Consistent signs of compromised inhibition found in a study of concussion sufferers were mirrored in separate tests on Canadian university football players.
These findings open new doors to predicting the impact of the often debilitating injury, as well as raise questions about the long-term impact of contact sports, according to Western researchers.
The study, “Concussion related deficits in the general population predict impairments in varsity footballers,” was recently published in the Journal of Neurology.
Led by graduate student Clara Stafford, the Owen Lab at Western analyzed results of 12 cognitive tests from an online survey of nearly 20,000 people in the general population. Among the questions, participants were asked about their concussion history, if they’d ever been knocked out and, if so, how many times.
The study found that participants who had previously suffered a concussion performed well on 11 of 12 cognitive tests, but showed a strong impairment in the test of inhibitory control—which your ability to suppress a thought, action or feeling
The neuroscientists next used those results to successfully predict the cognitive performance of 74 Canadian university football players.
The researchers did not request concussion history from the players before they completed the survey.
Like the general population, the football players performed well on 11 of 12 cognitive tasks, however a specific impairment of inhibition was also identified in all of the players.
“If you have an impairment of inhibitory control, it means that you are likely to carry on doing something when perhaps you should have stopped,” said Adrian Owen, a Schulich School of Medicine & Dentistry professor.
“For example, running an amber light when it may have been safer to stop. On the field, it might mean a player would continue with a tackle long after they’ve heard the whistle to stop.”
Stafford, Owen and their collaborators from Western’s Brain and Mind Institute and the Department of Anatomy and Cell Biology were originally interested to know whether concussions in the general population led to long-term cognitive or brain deficits. So far, the study has proven largely positive in that area.
“Although we did see a lower performance on the task of inhibition, we did see very typical performances on tasks of memory and deductive reasoning-positive news for people who have sustained a concussion that they don’t have devastating effects on their cognition over the long term,” Stafford said.
The findings are timely as families and organizations are starting to evaluate the risks of contact sports for youth.
For example, some soccer organizations have banned heading and some hockey leagues have banned checking for athletes under a certain age.
“These are the sorts of moves we’re starting to see. Science is gradually trickling down to affect policy. That’s a good thing,” Owen said.
Stafford agreed, “Rowan’s Law is a great example of science being translated into a form of policy. Our results demonstrate the importance of recognizing that sports at a very high level come with benefits and some costs.”
Passed in 2018, Rowan’s Law requires all coaches and team trainers to review their sport organization’s concussion code of conduct each year before the start of the season. Those codes set out expectations and rules of behavior to minimize concussions while playing sport.
While this study shows there are risks associated with some contact sports, it doesn’t mean people should stop participating, Owen noted.
“Like everything, there are risks and benefits,” he said. “Playing sports keeps you fit and is a social activity. There are many good things about it.
But it also comes with risks. Highlighting those risks helps people to mitigate against their affects.”
Owen cannot say that playing contact sports causes these deficits—but the indications lead towards that conclusion.
“We compared these athletes very carefully with groups of athletes involved in other sports that don’t involve head impacts. Those groups weren’t impaired in the same way,” he said. “It’s extremely likely that many seasons of football is what’s caused the deficit we observed.”
Sport-related concussion, a form of mild traumatic brain injury (mTBI) induced by biomechanical forces, is a clinical diagnosis of abnormal brain function based on the presence of signs and symptoms without neuroimaging evidence of structural injury (1).
The neurometabolic cascade associated with mTBI has been well-described from two decades of research (2), but neither this work nor more recent investigations employing electroencephalography (EEG) in humans have yet produced an objective measure that can confirm or refute the concussion diagnosis (3).
The lack of a physiological definition of concussion to guide diagnostic and prognostic criteria has also contributed to growing concern for contact sport athletes who are likely to sustain repeated head impacts capable of producing brain injury, but who have neither symptoms nor professional evaluation.
Water polo is one such contact sport that carries a risk of head and face injury in international competition (4, 5). Recent survey data reveal that 36% of USA Water Polo members report sustaining at least one concussion during their playing tenure (6), a lifetime incidence that is comparable to that observed in soccer (7).
However, the respondents also reported sustaining an average of two asymptomatic head impacts during a typical practice or game, a rate of exposure consistent with prospective data from in vivo monitoring over three competitive seasons (8).
In some contact sports (e.g., American football, soccer, hockey) the accumulation of these impacts is believed to contribute to clinically significant neurological dysfunction years after exposure has ceased (9, 10).
Though these injuries appear too subtle to be detected by cognitive testing after a single season of exposure, they become apparent when relating cognitive performance to neurobiological measures of injury and objective measures of head impact exposure (11).
Brain function arises from activity-based coupling across distributed neural networks that represent the brain’s hierarchical (i.e., small-world) organization (12, 13), a balance between segregated and integrated information processing (14–16). Brain functional networks are altered after a concussion (17–19), an effect that can persist even after symptoms have abated (20).
A few studies have observed similar changes after a single season of head impact exposure in football and rugby players, but these studies stratified their sample using a controversial method (21) of assessing injury thresholds based on head kinematic measures, (22) did not analyze individual differences in exposure, (23) and/or did not report objective measures of head impact exposure (24). These studies also used functional MRI methods that, despite offering useful insight into the pathophysiology of brain injury, are difficult to implement in prospective research designs and are not readily accessible to athletic training staff or even clinicians.
EEG represents a low-cost imaging method capable of measuring functional connectivity (FC) at fast time-scales not easily captured by MRI. Focal neural activity is governed by high-frequency oscillations (>20 Hz), whereas long-range, polysynaptic synchronization is instantiated in correlated slow-frequency oscillations (<7 Hz) (25, 26). Accordingly, fast-rhythm networks tend to be sparser and more clustered, and slow-rhythm networks tend to be denser with more synchronous activity (27).
Affective and cognitive dysfunction across a range of neurological disorders has been attributed to disrupted brain network organization arising from aberrant synchronization in thalamocortical circuits (28, 29). Graph theoretic measures have gained popularity as a means to summarize quantitatively these organizational properties (i.e., density, clustering, efficiency) of large-scale brain networks, in health and disease (30, 31).
Several studies have used EEG to examine functional network properties in athletes after mTBI. Teel and colleagues observed increased slow-rhythm synchrony in recovered (i.e., asymptomatic) athletes post-concussion relative to healthy, non-concussed control athletes, but did not use graph measures to support their inferences about the meaning of these patterns for brain network organization (32).
However, this pattern is consistent with reports of hyper-synchrony after mTBI using magnetoencephalography (33, 34) and is supported a recent review of 126 neuroimaging studies that concluded increased FC is a fundamental response to brain injury (35).
In contrast, Cao and Slobounov observed that athletes diagnosed with a concussion exhibited decreased long-range connectivity, and increased local connectivity, seven days post-injury relative to non-injured athletes (36). Using graph theoretical measures, the authors interpreted these changes as a loss of network small-worldness and a shift toward network “randomness.”
However, the control athletes in this study were all engaged in contact and collision sports (football, rugby, hockey) and thus had likely been exposed to repeated, asymptomatic head impacts, potentially confounding the interpretation of these group differences. Additionally, graph measures are frequency band-specific and can be influenced by the amplitude- and phase-dependence of the connectivity measures (37, 38). Neither were accounted for in this study.
To better understand the effects of repeated head impact exposure on brain FC, we monitored intercollegiate water polo athletes for head-impact frequency and magnitude during a season of competition. Specifically, we sought to test a dose relationship between head-impact exposure and changes in EEG-derived, whole-brain FC and small-world network characteristics.
To provide context for the potential clinical significance of these effects, we used a multivariate modeling technique to characterize the relationship between changes in spontaneous brain activity and performance on computerized tests of inhibitory control. Then, we determined whether head impact exposure contributed to changes in this brain-behavior relationship.
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