Quantitative relationship between axonal injury and mechanical response in a rodent head impact acceleration model

Establishing a 3-Tesla Magnetic Resonance Imaging Method for Assessing Diffuse Axonal Brain Injury in Rats

Traumatic brain injury (TBI) significantly contributes to death and disability worldwide. However, treatment options remain limited. Here, we focus on a specific pathology of TBI, diffuse axonal brain injury (DABI), which describes the process of the tearing of nerve fibers in the brain after blunt injury. Most protocols to study DABI do not incorporate a specific model for that type of pathology, limiting their ability to identify mechanisms and comorbidities of DABI. In this study, we developed a magnetic resonance imaging (MRI) protocol for DABI in a rat model using a 3-T clinical scanner. We compared the neuroimaging outcomes with histologic and neurologic assessments. In a sample size of 10 rats in the sham group and 10 rats in the DABI group, we established neurological severity scores before the intervention and at 48 h following DABI induction. After the neurological evaluation after DABI, all rats underwent MRI scans and were subsequently euthanized for histological evaluation. As expected, the neurological assessment showed a high sensitivity for DABI lesions indicated using the β-APP marker. Surprisingly, however, we found that the MRI method had greater sensitivity in assessing DABI lesions compared to histological methods. Out of the five MRI parameters with pathological changes in the DABI model, we found significant changes compared to sham rats in three parameters, and, as shown using comparative tests with other models, MRI was the most sensitive parameter, being even more sensitive than histology. We anticipate that this DABI protocol will have a significant impact on future TBI and DABI studies, advancing research on treatments specifically targeted towards improving patient quality of life and long-term outcomes.

Insights from Rodent Models for Improving Bench-to-Bedside Translation in Traumatic Brain Injury

Road accidents, domestic falls, and persons associated with sports and military services exhibited the concussion or contusion type of traumatic brain injury (TBI) that resulted in chronic traumatic encephalopathy. In some instances, these complex neurological aberrations pose severe brain damage and devastating long-term neurological sequelae. Several preclinical (rat and mouse) TBI models simulate the clinical TBI endophenotypes. Moreover, many investigational neuroprotective candidates showed promising effects in these models; however, the therapeutic success of these screening candidates has been discouraging at various stages of clinical trials. Thus, a correct selection of screening model that recapitulates the clinical neurobiology and endophenotypes of concussion or contusion is essential. Herein, we summarize the advantages and caveats of different preclinical models adopted for TBI research. We suggest that an accurate selection of experimental TBI models may improve the translational viability of the investigational entity.

Traumatic Brain Injury in Alpine Winter Sports: Comparison of Two Case Series from a Swiss Trauma Center 30 Years Apart

BACKGROUND

 Between 3 and 15% of winter sports-related injuries are related to head injuries, which are the primary cause of mortality and disability among skiers. Despite the widespread adoption of helmets in winter sports, which has reduced the incidence of direct head injury, there is a paradoxical trend of an increasing number of individuals wearing helmets sustaining diffuse axonal injuries (DAI), which can result in severe neurologic sequelae.

METHODS

 We retrospectively reviewed 100 cases collected by the senior author of this work from 13 full winter seasons during the period from 1981 to 1993 and compared them with 17 patients admitted during the more shortened 2019 to 2020 ski season due to COVID-19. All data analyzed come from a single institution. Population characteristics, mechanism of injury, helmet use, need for surgical treatment, diagnosis, and outcome were collected. Descriptive statistics were used to compare the two databases.

RESULTS

 From February 1981 to January 2020, most skiers with head injuries were men (76% for the 1981-1993 and 85% for 2020). The proportion of patients aged over 50 increased from <20% in 1981 to 65% in 2020 (p < 0.01), with a median age of 60 years (range: 22-83 years). Low- to medium-velocity injuries were identified in 76% (13) of cases during the 2019 to 2020 season against 38% (28/74) during the 1981 to 1993 seasons (p < 0.01). All injured patients during the 2020 season wore a helmet, whereas none of the patients between 1981 and 1993 wore one (p < 0.01). DAI was observed in six cases (35%) for the 2019 to 2020 season against nine cases (9%) for the 1981 to 1993 season (p < 0.01). Thirty-four percent (34) of patients during the 1981 to 1993 seasons and 18% (3) of patients during the 2019 to 2020 season suffered skeletal fractures (p = 0.02). Among the 100 patients of the 1981 to 1993 seasons, 13 (13%) died against 1 (6%) from the recent season during care at the hospital (p = 0.15). Neurosurgical intervention was performed in 30 (30%) and 2 (12%) patients for the 1981 to 1993 and 2019 to 2020 seasons, respectively (p = 0.003). Neuropsychological sequelae were reported in 17% (7/42) of patients from the 1981 to 1993 seasons and cognitive evaluation before discharge detected significant impairments in 24% (4/17) of the patients from the 2019 to 2020 season (p = 0.29).

CONCLUSION

 Helmet use among skiers sustaining head trauma has increased from none in the period from 1981 to 1993 to 100% during the 2019 to 2020 season, resulting in a reduction in the number of skull fractures and deaths. However, our observations suggest a marked shift in the type of intracranial injuries sustained, including a rise in the number of skiers experiencing DAI, sometimes with severe neurologic outcomes. The reasons for this paradoxical trend can only be speculated upon, leading to the question of whether the perceived benefits of helmet use in winter sports are actually misinterpreted.

Loss of Consciousness and Righting Reflex Following Traumatic Brain Injury: Predictors of Post-Injury Symptom Development (A Narrative Review)

Identifying predictors for individuals vulnerable to the adverse effects of traumatic brain injury (TBI) remains an ongoing research pursuit. This is especially important for patients with mild TBI (mTBI), whose condition is often overlooked. TBI severity in humans is determined by several criteria, including the duration of loss of consciousness (LOC): LOC < 30 min for mTBI and LOC > 30 min for moderate-to-severe TBI. However, in experimental TBI models, there is no standard guideline for assessing the severity of TBI. One commonly used metric is the loss of righting reflex (LRR), a rodent analogue of LOC. However, LRR is highly variable across studies and rodents, making strict numeric cutoffs difficult to define. Instead, LRR may best be used as predictor of symptom development and severity. This review summarizes the current knowledge on the associations between LOC and outcomes after mTBI in humans and between LRR and outcomes after experimental TBI in rodents. In clinical literature, LOC following mTBI is associated with various adverse outcome measures, such as cognitive and memory deficits; psychiatric disorders; physical symptoms; and brain abnormalities associated with the aforementioned impairments. In preclinical studies, longer LRR following TBI is associated with greater motor and sensorimotor impairments; cognitive and memory impairments; peripheral and neuropathology; and physiologic abnormalities. Because of the similarities in associations, LRR in experimental TBI models may serve as a useful proxy for LOC to contribute to the ongoing development of evidence-based personalized treatment strategies for patients sustaining head trauma. Analysis of highly symptomatic rodents may shed light on the biological underpinnings of symptom development after rodent TBI, which may translate to therapeutic targets for mTBI in humans.

Spatial and temporal dynamics of HDACs class IIa following mild traumatic brain injury in adult rats

The fundamental role of epigenetic regulatory mechanisms involved in neuroplasticity and adaptive responses to traumatic brain injury (TBI) is gaining increased recognition. TBI-induced neurodegeneration is associated with several changes in the expression-activity of various epigenetic regulatory enzymes, including histone deacetylases (HDACs). In this study, PET/CT with 6-([¹⁸F]trifluoroacetamido)-1- hexanoicanilide ([¹⁸F]TFAHA) to image spatial and temporal dynamics of HDACs class IIa expression-activity in brains of adult rats subjected to a weight drop model of diffuse, non-penetrating, mild traumatic brain injury (mTBI). The mTBI model was validated by histopathological and immunohistochemical analyses of brain tissue sections for localization and magnitude of expression of heat-shock protein-70 kDa (HSP70), amyloid precursor protein (APP), cannabinoid receptor-2 (CB2), ionized calcium-binding adapter protein-1 (IBA1), histone deacetylase-4 and -5 (HDAC4 and HDAC5). In comparison to baseline, the expression-activities of HDAC4 and HDAC5 were downregulated in the hippocampus, nucleus accumbens, peri-3rd ventricular part of the thalamus, and substantia nigra at 1-3 days post mTBI, and remained low at 7-8 days post mTBI. Reduced levels of HDAC4 and HDAC5 expression observed in neurons of these brain regions post mTBI were associated with the reduced nuclear and neuropil levels of HDAC4 and HDAC5 with the shift to perinuclear localization of these enzymes. These results support the rationale for the development of therapeutic strategies to upregulate expression-activity of HDACs class IIa post-TBI. PET/CT (MRI) with [¹⁸F]TFAHA can facilitate the development and clinical translation of unique therapeutic approaches to upregulate the expression and activity of HDACs class IIa enzymes in the brain after TBI.

Animal models of traumatic brain injury: a review of pathophysiology to biomarkers and treatments

Traumatic brain injury (TBI) is one of the main causes of death and disability in both civilian and military population. TBI may occur via a variety of etiologies, all of which involve trauma to the head. However, the neuroprotective drugs which were found to be very effective in animal TBI models failed in phase II or phase III clinical trials, emphasizing a compelling need to review the current status of animal TBI models and therapeutic strategies. No single animal model can adequately mimic all aspects of human TBI owing to the heterogeneity of clinical TBI. However, due to the ethical limitations, it is difficult to precisely emulate the TBI mechanisms that occur in humans. Therefore, many animal models with varying severity and mechanisms of brain injury have been developed, and each model has its own pros and cons in its implementation for TBI research. These challenges pose a need for study of continued TBI mechanisms, brain injury severity, duration, treatment strategies, and optimization of animal models across the neurotrauma research community. The aim of this review is to discuss (1) causes of TBI, (2) its prevalence in military and civilian population, (3) classification and pathophysiology of TBI, (4) biomarkers and detection methods, (5) animal models of TBI, and (6) the advantages and disadvantages of each model and the species used, as well as possible treatments.

Myelin basic protein and neurofilament H in postmortem cerebrospinal fluid as surrogate markers of fatal traumatic brain injury

The aim of this study was to investigate if the biomarkers myelin basic protein (MBP) and neurofilament-H (NF-H) yielded informative value in forensic diagnostics when examining cadaveric cerebrospinal fluid (CSF) biochemically via an enzyme-linked immunosorbent assay (ELISA) and comparing the corresponding brain tissue in fatal traumatic brain injury (TBI) autopsy cases by immunocytochemistry versus immunohistochemistry. In 21 trauma and 19 control cases, CSF was collected semi-sterile after suboccipital puncture and brain specimens after preparation. The CSF MBP (p = 0.006) and NF-H (p = 0.0002) levels after TBI were significantly higher than those in cardiovascular controls. Immunohistochemical staining against MBP and against NF-H was performed on cortical and subcortical samples from also biochemically investigated cases (5 TBI cases/5 controls). Compared to the controls, the TBI cases showed a visually reduced staining reaction against MBP or repeatedly ruptured neurofilaments against NF-H. Immunocytochemical tests showed MBP-positive phagocytizing macrophages in CSF with a survival time of > 24 h. In addition, numerous TMEM119-positive microglia could be detected with different degrees of staining intensity in the CSF of trauma cases. As a result, we were able to document that elevated levels of MBP and NF-H in the CSF should be considered as useful neuroinjury biomarkers of traumatic brain injury.

An Automated Kinematic Measurement System for Sagittal Plane Murine Head Impacts

Mild traumatic brain injuries are typically caused by nonpenetrating head impacts that accelerate the skull and result in deformation of the brain within the skull. The shear and compressive strains caused by these deformations damage neural and vascular structures and impair their function. Accurate head acceleration measurements are necessary to define the nature of the insult to the brain. A novel murine head tracking system was developed to improve the accuracy and efficiency of kinematic measurements obtained with high-speed videography. A three-dimensional (3D)-printed marker carrier was designed for rigid fixation to the upper jaw and incisors with an elastic strap around the snout. The system was evaluated by impacting cadaveric mice with the closed head impact model of engineered rotational acceleration (CHIMERA) system using an energy of 0.7 J (5.29 m/s). We compared the performance of the head-marker system to the previously used skin-tracking method and documented significant improvements in measurement repeatability (aggregate coefficient of variation (CV) within raters from 15.8 to 1.5 and between raters from 15.5 to 1.5), agreement (aggregate percentage error from 24.9 to 8.7), and temporal response (aggregate temporal curve agreement from 0.668 to 0.941). Additionally, the new system allows for automated software tracking, which dramatically decreases the analysis time required (74% reduction). This novel head tracking system for mice offers an efficient, reliable, and real-time method to measure head kinematics during high-speed impacts using CHIMERA or other rodent or small mammal head impact models.

Investigate the Variations of the Head and Brain Response in a Rodent Head Impact Acceleration Model by Finite Element Modeling

Diffuse axonal injury (DAI) is a severe form of traumatic brain injury and often induced by blunt trauma. The closed head impact acceleration (IA) model is the most widely used rodent DAI model. However, this model results in large variations of injury severity. Recently, the impact device/system was modified to improve the consistency of the impact energy, but variations of the head kinematics and subsequent brain injuries were still observed. This study was aimed to utilize a Finite Element (FE) model of a rat head/body and simulation to investigate the potential biomechanical factors influencing the impact energy transfer to the head. A detailed FE rat head model containing detailed skull and brain anatomy was developed based on the MRI, microCT and atlas data. The model consists of over 722,000 elements, of which 310,000 are in the brain. The white matter structures consisting of highly aligned axonal fibers were simulated with transversely isotropic material. The rat body was modeled to provide a realistic boundary at the spine-medulla junction. Rodent experiments including dynamic cortical deformation, brain-skull displacement, and IA kinematics were simulated to validate the FE model. The model was then applied to simulate the rat IA experiments. Parametric studies were conducted to investigate the effect of the helmet inclination angles (0°-5°) and skull stiffness (varied 20%) on the resulting head kinematics and maximum principal strain in the brain. The inclination angle of the helmet at 5° could vary head linear acceleration by 8-31%. The change in head rotational velocity was inversely related to the change in linear acceleration. Varying skull stiffness resulted in changes in head linear acceleration by 3% but with no effect on rotational velocity. The brain strain in the corpus callosum was only affected by head rotation while the strain in the brainstem was influenced by the combined head kinematics, local skull deformation, and head-neck position. Validated FE models of rat impact head injury can assist in exploring various biomechanical factors influencing the head impact response and internal brain response. Identification of these variables may help explain the variability of injury severity observed among experiments and across different labs.

Characterization of Ionic and Lipid Gradients within Corpus Callosum White Matter after Diffuse Traumatic Brain Injury in the Rat

There is increased recognition of the effects of diffuse traumatic brain injury (dTBI), which can initiate yet unknown biochemical cascades, resulting in delayed secondary brain degeneration and long-term neurological sequela. There is limited availability of therapies that minimize the effect of secondary brain damage on the quality of life of people who have suffered TBI, many of which were otherwise healthy adults. Understanding the cascade of biochemical events initiated in specific brain regions in the acute phase of dTBI and how this spreads into adjacent brain structures may provide the necessary insight into drive development of improved therapies. In this study, we have used direct biochemical imaging techniques (Fourier transform infrared spectroscopic imaging) and elemental mapping (X-ray fluorescence microscopy) to characterize biochemical and elemental alterations that occur in corpus callosum white matter in the acute phase of dTBI. The results provide direct visualization of differential biochemical and ionic changes that occur in the highly vulnerable medial corpus callosum white matter relative to the less vulnerable lateral regions of the corpus callosum. Specifically, the results suggest that altered ionic gradients manifest within mechanically damaged medial corpus callosum, potentially spreading to and inducing lipid alterations to white matter structures in lateral brain regions.

Targeting toll-like receptor 4 to modulate neuroinflammation in central nervous system disorders

Introduction: Adverse immune activation contributes to many central nervous system (CNS) disorders. All main CNS cell types express toll-like receptor 4 (TLR 4). This receptor is critical for a myriad of immune functions such as cytokine secretion and phagocytic activity of microglia; however, imbalances in TLR 4 activation can contribute to the progression of neurodegenerative diseases. Areas covered: We considered available evidence implicating TLR 4 activation in the following CNS pathologies: Alzheimer's disease, Parkinson's disease, ischemic stroke, traumatic brain injury, multiple sclerosis, multiple systems atrophy, and Huntington's disease. We reviewed studies reporting effects of TLR 4-specific antagonists and agonists in models of peripheral and CNS diseases from the perspective of possible future use of TLR 4 ligands in CNS disorders. Expert opinion: TLR 4-specific antagonists could suppress neuroinflammation by reducing overproduction of inflammatory mediators; however, they may interfere with protein clearance mechanisms and myelination. Agonists that specifically activate myeloid differentiation primary-response protein 88 (MyD88)-independent pathway of TLR 4 signaling could facilitate beneficial glial phagocytic activity with limited activity as inducers of proinflammatory mediators. Deciphering the disease stage-specific involvement of TLR 4 in CNS pathologies is crucial for the future clinical development of TLR 4 agonists and antagonists.

Models used in the study of traumatic brain injury

Traumatic brain injury (TBI) is a contemporary health problem and a leading cause of mortality and morbidity worldwide. Survivors of TBI frequently experience disabling long-term changes in cognition, sensorimotor function, and personality. A crucial step in understanding TBI and providing better treatment has been the use of models to mimic the event under controlled conditions. Here, we describe the known head injury models, which can be classified as whole animal (in vivo), in vitro, and mathematical models. We will also review the ways in which these models have advanced the knowledge of TBI.

Profiling biomarkers of traumatic axonal injury: From mouse to man

Traumatic brain injury (TBI) poses a major public health problem on a global scale. Its burden results from high mortality and significant morbidity in survivors. This stems, in part, from an ongoing inadequacy in diagnostic and prognostic indicators despite significant technological advances. Traumatic axonal injury (TAI) is a key driver of the ongoing pathological process following TBI, causing chronic neurological deficits and disability. The science underpinning biomarkers of TAI has been a subject of many reviews in recent literature. However, in this review we provide a comprehensive account of biomarkers from animal models to clinical studies, bridging the gap between experimental science and clinical medicine. We have discussed pathogenesis, temporal kinetics, relationships to neuro-imaging, and, most importantly, clinical applicability in order to provide a holistic perspective of how this could improve TBI diagnosis and predict clinical outcome in a real-life setting. We conclude that early and reliable identification of axonal injury post-TBI with the help of body fluid biomarkers could enhance current care of TBI patients by (i) increasing speed and accuracy of diagnosis, (ii) providing invaluable prognostic information, (iii) allow efficient allocation of rehabilitation services, and (iv) provide potential therapeutic targets. The optimal model for assessing TAI is likely to involve multiple components, including several blood biomarkers and neuro-imaging modalities, at different time points.

Mild and Mild-to-Moderate Traumatic Brain Injury-Induced Significant Progressive and Enduring Multiple Comorbidities

Traumatic brain injury (TBI) can produce life-long disabilities, including anxiety, cognitive, balance, and motor deficits. The experimental model of closed head TBI (cTBI) induced by weight drop/impact acceleration is known to produce hallmark TBI injuries. However, comprehensive long-term characterization of comorbidities induced by graded mild-to- mild/moderate intensities using this experimental cTBI model has not been reported. The present study used two intensities of weight drop (1.0 m and 1.25 m/450 g) to produce cTBI in a rat model to investigate initial and long-term disability of four comorbidities: anxiety, cognitive, vestibulomotor, and spinal reflex that related to spasticity. TBI and sham injuries were produced under general anesthesia. Time for righting recoveries post-TBI recorded to estimate duration of unconsciousness, revealed that the TBI mild/moderate group required a mean of 1 min 27 sec longer than the values observed for noninjured sham animals. Screening magnetic resonance imaging images revealed no anatomical changes, mid-line shifts, or hemorrhagic volumes. However, compared to sham injuries, significant long-term anxiety, cognitive, balance, and physiological changes in motor reflex related to spasticity were observed post-TBI for both TBI intensities. The longitudinal trajectory of anxiety and balance disabilities tested at 2, 4, 8, and 18 weeks revealed progressively worsening disabilities. In general, disability magnitudes were proportional to injury intensity for three of the four measures. A natural hypothesis would pose that all disabilities would increase incrementally relative to injury severity. Surprisingly, anxiety disability progressed over time to be greater in the mildest injury. Collectively, translational implications of these observations suggest that patients with mild TBI should be evaluated longitudinally at multiple time points, and that anxiety disorder could potentially have a particularly low threshold for appearance and progressively worsen post-injury.

Relationship of mechanical impact magnitude to neurologic dysfunction severity in a rat traumatic brain injury model

Objective

Traumatic brain injury (TBI) is a major brain injury type commonly caused by traffic accidents, falls, violence, or sports injuries. To obtain mechanistic insights about TBI, experimental animal models such as weight-drop-induced TBI in rats have been developed to mimic closed-head injury in humans. However, the relationship between the mechanical impact level and neurological severity following weight-drop-induced TBI remains uncertain. In this study, we comprehensively investigated the relationship between physical impact and graded severity at various weight-drop heights.

Approach

The acceleration, impact force, and displacement during the impact were accurately measured using an accelerometer, a pressure sensor, and a high-speed camera, respectively. In addition, the longitudinal changes in neurological deficits and balance function were investigated at 1, 4, and 7 days post TBI lesion. The inflammatory expression markers tested by Western blot analysis, including glial fibrillary acidic protein, beta-amyloid precursor protein, and bone marrow tyrosine kinase gene in chromosome X, in the frontal cortex, hippocampus, and corpus callosum were investigated at 1 and 7 days post-lesion.

Results

Gradations in impact pressure produced progressive degrees of injury severity in the neurological score and balance function. Western blot analysis demonstrated that all inflammatory expression markers were increased at 1 and 7 days post-impact injury when compared to the sham control rats. The severity of neurologic dysfunction and induction in inflammatory markers strongly correlated with the graded mechanical impact levels.

Conclusions

We conclude that the weight-drop-induced TBI model can produce graded brain injury and induction of neurobehavioral deficits and may have translational relevance to developing therapeutic strategies for TBI.

A multi-body dynamics study on a weight-drop test of rat brain injury

Traumatic brain injury (TBI), induced by impact of an object with the head, is a major health problem worldwide. Rats are a well-established animal analogue for study of TBI and the weight-drop impact-acceleration (WDIA) method is a well-established model in rats for creating diffuse TBI, the most common form of TBI seen in humans. However, little is known of the biomechanics of the WDIA method and, to address this, we have developed a four-degrees-of-freedom multi-body mass-spring-damper model for the WDIA test in rats. An analytical expression of the maximum skull acceleration, one of the important head injury predictor, was derived and it shows that the maximum skull acceleration is proportional to the impact velocity but independent of the impactor mass. Furthermore, a dimensional analysis disclosed that the maximum force on the brain and maximum relative displacement between brain and skull are also linearly proportional to impact velocity. Additionally, the effects of the impactor mass were examined through a parametric study from the developed multi-body dynamics model. It was found that increasing impactor mass increased these two brain injury predictors.

Effects of concussion on the blood-brain barrier in humans and rodents

Traumatic brain injury and the long-term consequences of repeated concussions constitute mounting concerns in the United States, with 5.3 million individuals living with a traumatic brain injury-related disability. Attempts to understand mechanisms and possible therapeutic approaches to alleviate the consequences of repeat mild concussions or traumatic brain injury on cerebral vasculature depend on several aspects of the trauma, including: (1) the physical characteristics of trauma or insult that result in damage; (2) the time “window” after trauma in which neuropathological features develop; (3) methods to detect possible breakdown of the blood–brain barrier; and (4) understanding different consequences of a single concussion as compared with multiple concussions. We review the literature to summarize the current understanding of blood–brain barrier and endothelial cell changes post-neurotrauma in concussions and mild traumatic brain injury. Attention is focused on concussion and traumatic brain injury in humans, with a goal of pointing out the gaps in our knowledge and how studies of rodent model systems of concussion may help in filling these gaps. Specifically, we focus on disruptions that concussion causes to the blood–brain barrier and its multifaceted consequences. Importantly, the magnitude of post-concussion blood–brain barrier dysfunction may influence the time course and extent of neuronal recovery; hence, we include in this review comparisons of more severe traumatic brain injury to concussion where appropriate. Finally, we address the important, and still unresolved, issue of how best to detect possible breakdown in the blood–brain barrier following neurotrauma by exploring intravascular tracer injection in animal models to examine leakage into the brain parenchyma.

Short and Long Term Behavioral and Pathological Changes in a Novel Rodent Model of Repetitive Mild Traumatic Brain Injury

A history of concussion, particularly repeated injury, has been linked to an increased risk for the development of neurodegenerative diseases, particularly chronic traumatic encephalopathy (CTE). CTE is characterized by abnormal accumulation of hyperphosphorylated tau and deficits in learning and memory. As yet the mechanisms associated with the development of CTE are unknown. Accordingly, the aim of the current study was to develop and characterize a novel model of repetitive mTBI that accurately reproduces the key short and long-term functional and histopathological features seen clinically. Forty male Sprague-Dawley rats were randomly assigned to receive 0, 1 or 3x mTBI spaced five days apart using a modified version of the Marmarou impact-acceleration diffuse-TBI model to deliver 110G of linear force. Functional outcomes were assessed six and twelve weeks post-injury, with histopathology assessed twenty-four hours and twelve weeks post-injury. Repetitive mTBI resulted in mild spatial and recognition memory deficits as reflected by increased escape latency on the Barnes maze and decreased time spent in the novel arm of the Y maze. There was a trend towards increased anxiety-like behavior, with decreased time spent in the inner portion of the open field. At 24 hours and 12 weeks post injury, repetitive mTBI animals showed increased tau phosphorylation and microglial activation within the cortex. Increases in APP immunoreactivity were observed in repetitive mTBI animals at 12 weeks indicating long-term changes in axonal integrity. This novel model of repetitive mTBI with its persistent cognitive deficits, neuroinflammation, axonal injury and tau hyperphosphorylation, thus represents a clinically relevant experimental approach to further explore the underlying pathogenesis of CTE.

Differential susceptibility of cortical and subcortical inhibitory neurons and astrocytes in the long term following diffuse traumatic brain injury

Long-term diffuse traumatic brain injury (dTBI) causes neuronal hyperexcitation in supragranular layers in sensory cortex, likely through reduced inhibition. Other forms of TBI affect inhibitory interneurons in subcortical areas but it is unknown if this occurs in cortex, or in any brain area in dTBI. We investigated dTBI effects on inhibitory neurons and astrocytes in somatosensory and motor cortex, and hippocampus, 8 weeks post-TBI. Brains were labeled with antibodies against calbindin (CB), parvalbumin (PV), calretinin (CR) and neuropeptide Y (NPY), and somatostatin (SOM) and glial fibrillary acidic protein (GFAP), a marker for astrogliosis during neurodegeneration. Despite persistent behavioral deficits in rotarod performance up to the time of brain extraction (TBI = 73.13 ± 5.23% mean ± SEM, Sham = 92.29 ± 5.56%, P < 0.01), motor cortex showed only a significant increase, in NPY neurons in supragranular layers (mean cells/mm² ± SEM, Sham = 16 ± 0.971, TBI = 25 ± 1.51, P = 0.001). In somatosensory cortex, only CR⁺ neurons showed changes, being decreased in supragranular (TBI = 19 ± 1.18, Sham = 25 ± 1.10, P < 0.01) and increased in infragranular (TBI = 28 ± 1.35, Sham = 24 ± 1.07, P < 0.05) layers. Heterogeneous changes were seen in hippocampal staining: CB⁺ decreased in dentate gyrus (TBI = 2 ± 0.382, Sham = 4 ± 0.383, P < 0.01), PV⁺ increased in CA1 (TBI = 39 ± 1.26, Sham = 33 ± 1.69, P < 0.05) and CA2/3 (TBI = 26 ± 2.10, Sham = 20 ± 1.49, P < 0.05), and CR⁺ decreased in CA1 (TBI = 10 ± 1.02, Sham = 14 ± 1.14, P < 0.05). Astrogliosis significantly increased in corpus callosum (TBI = 6.7 ± 0.69, Sham = 2.5 ± 0.38; P = 0.007). While dTBI effects on inhibitory neurons appear region- and type-specific, a common feature in all cases of decrease was that changes occurred in dendrite targeting interneurons involved in neuronal integration. J. Comp. Neurol. 524:3530-3560, 2016. © 2016 Wiley Periodicals, Inc.

Radiological-pathological correlation of diffusion tensor and magnetization transfer imaging in a closed head traumatic brain injury model

OBJECTIVE

Metrics of diffusion tensor imaging (DTI) and magnetization transfer imaging (MTI) can detect diffuse axonal injury in traumatic brain injury (TBI). The relationship between the changes in these imaging measures and the underlying pathologies is still relatively unknown. This study investigated the radiological-pathological correlation between these imaging techniques and immunohistochemistry using a closed head rat model of TBI.

METHODS

TBI was performed on female rats followed longitudinally by magnetic resonance imaging (MRI) out to 30 days postinjury, with a subset of animals selected for histopathological analyses. A MRI-based finite element analysis was generated to characterize the pattern of the mechanical insult and estimate the extent of brain injury to direct the pathological correlation with imaging findings.

RESULTS

DTI axial diffusivity and fractional anisotropy (FA) were sensitive to axonal integrity, whereas radial diffusivity showed significant correlation to the myelin compactness. FA was correlated with astrogliosis in the gray matter, whereas mean diffusivity was correlated with increased cellularity. Secondary inflammatory responses also partly affected the changes of these DTI metrics. The magnetization transfer ratio (MTR) at 3.5ppm demonstrated a strong correlation with both axon and myelin integrity. Decrease in MTR at 20ppm correlated with the extent of astrogliosis in both gray and white matter.

INTERPRETATION

Although conventional T2-weighted MRI did not detect abnormalities following TBI, DTI and MTI afforded complementary insight into the underlying pathologies reflecting varying injury states over time, and thus may substitute for histology to reveal diffusive axonal injury pathologies in vivo. This correlation of MRI and histology furthers understanding of the microscopic pathology underlying DTI and MTI changes in TBI. Ann Neurol 2016;79:907-920.

A Novel Closed-Head Model of Mild Traumatic Brain Injury Using Focal Primary Overpressure Blast to the Cranium in Mice

Mild traumatic brain injury (TBI) from focal head impact is the most common form of TBI in humans. Animal models, however, typically use direct impact to the exposed dura or skull, or blast to the entire head. We present a detailed characterization of a novel overpressure blast system to create focal closed-head mild TBI in mice. A high-pressure air pulse limited to a 7.5 mm diameter area on the left side of the head overlying the forebrain is delivered to anesthetized mice. The mouse eyes and ears are shielded, and its head and body are cushioned to minimize movement. This approach creates mild TBI by a pressure wave that acts on the brain, with minimal accompanying head acceleration-deceleration. A single 20-psi blast yields no functional deficits or brain injury, while a single 25-40 psi blast yields only slight motor deficits and brain damage. By contrast, a single 50-60 psi blast produces significant visual, motor, and neuropsychiatric impairments and axonal damage and microglial activation in major fiber tracts, but no contusive brain injury. This model thus reproduces the widespread axonal injury and functional impairments characteristic of closed-head mild TBI, without the complications of systemic or ocular blast effects or head acceleration that typically occur in other blast or impact models of closed-skull mild TBI. Accordingly, our model provides a simple way to examine the biomechanics, pathophysiology, and functional deficits that result from TBI and can serve as a reliable platform for testing therapies that reduce brain pathology and deficits.

Traumatic Brain Injury by a Closed Head Injury Device Induces Cerebral Blood Flow Changes and Microhemorrhages

OBJECTIVES

Traumatic brain injury is a poly-pathology characterized by changes in the cerebral blood flow, inflammation, diffuse axonal, cellular, and vascular injuries. However, studies related to understanding the temporal changes in the cerebral blood flow following traumatic brain injury extending to sub-acute periods are limited. In addition, knowledge related to microhemorrhages, such as their detection, localization, and temporal progression, is important in the evaluation of traumatic brain injury.

MATERIALS AND METHODS

Cerebral blood flow changes and microhemorrhages in male Sprague Dawley rats at 4 h, 24 h, 3 days, and 7 days were assessed following a closed head injury induced by the Marmarou impact acceleration device (2 m height, 450 g brass weight). Cerebral blood flow was measured by arterial spin labeling. Microhemorrhages were assessed by susceptibility-weighted imaging and Prussian blue histology.

RESULTS

Traumatic brain injury rats showed reduced regional and global cerebral blood flow at 4 h and 7 days post-injury. Injured rats showed hemorrhagic lesions in the cortex, corpus callosum, hippocampus, and brainstem in susceptibility-weighted imaging. Injured rats also showed Prussian blue reaction products in both the white and gray matter regions up to 7 days after the injury. These lesions were observed in various areas of the cortex, corpus callosum, hippocampus, thalamus, and midbrain.

CONCLUSIONS

These results suggest that changes in cerebral blood flow and hemorrhagic lesions can persist for sub-acute periods after the initial traumatic insult in an animal model. In addition, microhemorrhages otherwise not seen by susceptibility-weighted imaging are present in diverse regions of the brain. The combination of altered cerebral blood flow and microhemorrhages can potentially be a source of secondary injury changes following traumatic brain injury and may need to be taken into consideration in the long-term care of these cases.

Correlation of mechanical impact responses and biomarker levels: A new model for biomarker evaluation in TBI

A modified Marmarou impact acceleration model was used to help screen biomarkers to assess brain injury severity. Anesthetized male Sprague-Dawley rats were subjected to a closed head injury from 1.25, 1.75 and 2.25 m drop heights. Linear and angular responses of the head were measured in vivo. 24h after impact, cerebrospinal fluid (CSF) and serum were collected. CSF and serum levels of phosphorylated neurofilament heavy (pNF-H), glial fibrillary acidic protein (GFAP), interleukin 6 (IL-6), and amyloid beta (Aβ) 1-42 were assessed by enzyme-linked immunosorbent assay (ELISA). Compared to controls, significantly higher CSF and serum pNF-H levels were observed in all impact groups, except between 1.25 m and control in serum. Furthermore, CSF and serum pNF-H levels were significantly different between the impact groups. For GFAP, both CSF and serum levels were significantly higher at 2.25 m compared to 1.75 m, 1.25 m and controls. There was no significant difference in CSF and serum GFAP levels between 1.75 m and 1.25 m, although both groups were significantly higher than control. TBI rats also showed significantly higher levels of IL-6 versus control in both CSF and serum, but no significant difference was observed between each impact group. Levels of Aβ were not significantly different between groups. Pearson's correlation analysis showed pNF-H and GFAP levels in CSF and serum had positive correlation with power (rate of impact energy), followed by average linear acceleration and surface righting (p<0.01), which were good predictors for traumatic axonal injury according to histologic assessment in our previous study, suggesting that they are directly related to the injury mechanism. The model used in this study showed a unique ability in elucidating the relationship between biomarker levels and severity of the mechanical trauma to the brain.

Animal models of traumatic brain injury

Traumatic brain injury (TBI) is a major health issue comprising a heterogeneous and complex array of pathologies. Over the last several decades, numerous animal models have been developed to address the diverse nature of human TBI. The clinical relevance of these models has been a major point of reflection given the poor translation of pharmacologic TBI interventions to the clinic. While previously characterized broadly as either focal or diffuse, this classification is falling out of favor with increased awareness of the overlap in pathologic outcomes between models and an emerging consensus that no one model is sufficient. Moreover, an appreciation of injury biomechanics is essential in recapitulating and interpreting the spectrum of TBI neuropathology observed in various established models of dynamic closed-head TBI. While these models have replicated many specific features of human TBI, an enhanced context with clinical relevancy will facilitate the further elucidation of the mechanisms and treatment of injury.

Merging pathology with biomechanics using CHIMERA (Closed-Head Impact Model of Engineered Rotational Acceleration): a novel, surgery-free model of traumatic brain injury

Background

Traumatic brain injury (TBI) is a major health care concern that currently lacks any effective treatment. Despite promising outcomes from many preclinical studies, clinical evaluations have failed to identify effective pharmacological therapies, suggesting that the translational potential of preclinical models may require improvement. Rodents continue to be the most widely used species for preclinical TBI research. As most human TBIs result from impact to an intact skull, closed head injury (CHI) models are highly relevant, however, traditional CHI models suffer from extensive experimental variability that may be due to poor control over biomechanical inputs. Here we describe a novel CHI model called CHIMERA (Closed-Head Impact Model of Engineered Rotational Acceleration) that fully integrates biomechanical, behavioral, and neuropathological analyses. CHIMERA is distinct from existing neurotrauma model systems in that it uses a completely non-surgical procedure to precisely deliver impacts of prescribed dynamic characteristics to a closed skull while enabling kinematic analysis of unconstrained head movement. In this study, we characterized head kinematics as well as functional, neuropathological, and biochemical outcomes up to 14d following repeated TBI (rTBI) in adult C57BL/6 mice using CHIMERA.

Results

Head kinematic analysis showed excellent repeatability over two closed head impacts separated at 24h. Injured mice showed significantly prolonged loss of righting reflex and displayed neurological, motor, and cognitive deficits along with anxiety-like behavior. Repeated TBI led to diffuse axonal injury with extensive microgliosis in white matter from 2-14d post-rTBI. Injured mouse brains also showed significantly increased levels of TNF-α and IL-1β and increased endogenous tau phosphorylation.

Conclusions

Repeated TBI using CHIMERA mimics many of the functional and pathological characteristics of human TBI with a reliable biomechanical response of the head. This makes CHIMERA well suited to investigate the pathophysiology of TBI and for drug development programs.

Electronic supplementary material

The online version of this article (doi:10.1186/1750-1326-9-55) contains supplementary material, which is available to authorized users.

Viscoelastic properties of the P17 and adult rat brain from indentation in the coronal plane

This technical brief serves as an update to our previous work characterizing the region-dependence of viscoelastic mechanical properties of the P17 and adult rat brain in the coronal plane (Elkin et al., 2011, "A Detailed Viscoelastic Characterization of the P17 and Adult Rat Brain," J. Neurotrauma, 28, pp. 2235-2244.). Here, modifications to the microindentation device provided for the reliable measurement of load during the ramp portion of load relaxation microindentation tests. In addition, a correction factor for finite sample thickness was incorporated to more accurately assess the intrinsic mechanical properties of the tissue.The shear relaxation modulus was significantly dependent on the anatomic region and developmental age, with a general increase in stiffness with age and increased stiffness in the hippocampal and cortical regions compared with the white matter and cerebellar regions of the brain. The shear modulus ranged from ∼0.2 kPa to ∼2.6 kPa depending on region, age, and time scale. Best-fit Prony series parameters from least squares fitting to the indentation data from each region are reported, which describe the shear relaxation behavior for each anatomic region within each age group at both short (&lt;10 ms) and long (∼20 s) time scales. These data will be useful for improving the biofidelity of finite element models of rat brain deformation at short time scales, such as models of traumatic brain injury.

The mechanics of traumatic brain injury: a review of what we know and what we need to know for reducing its societal burden

Traumatic brain injury (TBI) is a significant public health problem, on pace to become the third leading cause of death worldwide by 2020. Moreover, emerging evidence linking repeated mild traumatic brain injury to long-term neurodegenerative disorders points out that TBI can be both an acute disorder and a chronic disease. We are at an important transition point in our understanding of TBI, as past work has generated significant advances in better protecting us against some forms of moderate and severe TBI. However, we still lack a clear understanding of how to study milder forms of injury, such as concussion, or new forms of TBI that can occur from primary blast loading. In this review, we highlight the major advances made in understanding the biomechanical basis of TBI. We point out opportunities to generate significant new advances in our understanding of TBI biomechanics, especially as it appears across the molecular, cellular, and whole organ scale.

Towards clinical management of traumatic brain injury: a review of models and mechanisms from a biomechanical perspective

Traumatic brain injury (TBI) is a major worldwide healthcare problem. Despite promising outcomes from many preclinical studies, the failure of several clinical studies to identify effective therapeutic and pharmacological approaches for TBI suggests that methods to improve the translational potential of preclinical studies are highly desirable. Rodent models of TBI are increasingly in demand for preclinical research, particularly for closed head injury (CHI), which mimics the most common type of TBI observed clinically. Although seemingly simple to establish, CHI models are particularly prone to experimental variability. Promisingly, bioengineering-oriented research has advanced our understanding of the nature of the mechanical forces and resulting head and brain motion during TBI. However, many neuroscience-oriented laboratories lack guidance with respect to fundamental biomechanical principles of TBI. Here, we review key historical and current literature that is relevant to the investigation of TBI from clinical, physiological and biomechanical perspectives, and comment on how the current challenges associated with rodent TBI models, particularly those involving CHI, could be improved.

Characterising effects of impact velocity on brain and behaviour in a model of diffuse traumatic axonal injury

The velocity of impact between an object and the human head is a critical factor influencing brain injury outcomes but has not been explored in any detail in animal models. Here we provide a comprehensive overview of the interplay between impact velocity and injury severity in a well-established weight-drop impact acceleration (WDIA) model of diffuse brain injury in rodents. We modified the standard WDIA model to produce impact velocities of 5.4, 5.85 and 6.15 m/s while keeping constant the weight and the drop height. Gradations in impact velocity produced progressive degrees of injury severity measured behaviourally, electrophysiologically and anatomically, with the former two methods showing greater sensitivity to changes in impact velocity. There were impact velocity-dependent reductions in sensorimotor performance and in cortical depth-related depression of sensory cortex responses; however axonal injury (demonstrated by immunohistochemistry for β-amyloid precursor protein and neurofilament heavy-chain) was discernible only at the highest impact velocity. We conclude that the WDIA model is capable of producing graded axonal injury in a repeatable manner, and as such will prove useful in the study of the biomechanics, pathophysiology and potential treatment of diffuse axonal injury.

Cortical hypoexcitation defines neuronal responses in the immediate aftermath of traumatic brain injury

Traumatic brain injury (TBI) from a blow to the head is often associated with complex patterns of brain abnormalities that accompany deficits in cognitive and motor function. Previously we reported that a long-term consequence of TBI, induced with a closed-head injury method modelling human car and sporting accidents, is neuronal hyper-excitation in the rat sensory barrel cortex that receives tactile input from the face whiskers. Hyper-excitation occurred only in supra-granular layers and was stronger to complex than simple stimuli. We now examine changes in the immediate aftermath of TBI induced with same injury method. At 24 hours post-trauma significant sensorimotor deficits were observed and characterisation of the cortical population neuronal responses at that time revealed a depth-dependent suppression of neuronal responses, with reduced responses from supragranular layers through to input layer IV, but not in infragranular layers. In addition, increased spontaneous firing rate was recorded in cortical layers IV and V. We postulate that this early post-injury suppression of cortical processing of sensory input accounts for immediate post-trauma sensory morbidity and sets into train events that resolve into long-term cortical hyper-excitability in upper sensory cortex layers that may account for long-term sensory hyper-sensitivity in humans with TBI.

Traumatic axonal injury in the mouse is accompanied by a dynamic inflammatory response, astroglial reactivity and complex behavioral changes

BACKGROUND

Diffuse traumatic axonal injury (TAI), a common consequence of traumatic brain injury, is associated with high morbidity and mortality. Inflammatory processes may play an important role in the pathophysiology of TAI. In the central fluid percussion injury (cFPI) TAI model in mice, the neuroinflammatory and astroglial response and behavioral changes are unknown.

METHODS

Twenty cFPI-injured and nine sham-injured mice were used, and the neuroinflammatory and astroglial response was evaluated by immunohistochemistry at 1, 3 and 7 days post-injury. The multivariate concentric square field test (MCSF) was used to compare complex behavioral changes in mice subjected to cFPI (n = 16) or sham injury (n = 10). Data was analyzed using non-parametric statistics and principal component analysis (MCSF data).

RESULTS

At all post-injury time points, β-amyloid precursor protein (β-APP) immunoreactivity revealed widespread bilateral axonal injury and IgG immunostaining showed increased blood-brain barrier permeability. Using vimentin and glial fibrillary acidic protein (GFAP) immunohistochemistry, glial cell reactivity was observed in cortical regions and important white matter tracts peaking at three days post-injury. Only vimentin was increased post-injury in the internal capsule and only GFAP in the thalamus. Compared to sham-injured controls, an increased number of activated microglia (MAC-2), infiltrating neutrophils (GR-1) and T-cells (CD3) appearing one day after TAI (P<0.05 for all cell types) was observed in subcortical white matter. In the MCSF, the behavioral patterns including general activity and exploratory behavior differed between cFPI mice and sham-injured controls.

CONCLUSIONS

Traumatic axonal injury TAI resulted in marked bilateral astroglial and neuroinflammatory responses and complex behavioral changes. The cFPI model in mice appears suitable for the study of injury mechanisms, including neuroinflammation, and the development of treatments targeting TAI.

The liver X receptor agonist GW3965 improves recovery from mild repetitive traumatic brain injury in mice partly through apolipoprotein E

Traumatic brain injury (TBI) increases Alzheimer’s disease (AD) risk and leads to the deposition of neurofibrillary tangles and amyloid deposits similar to those found in AD. Agonists of Liver X receptors (LXRs), which regulate the expression of many genes involved in lipid homeostasis and inflammation, improve cognition and reduce neuropathology in AD mice. One pathway by which LXR agonists exert their beneficial effects is through ATP-binding cassette transporter A1 (ABCA1)-mediated lipid transport onto apolipoprotein E (apoE). To test the therapeutic utility of this pathway for TBI, we subjected male wild-type (WT) and apoE−/− mice to mild repetitive traumatic brain injury (mrTBI) followed by treatment with vehicle or the LXR agonist GW3965 at 15 mg/kg/day. GW3965 treatment restored impaired novel object recognition memory in WT but not apoE−/− mice. GW3965 did not significantly enhance the spontaneous recovery of motor deficits observed in all groups. Total soluble Aβ₄₀ and Aβ₄₂ levels were significantly elevated in WT and apoE−/− mice after injury, a response that was suppressed by GW3965 in both genotypes. WT mice showed mild but significant axonal damage at 2 d post-mrTBI, which was suppressed by GW3965. In contrast, apoE−/− mice showed severe axonal damage from 2 to 14 d after mrTBI that was unresponsive to GW3965. Because our mrTBI model does not produce significant inflammation, the beneficial effects of GW3965 we observed are unlikely to be related to reduced inflammation. Rather, our results suggest that both apoE-dependent and apoE-independent pathways contribute to the ability of GW3965 to promote recovery from mrTBI.

Sensory cortex underpinnings of traumatic brain injury deficits

Traumatic brain injury (TBI) can result in persistent sensorimotor and cognitive deficits including long-term altered sensory processing. The few animal models of sensory cortical processing effects of TBI have been limited to examination of effects immediately after TBI and only in some layers of cortex. We have now used the rat whisker tactile system and the cortex processing whisker-derived input to provide a highly detailed description of TBI-induced long-term changes in neuronal responses across the entire columnar network in primary sensory cortex. Brain injury (n = 19) was induced using an impact acceleration method and sham controls received surgery only (n = 15). Animals were tested in a range of sensorimotor behaviour tasks prior to and up to 6 weeks post-injury when there were still significant sensorimotor behaviour deficits. At 8–10 weeks post-trauma, in terminal experiments, extracellular recordings were obtained from barrel cortex neurons in response to whisker motion, including motion that mimicked whisker motion observed in awake animals undertaking different tasks. In cortex, there were lamina-specific neuronal response alterations that appeared to reflect local circuit changes. Hyper-excitation was found only in supragranular layers involved in intra-areal processing and long-range integration, and only for stimulation with complex, naturalistic whisker motion patterns and not for stimulation with simple trapezoidal whisker motion. Thus TBI induces long-term directional changes in integrative sensory cortical layers that depend on the complexity of the incoming sensory information. The nature of these changes allow predictions as to what types of sensory processes may be affected in TBI and contribute to post-trauma sensorimotor deficits.

Attenuation of axonal injury and oxidative stress by edaravone protects against cognitive impairments after traumatic brain injury

Traumatic axonal injury (TAI), a feature of traumatic brain injury (TBI), progressively evolves over hours through impaired axonal transport and is thought to be a major contributor to cognitive dysfunction. In spite of various studies suggesting that pharmacologic or physiologic interventions might reduce TAI, clinical neuroprotective treatments are still unavailable. Edaravone, a free radical scavenger, has been shown to exert neuroprotective effects in animal models of several brain disorders. In this study, to evaluate whether edaravone suppresses TAI following TBI, mice were subjected to weight drop injury and had either edaravone (3.0mg/kg) or saline administered intravenously immediately after impact. Axonal injury and oxidative stress were assessed using immunohistochemistry with antibodies against amyloid precursor protein, a marker of impaired axonal transport, and with 8-hydroxy-2'-deoxyguanosine, a marker of oxidative DNA damage. Edaravone significantly suppressed axonal injury and oxidative stress in the cortex, corpus callosum, and hippocampus 24h after injury. The neuroprotective effects of edaravone were observed in mice receiving 1.0, 3.0, or 10mg/kg of edaravone immediately after impact, but not after 0.3mg/kg of edaravone. With treatment 1h after impact, axonal injury was also significantly suppressed and this therapeutic effect persisted up to 6h after impact. Furthermore, behavioral tests performed 9 days after injury showed memory deficits in saline-treated traumatized mice, which were not evident in the edaravone-treated group. These results suggest that edaravone protects against memory deficits following TBI and that this protection is mediated by suppression of TAI and oxidative stress.

Diffusion tensor imaging studies of mild traumatic brain injury: a meta-analysis

OBJECTIVES

To assess the possibility that diffusion tensor imaging (DTI) can detect white matter damage in mild traumatic brain injury (mTBI) patients via systematic review and meta-analysis.

METHODS

DTI studies that compared mTBI patients and controls were searched using MEDLINE, Web of Science, and EMBASE, (1980 through April 2012).

RESULTS

A comprehensive literature search identified 28 DTI studies, of which 13 independent DTI studies of mTBI patients were eligible for the meta-analysis. Random effect model demonstrated significant fractional anisotropy (FA) reduction in the corpus callosum (CC) (p=0.023, 95% CIs -0.466 to -0.035, 280 mTBIs and 244 controls) with no publication bias and minimum heterogeneity, and a significant increase in mean diffusivity (MD) (p=0.015, 95% CIs 0.062 to 0.581, 154 mTBIs and 100 controls). Meta-analyses of the subregions of the CC demonstrated in the splenium FA was significantly reduced (p=0.025, 95% CIs -0.689 to -0.046) and MD was significantly increased (p=0.013, 95% CIs 0.113 to 0.950). FA was marginally reduced in the midbody (p=0.099, 95% CIs -0.404 to 0.034), and no significant change in FA (p=0.421, 95% CIs -0.537 to 0.224) and MD (p=0.264, 95% CIs -0.120 to 0.438) in the genu of the CC.

CONCLUSIONS

Our meta-analysis revealed the posterior part of the CC was more vulnerable to mTBI compared with the anterior part, and suggested the potential utility of DTI to detect white matter damage in the CC of mTBI patients.

Temporal assessment of traumatic axonal injury in the rat corpus callosum and optic chiasm

Impaired axoplasmic transport (IAT) and neurofilament compaction (NFC), two common axonal pathology processes involved in traumatic axonal injury (TAI), have been well characterized. TAI is found clinically and in animal models in brainstem white matter (WM) tracts and in the corpus callosum (CC), optic chiasm (Och), and internal capsule. Previous published quantitative studies of the time course of TAI expression induced by the Marmarou impact acceleration model have been limited to the brainstem. Accordingly, this study assessed the extent of IAT and NFC in the CC and Och at 8h, 28 h, 3 days and 7 days after traumatic brain injury (TBI) induction by the Marmarou impact acceleration model. IAT peak density was observed at 8h in the CC and 28 h in the Och post-TBI. NFC peak density was observed at 28 h in both structures. The density of IAT and NFC decreased with increasing survival time in both structures. The NFC density time profile followed a similar trend in both the Och and CC, whereas the IAT density time profile was variable between the Och and CC. Furthermore, a strong linear relationship was observed between IAT and NFC in the CC but not in the Och. These findings highlight the heterogeneity of TAI as evidenced by variable IAT and NFC injury time profiles in each anatomical structure. This variability indicates the requirement of multiple markers for a comprehensive TAI evaluation and multiple targeted treatments for TAI polypathology within its therapeutic window time frame.

Impaired axoplasmic transport is the dominant injury induced by an impact acceleration injury device: an analysis of traumatic axonal injury in pyramidal tract and corpus callosum of rats

Traumatic axonal injury (TAI) involves neurofilament compaction (NFC) and impaired axoplasmic transport (IAT) in distinct populations of axons. Previous quantification studies of TAI focused on limited areas of pyramidal tract (Py) but not its entire length. Quantification of TAI in corpus callosum (CC) and its comparison to that in Py is also lacking. This study assessed and compared the extent of TAI in the entire Py and CC of rats following TBI. TBI was induced by a modified Marmarou impact acceleration device in 31 adult male Sprague Dawley rats by dropping a 450 gram impactor from either 1.25 m or 2.25 m. Twenty-four hours after TBI, TAI was assessed by beta amyloid precursor protein (β-APP-IAT) and RMO14 (NFC) immunocytochemistry. TAI density (β-APP and RMO14 axonal swellings, retraction balls and axonal profiles) was counted from panoramic images of CC and Py. Significantly high TAI was observed in 2.25 m impacted rats. β-APP immunoreactive axons were significantly higher in number than RMO14 immunoreactive axons in both the structures. TAI density in Py was significantly higher than in CC. Based on our parallel biomechanical studies, it is inferred that TAI in CC may be related to compressive strains and that in Py may be related to tensile strains. Overall, IAT appears to be the dominant injury type induced by this model and injury in Py predominates that in CC.

Injury predictors for traumatic axonal injury in a rodent head impact acceleration model

A modified Marmarou impact acceleration injury model was developed to study the kinematics of the rat head to quantify traumatic axonal injury (TAI) in the corpus callosum (CC) and brainstem pyramidal tract (Py), to determine injury predictors and to establish injury thresholds for severe TAI. Thirty-one anesthetized male Sprague-Dawley rats (392±13 grams) were impacted using a modified impact acceleration injury device from 2.25 m and 1.25 m heights. Beta-amyloid precursor protein (β-APP) immunocytochemistry was used to assess and quantify axonal changes in CC and Py. Over 600 injury maps in CC and Py were constructed in the 31 impacted rats. TAI distribution along the rostro-caudal direction in CC and Py was determined. Linear and angular responses of the rat head were monitored and measured in vivo with an attached accelerometer and angular rate sensor, and were correlated to TAI data. Logistic regression analysis suggested that the occurrence of severe TAI in CC was best predicted by average linear acceleration, followed by power and time to surface righting. The combination of average linear acceleration and time to surface righting showed an improved predictive result. In Py, severe TAI was best predicted by time to surface righting, followed by peak and average angular velocity. When both CC and Py were combined, power was the best predictor, and the combined average linear acceleration and average angular velocity was also found to have good injury predictive ability. Receiver operator characteristic curves were used to assess the predictive power of individual and paired injury predictors. TAI tolerance curves were also proposed in this study.