study shows that there is a need to record even mild head injuries because, when combined with genetic risk, they may lead to long-term health problems, such as brain diseases.
Alzheimer's is a disease that attacks the brain and gradually worsens. It is the most common form of dementia, a general term for mental decline that becomes serious enough to interfere with daily life.
It is the only disease among the top 10 causes of death in the United States that cannot be prevented, cured, or even slowed.
Currently, more than 5 million people in the U.S. are living with Alzheimer's disease. This number is expected to grow as more of the baby boom generation enters the age range at highest risk for the disease - 65 years and over.
In their study background, the researchers note that moderate-to-severe traumatic brain injury is one of the strongest environmental risk factors for late-onset Alzheimer's and other neurodegenerative diseases. However, it is not so clear whether the same applies to mild traumatic brain injury or concussion.
Concussion influences link to genetic risk
The team studied 160 war veterans between 19-58 years of age who had served in Iraq and Afghanistan, many of whom had been diagnosed with mild traumatic brain injury and post-traumatic stress disorder (PTSD).
Fast facts about Alzheimer's disease
- Every 66 seconds, someone in the U.S. develops Alzheimer's disease
- Alzheimer's and other forms of dementia cost the nation $236 billion in 2016
- The disease kills more U.S. adults than breast cancer and prostate cancer combined.
The participants underwent MRI scans, from which the researchers could measure thickness in cortical brain regions known to deteriorate in early stage Alzheimer's disease and compare them with measurements of control regions.
Researchers also calculated a genetic risk score for Alzheimer's disease for each participant, by comparing results from their genotyping tests with information on risk genes from a large Alzheimer's disease genome-wide association study.
The results show that concussion appears to influence the link between genetic risk for Alzheimer's disease and cortical thickness. Participants with concussion and high genetic risk had reduced cortical thickness in Alzheimer's disease-vulnerable regions.
The researchers also found that concussion and high genetic risk "indirectly influenced episodic memory performance through cortical thickness, suggesting that cortical thinning in Alzheimer's disease-vulnerable brain regions is a mechanism for reduced memory performance."
'Document all concussion events'
First author Jasmeet P. Hayes, assistant professor of psychiatry at BUSM, says: "We found that having a concussion was associated with lower cortical thickness in brain regions that are the first to be affected in Alzheimer's disease."
Prof. Hayes - who is also a research psychologist at the National Center for PTSD, VA Boston Healthcare System - also points out that they found these brain abnormalities in a relatively young group of veterans, with an average age of 32 years.
She says that the findings show promise as a way to detect the potential brain deterioration that concussion might inflict early in one's lifetime.
The results also highlight the importance of documenting concussion events and their symptoms, "even if the person reports only having their 'bell rung' and is able to shake it off fairly quickly," Prof. Hayes notes.
The researchers hope other teams can now take their findings further and explore the mechanisms that accelerate the onset of brain deterioration - not only Alzheimer's disease, but also in other neurodegenerative disorders, such as Parkinson's disease and chronic traumatic encephalopathy.
"Treatments may then one day be developed to target those mechanisms and delay the onset of neurodegenerative pathology."
Prof. Jasmeet P. Hayes