Portal de Periódicos da CAPES

It’s Not All Fun and Games: Sports, Concussions, and Neuroscience

2017; Cell Press; Volume: 94; Issue: 6 Linguagem: Inglês

10.1016/j.neuron.2017.05.003

1097-4199

Christopher C. Giza, Mayumi L. Prins, David A. Hovda,

Trauma and Emergency Care Studies

Few items grab the public's attention like sports, from extremes of great victory to injury and defeat. No injury currently arouses stronger interest than concussion. Giza et al., discuss how neuroscience can provide balance between physical activity and TBI, and guide thoughtful discourse and policy. Few items grab the public's attention like sports, from extremes of great victory to injury and defeat. No injury currently arouses stronger interest than concussion. Giza et al., discuss how neuroscience can provide balance between physical activity and TBI, and guide thoughtful discourse and policy. Headers in soccer, helmet-to-helmet hits in American football, fighting in ice hockey, intense combat in mixed martial arts—these potential mechanisms of brain injury are played out daily in sports and recreational activities all over the world. Traumatic brain injury (TBI), not long ago called the "silent epidemic," is increasingly in the public eye, playing in venues from local youth sports events to professional sports world championships to governmental hearings. The World Health Organization and others report that TBI is becoming one of the top global causes of acquired disability and represents a major public health issue. While impairments due to concussion or mild TBI are often not readily visible to an outside observer, the sheer number of those affected (80%–90% of all TBI are mTBI or concussion) as well as the vast number of individuals, including children, potentially exposed to repeated head impacts (over 250 million participants worldwide in soccer alone) warrant the increasing attention being brought to this arena. Here we will discuss how the medical concern over head injuries intersects with the world of neuroscience research. The field of sports neurotrauma has been hampered by qualitative definitions, oversimplification of injury and outcomes, pre-existing bias among stakeholders, and a reliance on retrospective observational studies. Nonetheless, innovative new technologies, when rigorously coupled with clinical correlates and biological mechanisms, offer tremendous opportunity for future advances. Bringing more rigorous experimental designs that include proper controls and prospective longitudinal follow-up, and incorporating preclinical translational studies, will more definitively prove or disprove our prior suppositions and lead to better, fact-based practice guidelines. Traumatic brain injury starts as biomechanical force transmitted to a soft, rubbery, wrinkled organ that constitutes 2% of the body's weight and hogs 20% of the body's energy. These forces disrupt cell membranes and damage intracellular scaffolding, releasing ionic floods and indiscriminate waves of neurotransmitters and triggering a cellular energy crisis. Axons undulate, synapses retract, and the cerebral internet disconnects, at least temporarily. Blood vessels become leaky and inflammation sets in (Giza and Hovda, 2014Giza C.C. Hovda D.A. Neurosurgery. 2014; 75: S24-S33Crossref PubMed Scopus (709) Google Scholar). Concurrently, cellular metabolism kicks into overdrive to help restore homeostasis, followed by a period of diminished neural activity and cerebral blood flow. It is likely that our brains evolved in settings where mild TBI (mTBI)/concussion events are quite common, and accordingly, various biological mechanisms work to restore things to normalcy after such injuries, although it may take hours, days, or even weeks for complete biological recovery. Furthermore, in the setting of greater injury severity, and quite likely also in settings of closely spaced repeated mTBI, biological homeostatic mechanisms may falter, resulting in more persistent functional impairments, lasting changes in connectivity, and even cell death (Prins et al., 2013Prins M.L. Alexander D. Giza C.C. Hovda D.A. J. Neurotrauma. 2013; 30: 30-38Crossref PubMed Scopus (236) Google Scholar). While reporting the neurobiology of concussion can seem rather dry, reporting bad behavior, violence, and death are riveting to the public's attention. In this increasingly shrill environment, how then do scientists and clinicians weigh in on the dialog about TBI and sports? To uncover the truth, one must first step back from any dogma, anecdote, intuition, or opinion. Four major areas of the sports concussion problem that would benefit enormously from the objective application of scientific principles include the following: (1) definition and distinctions of acute and chronic syndromes, (2) improved diagnostics, (3) evidence-based intervention and treatment, and (4) better delineating the beneficial biological effects of activity and sports participation. Not every acute blow to the head is a concussion. Diffuse cerebral edema with or without intracranial hemorrhage, sometimes termed "second impact syndrome," is a neurological catastrophe that has high mortality and morbidity. In 2006, Zackery Lystedt, a high school football player in Washington state, took a hit not unlike hundreds of player-to-player collisions that occur in contact sports every day. But after this, he experienced severe brain swelling that required neurosurgery and neurocritical care. His story and others have been motivating forces to the 50 state concussion laws in the U.S., starting with the Lystedt Law in Washington in 2009. These laws generally require (1) concussion/TBI education, (2) removal from play of athletes suspected of concussion/TBI, and (3) evaluation by a trained medical provider prior to return to sport. Similar legal and regulatory steps have started to spread internationally, with the recent passage of Rowan's Law in Canada and efforts to increase sports TBI education in the United Kingdom and Ireland. The terms mild, moderate, and severe TBI have historically been based on the Glasgow Coma Score. So in this nomenclature, mTBI refers to the mild end of the TBI spectrum, which may include concussion but may also include small intracranial hematomas or skull fractures. Concussion is a syndrome of neurological and pathophysiological dysfunction induced by biomechanical force. Symptoms are typically maximal early on and improve over time, a clinical characteristic mostly overlooked in the media. While mTBI and concussion are often used synonymously, concussion may be considered a clinical syndrome included in the category of mTBI. The term subconcussion is currently used to represent a presumably asymptomatic injury that occurs with repeated impacts but may have cumulative consequences. Its existence is supported by studies using highly sensitive advanced imaging, biochemical, or electrophysiological techniques showing evidence of microstructural, molecular, or functional changes in individuals exposed to repeated impacts but without a clinical diagnosis of concussion. In the media and the medical literature, this terminology is often confused. The clinical neurosciences must not only better define these acute syndromes, but then must carefully distinguish them when relaying information to peers and to the public. While it may seem beneficial to one's concussion grant to lump in numbers of thousands of annual severe TBI deaths, this only serves to obfuscate the real and important distinctions between different types and severity of TBI. These distinctions are almost certainly important for pathobiology, treatment, and outcome, so studies not taking this into consideration will continue to struggle to find meaningful effects and interventions that can successfully translate to improved human outcomes. Just as not every acute head impact results in a concussion, not every post-concussion symptom is a sign of dementia. For the vast majority of individuals experiencing a concussion, symptoms improve with time. In clinical studies, many of the risk factors for prolonged symptoms are related to comorbid conditions, like migraine, anxiety, depression, prior mTBI, or learning disabilities. Repetitive mTBI with a short impact interval is associated with more pronounced acute physiological changes in both animal models and human studies (Prins et al., 2013Prins M.L. Alexander D. Giza C.C. Hovda D.A. J. Neurotrauma. 2013; 30: 30-38Crossref PubMed Scopus (236) Google Scholar, Vagnozzi et al., 2008Vagnozzi R. Signoretti S. Tavazzi B. Floris R. Ludovici A. Marziali S. Tarascio G. Amorini A.M. Di Pietro V. Delfini R. Lazzarino G. Neurosurgery. 2008; 62: 1286-1296Crossref PubMed Scopus (276) Google Scholar). It is reasonably postulated that repetitive impacts with incomplete recovery could set the stage for chronic degenerative changes; however, the lay press is far ahead of the actual science here, where plausible mechanistic hypotheses remain to be proven or refuted. Clinical studies in cohorts of professional athletes have uncovered chronic neurocognitive impairment as measured by objective neuropsychological testing, and this impairment appears consistently related to the amount of impact exposure, suggesting a causal relationship. However, none of these clinical studies are longitudinal, and none have resulted in pathological or radiographic confirmation of neurodegeneration. As if that weren't complicated enough, more severe TBI in general is recognized as a likely environmental risk factor for some dementias, including associations with Alzheimer's disease, Parkinson's, and possibly CTE. Again, both basic and clinical neurosciences can lead the way to a clearer understanding of the sequelae of mild TBI or concussion by first better defining the outcomes, which then naturally lead to better understanding of the biological mechanisms underlying these outcomes. Only then can we proceed to develop sensible interventions to improve long-term consequences. If repetitive impacts result in metabolic vulnerability, then interventions can range from affecting the timing of return to contact risk, monitoring specific biological parameters, to more accurately delineate recovery or even providing alternative metabolic substrates or activating affected neurotransmitter systems to accelerate or enhance physiological recovery. Without better definitions, better diagnostics are not possible. However, with improved definitions, there are diagnostic elements where a science-based approach has great potential. Given the subjective nature of a concussion diagnosis and the range of clinical syndromes that can result after mTBI or concussion, it is clear that a single diagnostic is not likely to determine whether any brain injury occurred. It is important therefore to keep a broader perspective with emphasis on a diversity of diagnostic approaches. It's also imperative to rigorously test new tasks or devices for validity and reliability. Rushing technology to the mass market may be an effective business model, but in the world of concussion and TBI, inadequately tested declarations of efficacy can also cause harm and slow progress. Blood/fluid biomarkers are of great interest to deliver an objective measure for diagnosis of mTBI, in a setting where our most sensitive tools are inherently subjective symptom checklists, with their specificity dropping rapidly over time. Fluid-based biomarkers are already seeing some utility in acute care settings to determine individuals for whom diagnostic neuroimaging should be performed. One clinical scenario where these advances would be immediately applicable include improving on clinical decision rules for imaging mTBI in the emergency department to better identify and care for patients with intracranial lesions that affect medical care. Another would be in the setting of abusive head trauma in infants, where the presentation is often with nonspecific symptoms and an unclear medical history, but the consequences of missing the diagnosis can be severe. A myriad of impact sensors are being marketed directly to parents, athletes, and even school districts. While an attractive idea, reams of existing data already demonstrate that there is no easily delineated "concussion threshold," and thus the idea of a stop/go signaling device for general use in contact sports is not supported by science (Guskiewicz and Mihalik, 2011Guskiewicz K.M. Mihalik J.P. Exerc. Sport Sci. Rev. 2011; 39: 4-11Crossref PubMed Scopus (183) Google Scholar). Now, despite the inability of a biomechanical sensor to trigger a stop/go decision, one huge advantage of biomechanical sensors is that they obtain objective, quantifiable data. And while the impacts they record don't often result in clinical signs and symptoms, impact sensors remain a very powerful tool for researchers. Validated impact sensors should be able to objectively quantify forces regardless of whether they generate reportable symptoms. These devices can, over time and in properly designed studies, help determine whether a particular pattern or location or timing of impacts is more likely to result in a clinical concussion. More importantly, such devices might inform us as to the risk of short- or longer-term sequelae from concussive impacts. A final consideration is that there is great need for sensors that may be used in non-helmeted sports, including mouthguards, retainers, or earpieces. Most athletes, particularly female athletes, participate in sports not requiring helmets. At the same time, there is growing awareness that females are at increased risk of concussion or mTBI (O'Connor et al., 2017O'Connor K.L. Baker M.M. Dalton S.L. Dompier T.P. Broglio S.P. Kerr Z.Y. J. Athl. Train. 2017; 52: 175-185Crossref PubMed Scopus (123) Google Scholar), making it all the more important that we be able to measure and record these impacts. Advanced imaging is both a useful and problematic addition to the sports concussion picture. Currently expensive and immovable, these technologies provide exquisite detail and ever-increasing resolution to detect changes in brain. In fact, almost every study using diffusion tensor imaging or functional MRI detects significant differences from controls, whether the affected group is those with concussion/mTBI or attention problems or mood problems or pain. Furthermore, a particular signal may have different meanings in different contexts—type or severity of injury, age of subject, or time after injury may all affect the biological interpretation of a given imaging signal (Budde et al., 2011Budde M.D. Janes L. Gold E. Turtzo L.C. Frank J.A. Brain. 2011; 134: 2248-2260Crossref PubMed Scopus (289) Google Scholar). The challenge with these technologies, then, is to understand the biological meaning of the signals being detected, and with that, the clinical or functional relevance (Vagnozzi et al., 2008Vagnozzi R. Signoretti S. Tavazzi B. Floris R. Ludovici A. Marziali S. Tarascio G. Amorini A.M. Di Pietro V. Delfini R. Lazzarino G. Neurosurgery. 2008; 62: 1286-1296Crossref PubMed Scopus (276) Google Scholar, Kamins et al., 2017Kamins J. Bigler E. Covassin T. Henry L. Kemp S. Leddy J.J. Mayer A. McCrea M. Prins M. Schneider K.J. et al.Br. J. Sports Med. 2017; (Published online April 28, 2017)https://doi.org/10.1136/bjsports-2016-097464Crossref PubMed Scopus (189) Google Scholar). With great promise comes great responsibility—rushing advanced imaging into clinical service, where sample size is an n of 1 and the interpretation of the signal may be different based on many individual factors, risks a patient interpreting they have untreatable brain damage when that may be far from the case. Not only does neurotechnology offer opportunities for diagnosis of acute mTBI, but it may hold the key to monitoring recovery and risk for chronic problems. This is an area of substantial promise, but as yet with little data. There are reports of blood or fluid or genetic markers that predict worse acute outcomes, like acute cerebral swelling (CACNA1A) or chronic neurocognitive impairments after repeated impacts (APOE4). With time, will these become clinically useful? Electrophysiological measures have long been used both acutely and chronically after more severe TBI, and many efforts are now underway to translate these modalities to mTBI/concussion. In many cases, the physiological function of the signals detected remains unclear. Another area of promise includes ultrasound, where portable, noninvasive measurements of cerebral blood flow or cerebrovascular reactivity are showing changes after concussion and also after intervention, in small studies (Kamins et al., 2017Kamins J. Bigler E. Covassin T. Henry L. Kemp S. Leddy J.J. Mayer A. McCrea M. Prins M. Schneider K.J. et al.Br. J. Sports Med. 2017; (Published online April 28, 2017)https://doi.org/10.1136/bjsports-2016-097464Crossref PubMed Scopus (189) Google Scholar). Having objective predictors of outcome may signal a time period for safer return to contact sports, suggest mechanism-specific therapeutics or even affect personal choices with regards to participation in higher risk activities. While major challenges remain in management of post-concussion vulnerability, progress in this area is already seen, and it's one of the areas where basic science has informed clinical work in important ways. For instance, the basic science finding of neurometabolic dysfunction underlying a period of vulnerability to repeated concussion has been linked to clinical vulnerability in multiple studies. Clinical research showed increased risk for a second concussion during the first 10 days following the first concussion; more recently, clinical studies showed longer recovery in athletes who "played through" their concussions compared to those who were immediately removed from play. Metabolic and cerebrovascular dysfunction appear to occur in both animals and humans during this time window after concussion (Prins et al., 2013Prins M.L. Alexander D. Giza C.C. Hovda D.A. J. Neurotrauma. 2013; 30: 30-38Crossref PubMed Scopus (236) Google Scholar, Vagnozzi et al., 2008Vagnozzi R. Signoretti S. Tavazzi B. Floris R. Ludovici A. Marziali S. Tarascio G. Amorini A.M. Di Pietro V. Delfini R. Lazzarino G. Neurosurgery. 2008; 62: 1286-1296Crossref PubMed Scopus (276) Google Scholar), but the direct linkages between these measures of physiological dysfunction and cerebral vulnerability are still lacking and should be high-priority targets. Similar findings and loose associations exist for cytoskeletal disruption, deposition of damaged proteins (tau, amyloid, a-synuclein) and inflammation. The possibility that this acute vulnerability not only translates into worse/prolonged acute dysfunction but also initiates biological processes that lead to more permanent change or neurodegeneration is ominous (Giza and Hovda, 2014Giza C.C. Hovda D.A. Neurosurgery. 2014; 75: S24-S33Crossref PubMed Scopus (709) Google Scholar). Clinical studies to untangle these relationships prospectively will take years or decades, but work in animal models can greatly accelerate our understanding of these linkages and may be important to target the most promising points for intervention. Further investigation can better delineate this vulnerable period; for now, "when in doubt, sit it out" is the clinical mantra, but future studies will also permit us to answer "…but for how long"? The mainstay of concussion clinical management has been rest. This evolved from two concepts. First, protecting the brain from repeat injury during the vulnerable window—as already discussed, there is considerable evidence for this in both basic and clinical concussion/TBI research. The second concept is more challenging, namely the idea that the injured brain needs to be protected from any neural activity or stimulation. For this, the data are more complex. In non-TBI focal lesion models that result in limb weakness, forced overuse of the affected limb can result in a larger brain lesion, but with a nuance—enlargement of the brain lesion only occurred with forced overuse in the acute ( 7 days post-injury) voluntary wheel running enhances neurotrophins and other molecular markers of plasticity and results in improved cognition. This effect also appears to partially be dependent upon the severity of the initial injury. Interestingly, forced exercise results in a more sustained stress response, which may counteract the beneficial effects of post-TBI activity (Griesbach et al., 2011Griesbach G.S. Hovda D.A. Tio D.L. Taylor A.N. Neuroscience. 2011; 178: 147-158Crossref PubMed Scopus (80) Google Scholar). The pre-injury status of the individual also weighs heavily here; in a study where animals were exercising regularly prior to concussive injury, the greater the delay in returning to physical activity post-injury, the greater the deficit (Mychasiuk et al., 2016Mychasiuk R. Hehar H. Ma I. Candy S. Esser M.J. Eur. J. Neurosci. 2016; 44: 2407-2417Crossref PubMed Scopus (45) Google Scholar). Clinically, the original mandate to "sit it out" resulted in some practitioners instituting not only restrictions on contact sports participation (which is supported by evidence) but also prohibitions on non-contact physical activity, cognitive activity, and even use of electronics with screens (for which scientific evidence is lacking). This has been dubbed "cocoon therapy." Subsequent clinic studies have not supported severe restrictions on non-contact risk activity after concussion. In fact, prolonged inactivity certainly diminishes physical conditioning and function, likely interferes with mechanisms of repair and plasticity, and may result in worsened neurobehavioral status (i.e., lack of social interactions with teammates, anxiety, and depression). There is limited but promising clinical research suggesting that graded aerobic exercise may promote recovery in patients with post-concussion syndrome, further arguing against cocoon therapy and prompting more investigation to determine the "sweet spot" in concussion recovery for returning an individual to both cognitive and physical activity. No one ever got a concussion from doing homework. There are multiple other areas where basic neuroscience can lead the way for translational and clinical work in concussions. Metabolic perturbation is a hallmark of concussive injury, and while physiological recovery generally occurs fairly rapidly, laboratory studies suggest that provision of alternative energy sources such as ketones, fatty acids, monocarboxylates, and others may reduce injury or shorten duration of dysfunction. Synaptic transmission is affected following concussion/mTBI. Unlike severe TBI, where destruction of large portions of tissue may render synaptic reconnection impossible, synaptic dysfunction and neural inactivation after mTBI may be mitigated through judiciously timed use of activating pharmacotherapies. Inflammatory cascades are activated after TBI of all severities, and balancing the potential beneficial versus detrimental effects of modulating inflammatory systems is ripe for basic and translational investigation. The physiological effects of sleep and sleep-dependent activation of the glymphatic system in promoting recovery after brain injuries are just starting to be investigated. As happens for any common condition with limited proven interventions, many have jumped into the fray with their one-size-fits-all therapy for concussions, often held only to the standard that the treatment must have some consumer appeal. This is an area for great caution, as financial, media-related, reputational, and even scientific conflicts of interest can emerge overtly or insidiously. While new ideas are always welcome to push forward the frontiers of knowledge, hidden biases and agendas at best delay the quest for the truth, and at worst reverse the progress of years of painstaking careful science in an effort to find a quick fix, a simple answer (or make a quick buck). As in all rigorous science, anecdote and testimonials should be accorded the weight they deserve, and the hard work of having appropriate controls, randomizing, blinding, and objective outcome measures should not give way to short cuts, sloppy research, and inappropriate conclusions. Impacting one's brain repeatedly is not a good thing. However, equating sports participation with smoking or calling for a ban on all contact sports oversimplifies a complex equation. Similar or maybe greater risks accrue to the inactive brain, or the brain in an inactive body. While basic and clinical neuroscience studies examining biological adaptations in response to physical activity have clearly shown benefit, how to quantitate these benefits in a setting of risk for concussive or other injury is unclear. Another important area for investigation relates to the specific role of such physical activity after injury—what type, intensity, and timing of activity will result in maximal benefit? Where is the physiological "sweet spot" after concussion where increased demands of physical or mental exertion are associated with lasting functional improvement? For sports involving contact risk, what is the minimum level of training (particularly during brain development) to develop the physical skills necessary to play at the next level, and how does this balance with the potential risk savings obtained by simply delaying the age at which contact play is permitted? It is also important to keep the risk of concussion in perspective with all the other arguably greater risks faced by adolescents in the U.S., including obesity, tobacco/drugs, alcohol, and sexually transmitted diseases (Johnson et al., 2014Johnson N.B. Hayes L.D. Brown K. Hoo E.C. Ethier K.A. Centers for Disease Control and Prevention (CDC)MMWR Suppl. 2014; 63: 3-27PubMed Google Scholar).