A couple of years ago, Lynda Mainwaring was talking to her sport concussion class about the term subconcussion. They were having trouble wrapping their heads around what it actually means.
The concept has been garnering increasing attention ever since the surge of interest in chronic traumatic encephalopathy (CTE), a suspected long-term outcome of multiple repeated blows to the head – with or without concussion - that can manifest in different ways, from feeling depressed to being more irritable and impulsive.
“We knew it meant not a concussion, something that is under the threshold for the diagnosis of concussion, but what does that mean?” says Mainwaring, an associate professor at the University of Toronto’s Faculty of Kinesiology and Physical Education.
With the help of her graduate students, Kaleigh Ferdinand Pennock, Sandhya Mylabathula and Benjamin Alavie, Mainwaring embarked on a two year systematic review of all the literature on sport related concussion that examines the term subconcussion or repetitive hits to the head.
The first thing they noticed was a lack of consistency and clarity in defining and measuring variables related to the concept of subconcussion. Mainwaring and her group suggest replacing the term with subconcussive impact, which can be measured and operationally defined - putting everybody researching the phenomenon on the same page.
Next, Mainwaring and her team classified the literature into three main categories based on primary focus: neurobiological, neuropsychological and impact exposure metrics.
They found the neurobiological studies suggest that prolonged exposure to repetitive head impacts in sports is associated with both microstructural and functional changes in the brains of male athletes. This can manifest in many ways, including white matter changes, cortical thinning, decreased volume changes in some regions and transmission dysfunction in the corpus callosum-the major highway between the left and right hemispheres of the brain.
“We don’t have a direct one to one connection or evidence to say these kind of structural changes in the white matter will lead to cognitive impairment, depression or irritability, but we can tell that some of the areas of the brain that were affected - the limbic system and the dorsolateral prefrontal cortex - are areas of the brain that influence emotions.”
There was not enough evidence to suggest whether these changes are neuroprotective (a temporary shutting down of the nervous system to help the brain heal), transient or permanent. But, the message is clear, according to Mainwaring.
“It’s not a good idea to be exposed to any situation where you’re getting repeated blows to the head. Our brain is not meant to be knocked about like that.”
The neuropsychological studies didn't show clear evidence for a relationship between repetitive hits to the head and deterioration in neurocognitive performance, such as memory and attention.
“This may be either because that’s not happening or because our measures aren’t sensitive enough to pick it up,” says Mainwaring.
Finally, there was insufficient evidence to determine a minimal threshold for injury from repetitive hits to the head. According to Mainwaring, each individual is going to respond to a blow to the head differently, because everybody’s brain is different. Differences in time between impacts, hit location (top or front of the head), frequency of impacts, age, history of concussions, type and magnitude of force and history of depression, for example, all play a role in outcome.
“Whereas you may not get a concussion from an impact of 60 g, I may get one from 50 g. So, we can’t say with certainty that a particular amount of force will necessarily give you a concussion. Similarly, we cannot say that a certain force will not lead to a concussion, but may lead to a problem down the road. We need standardized measures and protocols in relation to each dimension of impact exposure to help us understand what repetitive hits to the head are doing,” says Mainwaring.
Mainwaring hopes this review serves as another warning to athletes to avoid banging their heads, but also to encourage modifications in sports that will prevent hits to the head. She also wants people to realize that you don’t need to have a diagnosis of concussion, but if you’ve been exposed to a number of hits to the head that result in some changes in the brain, you may face problems down the road.
The good news, according to Mainwaring, is that this research review shows that repetitive hits to the head do not necessarily lead to neuropsychological problems - thinking and emotional problems. That’s because the brain is much more plastic than we realize and we’re developing new brain cells throughout the lifespan. And, there is plenty of evidence of how important exercise is for neurogenesis (the growth of new neurons). So, we can recover if we don’t expose ourselves to repeated opportunities in which we’re going to be vulnerable to further damage. In other words, we can do something to help ourselves.
“Your brain is going to heal and you will go on to be a healthy functioning individual,” says Mainwaring. “I would say that most of the people I know have had at least one concussion and we’re all doing okay.”
Subconcussive head impacts in sport: A systematic review of the evidence was published on February 3 in the International Journal of Psychopysiology