A study published in The Lancet in April 2018 was one of the largest of its kind to look at the link between dementia risk and traumatic brain injury (TBI). A TBI is defined as any head injury that disrupts brain function brain, with leading examples including motor vehicle accidents, assaults, and falls.
After studying the link between TBI and cognitive decline, the Danish and US researchers concluded that the younger a person is at the time of the injury, the greater their risk of developing Alzheimer’s and other forms of dementia. Individuals who had a TBI in their 20s were at a 63% greater risk of developing dementia over the next thirty years, compared with people who had no such injury. However, if the injury was sustained when they were in their thirties, this risk only increased by 37%.
Of importance in this study is that even though a concussion is classified as a mild TBI, these findings were apparent in moderate and severe TBI.
What is the risk of cognitive decline in those who have had a true concussion?
The long-term effects of concussions on the aging brain are poorly understood. Researchers from UCSF Weill Institute for Neurosciences and the San Francisco Veterans Affairs Health Care System, published in May 2018, tracked more than one-third of a million veterans and the likelihood of dementia was found to more than double following concussion. They found that concussion without loss of consciousness led to 2.36 times the risk for dementia. These risks were slightly elevated for those in the loss-of-consciousness bracket (2.51) and were nearly four times higher (3.77) for those with the more serious moderate-to-severe traumatic brain injury.
There are several mechanisms that may explain the association between traumatic brain injury and dementia. Trauma may hasten the development of neurodegenerative conditions. One theory is that brain injury induces or accelerates the accumulation of abnormal proteins that lead to neuronal death associated with conditions like Alzheimer’s disease.
A small subset of individuals who have experienced a concussion may experience persistent cognitive deficits more than a year following the head injury. Neuroimaging studies suggest that changes in frontal areas of the brain are exacerbated when loss of consciousness is experienced and indicate that these changes may be progressive in nature for some people. Analyses were run on Canadian Longitudinal Study on Aging (CLSA) neuropsychological data, consisting of individuals who experienced concussion with loss of consciousness more than a year prior, compared against no-head injury comparisons. These same individuals were re-assessed three years later. At three-years follow-up, those who experienced concussion with longer duration of unconsciousness were more likely to exhibit cognitive decline relative to those who experienced less unconsciousness or comparisons.
A different study done approximately 20 years after a concussive event, looked at differences in brain volume, white matter microstructure, and brain activity relative to those with no history of concussion. A prior concussion was associated with differences between groups in both temporal and frontal regions of the brain. In addition, brain volume atrophy was noted. Despite these group differences in brain structure and function, no concussion-associated group differences in cognitive performance were found!
Together, these results suggest that concussions result in long-term functional and structural alterations in the brain, but these differences do not always significantly impact cognitive performance. One reason may be related to the extent and severity of the alterations. While the changes on average were examined more than 20 years after a concussion, the sample may not have reached the critical stage of pathologic change where functional outcomes are affected.
While this study provides further evidence of long-term consequences that often mimic the acute and chronic phases of concussion, the results also somewhat support the regional specificity of neurodegeneration seen in Chronic Traumatic Encephalopathy and Alzheimer’s Disease. The clinical features of CTE show some resemblance to the progressive cognitive decline and neuropsychiatric presentation associated with Alzheimer disease, including an insidious onset with amnestic mild cognitive impairment, and similar hallmark pathological features. However, CTE has been described as a separate condition, although the pathology is potentially overlapping with that of Alzheimer diseases in up to 25% of the cases.
While it is not clear if a concussion injury, or multiple injuries, can lead to cognitive decline, traumatic impacts to the brain may lead to the accumulation of abnormal proteins often seen in Alzheimer’s disease. It may be that other lifestyle factors may then change the risk profile to determine whether dementia develops at some stage in the individual’s life.
References
https://jnnp.bmj.com/content/91/5/455
https://www.ucsf.edu/news/2018/05/410361/dementia-risk-doubles-following-concussion-ucsf-study-shows
https://www.sciencedirect.com/science/article/pii/S1053811920306686
https://alz-journals.onlinelibrary.wiley.com/doi/abs/10.1002/alz.041309
https://www.news-medical.net/health/Can-Concussions-Increase-the-Risk-of-Dementia.aspx
Efan Gonsalves PT 