Considerations for animal models of blast-related traumatic brain injury and chronic traumatic encephalopathy

Navigating the Complexities of Traumatic Encephalopathy Syndrome (TES): Current State and Future Challenges

Chronic traumatic encephalopathy (CTE) is a unique neurodegenerative disease that is associated with repetitive head impacts (RHI) in both civilian and military settings. In 2014, the research criteria for the clinical manifestation of CTE, traumatic encephalopathy syndrome (TES), were proposed to improve the clinical identification and understanding of the complex neuropathological phenomena underlying CTE. This review provides a comprehensive overview of the current understanding of the neuropathological and clinical features of CTE, proposed biomarkers of traumatic brain injury (TBI) in both research and clinical settings, and a range of treatments based on previous preclinical and clinical research studies. Due to the heterogeneity of TBI, there is no universally agreed-upon serum, CSF, or neuroimaging marker for its diagnosis. However, as our understanding of this complex disease continues to evolve, it is likely that there will be more robust, early diagnostic methods and effective clinical treatments. This is especially important given the increasing evidence of a correlation between TBI and neurodegenerative conditions, such as Alzheimer's disease and CTE. As public awareness of these conditions grows, it is imperative to prioritize both basic and clinical research, as well as the implementation of necessary safe and preventative measures.

Understanding Traumatic Brain Injuries in Military Personnel: Investigating the Dynamic Interplay of the Cerebrospinal Fluid and Brain During Blasts

Background It is estimated that around 450,000 traumatic brain injury cases have occurred in the 21st century with possible under-reporting. Computational simulations are increasingly used to study the pathophysiology of traumatic brain injuries among US military personnel. This approach allows for investigation without ethical concerns surrounding live subject testing. Methodology The pertinent data on head acceleration is applied to a detailed 3D model of a patient-specific head, which encompasses all significant components of the brain and its surrounding fluid. The use of finite element analysis and smoothed-particle hydrodynamics serves to replicate the interaction between these elements during discharge through simulation of their fluid-structure dynamics. Results The stress levels of the brain are assessed at varying time intervals subsequent to the explosion. The regions where there is an intersection between the skull and brain are observed, along with the predominant orientations in which displacement of the brain occurs resulting in a brain injury. Conclusions It has been determined that the cerebrospinal fluid is inadequate in preventing brain damage caused by multiple abrupt directional shifts of the head. Accordingly, additional research must be undertaken to enhance our comprehension of the injury mechanisms linked with consecutive changes in acceleration impacting the head.

Applying the Bradford Hill Criteria for Causation to Repetitive Head Impacts and Chronic Traumatic Encephalopathy

Chronic traumatic encephalopathy (CTE) is a neurodegenerative disease associated with a history of repetitive head impacts (RHI). CTE was described in boxers as early as the 1920s and by the 1950s it was widely accepted that hits to the head caused some boxers to become "punch drunk." However, the recent discovery of CTE in American and Australian-rules football, soccer, rugby, ice hockey, and other sports has resulted in renewed debate on whether the relationship between RHI and CTE is causal. Identifying the strength of the evidential relationship between CTE and RHI has implications for public health and medico-legal issues. From a public health perspective, environmentally caused diseases can be mitigated or prevented. Medico-legally, millions of children are exposed to RHI through sports participation; this demographic is too young to legally consent to any potential long-term risks associated with this exposure. To better understand the strength of evidence underlying the possible causal relationship between RHI and CTE, we examined the medical literature through the Bradford Hill criteria for causation. The Bradford Hill criteria, first proposed in 1965 by Sir Austin Bradford Hill, provide a framework to determine if one can justifiably move from an observed association to a verdict of causation. The Bradford Hill criteria include nine viewpoints by which to evaluate human epidemiologic evidence to determine if causation can be deduced: strength, consistency, specificity, temporality, biological gradient, plausibility, coherence, experiment, and analogy. We explored the question of causation by evaluating studies on CTE as it relates to RHI exposure. Through this lens, we found convincing evidence of a causal relationship between RHI and CTE, as well as an absence of evidence-based alternative explanations. By organizing the CTE literature through this framework, we hope to advance the global conversation on CTE mitigation efforts.

Tau pathology, metal dyshomeostasis and repetitive mild traumatic brain injury: an unexplored link paving the way for neurodegeneration

Repetitive mild traumatic brain injury (r-mTBI), commonly experienced by athletes and military personnel, causes changes in multiple intracellular pathways, one of which involves the tau protein. Tau phosphorylation plays a role in several neurodegenerative conditions including chronic traumatic encephalopathy (CTE), a progressive neurodegenerative disorder linked to repeated head trauma. There is now mounting evidence suggesting that tau phosphorylation may be regulated by metal ions (such as iron, zinc and copper), which themselves are implicated in ageing and neurodegenerative disorders such as Alzheimer's disease (AD). Recent work has also shown that a single TBI can result in age-dependent and region-specific modulation of metal ions. As such, this review explores the link between TBI, CTE, ageing and neurodegeneration with a specific focus on the involvement of (and interaction between) tau pathology and metal dyshomeostasis. The authors highlight that metal dyshomeostasis has yet to be investigated in the context of repeat head trauma or CTE. Given the evidence that metal dyshomeostasis contributes to the onset and/or progression of neurodegeneration, and that CTE itself is a neurodegenerative condition, this brings to light an uncharted link that should be explored. The development of adequate models of r-mTBI and/or CTE will be crucial in deepening our understanding of the pathological mechanisms that drive the clinical manifestations in these conditions and also in the development of effective therapeutics targeted towards slowing progressive neurodegenerative disorders.

Hyperacute Excitotoxic Mechanisms and Synaptic Dysfunction Involved in Traumatic Brain Injury

The biological response of brain tissue to biomechanical strain are of fundamental importance in understanding sequela of a brain injury. The time after impact can be broken into four main phases: hyperacute, acute, subacute and chronic. It is crucial to understand the hyperacute neural outcomes from the biomechanical responses that produce traumatic brain injury (TBI) as these often result in the brain becoming sensitized and vulnerable to subsequent TBIs. While the precise physical mechanisms responsible for TBI are still a matter of debate, strain-induced shearing and stretching of neural elements are considered a primary factor in pathology; however, the injury-strain thresholds as well as the earliest onset of identifiable pathologies remain unclear. Dendritic spines are sites along the dendrite where the communication between neurons occurs. These spines are dynamic in their morphology, constantly changing between stubby, thin, filopodia and mushroom depending on the environment and signaling that takes place. Dendritic spines have been shown to react to the excitotoxic conditions that take place after an impact has occurred, with a shift to the excitatory, mushroom phenotype. Glutamate released into the synaptic cleft binds to NMDA and AMPA receptors leading to increased Ca2+ entry resulting in an excitotoxic cascade. If not properly cleared, elevated levels of glutamate within the synaptic cleft will have detrimental consequences on cellular signaling and survival of the pre- and post-synaptic elements. This review will focus on the synaptic changes during the hyperacute phase that occur after a TBI. With repetitive head trauma being linked to devastating medium – and long-term maladaptive neurobehavioral outcomes, including chronic traumatic encephalopathy (CTE), understanding the hyperacute cellular mechanisms can help understand the course of the pathology and the development of effective therapeutics.

Repetitive Blast Exposure Increases Appetitive Motivation and Behavioral Inflexibility in Male Mice

Blast exposure (via detonation of high explosives) represents a major potential trauma source for Servicemembers and Veterans, often resulting in mild traumatic brain injury (mTBI). Executive dysfunction (e.g., alterations in memory, deficits in mental flexibility, difficulty with adaptability) is commonly reported by Veterans with a history of blast-related mTBI, leading to impaired daily functioning and decreased quality of life, but underlying mechanisms are not fully understood and have not been well studied in animal models of blast. To investigate potential underlying behavioral mechanisms contributing to deficits in executive functioning post-blast mTBI, here we examined how a history of repetitive blast exposure in male mice affects anxiety/compulsivity-like outcomes and appetitive goal-directed behavior using an established mouse model of blast mTBI. We hypothesized that repetitive blast exposure in male mice would result in anxiety/compulsivity-like outcomes and corresponding performance deficits in operant-based reward learning and behavioral flexibility paradigms. Instead, results demonstrate an increase in reward-seeking and goal-directed behavior and a congruent decrease in behavioral flexibility. We also report chronic adverse behavioral changes related to anxiety, compulsivity, and hyperarousal. In combination, these data suggest that potential deficits in executive function following blast mTBI are at least in part related to enhanced compulsivity/hyperreactivity and behavioral inflexibility and not simply due to a lack of motivation or inability to acquire task parameters, with important implications for subsequent diagnosis and treatment management.

Dependence of visual and cognitive outcomes on animal holder configuration in a rodent model of blast overpressure exposure

Blast-induced traumatic brain injury is the signature injury of modern military conflicts. To more fully understand the effects of blast exposure, we placed rats in different holder configurations, exposed them to blast overpressure, and assessed the degree of eye and brain injury. Anesthetized Long-Evans rats received blast exposures directed at the head (63 kPa, 195 dB-SPL) in either an "open holder" (head and neck exposed; n = 7), or an "enclosed holder" (window for blast exposure to eye; n = 15) and were compared to non-blast exposed (control) rats (n = 22). Outcomes included optomotor response (OMR), electroretinography (ERG), and spectral domain optical coherence tomography (SD-OCT) at 2, 4, and 6 months post-blast, and cognitive function (Y-maze) at 3 months. Spatial frequency and contrast sensitivity were reduced in ipsilateral blast-exposed eyes in both holders (p < 0.01), while contralateral eyes showed greater deficits with the enclosed holder (p < 0.05). Thinner retinas (p < 0.001) and reduced ERG a- and b- wave amplitudes (p < 0.05) were observed for both ipsilateral and contralateral eyes with the enclosed, but not the open, holder. Rats in the open holder showed cognitive deficits compared to rats in the enclosed holder (p < 0.05). Overall, the animal holder configuration used in blast exposure studies can significantly affect outcomes. Enclosed holders may cause secondary damage to the contralateral eye by concussive injury or blast wave reflection off the holder wall. Open holders may damage the brain via rapid head movement (contrecoup injury). These results highlight additional factors to be considered when evaluating patients with blast exposure or developing models of blast injury.

Expression of GFAP and Tau Following Blast Exposure in the Cerebral Cortex of Ferrets

Blast exposures are a hallmark of contemporary military conflicts. We need improved preclinical models of blast traumatic brain injury for translation of pharmaceutical and therapeutic protocols. Compared with rodents, the ferret brain is larger, has substantial sulci, gyri, a higher white to gray matter ratio, and the hippocampus in a ventral position; these attributes facilitate comparison with the human brain. In this study, ferrets received compressed air shock waves and subsequent evaluation of glia and forms of tau following survival of up to 12 weeks. Immunohistochemistry and Western blot demonstrated altered distributions of astrogliosis and tau expression after blast exposure. Many aspects of the astrogliosis corresponded to human pathology: increased subpial reactivity, gliosis at gray-white matter interfaces, and extensive outlining of blood vessels. MRI analysis showed numerous hypointensities occurring in the 12-week survival animals, appearing to correspond to luminal expansions of blood vessels. Changes in forms of tau, including phosphorylated tau, and the isoforms 3R and 4R were noted using immunohistochemistry and Western blot in specific regions of the cerebral cortex. Of particular interest were the 3R and 4R isoforms, which modified their ratio after blast. Our data strongly support the ferret as an animal model with highly translational features to study blast injury.

Repetitive blast mild traumatic brain injury increases ethanol sensitivity in male mice and risky drinking behavior in male combat veterans

BACKGROUND

Mild traumatic brain injury (mTBI) is common in civilians and highly prevalent among military service members. mTBI can increase health risk behaviors (e.g., sensation seeking, impulsivity) and addiction risk (e.g., for alcohol use disorder (AUD)), but how mTBI and substance use might interact to promote addiction risk remains poorly understood. Likewise, potential differences in single vs. repetitive mTBI in relation to alcohol use/abuse have not been previously examined.

METHODS

Here, we examined how a history of single (1×) or repetitive (3×) blast exposure (blast-mTBI) affects ethanol (EtOH)-induced behavioral and physiological outcomes using an established mouse model of blast-mTBI. To investigate potential translational relevance, we also examined self-report responses to the Alcohol Use Disorders Identification Test-Consumption questions (AUDIT-C), a widely used measure to identify potential hazardous drinking and AUD, and used a novel unsupervised machine learning approach to investigate whether a history of blast-mTBI affected drinking behaviors in Iraq/Afghanistan Veterans.

RESULTS

Both single and repetitive blast-mTBI in mice increased the sedative properties of EtOH (with no change in tolerance or metabolism), but only repetitive blast potentiated EtOH-induced locomotor stimulation and shifted EtOH intake patterns. Specifically, mice exposed to repetitive blasts showed increased consumption "front-loading" (e.g., a higher rate of consumption during an initial 2-h acute phase of a 24-h alcohol access period and decreased total daily intake) during an intermittent 2-bottle choice condition. Examination of AUDIT-C scores in Iraq/Afghanistan Veterans revealed an optimal 3-cluster solution: "low" (low intake and low frequency), "frequent" (low intake and high frequency), and "risky" (high intake and high frequency), where Veterans with a history of blast-mTBI displayed a shift in cluster assignment from "frequent" to "risky," as compared to Veterans who were deployed to Iraq/Afghanistan but had no lifetime history of TBI.

CONCLUSIONS

Together, these results offer new insight into how blast-mTBI may give increase AUD risk and highlight the increased potential for adverse health risk behaviors following repetitive blast-mTBI.

An Overview and Therapeutic Promise of Nutraceuticals against Sports-Related Brain Injury

Sports-related traumatic brain injury (TBI) is one of the common neurological maladies experienced by athletes. Earlier the term 'punch drunk syndrome' was used in the case TBI of boxers and now this term is replaced by chronic traumatic encephalopathy (CTE). Sports-related brain injury can either be short term or long term. A common instance of brain injury encompasses subdural hematoma, concussion, cognitive dysfunction, amnesia, headache, vision issue, axonopathy, or even death if remain undiagnosed or untreated. Further, chronic TBI may lead to pathogenesis of neuroinflammation and neurodegeneration via tauopathy, formation of neurofibrillary tangles, and damage to the blood-brain barrier, microglial, and astrocyte activation. Thus, altered pathological, neurochemical, and neurometabolic attributes lead to the modulation of multiple signaling pathways and cause neurological dysfunction. Available pharmaceutical interventions are based on one drug one target hypothesis and thereby unable to cover altered multiple signaling pathways. However, in recent time's pharmacological intervention of nutrients and nutraceuticals have been explored as they exert a multifactorial mode of action and maintain over homeostasis of the body. There are various reports available showing the positive therapeutic effect of nutraceuticals in sport-related brain injury. Therefore, in the current article we have discussed the pathology, neurological consequence, sequelae, and perpetuation of sports-related brain injury. Further, we have discussed various nutraceutical supplements as well as available animal models to explore the neuroprotective effect/ upshots of these nutraceuticals in sports-related brain injury.

Risk Factors for Chronic Traumatic Encephalopathy: A Proposed Framework

Chronic traumatic encephalopathy (CTE) is a progressive neurodegenerative disease that has been neuropathologically diagnosed in contact and collision sport athletes, military veterans, and others with a history of exposure to repetitive head impacts (RHI). Identifying methods to diagnose and prevent CTE during life is a high priority. Timely diagnosis and implementation of treatment and preventative strategies for neurodegenerative diseases, including CTE, partially hinge upon early and accurate risk characterization. Here, we propose a framework of risk factors that influence the neuropathological development of CTE. We provide an up-to-date review of the literature examining cumulative exposure to RHI as the environmental trigger for CTE. Because not all individuals exposed to RHI develop CTE, the direct and/or indirect influence of nonhead trauma exposure characteristics (e.g., age, sex, race, genetics) on the pathological development of CTE is reviewed. We conclude with recommendations for future directions, as well as opinions for preventative strategies that could mitigate risk.

Persistent post-traumatic headache: a migrainous loop or not? The preclinical evidence

Background

According to the International Classification of Headache Disorders 3, post-traumatic headache (PTH) attributed to traumatic brain injury (TBI) is a secondary headache reported to have developed within 7 days from head injury, regaining consciousness following the head injury, or discontinuation of medication(s) impairing the ability to sense or report headache following the head injury. It is one of the most common secondary headache disorders, and it is defined as persistent when it lasts more than 3 months.

Main body

Currently, due to the high prevalence of this disorder, several preclinical studies have been conducted using different animal models of mild TBI to reproduce conditions that engender PTH. Despite representing a simplification of a complex disorder and displaying different limitations concerning the human condition, animal models are still a mainstay to study in vivo the mechanisms of PTH and have provided valuable insight into the pathophysiology and possible treatment strategies. Different models reproduce different types of trauma and have been ideated in order to ensure maximal proximity to the human condition and optimal experimental reproducibility.

Conclusion

At present, despite its high prevalence, PTH is not entirely understood, and the differential contribution of pathophysiological mechanisms, also observed in other conditions like migraine, has to be clarified. Although facing limitations, animal models are needed to improve understanding of PTH. The knowledge of currently available models is necessary to all researchers who want to investigate PTH and contribute to unravel its mechanisms.

Blast-Related Traumatic Brain Injury: Current Concepts and Research Considerations

Traumatic brain injury (TBI) is a well-known consequence of participation in activities such as military combat or collision sports. But the wide variability in eliciting circumstances and injury severities makes the study of TBI as a uniform disease state impossible. Military Service members are under additional, unique threats such as exposure to explosive blast and its unique effects on the body. This review is aimed toward TBI researchers, as it covers important concepts and considerations for studying blast-induced head trauma. These include the comparability of blast-induced head trauma to other mechanisms of TBI, whether blast overpressure induces measureable biomarkers, and whether a biodosimeter can link blast exposure to health outcomes, using acute radiation exposure as a corollary. This examination is contextualized by the understanding of concussive events and their psychological effects throughout the past century’s wars, as well as the variables that predict sustaining a TBI and those that precipitate or exacerbate psychological conditions.

Disclaimer: The views expressed in this article are solely the views of the authors and not those of the Department of Defense Blast Injury Research Coordinating Office, US Army Medical Research and Development Command, US Army Futures Command, US Army, or the Department of Defense.

Low-Level Primary Blast Induces Neuroinflammation and Neurodegeneration in Rats

OBJECTIVE

Mild blast traumatic brain injury is commonly prevalent in modern combat casualty care and has been associated with the development of neurodegenerative conditions. However, whether primary lower level blast overpressure (LBOP) causes neurodegeneration and neuroinflammation remains largely unknown. The aim of our present study was to determine whether LBOP can cause neuroinflammation and neurodegeneration.

METHODS

Anesthetized rats were randomly assigned to LBOP group (70 kPa, n = 5) or sham group (without blast, n = 5). Histopathological and cytokine changes in brain tissue at 5 days post-injury were evaluated by hematoxylin-eosin staining and Bioplex assay, respectively.

RESULTS

Histopathological assessment revealed neuronal degeneration and increased density of inflammatory cells in frontal and parietal cortex, hippocampus and thalamus in rats exposed to LBOP. LBOP exposure significantly elevated levels of pro-inflammatory cytokines (EPO, IL-1β, IL-6, IL-12, IL-18, and TNF-α) and chemokines (GRO and RANTES) as well as of an anti-inflammatory cytokine (IL-13) in the frontal cortex.

CONCLUSIONS

This study reveals a role of neuroinflammation in neurodegeneration after mild blast traumatic brain injury. Therapies that target this process might in warfighters might function either by attenuating the development of post-traumatic stress disorder, chronic traumatic encephalopathy and Alzheimer's disease, or by slowing their progression.

Pathophysiology of Concussion

Although concussion has been a subject of interest for centuries, this condition remains poorly understood. The mechanistic underpinnings and accepted definition of concussion remain elusive. To make sense of these issues, this article presents a brief history of concussion studies, detailing the evolution of motivations and experimental conclusions over time. Interest in concussion as a subject of scientific inquiry has increased with growing concern about the long-term consequences of mild traumatic brain injury (TBI). Although concussion is often associated with mild TBI, these conditions-the former a neurological syndrome, the latter a neurological event-are distinct, both mechanistically and pathobiologically. Modern research primarily focuses on the study of the biomechanics, pathophysiology, potential biomarkers and neuroimaging to distinguish concussion from mild TBI. In addition, mild TBI and concussion outcomes are influenced by age, sex, and genetic differences in people. With converging experimental objectives and methodologies, future concussion research has the potential to improve clinical assessment, treatment, and preventative measures.

Modeling sports-related mild traumatic brain injury in animals-A systematic review

Sports-related head trauma has emerged as an important public health issue, as mild traumatic brain injuries (mTBIs) may result in neurodegenerative disorders such as chronic traumatic encephalopathy (CTE). Research into mTBI and CTE pathophysiology are difficult to undertake in athletes, with observational trials and post-mortem analysis the current mainstays. Thus, animal models play an important role in the study of mTBI, however, traditional animal models have focused on acute, severe injuries rather than the more typical mTBI's seen in sport injuries. Recently, a number of animal models have been developed that are both appropriately scaled and biomechanically relevant to the forces sustained by athletes. This review aimed to examine the literature for variables included in these animal models, and the resulting neurotrauma as evidenced by pathology and behavioral deficits. A systematic search of the literature was performed in multiple electronic databases. The inclusion criteria required mimicry of athlete mTBI conditions: freedom of head movement, lack of surgical alteration of the skull, and application of direct contact force. Studies were analyzed for variables including apparatus design features (impact force, change in animal head velocity, and kinetic energy transfer to the head), demonstrated pathology (phosphorylated tau, TDP-43 aggregation, diffuse axonal injury, gliosis, cytokine inflammation response, and genetic integrity), and behavioral changes. These studies suggested that appropriate animal models can assist in understanding the pathological and functional outcomes of athlete mTBI, and could be used as a platform for future studies of diagnostic/prognostic markers and in the development of treatment interventions.

The long-term consequences of repetitive head impacts: Chronic traumatic encephalopathy

Chronic traumatic encephalopathy (CTE) is a neurodegenerative disease associated with exposure to repetitive head impacts (RHI). Although described in boxers for almost a century, scientific and public interest in CTE grew tremendously following a report of postmortem evidence of CTE in the first former professional American football player in 2005. Neuropathologic diagnostic criteria for CTE have been defined, with abnormal perivascular deposition of hyperphosphorylated tau at the sulcal depths as the pathognomonic feature. CTE can currently only be diagnosed postmortem, but clinical research criteria for the in vivo diagnosis of CTE have been proposed. The clinical phenotype of CTE is still ill-defined and there are currently no validated biomarkers to support an in-life diagnosis of "Probable CTE." Many knowledge gaps remain regarding the neuropathologic and clinical make-up of CTE. An increased understanding of CTE is critical given the millions that could potentially be impacted by this disease. This chapter describes the state of the literature on CTE. The historical origins of CTE are first presented, followed by a comprehensive description of the neuropathologic and clinical features. The chapter concludes with discussion on future research directions, emphasizing the importance of diagnosing CTE during life to facilitate development of preventative and intervention strategies.

Strides Toward Better Understanding of Post-Traumatic Headache Pathophysiology Using Animal Models

PURPOSE OF REVIEW

In recent years, the awareness of the detrimental impact of concussion and mild traumatic brain injuries (mTBI) is becoming more apparent. Concussive head trauma results in a constellation of cognitive and somatic symptoms of which post-traumatic headache is the most common. Our understanding of post-traumatic headache is limited by the paucity of well validated, characterized, and clinically relevant animal models with strong predictive validity. In this review, we aim to summarize and discuss current animal models of concussion/mTBI and related data that start to shed light on the pathophysiology of post-traumatic headache.

RECENT FINDINGS

Each of the models will be discussed in terms of their face, construct, and predictive validity as well as overall translational relevance to concussion, mTBI, and post-traumatic headache. Significant contributions to the pathophysiology of PTH garnered from these models are discussed as well as potential contributors to the development of chronic post-traumatic headache. Although post-traumatic headache is one of the most common symptoms following mild head trauma, there remains a disconnect between the study of mild traumatic brain injury and headache in the pre-clinical literature. A greater understanding of the relationship between these phenomena is currently needed to provide more insight into the increasing frequency of this debilitating condition in both military and civilian populations.

The Biological Basis of Chronic Traumatic Encephalopathy following Blast Injury: A Literature Review

The United States Department of Defense Blast Injury Research Program Coordinating Office organized the 2015 International State-of-the-Science meeting to explore links between blast-related head injury and the development of chronic traumatic encephalopathy (CTE). Before the meeting, the planning committee examined articles published between 2005 and October 2015 and prepared this literature review, which summarized broadly CTE research and addressed questions about the pathophysiological basis of CTE and its relationship to blast- and nonblast-related head injury. It served to inform participants objectively and help focus meeting discussion on identifying knowledge gaps and priority research areas. CTE is described generally as a progressive neurodegenerative disorder affecting persons exposed to head injury. Affected individuals have been participants primarily in contact sports and military personnel, some of whom were exposed to blast. The symptomatology of CTE overlaps with Alzheimer's disease and includes neurological and cognitive deficits, psychiatric and behavioral problems, and dementia. There are no validated diagnostic criteria, and neuropathological evidence of CTE has come exclusively from autopsy examination of subjects with histories of exposure to head injury. The perivascular accumulation of hyperphosphorylated tau (p-tau) at the depths of cortical sulci is thought to be unique to CTE and has been proposed as a diagnostic requirement, although the contribution of p-tau and other reported pathologies to the development of clinical symptoms of CTE are unknown. The literature on CTE is limited and is focused predominantly on head injuries unrelated to blast exposure (e.g., football players and boxers). In addition, comparative analyses of clinical case reports has been challenging because of small case numbers, selection biases, methodological differences, and lack of matched controls, particularly for blast-exposed individuals. Consequently, the existing literature is not sufficient to determine whether the development of CTE is associated with head injury frequency (e.g., single vs. multiple exposures) or head injury type (e.g., impact, nonimpact, blast-related). Moreover, the incidence and prevalence of CTE in at-risk populations is unknown. Future research priorities should include identifying additional risk factors, pursuing population-based longitudinal studies, and developing the ability to detect and diagnose CTE in living persons using validated criteria.

Concussion, microvascular injury, and early tauopathy in young athletes after impact head injury and an impact concussion mouse model

The mechanisms underpinning concussion, traumatic brain injury, and chronic traumatic encephalopathy, and the relationships between these disorders, are poorly understood. We examined post-mortem brains from teenage athletes in the acute-subacute period after mild closed-head impact injury and found astrocytosis, myelinated axonopathy, microvascular injury, perivascular neuroinflammation, and phosphorylated tau protein pathology. To investigate causal mechanisms, we developed a mouse model of lateral closed-head impact injury that uses momentum transfer to induce traumatic head acceleration. Unanaesthetized mice subjected to unilateral impact exhibited abrupt onset, transient course, and rapid resolution of a concussion-like syndrome characterized by altered arousal, contralateral hemiparesis, truncal ataxia, locomotor and balance impairments, and neurobehavioural deficits. Experimental impact injury was associated with axonopathy, blood-brain barrier disruption, astrocytosis, microgliosis (with activation of triggering receptor expressed on myeloid cells, TREM2), monocyte infiltration, and phosphorylated tauopathy in cerebral cortex ipsilateral and subjacent to impact. Phosphorylated tauopathy was detected in ipsilateral axons by 24 h, bilateral axons and soma by 2 weeks, and distant cortex bilaterally at 5.5 months post-injury. Impact pathologies co-localized with serum albumin extravasation in the brain that was diagnostically detectable in living mice by dynamic contrast-enhanced MRI. These pathologies were also accompanied by early, persistent, and bilateral impairment in axonal conduction velocity in the hippocampus and defective long-term potentiation of synaptic neurotransmission in the medial prefrontal cortex, brain regions distant from acute brain injury. Surprisingly, acute neurobehavioural deficits at the time of injury did not correlate with blood-brain barrier disruption, microgliosis, neuroinflammation, phosphorylated tauopathy, or electrophysiological dysfunction. Furthermore, concussion-like deficits were observed after impact injury, but not after blast exposure under experimental conditions matched for head kinematics. Computational modelling showed that impact injury generated focal point loading on the head and seven-fold greater peak shear stress in the brain compared to blast exposure. Moreover, intracerebral shear stress peaked before onset of gross head motion. By comparison, blast induced distributed force loading on the head and diffuse, lower magnitude shear stress in the brain. We conclude that force loading mechanics at the time of injury shape acute neurobehavioural responses, structural brain damage, and neuropathological sequelae triggered by neurotrauma. These results indicate that closed-head impact injuries, independent of concussive signs, can induce traumatic brain injury as well as early pathologies and functional sequelae associated with chronic traumatic encephalopathy. These results also shed light on the origins of concussion and relationship to traumatic brain injury and its aftermath.awx350media15713427811001.

Lifelong behavioral and neuropathological consequences of repetitive mild traumatic brain injury

Objective

Exposure to repetitive concussion, or mild traumatic brain injury (mTBI), has been linked with increased risk of long‐term neurodegenerative changes, specifically chronic traumatic encephalopathy (CTE). To date, preclinical studies largely have focused on the immediate aftermath of mTBI, with no literature on the lifelong consequences of mTBI in these models. This study provides the first account of lifelong neurobehavioral and histological consequences of repetitive mTBI providing unique insight into the constellation of evolving and ongoing pathologies with late survival.

Methods

Male C57BL/6J mice (aged 2–3 months) were exposed to either single or repetitive mild TBI or sham procedure. Thereafter, animals were monitored and assessed at 24 months post last injury for measures of motor coordination, learning deficits, cognitive function, and anxiety‐like behavior prior to euthanasia and preparation of the brains for detailed neuropathological and protein biochemical studies.

Results

At 24 months survival animals exposed to r‐mTBI showed clear evidence of learning and working memory impairment with a lack of spatial memory and vestibule‐motor vestibulomotor deficits compared to sham animals. Associated with these late behavioral deficits there was evidence of ongoing axonal degeneration and neuroinflammation in subcortical white matter tracts. Notably, these changes were also observed after a single mTBI, albeit to a lesser degree than repetitive mTBI.

Interpretation

In this context, our current data demonstrate, for the first time, that rather than an acute, time limited event, mild TBI can precipitate a lifelong degenerative process. These data therefore suggest that successful treatment strategies should consider both the acute and chronic nature of mTBI.

Neurometabolic indicators of mitochondrial dysfunction in repetitive mild traumatic brain injury

Mild traumatic brain injury (mTBI) is a significant national health concern and there is growing evidence that repetitive mTBI (rmTBI) can cause long-term change in brain structure and function. The mitochondrion has been suggested to be involved in the mechanism of TBI. There are noninvasive methods of determining mitochondrial dysfunction through biomarkers and spectroscopy. Mitochondrial dysfunction has been implicated in a variety of neurological consequences secondary to rmTBI through activation of caspases and calpains. The purpose of this review is to examine the mechanism of mitochondrial dysfunction in rmTBI and its downstream effects on neuronal cell death, axonal injury and blood–brain barrier compromise.

Concussion in Ice Hockey: Current Gaps and Future Directions in an Objective Diagnosis

OBJECTIVE

This review provides an update on sport-related concussion (SRC) in ice hockey and makes a case for changes in clinical concussion evaluation. Standard practice should require that concussions be objectively diagnosed and provide quantitative measures of the concussion injury that will serve as a platform for future evidence-based treatment.

METHODS

The literature was surveyed to address several concussion-related topics: research in ice hockey-related head trauma, current subjective diagnosis, promising components of an objective diagnosis, and current and potential treatments.

MAIN RESULTS

Sport-related head trauma has marked physiologic, pathologic, and psychological consequences for athletes. Although animal models have been used to simulate head trauma for pharmacologic testing, the current diagnosis and subsequent treatment in athletes still rely on an athlete's motivation to report or deny symptoms. Bias-free, objective diagnostic measures are needed to guide quantification of concussion severity and assessment of treatment effects. Most of the knowledge and management guidelines of concussion in ice hockey are generalizable to other contact sports.

CONCLUSIONS

There is a need for an objective diagnosis of SRC that will quantify severity, establish a prognosis, and provide effective evidence-based treatment. Potential methods to improve concussion diagnosis by health care providers include a standardized concussion survey, the King-Devick test, a quantified electroencephalogram, and blood analysis for brain cell-specific biomarkers.

What is the Relationship of Traumatic Brain Injury to Dementia?

There is a long history linking traumatic brain injury (TBI) with the development of dementia. Despite significant reservations, such as recall bias or concluding causality for TBI, a summary of recent research points to several conclusions on the TBI-dementia relationship. 1) Increasing severity of a single moderate-to-severe TBI increases the risk of subsequent Alzheimer's disease (AD), the most common type of dementia. 2) Repetitive, often subconcussive, mild TBIs increases the risk for chronic traumatic encephalopathy (CTE), a degenerative neuropathology. 3) TBI may be a risk factor for other neurodegenerative disorders that can be associated with dementia. 4) TBI appears to lower the age of onset of TBI-related neurocognitive syndromes, potentially adding "TBI cognitive-behavioral features". The literature further indicates several specific risk factors for TBI-associated dementia: 5) any blast or blunt physical force to the head as long as there is violent head displacement; 6) decreased cognitive and/or neuronal reserve and the related variable of older age at TBI; and 7) the presence of apolipoprotein E ɛ4 alleles, a genetic risk factor for AD. Finally, there are neuropathological features relating TBI with neurocognitive syndromes: 8) acute TBI results in amyloid pathology and other neurodegenerative proteinopathies; 9) CTE shares features with neurodegenerative dementias; and 10) TBI results in white matter tract and neural network disruptions. Although further research is needed, these ten findings suggest that dose-dependent effects of violent head displacement in vulnerable brains predispose to dementia; among several potential mechanisms is the propagation of abnormal proteins along damaged white matter networks.

Visual system pathology in humans and animal models of blast injury

Injury from blast exposure is becoming a more prevalent cause of death and disability worldwide. The devastating neurological impairments that result from blasts are significant and lifelong. Progress in the development of effective therapies to treat injury has been slowed by its heterogeneous pathology and the dearth of information regarding the cellular mechanisms involved. Within the last decade, a number of studies have documented visual dysfunction following injury. This brief review examines damage to the visual system in both humans and animal models of blast injury. The in vivo use of the retina as a surrogate to evaluate brain injury following exposure to blast is also highlighted.

Exendin-4 attenuates blast traumatic brain injury induced cognitive impairments, losses of synaptophysin and in vitro TBI-induced hippocampal cellular degeneration

Mild blast traumatic brain injury (B-TBI) induced lasting cognitive impairments in novel object recognition and less severe deficits in Y-maze behaviors. B-TBI significantly reduced the levels of synaptophysin (SYP) protein staining in cortical (CTX) and hippocampal (HIPP) tissues. Treatment with exendin-4 (Ex-4) delivered by subcutaneous micro-osmotic pumps 48 hours prior to or 2 hours immediately after B-TBI prevented the induction of both cognitive deficits and B-TBI induced changes in SYP staining. The effects of a series of biaxial stretch injuries (BSI) on a neuronal derived cell line, HT22 cells, were assessed in an in vitro model of TBI. Biaxial stretch damage induced shrunken neurites and cell death. Treatment of HT22 cultures with Ex-4 (25 to 100 nM), prior to injury, attenuated the cytotoxic effects of BSI and preserved neurite length similar to sham treated cells. These data imply that treatment with Ex-4 may represent a viable option for the management of secondary events triggered by blast-induced, mild traumatic brain injury that is commonly observed in militarized zones.

Considerations for Experimental Animal Models of Concussion, Traumatic Brain Injury, and Chronic Traumatic Encephalopathy-These Matters Matter

Animal models of concussion, traumatic brain injury (TBI), and chronic traumatic encephalopathy (CTE) are widely available and routinely deployed in laboratories around the world. Effective animal modeling requires careful consideration of four basic principles. First, animal model use must be guided by clarity of definitions regarding the human disease or condition being modeled. Concussion, TBI, and CTE represent distinct clinical entities that require clear differentiation: concussion is a neurological syndrome, TBI is a neurological event, and CTE is a neurological disease. While these conditions are all associated with head injury, the pathophysiology, clinical course, and medical management of each are distinct. Investigators who use animal models of these conditions must take into account these clinical distinctions to avoid misinterpretation of results and category mistakes. Second, model selection must be grounded by clarity of purpose with respect to experimental questions and frame of reference of the investigation. Distinguishing injury context ("inputs") from injury consequences ("outputs") may be helpful during animal model selection, experimental design and execution, and interpretation of results. Vigilance is required to rout out, or rigorously control for, model artifacts with potential to interfere with primary endpoints. The widespread use of anesthetics in many animal models illustrates the many ways that model artifacts can confound preclinical results. Third, concordance between key features of the animal model and the human disease or condition being modeled is required to confirm model biofidelity. Fourth, experimental results observed in animals must be confirmed in human subjects for model validation. Adherence to these principles serves as a bulwark against flawed interpretation of results, study replication failure, and confusion in the field. Implementing these principles will advance basic science discovery and accelerate clinical translation to benefit people affected by concussion, TBI, and CTE.

The Current Status of Research on Chronic Traumatic Encephalopathy

Chronic traumatic encephalopathy (CTE) evolved from the term dementia pugilistica describing the dementia found in many boxers to its current use in describing the dementia and depression sometimes found in athletes subjected to multiple concussions or subconcussive blows to the head. Concurrently, the neuropathology evolved to specify a unique type of tauopathy found in perivascular spaces at the depth of sulci and other features not typically seen in neurodegenerative tauopathies. Four stages of CTE have been proposed, with 4 corresponding clinical syndromes of traumatic encephalopathy syndrome. However, it remains unclear whether this is a syndrome unique to repetitive head trauma, especially in contact sports, because the epidemiology has been difficult to establish. In particular, research to date has had a denominator problem in not establishing the total number of potential cases at risk for developing CTE. The current review examines the evidence to date for these syndromes and contributing or complicating factors affecting the neuropathology, neuroimaging, and clinical presentations associated with them.

A Repetitive Concussive Head Injury Model in Mice

Despite the concussion/ mild traumatic brain injury (mTBI) being the most frequent occurrence of traumatic brain injury, there is still a lack of knowledge on the injury and its effects. To develop a better understanding of concussions, animals are often used because they provide a controlled, rigorous, and efficient model. Studies have adapted traditional animal models to perform mTBI to stimulate mild injury severity by changing the injury parameters. These models have been used because they can produce morphologically similar brain injuries to the clinical condition and provide a spectrum of injury severities. However, they are limited in their ability to present the identical features of injuries in patients. Using a traditional impact system, a repetitive concussive injury (rCHI) model can induce mild to moderate human-like concussion. The injury degree can be determined by measuring the period of loss of consciousness (LOC) with a sign of a transient termination of breathing. The rCHI model is beneficial to use for its accuracy and simplicity in determining mTBI effects and potential treatments.

Cumulative Head Impact Exposure Predicts Later-Life Depression, Apathy, Executive Dysfunction, and Cognitive Impairment in Former High School and College Football Players

The term "repetitive head impacts" (RHI) refers to the cumulative exposure to concussive and subconcussive events. Although RHI are believed to increase risk for later-life neurological consequences (including chronic traumatic encephalopathy), quantitative analysis of this relationship has not yet been examined because of the lack of validated tools to quantify lifetime RHI exposure. The objectives of this study were: 1) to develop a metric to quantify cumulative RHI exposure from football, which we term the "cumulative head impact index" (CHII); 2) to use the CHII to examine the association between RHI exposure and long-term clinical outcomes; and 3) to evaluate its predictive properties relative to other exposure metrics (i.e., duration of play, age of first exposure, concussion history). Participants included 93 former high school and collegiate football players who completed objective cognitive and self-reported behavioral/mood tests as part of a larger ongoing longitudinal study. Using established cutoff scores, we transformed continuous outcomes into dichotomous variables (normal vs. impaired). The CHII was computed for each participant and derived from a combination of self-reported athletic history (i.e., number of seasons, position[s], levels played), and impact frequencies reported in helmet accelerometer studies. A bivariate probit, instrumental variable model revealed a threshold dose-response relationship between the CHII and risk for later-life cognitive impairment (p < 0.0001), self-reported executive dysfunction (p < 0.0001), depression (p < 0.0001), apathy (p = 0.0161), and behavioral dysregulation (p < 0.0001). Ultimately, the CHII demonstrated greater predictive validity than other individual exposure metrics.

Synaptic Mechanisms of Blast-Induced Brain Injury

Blast wave-induced traumatic brain injury (TBI) is one of the most common injuries to military personnel. Brain tissue compression/tension due to blast-induced cranial deformations and shear waves due to head rotation may generate diffuse micro-damage to neuro-axonal structures and trigger a cascade of neurobiological events culminating in cognitive and neurodegenerative disorders. Although diffuse axonal injury is regarded as a signature wound of mild TBI (mTBI), blast loads may also cause synaptic injury wherein neuronal synapses are stretched and sheared. This synaptic injury may result in temporary disconnect of the neural circuitry and transient loss in neuronal communication. We hypothesize that mTBI symptoms such as loss of consciousness or dizziness, which start immediately after the insult, could be attributed to synaptic injury. Although empirical evidence is beginning to emerge; the detailed mechanisms underlying synaptic injury are still elusive. Coordinated in vitro-in vivo experiments and mathematical modeling studies can shed light into the synaptic injury mechanisms and their role in the potentiation of mTBI symptoms.

The role of biomarkers and MEG-based imaging markers in the diagnosis of post-traumatic stress disorder and blast-induced mild traumatic brain injury

BACKGROUND

Pervasive use of improvised explosive devices (IEDs), rocket-propelled grenades, and land mines in the recent conflicts in Iraq and Afghanistan has brought traumatic brain injury (TBI) and its impact on health outcomes into public awareness. Blast injuries have been deemed signature wounds of these wars. War-related TBI is not new, having become prevalent during WWI and remaining medically relevant in WWII and beyond. Medicine's past attempts to accurately diagnose and disentangle the pathophysiology of war-related TBI parallels current lines of inquiry and highlights limitations in methodology and attribution of symptom etiology, be it organic, psychological, or behavioral. New approaches and biomarkers are needed.

PRECLINICAL

Serological biomarkers and biomarkers of injury obtained with imaging techniques represent cornerstones in the translation between experimental data and clinical observations. Experimental models for blast related TBI and PTSD can generate critical data on injury threshold, for example for white matter injury from acceleration. Carefully verified and validated models can be evaluated with gene expression arrays and proteomics to identify new candidates for serological biomarkers. Such models can also be analyzed with diffusion MRI and microscopy in order to identify criteria for detection of diffuse white matter injuries, such as DAI (diffuse axonal injury). The experimental models can also be analyzed with focus on injury outcome in brain stem regions, such as locus coeruleus or nucleus raphe magnus that can be involved in response to anxiety changes.

CLINICAL

Mild (and some moderate) TBI can be difficult to diagnose because the injuries are often not detectable on conventional MRI or CT. There is accumulating evidence that injured brain tissues in TBI patients generate abnormal low-frequency magnetic activity (ALFMA, peaked at 1-4Hz) that can be measured and localized by magnetoencephalography (MEG). MEG imaging detects TBI abnormalities at the rates of 87% for the mild TBI, group (blast-induced plus non-blast causes) and 100% for the moderate group. Among the mild TBI patients, the rates of abnormalities are 96% and 77% for the blast and non-blast TBI groups, respectively. There is emerging evidence based on fMRI and MEG studies showing hyper-activity in the amygdala and hypo-activity in pre-frontal cortex in individuals with PTSD. MEG signal may serve as a sensitive imaging marker for mTBI, distinguishable from abnormalities generated in association with PTSD. More work is needed to fully describe physiological mechanisms of post-concussive symptoms.

Challenging the Paradigms of Experimental TBI Models: From Preclinical to Clinical Practice

Despite prodigious advances in TBI neurobiology research and a broad arsenal of animal models mimicking different aspects of human brain injury, this field has repeatedly experienced collective failures to translate from animals to humans, particularly in the area of therapeutics. This lack of success stems from variability and inconsistent standardization across models and laboratories, as well as insufficient objective and quantifiable diagnostic measures (biomarkers, high-resolution imaging), understanding of the vast clinical heterogeneity, and clinically centered conception of the TBI animal models. Significant progress has been made by establishing well-defined standards for reporting animal studies with "preclinical common data elements" (CDE), and for the reliability and reproducibility in preclinical TBI therapeutic research with the Operation Brain Trauma Therapy (OBTT) consortium. However, to break the chain of failures and achieve a therapeutic breakthrough in TBI will probably require the use of higher species models, specific mechanism-based injury models by which to theranostically targeted treatment portfolios are tested, more creative concepts of therapy intervention including combination therapy and regeneration neurobiology strategies, and the adoption of dosing regimens based upon pharmacokinetic-pharmacodynamic (PK-PD) studies and guided by the injury severity and TBI recovery process.

A critical review of chronic traumatic encephalopathy

Chronic traumatic encephalopathy (CTE) has been described in the literature as a neurodegenerative disease with: (i) localized neuronal and glial accumulations of phosphorylated tau (p-tau) involving perivascular areas of the cerebral cortex, sulcal depths, and with a preference for neurons within superficial cortical laminae; (ii) multifocal axonal varicosities and axonal loss involving deep cortex and subcortical white matter; (iii) relative absence of beta-amyloid deposits; (iv) TDP-43 immunoreactive inclusions and neurites; and (v) broad and diverse clinical features. Some of the pathological findings reported in the literature may be encountered with age and other neurodegenerative diseases. However, the focality of the p-tau cortical findings in particular, and the regional distribution, are believed to be unique to CTE. The described clinical features in recent cases are very similar to how depression manifests in middle-aged men and with frontotemporal dementia as the disease progresses. It has not been established that the described tau pathology, especially in small amounts, can cause complex changes in behavior such as depression, substance abuse, suicidality, personality changes, or cognitive impairment. Future studies will help determine the extent to which the neuropathology is causally related to the diverse clinical features.

Repetitive head trauma, chronic traumatic encephalopathy and tau: Challenges in translating from mice to men

Chronic traumatic encephalopathy (CTE) is a neurological and psychiatric condition marked by preferential perivascular foci of neurofibrillary and glial tangles (composed of hyperphosphorylated-tau proteins) in the depths of the sulci. Recent retrospective case series published over the last decade on athletes and military personnel have added considerably to our clinical and histopathological knowledge of CTE. This has marked a vital turning point in the traumatic brain injury (TBI) field, raising public awareness of the potential long-term effects of mild and moderate repetitive TBI, which has been recognized as one of the major risk factors associated with CTE. Although these human studies have been informative, their retrospective design carries certain inherent limitations that should be cautiously interpreted. In particular, the current overriding issue in the CTE literature remains confusing in regard to appropriate definitions of terminology, variability in individual pathologies and the potential case selection bias in autopsy based studies. There are currently no epidemiological or prospective studies on CTE. Controlled preclinical studies in animals therefore provide an alternative means for specifically interrogating aspects of CTE pathogenesis. In this article, we review the current literature and discuss difficulties and challenges of developing in-vivo TBI experimental paradigms to explore the link between repetitive head trauma and tau-dependent changes. We provide our current opinion list of recommended features to consider for successfully modeling CTE in animals to better understand the pathobiology and develop therapeutics and diagnostics, and critical factors, which might influence outcome. We finally discuss the possible directions of future experimental research in the repetitive TBI/CTE field.

Vascular and inflammatory factors in the pathophysiology of blast-induced brain injury

Blast-related traumatic brain injury (TBI) has received much recent attention because of its frequency in the conflicts in Iraq and Afghanistan. This renewed interest has led to a rapid expansion of clinical and animal studies related to blast. In humans, high-level blast exposure is associated with a prominent hemorrhagic component. In animal models, blast exerts a variety of effects on the nervous system including vascular and inflammatory effects that can be seen with even low-level blast exposures which produce minimal or no neuronal pathology. Acutely, blast exposure in animals causes prominent vasospasm and decreased cerebral blood flow along with blood-brain barrier breakdown and increased vascular permeability. Besides direct effects on the central nervous system, evidence supports a role for a thoracically mediated effect of blast; whereby, pressure waves transmitted through the systemic circulation damage the brain. Chronically, a vascular pathology has been observed that is associated with alterations of the vascular extracellular matrix. Sustained microglial and astroglial reactions occur after blast exposure. Markers of a central and peripheral inflammatory response are found for sustained periods after blast injury and include elevation of inflammatory cytokines and other inflammatory mediators. At low levels of blast exposure, a microvascular pathology has been observed in the presence of an otherwise normal brain parenchyma, suggesting that the vasculature may be selectively vulnerable to blast injury. Chronic immune activation in brain following vascular injury may lead to neurobehavioral changes in the absence of direct neuronal pathology. Strategies aimed at preventing or reversing vascular damage or modulating the immune response may improve the chronic neuropsychiatric symptoms associated with blast-related TBI.

Post-traumatic neurodegeneration and chronic traumatic encephalopathy

Traumatic brain injury (TBI) is a leading cause of mortality and morbidity around the world. Concussive and subconcussive forms of closed-head injury due to impact or blast neurotrauma represent the most common types of TBI in civilian and military settings. It is becoming increasingly evident that TBI can lead to persistent, long-term debilitating effects, and in some cases, progressive neurodegeneration and chronic traumatic encephalopathy (CTE). The epidemiological literature suggests that a single moderate-to-severe TBI may be associated with accelerated neurodegeneration and increased risk of Alzheimer's disease, Parkinson's disease, or motor neuron disease. However, the pathologic phenotype of these post-traumatic neurodegenerations is largely unknown and there may be pathobiological differences between post-traumatic disease and the corresponding sporadic disorder. By contrast, the pathology of CTE is increasingly well known and is characterized by a distinctive pattern of progressive brain atrophy and accumulation of hyperphosphorylated tau neurofibrillary and glial tangles, dystrophic neurites, 43 kDa TAR DNA-binding protein (TDP-43) neuronal and glial aggregates, microvasculopathy, myelinated axonopathy, neuroinflammation, and white matter degeneration. Clinically, CTE is associated with behavioral changes, executive dysfunction, memory deficits, and cognitive impairments that begin insidiously and most often progress slowly over decades. Although research on the long-term effects of TBI is advancing quickly, the incidence and prevalence of post-traumatic neurodegeneration and CTE are unknown. Critical knowledge gaps include elucidation of pathogenic mechanisms, identification of genetic risk factors, and clarification of relevant variables-including age at exposure to trauma, history of prior and subsequent head trauma, substance use, gender, stress, and comorbidities-all of which may contribute to risk profiles and the development of post-traumatic neurodegeneration and CTE. This article is part of a Special Issue entitled 'Traumatic Brain Injury'.

Ice Hockey Summit II: zero tolerance for head hits and fighting

OBJECTIVE

To present currently known basic science and on-ice influences of sport-related concussion (SRC) in hockey, building on the Ice Hockey Summit I action plan (2011) to reduce SRC.

METHODS

The prior summit proceedings included an action plan intended to reduce SRC. As such, the proceedings from Summit I served as a point of departure, for the science and discussion held during Summit II (Mayo Clinic, Rochester MN, October 2013). Summit II focused on (1) Basic Science of Concussions in Ice Hockey: Taking Science Forward; (2) Acute and Chronic Concussion Care: Making a Difference; (3) Preventing Concussions via Behavior, Rules, Education and Measuring Effectiveness; (4) Updates in Equipment: their Relationship to Industry Standards; and (5) Policies and Plans at State, National and Federal Levels to reduce SRC. Action strategies derived from the presentations and discussion described in these sectors were subsequently voted on for purposes of prioritization. The following proceedings include knowledge and research shared by invited faculty, many of whom are health care providers and clinical investigators.

RESULTS

The Summit II evidence-based action plan emphasizes the rapidly evolving scientific content of hockey SRC. It includes the most highly prioritized strategies voted on for implementation to decrease concussion.

CONCLUSIONS

The highest priority action items identified from the Summit includes the following: (1) eliminate head hits from all levels of ice hockey, (2) change body-checking policies, and (3) eliminate fighting in all amateur and professional hockey.