This is a shortened version of a primer I originally wrote for St John's Ambulance via the Brain Health Research Centre. It is very dry, but still handy if you want to get up to speed with current concussion research.
Concussions aren't too bad if you only ever have one, but even minor knocks to the head after that are risky. Be really careful with your brain, it is very squishy and sensitive.
Concussion is the result of force on the brain. This can either be direct (from a hit to the head) or indirect (from sudden acceleration and deceleration). As the brain is a soft structure, made of billions of thin cells meshed together, it is easily affected by these forces. The force stretches the axons, reducing their flexibility and increasing their risk of tearing. The thinner an axon is the more likely this force will be to break it, or strip it of myelination. This stretching also allows potassium to flood out of the cells, disturbing the electrochemical gradient. To complicate matters further, research suggests that mitochondria may also be damaged by the force of concussion, which disturbs the neuron’s ability to produce ATP to re-establish the concentration of potassium on the inside of the cell. The combination of these factors results in short-term concussive symptoms and potentially also long-term structural and functional damage to the brain.
Depending on the area and severity of damage, concussion can present as a number of different symptoms. These symptoms are exacerbated by intoxication, with intoxicated sufferers more likely to score poorly on the Glasgow Coma Scale and be hospitalised for their injuries.
In the case of traumatic accidents, such as car accidents, a concussion can be misattributed as an acute stress response (psychological shock). However, while confusion and amnesia do occur in both conditions, concussion also presents with a number of observable physiological symptoms which are noted below.
Minutes to hours after the initial injury
Days to weeks after the initial injury
The symptoms of confusion experienced by those with concussion should be gone after 24 hours, however other symptoms can last for up to 12 weeks. Health prior to the incident appears to improve recovery outcomes.
While sleeping post-concussion cannot increase the chance of coma, it can obscure the onset of a coma or significant deterioration. Swelling should reduce after six hours, but prior to this the individual should be roused every hour or so to determine whether they are still able to regain consciousness.
The effects of concussion on the brain There are two types of axonal injury which occur as a result of concussion: micro-tearing, and diffuse axonal injury.
Diffuse axonal injury, or DAI, is a misleading name as the damage is not diffuse, but rather stems from one or more focal regions. This damage tends to be concentrated around the central white matter tracts of the brain: the corpus callosum, internal capsules, cerebellar peduncles and brainstem. DAI is believed to be caused by damage to axonal transport as a result of acceleration. This leads to swelling of the axon, and then detachment of the axon from those points of swelling.
Micro-tearing is incomplete tearing of the axonal plasma membrane, which damages the structural integrity of the axon but does not result in axonal detachment.
Diffusion tensor imaging (DTI), which is a scanning technique that uses the pattern of water diffusion throughout the brain to identify fibre tracts, has confirmed that white matter tracts are significantly damaged as a result of concussion via both DAI and micro-tearing. DTI scans also point to reduced structural integrity of axons. Differences have been detected relating to how the concussion is triggered, for instance sports-related concussions tend to result in damage to the axonal tracts which extend from the front of the brain to the back of the brain, whereas blast-related concussions received by military servicemen tend to have diffuse axonal damage with multiple focal areas of injury.
Structural damage, however, is not the only damage caused by concussion. Resting state fMRI studies have demonstrated changes in the functional connectivity of the brains of people with concussion. These changes persist even after symptoms have resolved. Functional connectivity is the ability of the brain to use two or more physically separated regions for the same task. This is achieved by having tracts of axons connecting these different regions of the brain in order to allow them to communicate with one another effectively.
Damage to the axons, which allow for this functional connection, can lessen the connectivity of the regions and impact neurological performance.
Individuals who did not experience lingering symptoms of concussion, or who had been believed to have recovered from concussion, showed no changes to functional connectivity during rest but presented with deficits after physical exertion. This suggests that even after symptoms have resolved underlying physical damage remains.
Repeated concussions, in particular those which occur in contact sports, have been associated with disorders such as post-concussive syndrome and chronic traumatic encephalopathy.
Post-concussive syndrome is the more common of the two to be observed in non-sport playing individuals, and is the persistence of concussion symptoms. Children and the elderly are particularly at risk of developing this syndrome, though it is unclear why. Evidence suggests that the functional connectivity of individuals with persistent post-concussive syndrome (lasting 6 months or longer) is significantly reduced even when compared to the functional connectivity of individuals with recent concussion.
Chronic traumatic encephalopathy occurs almost exclusively in soldiers and professional contact-sportsmen. The individual develops concussion-like symptoms as well as issues related to regulating mood, memory deficits, and problems with verbal communication. Autopsies have revealed a build-up of the protein Tau in the brains of these individuals, in some cases, resulting in severe dementia.
Concussion, though a primarily subjective illness, is much better understood now than it was 20 years ago. The ramifications of concussion, especially repeat concussion, are still up for debate but evidence suggests that these events result in observable damage to the brain and as such should be taken seriously.