White matter changes in patients with mild traumatic brain injury: MRI perspective

Magnetic Resonance Spectroscopy following Mild Traumatic Brain Injury: A Systematic Review and Meta-Analysis on the Potential to Detect Posttraumatic Neurodegeneration

INTRODUCTION

Traumatic brain injury (TBI) is the most relevant external risk factor for dementia and a major global health burden. Mild TBI (mTBI) contributes to up to 90% of all TBIs, and the classification "mild" often misrepresents the patient's burden who suffer from neuropsychiatric long-term sequelae. Magnetic resonance spectroscopy (MRS) allows in vivo detection of compromised brain metabolism although it is not routinely used after TBI.

OBJECTIVE

Thus, we performed a systematic review and meta-analysis to elucidate if MRS has the potential to identify changes in brain metabolism in adult patients after a single mTBI with a negative routine brain scan (CCT and/or MRI scan) compared to aged- and sex-matched healthy controls (HC) during the acute or subacute postinjury phase (≤90 days after mTBI).

METHODS

A comprehensive literature search was conducted from the first edition of electronic databases until January 31, 2020. Group analyses were performed per metabolite using a random-effects model.

RESULTS

Four and 2 out of 5,417 articles met the inclusion criteria for the meta-analysis and systematic review, respectively. For the meta-analysis, 50 mTBI patients and 51 HC with a mean age of 31 and 30 years, respectively, were scanned using N-acetyl-aspartate (NAA), a marker for neuronal integrity. Glutamate (Glu), a marker for disturbed brain metabolism, choline (Cho), a marker for increased cell membrane turnover, and creatine (Cr) were used in 2 out of the 4 included articles. Regions of interests were the frontal lobe, the white matter around 1 cm above the lateral ventricles, or the whole brain. NAA was decreased in patients compared to HC with an effect size (ES) of -0.49 (95% CI -1.08 to 0.09), primarily measured in the frontal lobe. Glu was increased in the white matter in 22 mTBI patients compared to 22 HC (ES 0.79; 95% CI 0.17-1.41). Cho was decreased in 31 mTBI patients compared to 31 HC (ES -0.31; 95% CI -0.81 to 0.19). Cr was contradictory and, therefore, potentially not suitable as a reference marker after mTBI.

CONCLUSIONS

MRS pinpoints changes in posttraumatic brain metabolism that correlate with cognitive dysfunction and, thus, might possibly help to detect mTBI patients at risk for unfavorable outcome or posttraumatic neurodegeneration early.

Separate N-acetyl aspartyl glutamate, N-acetyl aspartate, aspartate, and glutamate quantification after pediatric mild traumatic brain injury in the acute phase

PURPOSE

To separately measure N-acetyl aspartul glutamate (NAAG), N-acetyl aspartate (NAA), aspartate (Asp), and glutamate (Glu) concentrations in white matter (WM) using J-editing techniques in patients with mild traumatic brain injury (mTBI) in the acute phase.

METHODS

Twenty-four patients with closed concussive head injury and 29 healthy volunteers were enrolled in the current study. For extended ¹ H MRS examination, patients and controls were equally divided into two subgroups. In subgroup 1 (12 patients/15 controls), NAAG and NAA concentrations were measured in WM separately with MEGA-PRESS (echo time/repetition time [TE/TR] = 140/2000 ms; δ ON NAA / δ OFF NAA = 4.84/4.38 ppm, δ ON NAAG / δ OFF NAAG = 4.61/4.15 ppm). In subgroup 2 (12 patients/14 controls), Asp and Glu concentrations were acquired with MEGA-PRESS (TE/TR = 90/2000 ms; δ ON Asp / δ OFF Asp = 3.89/5.21 ppm) and TE-averaged PRESS (TE from 35 ms to 185 ms with 2.5-ms increments; TR = 2000 ms) pulse sequences, respectively.

RESULTS

tNAA and NAAG concentrations were found to be reduced, while NAA concentrations were unchanged, after mild mTBI. Reduced Asp and elevated myo-inositol (mI) concentrations were also found.

CONCLUSION

The main finding of the study is that the tNAA signal reduction in WM after mTBI is associated with a decrease in the NAAG concentration rather than a decrease in the NAA concentration, as was thought previously. This finding highlights the importance of separating these signals, at least for WM studies, to avoid misinterpretation of the results. NAAG plays an important role in selectively activating mGluR3 receptors, thus providing neuroprotective and neuroreparative functions immediately after mTBI. NAAG shows potential for the development of new therapeutic strategies for patients with injuries of varying severity.

Examining the Progressive Behavior and Neuropathological Outcomes Associated with Chronic Repetitive Mild Traumatic Brain Injury in Rats

While the physical and behavioral symptomologies associated with a single mild traumatic brain injury (mTBI) are typically transient, repetitive mTBIs (RmTBI) have been associated with persisting neurological deficits. Therefore, this study examined the progressive changes in behavior and the neuropathological outcomes associated with chronic RmTBI through adolescence and adulthood in male and female Sprague Dawley rats. Rats experienced 2 mTBIs/week for 15 weeks and were periodically tested for changes in motor behavior, cognitive function, emotional disturbances, and aggression. Brain tissue was examined for neuropathological changes in ventricle size and presentation of Iba1 and GFAP. We did not see progressively worse behavioral impairments with the accumulation of injuries or time, but did find evidence for neurological and functional change (motor disturbance, reduced exploration, reduced aggression, alteration in depressive-like behavior, deficits in short-term working memory). Neuropathological assessment of RmTBI animals identified an increase in ventricle size, prolonged changes in GFAP, and sex differences in Iba1, in the corpus callosum, thalamus, and medial prefrontal cortex. Telomere length reduced exponentially as the injury load increased. Overall, chronic RmTBI did not result in accumulating behavioral impairment, and there is a need to further investigate progressive behavioral changes associated with repeated injuries in adolescence and young adulthood.

Altered corticomotor latencies but normal motor neuroplasticity in concussed athletes

Persistent cognitive, affective, and motor symptoms have been associated with sports-related concussions including several neurophysiological changes in the primary motor cortex. In particular, previous research has provided some evidence of altered latencies of the corticomotor pathway and altered motor neuroplasticity. However, to date, no studies have assessed these neurophysiological metrics in a common group of athletes across different phases of injury and recovery. In this study corticomotor latencies and neuroplasticity were assessed in collegiate athletes with or without a history of prior concussion across two different phases of injury: either in an acute state of concussion (within 2 wk of injury) or in a chronic state of concussion (more than 1 yr after injury). Corticomotor latencies were determined by measuring the motor evoked potential (MEP) onset time, and motor neuroplasticity was assessed by measuring MEP amplitudes following application of anodal transcranial direct current stimulation (tDCS) over the primary motor cortex (M1). We found that concussed athletes had slower corticomotor latencies than nonconcussed athletes, and corticomotor latency was also positively correlated with the number of prior concussions. In contrast, there was no evidence of altered motor neuroplasticity in athletes regardless of concussion history. These findings suggest concussions may lead to permanent changes in the corticospinal tract that are exacerbated by repeated injury.NEW & NOTEWORTHY We are the first to assess corticomotor latencies and motor neuroplasticity in a common group of collegiate athletes across different phases of injury and recovery. We found that the number of concussions an individual sustains negatively impacts corticomotor latencies with a higher number of prior concussions correlating positively with longer latencies. Our findings indicate that concussions may lead to permanent changes in the corticospinal tract that are exacerbated by repeated injury.

IL-13 Ameliorates Neuroinflammation and Promotes Functional Recovery after Traumatic Brain Injury

Microglia play essential roles in neuroinflammatory responses after traumatic brain injury (TBI). Our previous studies showed that phenotypes of microglia, as well as infiltrating macrophages, altered at different stages after CNS injury, which was correlated to functional outcomes. IL-13 is an anti-inflammatory cytokine that has been reported to protect against demyelination and spinal cord injury through immunomodulation. The effects of IL-13 in microglia/macrophage-mediated immune responses after TBI remain unknown. In this study, we showed that intranasal administration of IL-13 in male C57BL/6J mice accelerated functional recovery in the controlled cortical impact model of TBI. IL-13 treatment increased the time to fall off in the Rotarod test, reduced the number of foot faults in the foot fault test, and improved the score in the wire hang test up to 28 d after TBI. Consistent with functional improvement, IL-13 reduced neuronal tissue loss and preserved white matter integrity 6 d after TBI. Furthermore, IL-13 ameliorated the elevation of proinflammatory factors and reduced the number of proinflammatory microglia/macrophages 6 d after TBI. Additionally, IL-13 enhanced microglia/macrophage phagocytosis of damaged neurons in the peri-lesion areas. In vitro studies confirmed that IL-13 treatment inhibited the production of proinflammatory cytokines in rat primary microglia in response to LPS or dead neuron stimulation and increased the ability of microglia to engulf fluorophore-labeled latex beads or dead neurons. Collectively, we demonstrated that IL-13 treatment improved neurologic outcomes after TBI through adjusting microglia/macrophage phenotypes and inhibiting inflammatory responses. IL-13 may represent a potential immunotherapy to promote long-term recovery from TBI.

C-terminal binding proteins 1 and 2 in traumatic brain injury-induced inflammation and their inhibition as an approach for anti-inflammatory treatment

Traumatic brain injury (TBI) induces an acute inflammatory response in the central nervous system that involves both resident and peripheral immune cells. The ensuing chronic neuroinflammation causes cell death and tissue damage and may contribute to neurodegeneration. The molecular mechanisms involved in the maintenance of this chronic inflammation state remain underexplored. C-terminal binding protein (CtBP) 1 and 2 are transcriptional coregulators that repress diverse cellular processes. Unexpectedly, we find that the CtBPs can transactivate a common set of proinflammatory genes both in lipopolysaccharide-activated microglia, astrocytes and macrophages, and in a mouse model of the mild form of TBI. We also find that the expression of these genes is markedly enhanced by a single mild injury in both brain and peripheral blood leukocytes in a severity- and time-dependent manner. Moreover, we were able to demonstrate that specific inhibitors of the CtBPs effectively suppress the expression of the CtBP target genes and thus improve neurological outcome in mice receiving single and repeated mild TBIs. This discovery suggests new avenues for therapeutic modulation of the inflammatory response to brain injury.

Aberrant ER Stress Induced Neuronal-IFNβ Elicits White Matter Injury Due to Microglial Activation and T-Cell Infiltration after TBI

Persistent endoplasmic reticulum (ER) stress in neurons is associated with activation of inflammatory cells and subsequent neuroinflammation following traumatic brain injury (TBI); however, the underlying mechanism remains elusive. We found that induction of neuronal-ER stress, which was mostly characterized by an increase in phosphorylation of a protein kinase R-like ER kinase (PERK) leads to release of excess interferon (IFN)β due to atypical activation of the neuronal-STING signaling pathway. IFNβ enforced activation and polarization of the primary microglial cells to inflammatory M1 phenotype with the secretion of a proinflammatory chemokine CXCL10 due to activation of STAT1 signaling. The secreted CXCL10, in turn, stimulated the T-cell infiltration by serving as the ligand and chemoattractant for CXCR3⁺ T-helper 1 (Th1) cells. The activation of microglial cells and infiltration of Th1 cells resulted in white matter injury, characterized by impaired myelin basic protein and neurofilament NF200, the reduced thickness of corpus callosum and external capsule, and decline of mature oligodendrocytes and oligodendrocyte precursor cells. Intranasal delivery of CXCL10 siRNA blocked Th1 infiltration but did not fully rescue microglial activation and white matter injury after TBI. However, impeding PERK-phosphorylation through the administration of GSK2656157 abrogated neuronal induction of IFNβ, switched microglial polarization to M2 phenotype, prevented Th1 infiltration, and increased Th2 and Treg levels. These events ultimately attenuated the white matter injury and improved anxiety and depressive-like behavior following TBI.SIGNIFICANCE STATEMENT A recent clinical study showed that human brain trauma patients had enhanced expression of type-1 IFN; suggests that type-1 IFN signaling may potentially influence clinical outcome in TBI patients. However, it was not understood how TBI leads to an increase in IFNβ and whether induction of IFNβ has any influence on neuroinflammation, which is the primary reason for morbidity and mortality in TBI. Our study suggests that induction of PERK phosphorylation, a characteristic feature of ER stress is responsible for an increase in neuronal IFNβ, which, in turn, activates microglial cells and subsequently manifests the infiltration of T cells to induce neuroinflammation and subsequently white matter injury. Blocking PERK phosphorylation using GSK2656157 (or PERK knockdown) the whole cascade of neuroinflammation was attenuated and improved cognitive function after TBI.

Multi-Modal Signatures of Tau Pathology, Neuronal Fiber Integrity, and Functional Connectivity in Traumatic Brain Injury

[¹⁸F]AV-1451 (aka ¹⁸F-Flortaucipir, [¹⁸F]T807) was developed for positron-emission tomography (PET) imaging of paired helical filaments of hyperphosphorylated tau, which are of interest in a range of neuropathologies, including traumatic brain injury (TBI). Magnetic resonance imaging (MRI) techniques like diffusion tensor imaging (DTI) and resting state functional connectivity assess structural and functional characteristics of the brain, complementing the molecular information that can be obtained by PET. The goal herein was to explore the utility of such multi-modal imaging in a case series based on a population of TBI subjects. This study probes the interrelationship between tau deposition, white matter integrity, and gray matter functional connectivity across the spectrum of TBI. Nineteen subjects (11 controls, five former contact sports athletes, one automotive accident, and two with military-related injury) underwent [¹⁸F]AV-1451 PET and magnetic resonance scanning procedures. [¹⁸F]AV-1451 distribution volume ratio (DVR) was estimated using the Logan method and the cerebellum as a reference region. Diffusion tractography images and fractional anisotropy (FA) images were generated using diffusion toolkit and FSL. Resting-state functional MRI (fMRI) analysis was based on a graph theory metric, namely weighted degree centrality. TBI subjects showed greater heterogeneity in [¹⁸F]AV-1451 DVR when compared with control subjects. In a subset of TBI subjects, areas with high [¹⁸F]AV-1451 binding corresponded with increased FA and diminished white matter tract density in DTI. Functional MRI results exhibited an increase in functional connectivity, particularly among local connections, in the areas where tau aggregates were more prevalent. In a case series of a diverse group of TBI subjects, brain regions with elevated tau burden exhibited increased functional connectivity as well as decreased white matter integrity. These findings portray molecular, microstructural, and functional corollaries of TBI that spatially coincide and can be measured in the living human brain using noninvasive neuroimaging techniques.

A common neural signature of brain injury in concussion and subconcussion

The midbrain is biomechanically susceptible to force loading from repetitive subconcussive head impacts (RSHI), is a site of tauopathy in chronic traumatic encephalopathy (CTE), and regulates functions (e.g., eye movements) often disrupted in concussion. In a prospective longitudinal design, we demonstrate there are reductions in midbrain white matter integrity due to a single season of collegiate football, and that the amount of reduction in midbrain white matter integrity is related to the amount of rotational acceleration to which players' brains are exposed. We then replicate the observation of reduced midbrain white matter integrity in a retrospective cohort of individuals with frank concussion, and further show that variance in white matter integrity is correlated with levels of serum-based tau, a marker of blood-brain barrier disruption. These findings mean that noninvasive structural MRI of the midbrain is a succinct index of both clinically silent white matter injury as well as frank concussion.

The Importance of Therapeutic Time Window in the Treatment of Traumatic Brain Injury

Traumatic brain injury (TBI) is a major cause of death and disability. Despite its importance in public health, there are presently no drugs to treat TBI. Many reasons underlie why drugs have failed clinical trials, one reason is that most drugs to treat TBI lose much of their efficacy before patients are first treated. This review discusses the importance of therapeutic time window; the time interval between TBI onset and the initiation of treatment. Therapeutic time window is complex, as brain injury is both acute and chronic, resulting in multiple drug targets that appear and disappear with differing kinetics. The speed and increasing complexity of TBI pathophysiology is a major reason why drugs lose efficacy as time to first dose increases. Recent Phase III clinical trials treated moderate to severe TBI patients within 4–8 h after injury, yet they turned away many potential patients who could not be treated within these time windows. Additionally, most head trauma is mild TBI. Unlike moderate to severe TBI, patients with mild TBI often delay treatment until their symptoms do not abate. Thus, drugs to treat moderate to severe TBI likely will need to retain high efficacy for up to 12 h after injury; drugs for mild TBI, however, will likely need even longer windows. Early pathological events following TBI progress with similar kinetics in humans and animal TBI models suggesting that preclinical testing of time windows assists the design of clinical trials. We reviewed preclinical studies of drugs first dosed later than 4 h after injury. This review showed that therapeutic time window can differ depending upon the animal TBI model and the outcome measure. We identify the few drugs (methamphetamine, melanocortin, minocycline plus N-acetylcysteine, and cycloserine) that demonstrated good therapeutic windows with multiple outcome measures. On the basis of their therapeutic window, these drugs appear to be excellent candidates for clinical trials. In addition to further testing of these drugs, we recommend that the assessment of therapeutic time window with multiple outcome measures becomes a standard component of preclinical drug testing.

Analytic Tools for Post-traumatic Epileptogenesis Biomarker Search in Multimodal Dataset of an Animal Model and Human Patients

Epilepsy is among the most common serious disabling disorders of the brain, and the global burden of epilepsy exerts a tremendous cost to society. Most people with epilepsy have acquired forms of the disorder, and the development of antiepileptogenic interventions could potentially prevent or cure epilepsy in many of them. However, the discovery of potential antiepileptogenic treatments and clinical validation would require a means to identify populations of patients at very high risk for epilepsy after a potential epileptogenic insult, to know when to treat and to document prevention or cure. A fundamental challenge in discovering biomarkers of epileptogenesis is that this process is likely multifactorial and crosses multiple modalities. Investigators must have access to a large number of high quality, well-curated data points and study subjects for biomarker signals to be detectable above the noise inherent in complex phenomena, such as epileptogenesis, traumatic brain injury (TBI), and conditions of data collection. Additionally, data generating and collecting sites are spread worldwide among different laboratories, clinical sites, heterogeneous data types, formats, and across multi-center preclinical trials. Before the data can even be analyzed, these data must be standardized. The Epilepsy Bioinformatics Study for Antiepileptogenic Therapy (EpiBioS4Rx) is a multi-center project with the overarching goal that epileptogenesis after TBI can be prevented with specific treatments. The identification of relevant biomarkers and performance of rigorous preclinical trials will permit the future design and performance of economically feasible full-scale clinical trials of antiepileptogenic therapies. We have been analyzing human data collected from UCLA and rat data collected from the University of Eastern Finland, both centers collecting data for EpiBioS4Rx, to identify biomarkers of epileptogenesis. Big data techniques and rigorous analysis are brought to longitudinal data collected from humans and an animal model of TBI, epilepsy, and their interaction. The prolonged continuous data streams of intracranial, cortical surface, and scalp EEG from humans and an animal model of epilepsy span months. By applying our innovative mathematical tools via supervised and unsupervised learning methods, we are able to subject a robust dataset to recently pioneered data analysis tools and visualize multivariable interactions with novel graphical methods.

Diffusion Tensor Imaging Analysis of Mild Traumatic Brain Injury and Posttraumatic Stress Disorder

BACKGROUND

Debate exists over the extent to which dysfunctions arising from mild traumatic brain injury (mTBI) are distinct from posttraumatic stress disorder (PTSD).

METHODS

This study investigated 1) the white matter integrity of participants with either mTBI or PTSD, and 2) the relationship between white matter integrity and postconcussive syndrome. The sample comprised 110 civilians (mTBI group = 40; PTSD group = 32; age- and sex-matched trauma-exposed control subjects = 38) recruited from community advertising. Indicators of white matter abnormalities were fractional anisotropy, mean diffusivity, axial diffusivity, and radial diffusivity. PTSD symptoms were indexed by the Clinician-Administered PTSD Scale, and postconcussive symptoms were assessed using the Somatic and Psychological Health Report measure.

RESULTS

Fractional anisotropy was reduced in mTBI participants in the corpus callosum, tracts of the brainstem, projection fibers, association fibers, and limbic fibers compared with both PTSD and trauma-exposed control subjects. This decrease in fractional anisotropy was observed in the context of concurrent changes in radial diffusivity, axial diffusivity, and mean diffusivity. Postconcussive symptoms were largely explained by PTSD severity rather than by changes in brain white matter. mTBI appears to be characterized by distinct reductions in white matter integrity, and this cannot be attributed to PTSD.

CONCLUSIONS

PTSD symptoms appear to be more strongly associated with postconcussive syndrome than with white matter compromise. These findings extend epidemiological evidence of the relative associations of PTSD and mTBI with postconcussive syndrome.

The auditory comprehension changes over time after sport-related concussion can indicate multisensory processing dysfunctions

BACKGROUND

Although science findings and treatment approaches of a concussion have changed in recent years, there continue to be challenges in understanding the nature of the post-concussion behavior. There is growing a body of evidence that some deficits can be related to an impaired auditory processing.

PURPOSE

To assess auditory comprehension changes over time following sport-related concussion (SRC) in young athletes.

METHODS

A prospective, repeated measures mixed-design was used. A sample of concussed athletes (n = 137) and the control group consisted of age-matched, non-concussed athletes (n = 143) were administered Subtest VIII of the Computerized-Revised Token Test (C-RTT). The 88 concussed athletes selected for final analysis (neither previous history of brain injury, neurological, psychiatric problems, nor auditory deficits) were evaluated after injury during three sessions (PC1, PC2, and PC3); controls were tested once. Between- and within-group comparisons using RMANOVA were performed on the C-RTT Efficiency Score (ES).

RESULTS

ES of the SRC athletes group improved over consecutive testing sessions (F = 14.7, p < .001), while post-hoc analysis showed that PC1 results differed from PC2 and PC3 (ts ≥ 4.0, ps < .001), but PC2 and PC3 C-RTT ES did not change statistically (t = 0.6, p = .557). The SRC athletes demonstrated lower ES for all test session when compared to the control group (ts > 2.0, Ps<.01).

CONCLUSION

Dysfunctional auditory comprehension performance following a concussion improved over time, but after the second testing session improved performance slowed, especially in terms of its timing. Yet, not only auditory processing but also sensorimotor integration and/or motor execution can be compromised after a concussion.