Over the last decade or so, we have seen an exponential rise in research and media attention pertaining to sports-related concussions (SRCs). And for good reason, as it is estimated that upwards of 4 million SRCs occur each year in the United States alone. I’ve previously written about the newfound association between SRC and lower extremity injuries (https://rebelmovement.org/2018/11/15/sport-related-concussion-and-lower-extremity-injury/), but in this blog series I want to take a much deeper dive into SRCs themselves, particularly on the latest research and clinical practices. Fair warning: SRCs are a difficult injury to study and manage due to the heterogeneous nature of symptomology and recovery. Simply put, there are still a lot of unknowns, but I hope this blog post gives you an idea of our current knowledge base and where we are heading in the future. Feel free to reach out via Twitter (@JasonAvedesian) or email (firstname.lastname@example.org) in you want to talk more about SRCs!
SRC Injury Incidence
A good starting point is to discuss the incidence of SRCs. Contact-centric sports such as football, rugby, ice hockey, and soccer make up the majority of SRCs across all levels of competition. Recent reports suggest that SRCs account for 9.6% and 4.0% of total injuries in youth and high school football athletes, respectively.10 Overall, SRCs comprise approximately 6.2% of total injuries sustained in NCAA athletes,24 and certain sports such as basketball and lacrosse have seen the rates of SRCs nearly double compared to the previous 15 years.7 Athletes appear to be at the greatest risk for SRC during competitions, as recently it has been reported that male and female collegiate soccer athletes were at a 5.54 and 9.05 times greater risk for an in-game SRC versus one sustained in practice.24 Across 20 high school sports, investigators recently reported an incidence rate of 10.37 SRCs per 10,000 athletic exposures in competition versus a 2.04 SRCs per 10,000 athletic exposures during practice.18 Furthermore, over three-quarters of collegiate athletes report an SRC during the in-season sport phase, with the majority (61.4%) occurring in competition.7 It is speculated that a more aggressive playing style and a higher frequency of head impacts during games, compared to practice, may lead to this increased risk for SRC.22
Physiological Mechanisms of SRC
An SRC is typically viewed as a functional injury rather than an injury with both functional and structural damages. Once an athlete sustains an SRC, a cascade of neurometabolic events occur in an attempt to restore ionic balance within the injured brain.12 A release of glutamine and aspartate may lead to cell permeability alterations that damage and ultimately kill the cell.13 The aforementioned amino acids lead to potassium ions exiting the cell, while a sodium and calcium influx occurs, thereby changing cellular pH levels and causing the blood vessels to constrict.11 An “energy crisis” occurs as the brain requires increased glucose metabolism to restore membrane potential, all while being in a state of reduced cerebral blood.11 This mismatch in energy supply and demand is thought to express itself through acute psychological and motor behavior changes commonly seen in concussed athletes.11
SRCs general reflect a pathophysiological disturbance rather than an injury readily seen on standard neuroimaging measures. As such, traditional medical imaging techniques (e.g. CT scans) may not demonstrate the sensitivity to detect micro alterations following a concussive event. However, recent medical advances have allowed researchers to gain further insight into the subtle, yet lingering physiological alterations that occur following an SRC. These techniques include functional magnetic resonance imaging (fMRI), diffusion tensor imaging (DTI), transcranial magnetic stimulation (TMS), fluid biomarkers, and brain metabolites. During verbal and visual memory tasks, symptomatic and asymptomatic male athletes demonstrated significantly less activation of the dorsolateral prefrontal cortex,5,6 a brain region associated with working memory performance. More recently, previously concussed athletes have demonstrated increased activation of various brain regions (right/left dorsolateral prefrontal cortex and cerebellum) during spatial processing tasks versus controls, potentially indicative of cortical compensation to match performance of those without a concussive history.23 Even in the absence of a diagnosed SRC, researchers have seen abnormal white matter characteristics within the brain that is linked to worse memory performance following a single season of high school football.8 Utilizing DTI and magnetic resonance, female athletes who sustained an SRC displayed abnormalities within the primary motor cortex, white matter tract, and corticospinal tract compared to non-concussed athletes.4 Most concerning, imaging was performed in symptom-free athlete on average 18.9 months post-SRC, suggesting chronic impairments in decision-making and motor execution.4 Various TMS studies have also shown that post-concussive athletes demonstrate an elevated cortical silent period, a mechanisms of motor cortex inhibition. Prolonged cortical silent period has been shown both acutely (i.e. 8 weeks)20 and chronically (i.e. 19 months)9 following a concussive event.
Overall, it does appear that post-concussive athletes demonstrate physiological alterations beyond traditional clinical resolution (i.e. symptom free, return to baseline on cognitive / balance assessments). While the aforementioned techniques offer promise for SRC diagnosis and management, much remains in terms of clinical validation and establishing appropriate monitoring protocols. For example, there is a severe lack of longitudinal study within the imaging literature that currently limits our understanding of when(if?) physiological recovery occurs following an SRC.17
Head Impact Biomechanics and SRC Risk
To ascertain biomechanical mechanisms of SRC, researchers have conducted head impact studies in a variety of sporting populations. The proposed rationale for analyzing variables such as linear and rotational head acceleration during impact events is that “thresholds” for a concussive event can be determined, along with the analysis of total “subconcussive” impacts that an athlete may sustain over the course of a practice, game, and/or season. We know with great certainty that many SRCs go unreported, therefore, measuring head accelerations in real-time may provide us with an objective tool to immediately pull an athlete from the field and conduct a clinical evaluation if an SRC is suspected. There are a few different impact sensor systems, including (1) multi-sensor units placed in the helmet; (2) a single sensor attached to the skin (forehead or neck); (3) sensor-equipped mouthguards. While these sound like a great devices to add to the SRC assessment toolbox, there are some significant limitations that I will address at the end of this section.
Unsurprisingly, football and soccer athletes sustain thousands of head impacts (defined as greater than 10g) each season.15,22 Most head impacts in collegiate athletes range between 20–30g,14 although significantly greater head accelerations, at a more frequent rate, are sustained in competition versus practice.22 While impacts located on the front, side, and top of the head are associated with higher risk for SRC,2 the frequency and location in which an athlete receives a head impact may be affected by their visual and sensory performance.16
There does appear to be a negative cumulative effect of head impacts in the absence of a diagnosed SRC. In non-concussed high school football athletes, those with demonstrated cognitive performance declines sustained a significantly greater amount of median head impacts versus counterparts without change in cognitive performance (1103 versus 438 head impacts).1 Researchers have also demonstrated a dose-response relationship between cumulative head impacts and cognitive impairments following the completion of an athletic career. In a cohort of former high school and collegiate football players, 1,800–2,400 cumulative head impacts were found to be significant thresholds for risk of developing depression, with an additional 2,800 impacts associated with a 2x risk for later life neurological consequences such as white matter brain changes.21
There are some strong limitations with the current head impact technology. First, helmet impact sensor systems assume that the helmet and skull move as a single body, therefore, improper helmet fit may overestimate true acceleratory values. Presently, there is limited data on the accuracy of single sensor systems; it’s unclear if these sensors can differentiate between received head impacts versus those due to purposeful neck motion. The biggest issue surrounding impact sensor technology is that researchers have been unable to determine a distinct head impact threshold leading to an SRC, as athletes may be concussed following a wide range of recorded head accelerations during sport.3 For example, Mihalik et al. (2017) found that “diagnosed concussion impacts ranged from 40.3g to 173.22g in linear acceleration and 163.35 to 15393.07 rad/s2 in rotational acceleration” while “noninjury impacts ranged from 10.00g to 350.00g in linear acceleration and 0.15 to 30,601.02 rad/s2.19 Simply put, there are many influential risk factors that these impact devices cannot account for, including head impact / SRC history, gender, and anthropometric measures.
As you can see, there is a lot to unpack with this complex injury. Hopefully this gives you a better understanding of SRC from an incidence and physiological / biomechanical perspective. This wraps up Part 1 in our SRC deep dive. Stay tuned for more coming soon!
- Breedlove EL, Robinson M, Talavage TM, et al. Biomechanical correlates of symptomatic and asymptomatic neurophysiological impairment in high school football. Journal of Biomechanics. 2012;45(7):1265-1272.
- Broglio SP, Schnebel B, Sosnoff JJ, et al. Biomechanical properties of concussions in high school football. Med Sci Sports Exerc. 2010;42(11):2064-2071.
- Brolinson PG, Manoogian S, McNeely D, Goforth M, Greenwald R, Duma S. Analysis of linear head accelerations from collegiate football impacts: Current Sports Medicine Reports. 2006;5(1):23-28.
- Chamard E, Lassonde M, Henry L, et al. Neurometabolic and microstructural alterations following a sports-related concussion in female athletes. Brain Injury. 2013;27(9):1038-1046.
- Chen J, Johnston KM, Collie A, McCrory P, Ptito A. A validation of the post-concussion symptom scale in the assessment of complex concussion using cognitive testing and functional MRI. J Neurol Neurosurg Psychiatry. 2007;78(11):1231-1238..
- Chen J-K, Johnston KM, Frey S, Petrides M, Worsley K, Ptito A. Functional abnormalities in symptomatic concussed athletes: an fMRI study. Neuroimage. 2004;22(1):68-82.
- Covassin T, Moran R, Elbin RJ. Sex differences in reported concussion injury rates and time loss from participation: An update of the National Collegiate Athletic Association Injury Surveillance Program from 2004–2005 through 2008–2009. Journal of Athletic Training. 2016;51(3):189-194.
- Davenport EM, Whitlow CT, Urban JE, et al. Abnormal white matter integrity related to head impact exposure in a season of high school varsity football. Journal of Neurotrauma. 2014;31(19):1617-1624.
- De Beaumont L, Mongeon D, Tremblay S, et al. Persistent motor system abnormalities in formerly concussed athletes. Journal of Athletic Training. 2011;46(3):234-240..
- Dompier TP, Kerr ZY, Marshall SW, et al. Incidence of concussion during practice and games in youth, high school, and collegiate American football players. JAMA Pediatr. 2015;169(7):659.
- Giza CC, Hovda DA. The new neurometabolic cascade of concussion: Neurosurgery. 2014;75:S24-S33.
- Giza CC, Kutcher JS. An introduction to sports concussions: Lifelong Learning in Neurology. 2014;20:1545-1551.
- Grady MF. Concussion in the Adolescent Athlete. Current Problems in Pediatric and Adolescent Health Care. 2010;40(7):154-169.
- Guskiewicz KM, Mihalik JP. Biomechanics of sport concussion: Quest for the elusive injury threshold. 2011;39(1):9.
- Gysland SM, Mihalik JP, Register-Mihalik JK, Trulock SC, Shields EW, Guskiewicz KM. The relationship between subconcussive impacts and concussion history on clinical measures of neurologic function in collegiate football players. Ann Biomed Eng. 2012;40(1):14-22.
- Harpham JA, Mihalik JP, Littleton AC, Frank BS, Guskiewicz KM. The effect of visual and sensory performance on head impact biomechanics in college football players. Ann Biomed Eng. 2014;42(1):1-10.
- Kamins J, Bigler E, Covassin T, et al. What is the physiological time to recovery after concussion? A systematic review. Br J Sports Med. 2017;51(12):935-940.
- Kerr ZY, Chandran A, Nedimyer AK, Arakkal A, Pierpoint LA, Zuckerman SL. Concussion incidence and trends in 20 high school sports. Pediatrics. 2019;144(5):e20192180.
- Mihalik JP, Lynall RC, Wasserman EB, Guskiewicz KM, Marshall SW. Evaluating the “threshold theory”: Can head impact indicators help? Med Sci Sports Exerc. 2017;49(2):247-253..
- Miller NR, Yasen AL, Maynard LF, Chou L-S, Howell DR, Christie AD. Acute and longitudinal changes in motor cortex function following mild traumatic brain injury. Brain Injury. 2014;28(10):1270-1276.
- Montenigro PH, Alosco ML, Martin BM, et al. Cumulative head impact exposure predicts later-life depression, apathy, executive dysfunction, and cognitive impairment in former high school and college football players. Journal of Neurotrauma. 2017;34(2):328-340.
- Reynolds BB, Patrie J, Henry EJ, et al. Effects of sex and event type on head impact in collegiate soccer. Orthopaedic Journal of Sports Medicine. 2017;5(4):232596711770170.
- Slobounov SM, Zhang K, Pennell D, Ray W, Johnson B, Sebastianelli W. Functional abnormalities in normally appearing athletes following mild traumatic brain injury: a functional MRI study. Exp Brain Res. 2010;202(2):341-354..
- Zuckerman SL, Kerr ZY, Yengo-Kahn A, Wasserman E, Covassin T, Solomon GS. Epidemiology of sports-related concussion in NCAA athletes from 2009-2010 to 2013-2014: Incidence, recurrence, and mechanisms. Am J Sports Med. 2015;43(11):2654-2662.