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Over 40 million people worldwide get concussion, or mild traumatic brain injury, a year. Concussions can tremendously alter your mental health and quality of life with significant social, relationship and working/ school life challenges. Concussion effects are dose-dependent: the more you endure, the worse the outcome for your brain health (find a list of concussion symptoms here). Exercise and sports, although beneficial to brain health, tend to be high-impact. In the US alone, 3.8 million cases of sport-related concussions occur annually with over half in youth (especially boxing, American football, soccer, rugby and ice hockey).

Athletes (young/ old; pros/ amateurs) who have suffered multiple concussions, even those spaced years apart, have been diagnosed with many, or all, of the following: degenerative brain disorders like dementia (including Alzheimer’s Disease and Chronic Traumatic Encephalopathy, CTE), depression, and other long-term brain function deficits (post-concussive syndrome/ epilepsy/ headaches).

Here we explain four ways scientists think concussion leads to altered brain health, including dementia and depression. If you want to know more about anything that we mention here then you can find out more in our more detailed blog.

1: Concussion is like a brain fracture

Strong concussive blows to the head, especially rotational, cause twisting, and stretching of white matter tracts - the nerve fiber bundles that carry electrical signals enabling different brain areas to communicate. Further blows can eventually lead them to break. To put it bluntly, a concussion is a fracture of your brain communication cables.

The tracts transmit messages across the brain, enabling high energy tasks that require a lot of different information. When they are damaged, functions such as attention, working memory, problem solving, being able to switch between tasks, and reasoning are compromised. The brain is split into different areas with different functions so concussion symptoms depend on what points along the tracts are damaged. Depression, for instance, is associated with alterations in tracts linking limbic, cortical, and subcortical brain regions (alongside neurochemicals in these areas).

This damage may also lead to neurodegenerative disorders like dementia, Alzheimer’s disease, Parkinson’s disease and ALS (amyotrophic lateral sclerosis) and CTE. Proteins, such as one called tau, help maintain the structure of white matter tracts, but the fractures may cause them to form tangles or plaques. When brain areas with specific functions, like memory, start to get tau tangles or even amyloid-β plaques, the function of that brain area is progressively diminished, resulting in dementia. Tangles of proteins, or plaques, tend to follow the pathways marked out by white matter tract connections so that seemingly unconnected, widespread areas of the brain can be affected. Although the presence of tau in abnormal forms, or in places where it shouldn’t be, is a strong predictor of degenerative brain diseases (such as dementia), what scientists are not completely clear on is whether tau plays a part in the brain degeneration or whether it is attempting to heal the damaged brain parts.

2: Concussion sends your brain into an energy crisis

Smaller connections between brain cells, called axons, also become stretched and damaged. This causes increased abnormal electrical conduction across the nerve axons. The brain cells then kind of short circuit, which leads to a shutdown of brain activity in far apart brain areas simultaneously leading to concussion symptoms, like amnesia and loss of consciousness. This (and other things) causes an energy crisis and energy-starved brain cells cannot perform at their peak, increasing the risk of detrimental effects of further concussions (further axonal damage/ cell death/ build-up of dementia-associated proteins, including tau and amyloid-β).

3: The brain fights concussion with an immune response

The first line of defence against damage to the brain is a type of brain immune cell called microglia. The microglia act like a brain referee and respond very quickly to brain cells damaged by trauma, a process known as inflammation. Brain inflammation persists after a concussion and can get worse.

Now the brain is on guard and further trauma amplifies future immune responses. Normally microglia work to clean up injured cells and debris from injured parts of the brain but chronic inflammation can lead to accumulation of tau associated with dementia. Interestingly, if the brain is in this “primed” inflammation state, any kind of impact on the brain immune system, not limited to a physical trauma, might cause deleterious effects. This would include an infection or even stress. So, concussion plus concussion; or concussion plus flu; or concussion plus exams… could lead to a greater risk of depression or dementia down the line.

4: Multiple concussions knockout brain cells

Multiple concussions lead to brain shrinkage, mainly due to loss of brain cells and connections (also known as atrophy) in brain areas including the amygdala (emotion), hippocampus (memory), cortex (attention, problem solving), and cerebellum (balance).

Loss of brain cells is dominant to progress of symptoms in neurodegenerative diseases like Alzheimer’s disease, Parkinson’s disease, ALS and CTE. Cortical thinning is one of the most sensitive markers of brain degeneration and is not only present in older professional athletes, but also in young athletes and children with a history of multiple concussions.

The Anderson lab at University of Arizona, has been tackling the problem of multiple concussions on developing brains. Using rodent models, they have found that young brains subjected to multiple concussions already show considerable cortical thinning with loss of brain cells. In other labs, brain scan studies in young athletes found cortical thinning (in cortical areas associated with dementia, other cognitive impairments and emotion regulation), with increasing severity with the number of concussions. This suggests that multiple concussions in young brains can set into motion brain changes to make them more prone to dementia and depression later in life.

Time out: what do I need to know about treating a concussion?

Guidelines state that the most important thing to do if you have concussion is rest. This includes physical (no sports etc.; “return to play”) and mental exertion (no school/ work, homework, screens, computer games; “return to learn”). It’s advised that brain rest is implemented for as long as it takes for all concussion related symptoms to be alleviated. Concussion behavioral symptoms can improve a long time before the brain has finished repairing itself. Sticking to the instructions for recovery is not quite as easy as donning a cast for a broken bone, especially for kids. However, more research is required to better understand what is best for the brain post-concussion and what return to play/ return to learn guidelines should be. See box “Towards better sport concussion management” here.

Game plan: how do I recognize or prevent a concussion?

Concussion symptoms can depend on age, gender, history of injuries; thus, diagnosis often depends on athletes reporting their symptoms (and many are reluctant to do so).

Athletes of high-impact sports and the general public are now more aware of the effects of multiple concussions. Major sporting organizations such as National Football League (NFL), Major League Baseball (MLB) and World Rugby Organization (WRO), as well as school authorities, have made major changes to regulations to improve identification of concussion (such as video footage in medical rooms), removing players from games and prescribing rest so that the brain has time to recover.

A number of systems are being developed to study and identify concussion objectively, including: technology like smart helmets and mouth guards that measure concussion in real time (note that helmets do not prevent concussions); biomarkers in the blood stream or brain fluid that can predict concussion severity or neurological disorders (e.g. presence of markers of damage to white matter tract); brain scans; and behavioral and neuropsychological tests.

Closing commentary: a-head of the game for our young

Sport and exercise is important for health. However, we owe it to the younger generation to take heed of what we know about the damage concussions can do to the brain. The best way to deal with concussion is to prevent it. However, in case of prevention failure, every athlete, young and experienced, should be prepared to recognize the signs of a brain injury, to speak up and not cover up their brain injury, and be given appropriate time to recover.

The Blog is a shorter version of one published earlier that can be found here. It was written by Carolyn Lacey, Scientific Outreach Manager at Neurexpert. To learn more about Carolyn and Neurexpert, please click here.

Special thanks to Trent Anderson, The University of Arizona, for answering many questions.

Scholarly reading list

Research articles:

Open access reviews:

Not open access reviews:

http://nro.sagepub.com/content/early/2015/05/06/1073858415585108.abstract http://www.cell.com/trends/neurosciences/abstract/S0166-2236(15)00177-0 http://www.ncbi.nlm.nih.gov/m/pubmed/24514870/