Subconcussive Blows Research Articles

A - 33 Examining Neurocognitive Effects of Contact Exposure in High School Athletes

Abstract Purpose Concerns have been raised about the effect of repetitive head impacts and subconcussive blows in contact sports, though studies have largely focused on the effect in late-life or retired professional athletes. We examined the effect of extended participation (months-to-years) in low/non-contact vs high contact/collision sports within a sample of high school athletes on the Immediate Post-Concussion Assessment and Cognitive Test (ImPACT). Method A multisport sample of 1650 athletes (34% Female; 1088 contact, 528 non-contact; age 14–18) without a history of concussion completed baseline ImPACT testing across two times points (time between administrations ranging from 7–45 months; M = 20.40), with active sport participation between administrations. Results Repeated mixed-measure ANOVA revealed increased performance across ImPACT administrations for all composites (p < 0.001; partial Œ∑2 = 0.02–0.23). With the exception of a very small effect on Impulse Control composites (partial Œ∑2 = 0.003), there was no statistically significant difference between high contact and low contact sport groups across composite scores (p < 0.05). Conclusions Our findings showed no meaningful changes in neurocognitive performance in high school athletes exposed to high degrees of contact sports for months/years, with both contact groups demonstrating a consistent mild–moderate practice effect over time. This also aligns with the recent systematic literature review finding no increased risk of neurological/neurocognitive disease in amateur contact athletes in later life (Iverson et al, 2023). Future prospective studies with longer follow up and incorporation of additional measures (e.g., mental health, expanded testing, and biomarkers) will be important to improve our understanding of the risk of contact exposure in athletes.

Neuroimaging, wearable sensors, and blood-based biomarkers reveal hyperacute changes in the brain after sub-concussive impacts

Impacts in mixed martial arts (MMA) have been studied mainly in regard to the long-term effects of concussions. However, repetitive sub-concussive head impacts at the hyperacute phase (minutes after impact), are not understood. The head experiences rapid acceleration similar to a concussion, but without clinical symptoms. We utilize portable neuroimaging technology – transcranial Doppler (TCD) ultrasound and functional near infrared spectroscopy (fNIRS) – to estimate the extent of pre- and post-differences following contact and non-contact sparring sessions in nine MMA athletes. In addition, the extent of changes in neurofilament light (NfL) protein biomarker concentrations, and neurocognitive/balance parameters were determined following impacts. Athletes were instrumented with sensor-based mouth guards to record head kinematics. TCD and fNIRS results demonstrated significantly increased blood flow velocity (p = 0.01) as well as prefrontal (p = 0.01) and motor cortex (p = 0.04) oxygenation, only following the contact sparring sessions. This increase after contact was correlated with the cumulative angular acceleration experienced during impacts (p = 0.01). In addition, the NfL biomarker demonstrated positive correlations with angular acceleration (p = 0.03), and maximum principal and fiber strain (p = 0.01). On average athletes experienced 23.9 ± 2.9 g peak linear acceleration, 10.29 ± 1.1 rad/s peak angular velocity, and 1,502.3 ± 532.3 rad/s2 angular acceleration. Balance parameters were significantly increased following contact sparring for medial-lateral (ML) center of mass (COM) sway, and ML ankle angle (p = 0.01), illustrating worsened balance. These combined results reveal significant changes in brain hemodynamics and neurophysiological parameters that occur immediately after sub-concussive impacts and suggest that the physical impact to the head plays an important role in these changes. Statement of significance: Brain injuries sustained during sport participation have received much attention since it is a common occurrence among participants. Although protective technologies have been developed over the years, the mechanism of injury is still unclear. There is less focus on the repetitive exposure to sub-concussive impacts on the functional integrity of the brain. Sub-concussive impacts are defined as a lesser impact force resulting in acceleration of the head without symptoms of concussion. Diminished neurocognitive performance has been associated with increased sparring exposure in amateur MMA/boxers suggesting that repeated sub-concussive blows may be just as harmful. However, no one has studied the potential effect of repeated sub-concussive head impacts at the hyperacute level defined as within minutes after impact. We apply novel mobile sensing tools such as head impact sensors and portable neuroimaging devices that allow us to examine possible physiological effects taking place within minutes of sub-concussive impacts which are generally transient, and have not been captured before due to limitations with clinical imaging. Based on previous studies, we developed a protocol to test real-world sub-concussive head impact effects on cerebral blood flow and activation patterns and demonstrate that significant changes can be observed immediately after impacts occur, which could lead to improved monitoring and management of injury risk in sport participation.

Years of play alter MRI measures of brain health in former Canadian Football League athletes: a pilot study

Introduction: Concussive and sub-concussive blows are commonly sustained during contact sports. Through a detailed neuroimaging analysis, this pilot study aimed to determine if a history of sport-related concussions exacerbated cognitive decline later in life. It was hypothesized that clinical health assessments and magnetic resonance imaging (MRI) techniques would provide insight into lasting health and well-being, structural, microstructural, and functional alterations caused by a history of concussive injuries. Materials and Methods: Twenty aging, retired Canadian Football League (rCFL) players (aged 56.9 ± 6.9) had clinical testing and MRI data acquired. Cortical thickness, voxel-wise diffusion tensor imaging (DTI), and Default Mode Network (DMN) connectivity data was collected for each subject and compared against healthy controls. Retired athlete age, playing position, and career length were also examined. Results: This study found widespread cortical thinning, significantly increased mean diffusivity, increased axial diffusivity, and both hyperactivity and hypoactivity within the DMN. Athlete age, position, and career length all influenced microstructural integrity. On average, retired athletes scored about 4 times greater depression-like and concussion-related symptoms and scored significantly lower in all health categories compared to healthy controls. Conclusions: These findings suggested that lasting signs of neurological injuries were present years after retiring from professional play.

A-122 Verbal Fluency Switching and Clustering Performance in Retired Professional Football Players

Abstract Introduction Repeated sports-related concussion has been associated with cognitive deficits, like other forms of traumatic brain injury. Football speed players (FSP; e.g., quarterbacks) are at greater risk of cognitive impairment compared to football non-speed players (FNP). Verbal fluency is typically comprised of two tasks: letter fluency (LF) and semantic fluency (SF). Verbal clustering (production of continuous words belonging to the same category or subcategory) and switching (abandoning an exhausted semantic cluster to a new one in order to produce more words) reflect executive control and strategy use. We examined LF, SF, as well as LF and SF switching and clustering performance in retired FSP, FNP, and healthy comparison (HC) participants. Methods The sample consisted of 28 HC, 17 retired FSP, and 53 retired FNP. ANOVAs were conducted to determine group differences on LF, SF, switching, and clustering. FSP and FNP did not differ in concussion frequency. Results We found the HC group outperformed the FSP group in LF, p = 0.042, ηp2 = 0.07. For SF, the HC and FNP groups outperformed the FSP group, p = 0.013, ηp2 = 0.09. Furthermore, we found the HC group outperformed both football groups in SF switching, p = 0.000, ηp2 = 0.17. Conclusion As expected, the HC group outperformed the FSP group on LF and SF. Also, the FNP group outperformed the FSP group on SF. Interestingly, FSP displayed generally worse performances, supporting the notion that their experience of sub-concussive blows puts them at greater risk for cognitive impairment. Further investigation is needed with larger sample sizes to evaluate SF and other cognitive deficits in the FSP participants.

Using Dynamics of Eye Movements, Speech Articulation and Brain Activity to Predict and Track mTBI Screening Outcomes

Repeated subconcussive blows to the head during sports or other contact activities may have a cumulative and long lasting effect on cognitive functioning. Unobtrusive measurement and tracking of cognitive functioning is needed to enable preventative interventions for people at elevated risk of concussive injury. The focus of the present study is to investigate the potential for using passive measurements of fine motor movements (smooth pursuit eye tracking and read speech) and resting state brain activity (measured using fMRI) to complement existing diagnostic tools, such as the Immediate Post-concussion Assessment and Cognitive Testing (ImPACT), that are used for this purpose. Thirty-one high school American football and soccer athletes were tracked through the course of a sports season. Hypotheses were that (1) measures of complexity of fine motor coordination and of resting state brain activity are predictive of cognitive functioning measured by the ImPACT test, and (2) within-subject changes in these measures over the course of a sports season are predictive of changes in ImPACT scores. The first principal component of the six ImPACT composite scores was used as a latent factor that represents cognitive functioning. This latent factor was positively correlated with four of the ImPACT composites: verbal memory, visual memory, visual motor speed and reaction speed. Strong correlations, ranging between r = 0.26 and r = 0.49, were found between this latent factor and complexity features derived from each sensor modality. Based on a regression model, the complexity features were combined across sensor modalities and used to predict the latent factor on out-of-sample subjects. The predictions correlated with the true latent factor with r = 0.71. Within-subject changes over time were predicted with r = 0.34. These results indicate the potential to predict cognitive performance from passive monitoring of fine motor movements and brain activity, offering initial support for future application in detection of performance deficits associated with subconcussive events.

Traumatic brain injury (TBI) in collision sports: Possible mechanisms of transformation into chronic traumatic encephalopathy (CTE)

Traumatic brain injury (TBI) is a leading cause of death and disability, contributing to ~30% of all injury-related deaths in the US. TBI occurs when a force transmitted to the head causes neuropathologic damage and impairment of brain function. TBI doubles risk of suicide and is the major determinant of acquired seizure disorders. TBI arising from closed head trauma (CHT) significantly increases the risk of developing Alzheimer's disease (AD), Parkinson's disease (PD) and chronic traumatic encephalopathy (CTE). Evidence for a possible role of TBI as a risk factor for sporadic amyotrophic lateral sclerosis (sALS) has been provided by studies of professional players of European football. Depending on age, genetic make-up (in particular, being a carrier of one or two ApoE4 alleles), the number of TBIs sustained, their severity, the time periods involved, and many other factors that affect vulnerability, decades may pass after occurrence of one or more TBIs before sequelae such as AD, PD, sALS or CTE become clinically evident. Among college and professional football players who experience repeated concussions and sub-concussive blows to the head, the risk of developing CTE increases with the number of years actively devoted to the sport, and the degree of exposure to physical impacts inherent in the position played. Following a moderate or severe concussion, or a series of mild blows to the head, the brain may undergo subtle pathophysiological changes that are unlikely to be detected with confidence using available diagnostic methods. Biomarkers are being sought that can help the attending physician infer the likely presence of an ongoing occult neurodegenerative process. One example of the adverse effect of collision on the brain is “heading” the soccer ball—a feat that, repeated over years of competition, has been found to produce severe brain damage in veteran players. CTE has attracted increasing national attention because of its devastating effects in a high proportion of retired professional players of American football. In a study of brains from deceased former football players, contributed mostly by family members, CTE was neuropathologically diagnosed in 110 of 111 of National Football League (NFL) veterans. In the CTE-positive subjects, the authors observed extensive brain atrophy, astrogliosis, myelinated axonopathy, microvascular injury, perivascular neuroinflammation, and phosphorylated tau protein pathology. CTE's neuropathology has been formally defined as a tauopathy characterized by a distinct perivascular accumulation of hyperphosphorylated tau in neurons and astrocytes within cerebral sulci. Although the mechanism that underlies the unforeseen emergence of CTE long after the occurrence of one or more closed head traumas is unknown, an explanation proposed by Albayram and associates is persuasive. They discovered TBI-induced neuronal production of the toxic compound cis P-tau, an abnormal and destructive isomer of the normal and benign trans P-tau, in mouse models of CTE. Cis P-tau produced a CTE-like syndrome via a process they termed cistauosis. Cistauosis can be blocked in laboratory animals by cis P-tau monoclonal antibody, which prevents later development of tau tangles, brain atrophy and virtual CTE. In a subsequent study, the same group found in human samples obtained post-TBI from a variety of causes, that cis P-tau is induced in cortical axons and cerebrospinal fluid and positively correlates with axonal injury and clinical outcome. Thus, cis P-tau appears to contribute to short-term and long-term sequelae after TBI, but may be subject to neutralization by cis-antibody treatment.

Mild Chronic Traumatic Encephalopathy Neuropathology in People With No Known Participation in Contact Sports or History of Repetitive Neurotrauma.

It has been asserted that chronic traumatic encephalopathy (CTE) pathology is only present in former athletes and others who have been exposed to repetitive concussions, subconcussive blows, or both. We hypothesized that CTE pathology would be present in men who had no known history of repetitive neurotrauma. Comprehensive medical record reviews and health surveys completed by a family member were available for the 8 men in this case series, none of whom had known exposure to repetitive neurotrauma but 2 of whom had a history of traumatic brain injury (TBI). Postmortem tissue was immunostained for hyperphosphorylated tau (p-tau) to assess for CTE pathology, Braak stage, and aging-related p-tau. The neuropathologist was blind to age, personal history, and clinical history. Six of the 8 cases (75%) showed p-tau in neurons, astrocytes, and cell processes around small blood vessels in an irregular pattern at the depths of the cortical sulci. The changes were focal and limited in terms of overall extent, and some of the cases had a clearer pattern of pathology and some could be considered equivocal. Two of the 8 cases had a history of TBI and one of them showed CTE pathology. Five of the 6 cases with no known history of neurotrauma appeared to meet consensus criteria for CTE. This study adds to the emerging literature indicating that CTE pathology is present in people not known to have experienced multiple concussions or subconcussive blows to the head.

Quantitative evaluation of impact attenuation by football helmets using a modal impulse hammer

Concussions and changes in neurophysiology due to sub-concussive blows are common in contact sports, motivating a need for improved safety systems. While there have been attempts to determine whether or not new helmet designs influence the incidence of concussion, the results to date have been inconclusive. Consequently, the goal of this study was to evaluate the ability of modern football helmets to mitigate linear impacts directed normal, as well as oblique, to the surface of the helmeted head using a system that quantifies both the input load and the resulting accelerations of a Hybrid III headform. The transfer function connecting the inputs and outputs of the system was developed using dimensional analysis to provide a means of comparing helmets across brands and sports. For translational accelerations, increased helmet mass and a secondary layer of padding helped mitigate the impacts. The attenuation of angular accelerations depended substantially on helmet model and impact location.

Advanced white matter mapping in the subconcussive brain

The topic of behavioral and structural deficits caused by concussions is an increasingly important 1 in the related research fields. With an incidence rate of 2.9 competition concussions per 1,000 athlete exposures (NCAA 2013) in collegiate football, the concussion risk to athletes is significant. However, even subconcussive blows, or blows that do not lead to a concussion diagnosis, appear to create health risks for athletes. These impacts appear to lead to significant neural changes, the severity of which may depend on the number of hits (McAllister et al., 2014). An anatomically informed, personalized-medicine tractography approach was used to determine which major white matter tracts showed the greatest degree of difference in white matter tensor measures between 17 Division I upperclassmen football players, 15 Division I upperclassman cross-country runners, and 9 socioeconomically-matched non-athlete controls. We determined the underlying microstructural white matter biomarkers, using a classic diffusion-tensor model (Pierpaoli and Basser, 1999) as well as Neurite Orientation Dispersion and Density Imaging (NODDI; Zhang et al., 2012), that predict differences across different white matter tracts in the groups of athletes. Results show widespread differences in white matter tissue properties in multiple tracts and among the 3 athletes groups, including decreased FA, increased ICVF, and OD in the football players vs the 2 control groups. These differences occurred more often in longer fiber tracts compared to shorter fiber tracts, suggesting a differential effect of head impacts based on the geometric properties of these tracts. We developed a fully automated processing pipeline for this study, available as open source code as well as open service at brainlife.io. These results support the hypothesis that multiple subconcussive blows can result in white matter structural changes, with differential effects based on the length of the fiber tract being investigated, that are detectable with diffusion MRI and tractography.

Correction: Evidence of Cognitive Dysfunction after Soccer Playing with Ball Heading Using a Novel Tablet-Based Approach.

[This corrects the article DOI: 10.1371/journal.pone.0057364.].

Concussion: A History of Science and Medicine, 1870‐2005

To review the intellectual history of concussion from the mid-19th century to the opening decade of the 21st century. Head injuries (HI) and their acute and long-term effects have been investigated for centuries, with major reviews of the topic appearing by 1870. Thus, while it has long been acknowledged that chronic traumatic encephalopathy was first described by Harrison Martland in 1928, an examination of the history of concussion research up to Martland's seminal report places his studies in a deeper historical context. This history makes clear that Martland's findings were one among many such studies showcasing the lasting dangers of blows to the head. In the years after Martland published his study, his paper was frequently cited in other papers that made clear that blows to the head, of all ranges of severity, were dangerous injuries with potentially life-changing consequences. The author has engaged in an historical analysis of the development and elaboration of concussion research in clinical medicine, neurology, neurosurgery, and those scientific disciplines related to clinical medicine. The author has found numerous primary sources from the history of medicine and science that describe the acute and chronic effects of single and repeated sub-concussive and concussive blows to the head. This study makes clear that evidence-based methodologies inevitably short-change the knowledge of past clinicians and scientists by holding these figures to normative standards of recent invention. What criticism of this kind fails to recognize is that past investigators, many of them pioneers in their fields, published their work in ways that matched the highest normative standards of their day for the presentation of evidence. It has been recognized for a long time that concussions are dangerous injuries with potentially life-changing consequences, ranging from permanent symptoms to degenerative neurological states. The intellectual history of medicine and science from 1870 to the recent past shows both a continuity of clinical observations about HI and a steady, incremental accumulation of knowledge refining our understanding of those observations from a remarkably wide sphere of scientific disciplines.

Chronic Traumatic Encephalopathy-like Neuropathological Findings without a History of Trauma

Chronic traumatic encephalopathy (CTE) is a neurodegenerative disease characterized by neurocognitive decline and abnormal neuronal and glial deposition of hyperphosphorylated-tau in a pattern distinct from other tauopathies. These changes are hypothesized to occur following repeated mild traumatic brain injury. We report a case of CTE-like neuropathological changes, and features of motor neuron disease in a patient without history of either head trauma or participation in activities associated with increased risk of concussive or sub-concussive blows.

Novel Method of Weighting Cumulative Helmet Impacts Improves Correlation with Brain White Matter Changes After One Football Season of Sub-Concussive Head Blows (P6.147)

Novel Method of Weighting Cumulative Helmet Impacts Improves Correlation with Brain White Matter Changes After One Football Season of Sub-Concussive Head Blows (P6.147)

The Pathophysiology of Concussion.

Concussion is a significant issue in medicine and the media today. With growing interest on the long-term effects of sports participation, it is important to understand what occurs in the brain after an impact of any degree. While some of the basic pathophysiology has been elucidated, much is still unknown about what happens in the brain after traumatic brain injury, particularly with milder injuries where no damage can be seen at the structural level on standard neuroimaging. Understanding the chain of events from a cellular level using studies investigating more severe injuries can help to drive research efforts in understanding the symptomatology that is seen in the acute phase after concussion, as well as point to mechanisms that may underlie persistent post-concussive symptoms. This review discusses the basic neuropathology that occurs after traumatic brain injury at the cellular level. We also present the pathology of chronic traumatic encephalopathy and its similarities to other neurodegenerative diseases. We conclude with recent imaging and biomarker findings looking at changes that may occur after repeated subconcussive blows, which may help to guide efforts in understanding if cumulative subconcussive mechanical forces upon the brain are detrimental in the long term or if concussive symptoms mark the threshold for brain injury.

Subconcussive Blows to the Head: A Formative Review of Short-term Clinical Outcomes.

Given questions about "lower thresholds" for concussion, as well as possible effects of repetitive concussion and chronic traumatic encephalopathy (CTE), and associated controversy, there is increasing interest in "subconcussive" blows and their potential significance. A formative review with critical examination of the developing literature on subconcussive blows in athletes with an emphasis on clinical outcomes. Studies of biomechanical, performance and/or symptom-based, and neuroimaging data were identified via PubMed search and critically reviewed. Five studies of symptom reporting/performance and 4 studies of neuroimaging were included. The relation between biomechanical parameters and diagnosed concussion is not straightforward (ie, it is not the case that greater and more force leads to more severe injury or cognitive/behavioral sequelae). Neuropsychological studies of subconcussive blows within a single athletic season have failed to demonstrate any strong and consistent relations between number and severity of subconcussive events and cognitive change. Recent studies using neuroimaging have demonstrated a potential cumulative effect of subconcussive blows, at least in a subset of individuals. Human studies of the neurological/neuropsychological impact of subconcussive blows are currently quite limited. Subconcussive blows, in the short-term, have not been shown to cause significant clinical effects. To date, findings suggest that any effect of subconcussive blows is likely to be small or nonexistent, perhaps evident in a subset of individuals on select measures, and maybe even beneficial in some cases. Longer-term prospective studies are needed to determine if there is a cumulative dose effect.

Chronic traumatic encephalopathy and athletes.

Recent case reports have described athletes previously exposed to repetitive head trauma while participating in contact sports who later in life developed mood disorders, headaches, cognitive difficulties, suicidal ideation, difficulties with speech, and aggressive behavior. Postmortem discoveries show that some of these athletes have pathologic findings that are collectively termed chronic traumatic encephalopathy (CTE). Current hypotheses suggest that concussions or perhaps blows to the head that do not cause the signs and symptoms necessary for making the diagnosis of concussion, so-called subconcussive blows, cause both the clinical and pathologic findings. There are, however, some athletes who participate in contact sports who do not develop the findings ascribed to CTE. Furthermore, there are people who have headaches, mood disorders, cognitive difficulties, suicidal ideation, and other clinical problems who have neither been exposed to repeated head trauma nor possessed the pathologic postmortem findings of those currently diagnosed with CTE. The current lack of prospective data and properly designed case-control studies limits the current understanding of CTE, leading to debate about the causes of the neuropathologic findings and the clinical observations. Given the potential for referral and recall bias in available studies, it remains unclear whether or not the pathologic findings made postmortem cause the presumed neurobehavioral sequela and whether the presumed risk factors, such as sports activity, cerebral concussions, and subconcussive blows, are solely causative of the clinical signs and symptoms. This article discusses the current evidence and the associated limitations.

Division III Collision Sports Are Not Associated with Neurobehavioral Quality of Life.

We sought to determine whether the exposure to the sub-concussive blows that occur during division III collegiate collision sports affect later life neurobehavioral quality-of-life measures. We conducted a cross-sectional study of alumni from four division III colleges, targeting those between the ages of 40-70 years, using several well-validated quality-of-life measures for executive function, general concerns, anxiety, depression, emotional and behavior dyscontrol, fatigue, positive affect, sleep disturbance, and negative consequences of alcohol use. We used multivariable linear regression to assess for associations between collision sport participation and quality-of-life measures while adjusting for covariates including age, gender, race, annual income, highest educational degree, college grades, exercise frequency, and common medical conditions. We obtained data from 3702 alumni, more than half of whom (2132) had participated in collegiate sports, 23% in collision sports, 23% in non-contact sports. Respondents with a history of concussion had worse self-reported health on several measures. When subjects with a history of concussion were removed from the analyses in order to assess for any potential effect of sub-concussive blows alone, negative consequences of alcohol use remained higher among collision sport athletes (β-coefficient 1.957, 95% CI 0.827-3.086). There were, however, no other significant associations between exposure to collision sports during college and any other quality-of-life measures. Our results suggest that, in the absence of a history of concussions, participation in collision sports at the Division III collegiate level is not a risk factor for worse long-term neurobehavioral outcomes, despite exposure to repeated sub-concussive blows.

Sub-Concussive Hit Characteristics Predict Deviant Brain Metabolism in Football Athletes

Magnetic resonance spectroscopy and helmet telemetry were used to monitor the neural metabolic response to repetitive head collisions in 25 high school American football athletes. Specific hit characteristics were determined highly predictive of metabolic alterations, suggesting that sub-concussive blows can produce biochemical changes and potentially lead to neurological problems.

Examining Neurocognitive Function in Previously Concussed Interscholastic Female Soccer Players

Awareness of sport-related concussions in soccer has gained recent attention in the medical community. Interestingly, purposeful heading—a unique yet strategic and inherent part of soccer—involves repeated subconcussive blows to the head. We divided 210 female interscholastic soccer players into control (CON [never concussed]) and experimental (EXP [previously concussed]) groups. We assessed neurocognitive performance using the Automated Neuropsychological Assessment Metrics computer program before and after the players' competitive season. Headers were recorded at all sanctioned matches. Data were analyzed using a series of one-way analyses of covariance and t tests. Both groups essentially played in the same number of games (EXP = 16.1 vs. CON = 16.1) and had an equal number of total headers (EXP = 24.9 vs. CON = 24.3). Additionally, headers per game were surprisingly low in both groups (1.4 in EXP vs. 1.3 in CON). Unexpectedly, there were no significant differences between the EXP and CON groups across all dependent variables measured (p > .05). This study suggests that although previously concussed players involve themselves in purposeful heading (i.e., subconcussive insults) throughout a competitive season, there appear to be no negative consequences on neuropsychological test performance or concussion-related symptoms. Additional research is needed to determine what may result during the course of a playing career.

The long-term effects of repetitive mild head injuries in sports.

At least 300 000 sports-related concussions occur in the United States annually. With millions of American athletes, the long-term effects of repeated concussion or mild traumatic brain injury are an important topic. Unfortunately, there is a lack of strong data on the causality or prevalence of long-term effects among athletes. Chronic traumatic encephalopathy (CTE), a progressive neurodegenerative tauopathy, with associated clinical, behavioral, and neuropathological findings, is an important clinical entity in need of further study. Diffusion tensor imaging can elucidate trauma-induced white matter damage, but the diagnosis of CTE cannot be proven until postmortem neuropathology shows characteristic neurofibrillary and astrocytic tangles. Concern exists that athletes subject to repeated concussive and even subconcussive blows may be at risk of CTE, but no definitive data exist due to the difficulty in diagnosis. Animal models suggest that mild traumatic brain injuries lead to primarily a metabolic derangement with increased excitotoxic neurotransmitter release, extracellular potassium, and intracellular calcium. Further understanding of the underlying pathophysiology may eventually lead to better therapeutic and diagnostic options for the treating clinician.