Why we need to know about sub-concussion

By Dr Andrew Lavender

A spate of recent head knocks in the AFL has put the issue of concussion back in the news, with some in the industry calling for harsher penalties for those who commit head-high bumps.

There is good reason for this. Concussion is an injury to the brain that results in temporary loss of normal brain function, often from a direct trauma through being hit by something, or hitting the head on the ground in a fall. This is relatively common in contact sports and the symptoms can be immediate with loss of consciousness, disorientation and reduced motor function and balance.

In the last few decades, concussion, also known as mild traumatic brain injury (mTBI), has attracted great interest in the sport science and health research community.

Contributing to this interest is the extensive coverage of a series of litigations in the US by retired NFL and NHL players experiencing early onset of ageing-related brain deficiencies similar to those seen in Parkinson’s and Alzheimer’s diseases. We have also seen famous retired athletes from other sports with severe symptoms, including boxer Muhammed Ali and Australian football legend Graham ‘Polly’ Farmer.

In high school and college level contact and combat sports in the US, the risk of concussion has been reported as accounting for between 0.74 to 7.92 per cent of injuries across various studies. This has prompted more research in concussion in sport and results show a clear link between concussive impacts and substantial neurophysiological and neurocognitive dysfunction evident immediately, and can be last minutes to hours after the incident.

The brains of some players after they had died were examined and many showed signs of chronic traumatic encephalopathy (CTE). In CTE, the brain becomes damaged when the person experiences multiple blows to the head over long periods of time – perhaps a career in sport lasting several years or even decades.

But not all head knocks are heavy enough to result in a concussion. While research into concussion is ongoing, some researchers have begun to focus more on the on the phenomenon of sub-concussion – injury to the brain that is very minimal with few, if any, noticeable acute signs or symptoms. Much of the research in sub-concussion has centred on heading the ball in soccer – a skill unique to the sport.

These sub-concussions can also contribute to CTE since Tau proteins, which have many functions including stabilising neurons in the healthy brain, form clumps with head knocks that cause damage to brain cells. One study of deceased NFL players found that 110 of the 111 brains under investigation exhibited symptoms of CTE. The age of death for these players ranged from 23 to 89 years. In fact, it has been shown that high school players who start tackle football at the age of five years are 10 times more likely to develop CTE than those who started at 14 years.

During a 90-minute competitive match, a player may head the ball six to 12 times, with more occurrences during practice. One study found that players head the ball 44 and 27 times (for men and women respectively) over a two-week period, and these actions are non-injurious. Another study of American footballers, who did not suffer a concussion, reported that disruption of white matter in the brain was directly proportional to number of tackles experienced, indicating a detrimental effect of sub-concussive impacts in American Football.

One group of researchers investigated a protein which is a marker of damage to the central nervous system called serum neurofilament light (NF-L) protein. They found an increase of 26 per cent of the protein immediately after 40 headers in 20 minutes which increased to 311 per cent 22 days later, indicating structural and functional impacts of sub-concussion on the brain.

Other studies have found short-term deficits in memory and long-term decrements in neuropsychological tests including memory, attention, verbal learning and reaction times.

While several previous studies have evaluated sub-concussion in rats, a recent article in Frontiers of Neurology: Neurotrauma assessed rats who received very light (10 per cent of the weight usually used in concussion studies in rats) repeated sub-concussive events equivalent to someone heading a soccer ball, three times per week for two weeks and found no change in neuromotor function which was assessed using specialised balance tests. However, after 12 weeks of repeated sub-concussion, the rats exhibited moderate neuromotor dysfunction. This is the first study to show neuromotor dysfunction after multiple sub-concussive impacts in rats.

Possible mechanisms

Evidence for reduced integrity of the blood brain barrier (BBB) as a result of multiple sub-concussive impacts is mounting. The BBB has tight junctions between cells and acts as a kind of filter system to keep the brain safe from large molecules. Disruption of the BBB is commonly observed in Alzheimer’s disease and new studies are suggesting a causal role to cognitive decline associated with the disease. Dysfunction of the BBB has been linked to mTBI and concussion and has been shown to occur in professional mixed martial artists and adolescent rugby players.

Compromised BBB integrity may result in toxic molecules leaking through to the brain. This can eventually cause inflammation through oxidative stress which compromises the function of neural cells ultimately causing the cells to die. This is evidenced by a marker called glial fibrillary protein (GFAP) which activates astrocytes and microglia which respond to damage of neural cells.

Changes in the activity of these molecules have been reported to last up to 12 months after mTBI in mouse experiments. And a study of 12 mTBI impacts over 12 days in mice found a significant increase in GFAP seven days after the final intervention a result confirmed by other similar studies.

The evidence for a deleterious effect of repeated sub-concussive impacts from human and animal models is mounting quickly. It is important for researchers, coaches, athletes, parents and board members of sporting organisations to understand the effects of seemingly non-injurious head impacts of the kind commonly seen in many sports to consider the research carefully.

The short and long-term impact on a person’s health must be at the forefront of consideration when developing rules in sports to ensure the safety of participants at all levels. Many sports including Australian Rules football and Rugby codes are already working towards making their sport safer by changing tackle rules and enforcing mandatory time away from matches and training following mTBI. This should continue to be reviewed and monitored at all levels of contact sports into the future.

Dr Andrew Lavender is a Senior Lecturer in the School of Science, Psychology and Sport and has published articles and current research projects looking at the effects of sub-concussion.