The Overlaps of Traumatic Brain Injury and Alzheimer’s Disease

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It’s estimated that around 69 million individuals sustain a traumatic brain injury (TBI) annually, and over 1.4 million people attend A&E in the UK every year with a TBI. TBIs usually result in altered brain structure and function, and in cognitive problems like memory deficits, impaired social function, and decision-making difficulties. The severity of this injury can be classed as mild, moderate, or severe based on neurological measures, such as loss of consciousness duration. Mild TBI (also known as a concussion) is the most common type and accounts for around 80% of all TBI cases. It is a known risk factor for Alzheimer’s disease, however previous studies are yet to quantify the degree to which these conditions share patterns of neural degeneration in the brain. And so, the researchers of this study came to the conclusion that comparing these patterns could not only show how the degenerative trajectories of the 2 conditions are similar, but could also show which features of brain atrophy (wasting away of brain cells) could predict Alzheimer’s risk following TBI.

Image 1: Information on Alzheimer’s disease.
Image 2: Information on traumatic brain injuries.

The study involved 180 participants in total – 33 individuals with TBIs due to a fall, 66 individuals with diagnosed Alzheimer’s disease, and 81 healthy individuals as the controls (they did not have Alzheimer’s or TBI). The researchers analysed MRIs of the participants’ brains and created additional computer-generated models for comparison of different brain structures, eventually mapping and outlining similarities and differences between the three different groups. In many brain areas of both TBI and Alzheimer’s participants, reduced cortical thickness was found when compared to the controls. Previous research has shown cortical thickness to be a useful measure of grey matter atrophy – and it is also roughly associated with brain age. Grey matter thinning is usually correlated with diminished attention, memory and verbal fluency, which translate into functional deficits such as a reduced ability to make decisions, put together new information and adapt one’s behaviour to new situations.

With the use of the MRIs, the researchers were able to identify significant similarities between TBI and Alzheimer’s disease in how the grey and white matter of the brain degrade following injury. Grey matter is the part of the brain where the neuronal cell bodies are found along with their short-range connections. In grey matter, the researchers found that the most widespread similarities were in areas involved in memory (such as the temporal lobes) and decision-making (such as the orbitofrontal cortices), where thinner cortices were observed. White matter is the part of the brain where the other parts of the neurone (such as the axon) is found, and it allows communication across longer distances by connecting different brain regions. In white matter, the researchers found similar patterns of degeneration in structures such as the fornix, corpus callosum, and corona radiata. Whilst the fornix is important in memory function, the corpus callosum helps with exchange of information between brain hemispheres. The corona radiata is involved in limb movement, so injury can result in poorer coordination and balance.

Image 3: Statistically significant differences in cortical thickness between (a) TBI and controls, (b) Alzheimer’s and controls, and (c) TBI and Alzheimer’s patients. In (a) and (b), blue regions represent areas where TBI and Alzheimer’s subjects, respectively, have significantly thinner cortex compared to controls. Regions coloured in red represent areas where controls have significantly thinner cortex. In (c), areas coloured in green represent regions whose mean cortical thickness is statistically indistinguishable across TBI and Alzheimer’s participants.

The researchers also used machine learning techniques to precisely predict the extent of Alzheimer’s-like brain changes observed during the chronic stage of mild TBI based on cognitive assessments conducted shortly following such injuries. Studies of TBI effects on brain structure have identified both amyloid plaques and neurofibrillary tangles (which are twisted fibres found inside the brain’s cells) which resemble those observed in Alzheimer’s disease. Despite this evidence, few studies have investigated whether TBI can alter brain trajectories toward Alzheimer’s, particularly at older ages. Although the new findings of this study do not establish a cause-and-effect relationship between TBI and Alzheimer’s disease, they do add to the evidence that the two conditions share common trajectories. The results of the study revealed Alzheimer’s-analogous patterns of neurodegeneration after TBI, which may be proportional to AD risk. Thus, the findings are relevant to continuous efforts to identify TBI patients at high risk for AD.

Original Paper: Rostowsky, K.A., Irimia, A. & for the Alzheimer’s Disease Neuroimaging Initiative. (2021). “Acute cognitive impairment after traumatic brain injury predicts the occurrence of brain atrophy patterns similar to those observed in Alzheimer’s disease.” GeroScience. https://doi.org/10.1007/s11357-021-00355-9

Original Link: https://link.springer.com/article/10.1007/s11357-021-00355-9

Image Sources:

Image 1 – https://drjockers.com/alzheimers-disease/

Image 2 – https://visual.ly/community/Infographics/health/traumatic-brain-injuries-tbi

Image 3 – https://link.springer.com/article/10.1007/s11357-021-00355-9

Featured Image – Photo by Kindel Media from Pexels

Edited by Cyrus Rohani-Shukla

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