MRI assessment of cerebral blood flow after experimental traumatic brain injury combined with hemorrhagic shock in mice

Effects of Low-Level Blast on Neurovascular Health and Cerebral Blood Flow: Current Findings and Future Opportunities in Neuroimaging

Low-level blast (LLB) exposure can lead to alterations in neurological health, cerebral vasculature, and cerebral blood flow (CBF). The development of cognitive issues and behavioral abnormalities after LLB, or subconcussive blast exposure, is insidious due to the lack of acute symptoms. One major hallmark of LLB exposure is the initiation of neurovascular damage followed by the development of neurovascular dysfunction. Preclinical studies of LLB exposure demonstrate impairment to cerebral vasculature and the blood-brain barrier (BBB) at both early and long-term stages following LLB. Neuroimaging techniques, such as arterial spin labeling (ASL) using magnetic resonance imaging (MRI), have been utilized in clinical investigations to understand brain perfusion and CBF changes in response to cumulative LLB exposure. In this review, we summarize neuroimaging techniques that can further our understanding of the underlying mechanisms of blast-related neurotrauma, specifically after LLB. Neuroimaging related to cerebrovascular function can contribute to improved diagnostic and therapeutic strategies for LLB. As these same imaging modalities can capture the effects of LLB exposure in animal models, neuroimaging can serve as a gap-bridging diagnostic tool that permits a more extensive exploration of potential relationships between blast-induced changes in CBF and neurovascular health. Future research directions are suggested, including investigating chronic LLB effects on cerebral perfusion, exploring mechanisms of dysautoregulation after LLB, and measuring cerebrovascular reactivity (CVR) in preclinical LLB models.

Advanced MRI to assess hippocampal injury after incomplete cerebral ischemia-reperfusion in rats

BACKGROUND AND PURPOSE

The purpose of this study was to evaluate advanced MRI findings in the bilateral hippocampus CA1 region of rats with hemorrhagic shock reperfusion (HSR) and their correlation with histopathological results. Additionally, this study aimed to identify effective MRI examination methods and detection indexes for assessing HSR.

METHODS

Rats were randomized into the HSR and the Sham groups with 24 rats in each group. MRI examination included diffusion kurtosis imaging (DKI) and 3-dimensional arterial spin labeling (3D-ASL). Apoptosis and pyroptosis were evaluated directly from tissue.

RESULTS

In the HSR group, cerebral blood flow (CBF) was significantly lower than that of the Sham group, while radial kurtosis (Kr), axial kurtosis (Ka), and mean kurtosis (MK) were all higher. In the HSR group, fractional anisotropy (FA) at 12 and 24 hours and radial diffusivity, axial diffusivity (Da), and mean diffusivity (MD) at 3 and 6 hours were lower than in the Sham group. MD and Da at 24 hours in the HSR group were significantly higher. The apoptosis rate and pyroptosis rate were also enhanced in the HSR group. CBF, FA, MK, Ka, and Kr values in the early stage were strongly correlated with apoptosis rate and pyroptosis rate. The metrics were obtained from DKI and 3D-ASL.

CONCLUSIONS

Advanced MRI metrics from DKI and 3D-ASL, including CBF, FA, Ka, Kr, and MK values, are useful to evaluate abnormal blood perfusion and microstructural changes in the hippocampus CA1 area in the setting of incomplete cerebral ischemia-reperfusion in rats induced by HSR.

Long period changes of hippocampal cerebral blood flow and its correlation with anxiety-like behavior and inflammation after incomplete cerebral ischemia reperfusion in rats

OBJECTIVE

This study was designed to summarize the changes of cerebral blood flow (CBF) in the bilateral hippocampal CA1 region of the hemorrhagic shock reperfusion (HSR) model of rats and their correlation with anxiety-like behavior and inflammation.

METHODS

Rats were randomly divided into the HSR group and the Sham group. 30 rats in each group were subdivided into 5 time points (1 w, 2 w, 4 w, 8 w, and 12 w) for examination. 3D-arterial spin labeling (3D-ASL) was performed. Long period anxiety-like behaviors were analyzed through the open field test. Histopathology was used to detect astrocytic activation in bilateral hippocampus. The concentrations of pro-inflammatory cytokines were analyzed by ELISA.

RESULTS

At 1, 2, 4, and 8 weeks, CBF in bilateral hippocampus CA1 area of the rats in the Sham group was significantly higher than the rats in the HSR group. The rats in the HSR group had significantly shorter total traveled distance, lower velocity, and less rearing counts than those in the Sham group at 1, 2, 4, 8, and 12 weeks after the surgery. The CBF at 1, 2, 4, 8, and 12 weeks after the surgery had positive correlation with the total traveled distance, velocity, and rearing counts in the open field test. The rats in the HSR group had significantly higher GFAP intensity and the concentrations of IL-6, IL-1β, and TNF-α than those in the Sham group at 1, 2, 4, 8, and 12 weeks after the surgery. The CBF at 1, 2, 4, 8 and 12 weeks after the surgery had significantly negative correlation with the GFAP intensity and the concentrations of IL-6, IL-1β, and TNF-α.

CONCLUSION

In conclusion, CBF in bilateral hippocampus CA1 area, spatial exploration ability in rats with HSR were decreased while the astrocyte activation was enhanced. During the long period after the induction of HSR, the value of CBF in bilateral hippocampus CA1 area was proved to have significant correlation with anxiety-like behaviors and astrocyte activation.

Cerebral Blood Flow Hemispheric Asymmetry in Comatose Adults Receiving Extracorporeal Membrane Oxygenation

Peripheral veno-arterial extracorporeal membrane oxygenation (ECMO) artificially oxygenates and circulates blood retrograde from the femoral artery, potentially exposing the brain to asymmetric perfusion. Though ECMO patients frequently experience brain injury, neurologic exams and imaging are difficult to obtain. Diffuse correlation spectroscopy (DCS) non-invasively measures relative cerebral blood flow (rBF) at the bedside using an optical probe on each side of the forehead. In this study we observed interhemispheric rBF differences in response to mean arterial pressure (MAP) changes in adult ECMO recipients. We recruited 13 subjects aged 21–78 years (7 with cardiac arrest, 4 with acute heart failure, and 2 with acute respiratory distress syndrome). They were dichotomized via Glasgow Coma Scale Motor score (GCS-M) into comatose (GCS-M ≤ 4; n = 4) and non-comatose (GCS-M > 4; n = 9) groups. Comatose patients had greater interhemispheric rBF asymmetry (ASYM_rBF) vs. non-comatose patients over a range of MAP values (29 vs. 11%, p = 0.009). ASYM_rBF in comatose patients resolved near a MAP range of 70–80 mmHg, while rBF remained symmetric through a wider MAP range in non-comatose patients. Correlations between post-oxygenator pCO₂ or pH vs. ASYM_rBF were significantly different between comatose and non-comatose groups. Our findings indicate that comatose patients are more likely to have asymmetric cerebral perfusion.

Hippocampal and rostral anterior cingulate blood flow is associated with affective symptoms in chronic traumatic brain injury

OBJECTIVE

The purpose of this study was to assess cerebral blood flow (CBF) and its association with self-reported symptoms in chronic traumatic brain injury (TBI).

PARTICIPANTS

Sixteen participants with mild to severe TBI and persistent self-reported neurological symptoms, 6 to 72 months post-injury were included. For comparison, 16 age- and gender-matched healthy normal control participants were also included.

MAIN MEASURES

Regional CBF and brain volume were assessed using pseudo-continuous Arterial Spin Labeling (PCASL) and T₁-weighted data respectively. Cognitive function and self-reported symptoms were assessed in TBI participants using the national institutes of health (NIH) Toolbox Cognition Battery and Patient-Reported Outcome Measurement Information System respectively. Associations between CBF and cognitive function, symptoms were assessed.

RESULTS

Global CBF and regional brain volumes were similar between groups, but region of interest (ROI) analysis revealed lower CBF bilaterally in the thalamus, hippocampus, left caudate, and left amygdala in the TBI group. Voxel-wise analysis revealed that CBF in the hippocampus, parahippocampus, rostral anterior cingulate, inferior frontal gyrus, and other temporal regions were negatively associated with self-reported anger, anxiety, and depression symptoms. Furthermore, region of interest (ROI) analysis revealed that hippocampal and rostral anterior cingulate CBF were negatively associated with symptoms of fatigue, anxiety, depression, and sleep issues.

CONCLUSION

Regional CBF deficit was observed in the group with chronic TBI compared to the normal control (NC) group despite similar volume of cerebral structures. The observed negative correlation between regional CBF and affective symptoms suggests that CBF-targeted intervention may potentially improve affective symptoms and quality of life after TBI, which needs to be assessed in future studies.

Choice of Whole Blood versus Lactated Ringer's Resuscitation Modifies the Relationship between Blood Pressure Target and Functional Outcome after Traumatic Brain Injury plus Hemorrhagic Shock in Mice

Civilian traumatic brain injury (TBI) guidelines recommend resuscitation of patients with hypotensive TBI with crystalloids. Increasing evidence, however, suggests that whole blood (WB) resuscitation may improve physiological and survival outcomes at lower resuscitation volumes, and potentially at a lower mean arterial blood pressure (MAP), than crystalloid after TBI and hemorrhagic shock (HS). The objective of this study was to assess whether WB resuscitation with two different MAP targets improved behavioral and histological outcomes compared with lactated Ringer's (LR) in a mouse model of TBI+HS. Anesthetized mice (n = 40) underwent controlled cortical impact (CCI) followed by HS (MAP = 25-27 mm Hg; 25 min) and were randomized to five groups for a 90 min resuscitation: LR with MAP target of 70 mm Hg (LR70), LR60, WB70, WB60, and monitored sham. Mice received a 20 mL/kg bolus of LR or autologous WB followed by LR boluses (10 mL/kg) every 5 min for MAP below target. Shed blood was reinfused after 90 min. Morris Water Maze testing was performed on days 14-20 post-injury. Mice were euthanized (21 d) to assess contusion and total brain volumes. Latency to find the hidden platform was greater versus sham for LR60 (p < 0.002) and WB70 (p < 0.007) but not LR70 or WB60. The WB resuscitation did not reduce contusion volume or brain tissue loss. The WB targeting a MAP of 60 mm Hg did not compromise function versus a 70 mm Hg target after CCI+HS, but further reduced fluid requirements (p < 0.03). Using LR, higher achieved MAP was associated with better behavioral performance (rho = -0.67, p = 0.028). Use of WB may allow lower MAP targets without compromising functional outcome, which could facilitate pre-hospital TBI resuscitation.

Mild traumatic brain injury induces microvascular injury and accelerates Alzheimer-like pathogenesis in mice

INTRODUCTION

Traumatic brain injury (TBI) is considered as the most robust environmental risk factor for Alzheimer's disease (AD). Besides direct neuronal injury and neuroinflammation, vascular impairment is also a hallmark event of the pathological cascade after TBI. However, the vascular connection between TBI and subsequent AD pathogenesis remains underexplored.

METHODS

In a closed-head mild TBI (mTBI) model in mice with controlled cortical impact, we examined the time courses of microvascular injury, blood-brain barrier (BBB) dysfunction, gliosis and motor function impairment in wild type C57BL/6 mice. We also evaluated the BBB integrity, amyloid pathology as well as cognitive functions after mTBI in the 5xFAD mouse model of AD.

RESULTS

mTBI induced microvascular injury with BBB breakdown, pericyte loss, basement membrane alteration and cerebral blood flow reduction in mice, in which BBB breakdown preceded gliosis. More importantly, mTBI accelerated BBB leakage, amyloid pathology and cognitive impairment in the 5xFAD mice.

DISCUSSION

Our data demonstrated that microvascular injury plays a key role in the pathogenesis of AD after mTBI. Therefore, restoring vascular functions might be beneficial for patients with mTBI, and potentially reduce the risk of developing AD.

Multifaceted Benefit of Whole Blood Versus Lactated Ringer's Resuscitation After Traumatic Brain Injury and Hemorrhagic Shock in Mice

BACKGROUND

Despite increasing use in hemorrhagic shock (HS), whole blood (WB) resuscitation for polytrauma with traumatic brain injury (TBI) is largely unexplored. Current TBI guidelines recommend crystalloid for prehospital resuscitation. Although WB outperforms lactated Ringer's (LR) in increasing mean arterial pressure (MAP) in TBI + HS models, effects on brain tissue oxygenation (PbtO₂), and optimal MAP remain undefined.

METHODS

C57BL/6 mice (n = 72) underwent controlled cortical impact followed by HS (MAP = 25-27 mmHg). Ipsilateral hippocampal PbtO₂ (n = 40) was measured by microelectrode. Mice were assigned to four groups (n = 18/group) for "prehospital" resuscitation (90 min) with LR or autologous WB, and target MAPs of 60 or 70 mmHg (LR₆₀, WB₆₀, LR₇₀, WB₇₀). Additional LR (10 ml/kg) was bolused every 5 min for MAP below target.

RESULTS

LR requirements in WB₆₀ (7.2 ± 5.0 mL/kg) and WB₇₀ (28.3 ± 9.6 mL/kg) were markedly lower than in LR₆₀ (132.8 ± 5.8 mL/kg) or LR₇₀ (152.2 ± 4.8 mL/kg; all p < 0.001). WB₇₀ MAP (72.5 ± 2.9 mmHg) was higher than LR₇₀ (59.8 ± 4.0 mmHg, p < 0.001). WB₆₀ MAP (68.7 ± 4.6 mmHg) was higher than LR₆₀ (53.5 ± 3.2 mmHg, p < 0.001). PbtO₂ was higher in WB₆₀ (43.8 ± 11.6 mmHg) vs either LR₆₀ (25.9 ± 13.0 mmHg, p = 0.04) or LR₇₀ (24.1 ± 8.1 mmHg, p = 0.001). PbtO₂ in WB₇₀ (40.7 ± 8.8 mmHg) was higher than in LR₇₀ (p = 0.007). Despite higher MAP in WB₇₀ vs WB₆₀ (p = .002), PbtO₂ was similar.

CONCLUSION

WB resuscitation after TBI + HS results in robust improvements in brain oxygenation while minimizing fluid volume when compared to standard LR resuscitation. WB resuscitation may allow for a lower prehospital MAP without compromising brain oxygenation when compared to LR resuscitation. Further studies evaluating the effects of these physiologic benefits on outcome after TBI with HS are warranted, to eventually inform clinical trials.

Alterations in Peripheral Organs following Combined Hypoxemia and Hemorrhagic Shock in a Rat Model of Penetrating Ballistic-Like Brain Injury

Polytrauma, with combined traumatic brain injury (TBI) and systemic damage are common among military and civilians. However, the pathophysiology of peripheral organs following polytrauma is poorly understood. Using a rat model of TBI combined with hypoxemia and hemorrhagic shock, we studied the status of peripheral redox systems, liver glycogen content, creatinine clearance, and systemic inflammation. Male Sprague-Dawley rats were subjected to hypoxemia and hemorrhagic shock insults (HH), penetrating ballistic-like brain injury (PBBI) alone, or PBBI followed by hypoxemia and hemorrhagic shock (PHH). Sham rats received craniotomy only. Biofluids and liver, kidney, and heart tissues were collected at 1 day, 2 days, 7 days, 14 days, and 28 days post-injury (DPI). Creatinine levels were measured in both serum and urine. Glutathione levels, glycogen content, and superoxide dismutase (SOD) and cytochrome C oxidase enzyme activities were quantified in the peripheral organs. Acute inflammation marker serum amyloid A-1 (SAA-1) level was quantified using western blot analysis. Urine to serum creatinine ratio in PHH group was significantly elevated on 7-28 DPI. Polytrauma induced a delayed disruption of the hepatic GSH/GSSG ratio, which resolved within 2 weeks post-injury. A modest decrease in kidney SOD activity was observed at 2 weeks after polytrauma. However, neither PBBI alone nor polytrauma changed the mitochondrial cytochrome C oxidase activity. Hepatic glycogen levels were reduced acutely following polytrauma. Acute inflammation marker SAA-1 showed a significant increase at early time-points following both systemic and brain injury. Overall, our findings demonstrate temporal cytological/tissue level damage to the peripheral organs due to combined PBBI and systemic injury.

Evaluation of B0-correction of relative CBF maps using tagging distance dependent Z-spectrum (TADDZ)

Arterial spin labeling (ASL) MRI, based on endogenous contrast from blood water, is used in research and diagnosis of cerebral vascular conditions. However, artifacts due to imperfect imaging conditions such as B₀-inhomogeneity (ΔB₀) could lead to variations in the quantification of relative cerebral blood flow (CBF). In this study, we evaluate a new approach using tagging distance dependent Z-spectrum (TADDZ) data, similar to the ΔB₀ corrections in the chemical exchange saturation transfer (CEST) experiments, to remove the imaging plane B₀ inhomogeneity induced CBF artifacts in ASL MRI. Our results indicate that imaging-plane B₀-inhomogeneity can lead to variations and errors in the relative CBF maps especially under small tagging distances. Along with an acquired B₀ map, TADDZ data helps to eliminate B₀-inhomogeneity induced artifacts in the resulting relative CBF maps. We demonstrated the effective use of TADDZ data to reduce variation while subjected to systematic changes in ΔB₀. In addition, TADDZ corrected ASL MRI, with improved consistency, was shown to outperform conventional ASL MRI by differentiating the subtle CBF difference in Alzheimer's disease (AD) mice brains with different APOE genotypes.

Novel Calibrated Short TR Recovery (CaSTRR) Method for Brain-Blood Partition Coefficient Correction Enhances Gray-White Matter Contrast in Blood Flow Measurements in Mice

The goal of the study was to develop a novel, rapid Calibrated Short TR Recovery (CaSTRR) method to measure the brain-blood partition coefficient (BBPC) in mice. The BBPC is necessary for quantifying cerebral blood flow (CBF) using tracer-based techniques like arterial spin labeling (ASL), but previous techniques required prohibitively long acquisition times so a constant BBPC equal to 0.9 mL/g is typically used regardless of studied species, condition, or disease. An accelerated method of BBPC correction could improve regional specificity in CBF maps particularly in white matter. Male C57Bl/6N mice (n = 8) were scanned at 7T using CaSTRR to measure BBPC determine regional variability. This technique employs phase-spoiled gradient echo acquisitions with varying repetition times (TRs) to estimate proton density in the brain and a blood sample. Proton density weighted images are then calibrated to a series of phantoms with known concentrations of deuterium to determine BBPC. Pseudo-continuous ASL was also acquired to quantify CBF with and without empirical BBPC correction. Using the CaSTRR technique we demonstrate that, in mice, white matter has a significantly lower BBPC (BBPC_white = 0.93 ± 0.05 mL/g) than cortical gray matter (BBPC_gray = 0.99 ± 0.04 mL/g, p = 0.03), and that when voxel-wise BBPC correction is performed on CBF maps the observed difference in perfusion between gray and white matter is improved by as much as 14%. Our results suggest that BBPC correction is feasible and could be particularly important in future studies of perfusion in white matter pathologies.

Acute Physiology and Neurologic Outcomes after Brain Injury in SCOP/PHLPP1 KO Mice

Suprachiasmatic nucleus circadian oscillatory protein (SCOP) (a.k.a. PHLPP1) regulates long-term memory consolidation in the brain. Using a mouse model of controlled cortical impact (CCI) we tested if (1) brain tissue levels of SCOP/PHLPP1 increase after a traumatic brain injury (TBI), and (2) if SCOP/PHLPP1 gene knockout (KO) mice have improved (or worse) neurologic outcomes. Blood chemistry (pH, pCO₂, pO₂, pSO₂, base excess, sodium bicarbonate, and osmolarity) and arterial pressure (MAP) differed in isoflurane anesthetized WT vs. KOs at baseline and up to 1 h post-injury. CCI injury increased cortical/hippocampal SCOP/PHLPP1 levels in WTs 7d and 14d post-injury. Injured KOs had higher brain tissue levels of phosphorylated AKT (pAKT) in cortex (14d post-injury), and higher levels of phosphorylated MEK (pMEK) in hippocampus (7d and 14d post-injury) and in cortex (7d post-injury). Consistent with an important role of SCOP/PHLPP1 on memory function, injured-KOs had near normal performance on the probe trial of the Morris water maze, whereas injured-WTs were impaired. CA1/CA3 hippocampal survival was lower in KOs vs. WTs 24 h post-injury but equivalent by 7d. No difference in 21d cortical lesion volume was detected. SCOP/PHLPP1 overexpression in cultured rat cortical neurons had no effect on 24 h cell death after a mechanical stretch-injury.

Polynitroxylated Pegylated Hemoglobin-A Novel, Small Volume Therapeutic for Traumatic Brain Injury Resuscitation: Comparison to Whole Blood and Dose Response Evaluation

Resuscitation with polynitroxylated pegylated hemoglobin (PNPH), a pegylated bovine hemoglobin decorated with nitroxides, eliminated the need for fluid administration, reduced intracranial pressure (ICP) and brain edema, and produced neuroprotection in vitro and in vivo versus Lactated Ringer's solution (LR) in experimental traumatic brain injury (TBI) plus hemorrhagic shock (HS). We hypothesized that resuscitation with PNPH would improve acute physiology versus whole blood after TBI+HS and would be safe and effective across a wide dosage range. Anesthetized mice underwent controlled cortical impact and severe HS to mean arterial pressure (MAP) of 25-27 mm Hg for 35 min, then were resuscitated with PNPH, autologous whole blood, or LR. Markers of acute physiology, including mean arterial blood pressure (MAP), heart rate (HR), blood gases/chemistries, and brain oxygenation (PbtO2), were monitored for 90 min on room air followed by 15 min on 100% oxygen. In a second experiment, the protocol was repeated, except mice were resuscitated with PNPH with doses between 2 and 100 mL/kg. ICP and 24 h %-brain water were evaluated. PNPH-resuscitated mice had higher MAP and lower HR post-resuscitation versus blood or LR (p < 0.01). PNPH-resuscitated mice, versus those resuscitated with blood or LR, also had higher pH and lower serum potassium (p < 0.05). Blood-resuscitated mice, however, had higher PbtO2 versus those resuscitated with LR and PNPH, although PNPH had higher PbtO2 versus LR (p < 0.05). PNPH was well tolerated across the dosing range and dramatically reduced fluid requirements in all doses-even 2 or 5 mL/kg (p < 0.001). ICP was significantly lower in PNPH-treated mice for most doses tested versus in LR-treated mice, although %-brain water did not differ between groups. Resuscitation with PNPH, versus resuscitation with LR or blood, improved MAP, HR, and ICP, reduced acidosis and hyperkalemia, and was well tolerated and effective across a wide dosing range, supporting ongoing pre-clinical development of PNPH for TBI resuscitation.

Improving on Laboratory Traumatic Brain Injury Models to Achieve Better Results

Experimental modeling of traumatic brain injury (TBI) in animals has identified several potential means and interventions that might have beneficial applications for treating traumatic brain injury clinically. Several of these interventions have been applied and tried with humans that are at different phases of testing (completed, prematurely terminated and others in progress). The promising results achieved in the laboratory with animal models have not been replicated with human trails as expected. This review will highlight some insights and significance attained via laboratory animal modeling of TBI as well as factors that require incorporation into the experimental studies that could help in translating results from laboratory to the bedside. Major progress has been made due to laboratory studies; in explaining the mechanisms as well as pathophysiological features of brain damage after TBI. Attempts to intervene in the cascade of events occurring after TBI all rely heavily on the knowledge from basic laboratory investigations. In looking to discover treatment, this review will endeavor to sight and state some central discrepancies between laboratory models and clinical scenarios.

Substantial Reduction of Parenchymal Cerebral Blood Flow in Mice with Bilateral Common Carotid Artery Stenosis

The bilateral common carotid artery stenosis (BCAS) mouse model, which replicates chronic cerebral hypoperfusion and white matter ischemic lesions, is considered to model some aspects of vascular cognitive impairment. Cerebral blood flow (CBF) changes in the brain surface post-BCAS have been demonstrated by laser speckle flowmetry, but CBF levels in the brain parenchyma remain unknown. Adult C57BL/6J male mice were subjected to BCAS using external microcoils. Brain magnetic resonance angiography (MRA) was conducted to visualize the intracranial main arteries while arterial spin labeling (ASL) was used to measure cortical and subcortical parenchymal CBF levels before and after BCAS. Brain MRA showed anterior circulation flow was substantially decreased until 14 days post-BCAS, which gradually but incompletely recovered over the following 14 days, with probable growth of collaterals from the posterior cerebral artery. ASL showed that cortical and subcortical parenchymal CBF remained decreased at approximately 50% of the baseline level during 1 and 14 days post-BCAS, recovering to approximately 70% at day 28. CBF levels in the parenchyma were lower than the cortical superficial region in the BCAS model and remained decreased without recovery during the first 2 weeks post-BCAS. These results suggest that the BCAS model reliably replicates chronic cerebral hypoperfusion.

Traumatic Brain Injury and Polytrauma in Theaters of Combat: The Case for Neurotrauma Resuscitation?

Polytrauma associated with traumatic brain injury (TBI) is defined as a concurrent injury to the brain and one or more body areas or organ systems that results in physical, cognitive, and psychosocial impairments. Consequently, polytrauma accompanied by TBI presents a unique challenge for emergency medicine, in particular, to those associated with the austere environments encountered in military theaters of operation and the logistics of en-route care. Here, we attempt to put needed focus on this medical emergency, specifically addressing the problem of an exsanguinating polytrauma requiring fluid resuscitation complicated by TBI. Critical questions to consider are the following: (1) What is the optimal resuscitation fluid for these patients? (2) In defining the resuscitation fluid, what considerations must be given with regard to the very specific logistics of military operations? and (3) Can treatment of the brain injury be initiated in parallel with resuscitation practices. Recognizing the immense clinical and experimental complexity of this problem, our goal was to encourage research that embraces with high-fidelity 'combined' animal models of polytrauma and TBI with an objective toward elucidating safe and effective neurotherapeutic resuscitation protocols.

Combined hypoxemic and hypotensive insults altered physiological responses and neurofunction in a severity-dependent manner following penetrating ballistic-like brain injury in rats

BACKGROUND

Traumatic brain injury often occurs with concomitant hypoxemia (HX) and hemorrhagic shock (HS), leading to poor outcomes. This study characterized the acute physiology and subacute behavioral consequences of these additional insults in a model of penetrating ballistic-like brain injury (PBBI).

METHODS

Rats were randomly assigned into sham control, HX + HS (HH), 5% PBBI alone, 5% PBBI + HH, 10% PBBI alone, and 10% PBBI + HH groups. Mean arterial pressure, heart rate, and breathing rate were monitored continuously. In the combined injury groups, animals were subjected to 30-minute HX (Pao2, 30-40 mm Hg) and then 30-min HS (mean arterial pressure, 40 mm Hg) followed by fluid resuscitation with lactated Ringer's solution after PBBI or sham PBBI. Motor function was assessed using the rotarod task at 7 days and 14 days after injury. Cognitive function was assessed in the Morris water maze task from 13 days to 17 days after injury.

RESULTS

Combined HH caused acute bradycardia that was reversed by fluid resuscitation. During HX phase, tachypnea was observed in all HH groups. Persistent bradypnea was detected in 10% PBBI + HH group during the resuscitation phase. PBBI produced significant decrements in motor performance (vs. sham and HH groups). Additional insults significantly worsened motor deficits following 5% PBBI but not 10% PBBI. Both 5% PBBI and 10% PBBI produced significant cognitive deficits in the Morris water maze task with worsened deficits evident following the more severe injury (i.e., 10% PBBI). Alternatively, rats subjected to 5% PBBI + HH exhibited cognitive impairment that was significantly worse compared with 5% PBBI alone, whereas this worsening effect was not detected in the 10% PBBI groups.

CONCLUSION

This study characterized the physiological responses and neurobehavioral profiles following combined PBBI and HH. Ten percent PBBI produces motor and cognitive deficits, which may exceed a sensitivity threshold capacity. In contrast, 5% PBBI produces a lower, albeit significant, magnitude of deficits and thus provides a more sensitive screen for evaluating the cumulative effects of additional insults, which were indeed demonstrated to significantly worsen outcome.

Combined Neurotrauma Models: Experimental Models Combining Traumatic Brain Injury and Secondary Insults

Patients with severe traumatic brain injury (TBI) frequently present with concomitant injuries that may cause secondary brain injury and impact outcomes. Animal models have been developed that combine contemporary models of TBI with a secondary neurologic insult such as hypoxia, shock, long bone fracture, and radiation exposure. Combined injury models may be particularly useful when modeling treatment strategies and in efforts to map basic research to a heterogeneous patient population. Here, we review these models and their collective contribution to the literature on TBI. In addition, we provide protocols and notes for two well-characterized models of TBI plus hemorrhagic shock.

Approach to Modeling, Therapy Evaluation, Drug Selection, and Biomarker Assessments for a Multicenter Pre-Clinical Drug Screening Consortium for Acute Therapies in Severe Traumatic Brain Injury: Operation Brain Trauma Therapy

Traumatic brain injury (TBI) was the signature injury in both the Iraq and Afghan wars and the magnitude of its importance in the civilian setting is finally being recognized. Given the scope of the problem, new therapies are needed across the continuum of care. Few therapies have been shown to be successful. In severe TBI, current guidelines-based acute therapies are focused on the reduction of intracranial hypertension and optimization of cerebral perfusion. One factor considered important to the failure of drug development and translation in TBI relates to the recognition that TBI is extremely heterogeneous and presents with multiple phenotypes even within the category of severe injury. To address this possibility and attempt to bring the most promising therapies to clinical trials, we developed Operation Brain Trauma Therapy (OBTT), a multicenter, pre-clinical drug screening consortium for acute therapies in severe TBI. OBTT was developed to include a spectrum of established TBI models at experienced centers and assess the effect of promising therapies on both conventional outcomes and serum biomarker levels. In this review, we outline the approach to TBI modeling, evaluation of therapies, drug selection, and biomarker assessments for OBTT, and provide a framework for reports in this issue on the first five therapies evaluated by the consortium.

Traumatic brain injury disrupts cerebrovascular tone through endothelial inducible nitric oxide synthase expression and nitric oxide gain of function

BACKGROUND

Traumatic brain injury (TBI) has been reported to increase the concentration of nitric oxide (NO) in the brain and can lead to loss of cerebrovascular tone; however, the sources, amounts, and consequences of excess NO on the cerebral vasculature are unknown. Our objective was to elucidate the mechanism of decreased cerebral artery tone after TBI.

METHODS AND RESULTS

Cerebral arteries were isolated from rats 24 hours after moderate fluid‐percussion TBI. Pressure‐induced increases in vasoconstriction (myogenic tone) and smooth muscle Ca2+ were severely blunted in cerebral arteries after TBI. However, myogenic tone and smooth muscle Ca2+ were restored by inhibition of NO synthesis or endothelium removal, suggesting that TBI increased endothelial NO levels. Live native cell NO, indexed by 4,5‐diaminofluorescein (DAF‐2 DA) fluorescence, was increased in endothelium and smooth muscle of cerebral arteries after TBI. Clamped concentrations of 20 to 30 nmol/L NO were required to simulate the loss of myogenic tone and increased (DAF‐2T) fluorescence observed following TBI. In comparison, basal NO in control arteries was estimated as 0.4 nmol/L. Consistent with TBI causing enhanced NO‐mediated vasodilation, inhibitors of guanylyl cyclase, protein kinase G, and large‐conductance Ca2+‐activated potassium (BK) channel restored function of arteries from animals with TBI. Expression of the inducible isoform of NO synthase was upregulated in cerebral arteries isolated from animals with TBI, and the inducible isoform of NO synthase inhibitor 1400W restored myogenic responses following TBI.

CONCLUSIONS

The mechanism of profound cerebral artery vasodilation after TBI is a gain of function in vascular NO production by 60‐fold over controls, resulting from upregulation of the inducible isoform of NO synthase in the endothelium.

The brain metabolic activity after resuscitation with liposome-encapsulated hemoglobin in a rat model of hypovolemic shock

We examined the effect of resuscitation with liposome-encapsulated hemoglobin (LEH) on cerebral bioenergetics in a rat model of 45% hypovolemia. The rats were resuscitated with isovolemic LEH or saline after 15 minutes of shock and followed up to 6 hours. Untreated hypovolemic rats received no fluid. The cerebral uptake of F-18-fluorodeoxyglucose (FDG) was measured by PET, and at 6 hours, the brain was collected for various assays. Hypovolemia decreased cellular adenosine triphosphate (ATP), phosphocreatine, nicotinamide adenine dinucleotide (NAD)/NADH ratio, citrate synthase activity, glucose-6-phosphate, and nerve growth factor (NGF), even when FDG uptake remained unchanged. The FDG uptake was reduced by saline, but not by LEH infusion. The reduced FDG uptake in saline group was associated with a decrease in hexokinase I expression. The LEH infusion effectively restored ATP content, NAD/NADH ratio, and NGF expression, and reduced the hypovolemia-induced accumulation of pyruvate and ubiquitinated proteins; in comparison, saline was significantly less effective. The LEH infusion was associated with low pH and high anion gap, indicating anionic gap acidosis. The results suggest that hypovolemic shock perturbs glucose metabolism at the level of pyruvate utilization, resulting in deranged cerebral energy stores. The correction of volume and oxygen deficits by LEH recovers the cerebral metabolism and creates a prosurvival phenotype.

Differential effects of early postinjury treatment with neuroprotective drugs in a mouse model using diffuse reflectance spectroscopy

The time required for the arrival of an ambulance crew and administration of first aid is critical to clinical outcome, particularly in the case of head injury victims requiring neuroprotective drugs following a car accident, falls, and assaults. Short response times of the medical team, together with proper treatment, can limit injury severity and even save a life before transportation to the nearest medical center. We present a comparative evaluation of five different neuroprotective drugs frequently used in intensive care and operating units in the early phase following traumatic brain injury (TBI): hypertonic saline (HTS), mannitol, morphine, melatonin, and minocycline. The effectiveness of these drugs in terms of changes in brain tissue morphology (cell organelle size, density, distribution, etc.) and biochemical tissue properties (chromophores' content) was experimentally evaluated through analysis of the spectral reduced scattering and optical absorption coefficient parameters in the near-infrared (NIR) optical range (650 to 1000 nm). Experiments were conducted on anesthetized male mice subjected to a noninvasive closed head weight-drop model of focal TBI ([Formula: see text] and [Formula: see text] control) and monitored using an NIR diffuse reflectance spectroscopy system utilizing independent source-detector separation and location. After 10 min of baseline measurement, focal TBI was induced and measurements were conducted for 20 min. Subsequently, a neuroprotective drug was administrated and measurements were recorded for another 30 min. This work's major findings are threefold: first, minocycline was found to improve hemodynamic outcome at the earliest time postinjury. Second, HTS decreased brain water content and inhibited the increase in intracranial pressure. Third, the efficacy of neuroprotective drugs can be monitored noninvasively with diffuse reflectance spectroscopy. The demonstrated ability to noninvasively detect cerebral physiological properties following early administration of neuroprotective drugs underlines the need for more extensive investigation of the combined use of clinical drugs in larger-scale preclinical experiments to find the most beneficial drug treatment for brain injury patients.

Transcriptional profiling in rat hair follicles following simulated Blast insult: a new diagnostic tool for traumatic brain injury

With wide adoption of explosive-dependent weaponry during military activities, Blast-induced neurotrauma (BINT)-induced traumatic brain injury (TBI) has become a significant medical issue. Therefore, a robust and accessible biomarker system is in demand for effective and efficient TBI diagnosis. Such systems will also be beneficial to studies of TBI pathology. Here we propose the mammalian hair follicles as a potential candidate. An Advanced Blast Simulator (ABS) was developed to generate shock waves simulating traumatic conditions on brains of rat model. Microarray analysis was performed in hair follicles to identify the gene expression profiles that are associated with shock waves. Gene set enrichment analysis (GSEA) and sub-network enrichment analysis (SNEA) were used to identify cell processes and molecular signaling cascades affected by simulated bomb blasts. Enrichment analyses indicated that genes with altered expression levels were involved in central nervous system (CNS)/peripheral nervous system (PNS) responses as well as signal transduction including Ca2+, K+-transportation-dependent signaling, Toll-Like Receptor (TLR) signaling and Mitogen Activated Protein Kinase (MAPK) signaling cascades. Many of the pathways identified as affected by shock waves in the hair follicles have been previously reported to be TBI responsive in other organs such as brain and blood. The results suggest that the hair follicle has some common TBI responsive molecular signatures to other tissues. Moreover, various TBI-associated diseases were identified as preferentially affected using a gene network approach, indicating that the hair follicle may be capable of reflecting comprehensive responses to TBI conditions. Accordingly, the present study demonstrates that the hair follicle is a potentially viable system for rapid and non-invasive TBI diagnosis.

Traumatic brain injury using mouse models

The use of mouse models in traumatic brain injury (TBI) has several advantages compared to other animal models including low cost of breeding, easy maintenance, and innovative technology to create genetically modified strains. Studies using knockout and transgenic mice demonstrating functional gain or loss of molecules provide insight into basic mechanisms of TBI. Mouse models provide powerful tools to screen for putative therapeutic targets in TBI. This article reviews currently available mouse models that replicate several clinical features of TBI such as closed head injuries (CHI), penetrating head injuries, and a combination of both. CHI may be caused by direct trauma creating cerebral concussion or contusion. Sudden acceleration-deceleration injuries of the head without direct trauma may also cause intracranial injury by the transmission of shock waves to the brain. Recapitulation of temporary cavities that are induced by high-velocity penetrating objects in the mouse brain are difficult to produce, but slow brain penetration injuries in mice are reviewed. Synergistic damaging effects on the brain following systemic complications are also described. Advantages and disadvantages of CHI mouse models induced by weight drop, fluid percussion, and controlled cortical impact injuries are compared. Differences in the anatomy, biomechanics, and behavioral evaluations between mice and humans are discussed. Although the use of mouse models for TBI research is promising, further development of these techniques is warranted.

Polynitroxylated-pegylated hemoglobin attenuates fluid requirements and brain edema in combined traumatic brain injury plus hemorrhagic shock in mice

Polynitroxylated-pegylated hemoglobin (PNPH), a bovine hemoglobin decorated with nitroxide and polyethylene glycol moieties, showed neuroprotection vs. lactated Ringer's (LR) in experimental traumatic brain injury plus hemorrhagic shock (TBI+HS).

HYPOTHESIS

Resuscitation with PNPH will reduce intracranial pressure (ICP) and brain edema and improve cerebral perfusion pressure (CPP) vs. LR in experimental TBI+HS. C57/BL6 mice (n=20) underwent controlled cortical impact followed by severe HS to mean arterial pressure (MAP) of 25 to 27 mm Hg for 35 minutes. Mice (n=10/group) were then resuscitated with a 20 mL/kg bolus of 4% PNPH or LR followed by 10 mL/kg boluses targeting MAP>70 mm Hg for 90 minutes. Shed blood was then reinfused. Intracranial pressure was monitored. Mice were killed and %brain water (%BW) was measured (wet/dry weight). Mice resuscitated with PNPH vs. LR required less fluid (26.0±0.0 vs. 167.0±10.7 mL/kg, P<0.001) and had a higher MAP (79.4±0.40 vs. 59.7±0.83 mm Hg, P<0.001). The PNPH-treated mice required only 20 mL/kg while LR-resuscitated mice required multiple boluses. The PNPH-treated mice had a lower peak ICP (14.5±0.97 vs. 19.7±1.12 mm Hg, P=0.002), higher CPP during resuscitation (69.2±0.46 vs. 45.5±0.68 mm Hg, P<0.001), and lower %BW vs. LR (80.3±0.12 vs. 80.9±0.12%, P=0.003). After TBI+HS, resuscitation with PNPH lowers fluid requirements, improves ICP and CPP, and reduces brain edema vs. LR, supporting its development.

Traumatic brain injury, neuroimaging, and neurodegeneration

Depending on severity, traumatic brain injury (TBI) induces immediate neuropathological effects that in the mildest form may be transient but as severity increases results in neural damage and degeneration. The first phase of neural degeneration is explainable by the primary acute and secondary neuropathological effects initiated by the injury; however, neuroimaging studies demonstrate a prolonged period of pathological changes that progressively occur even during the chronic phase. This review examines how neuroimaging may be used in TBI to understand (1) the dynamic changes that occur in brain development relevant to understanding the effects of TBI and how these relate to developmental stage when the brain is injured, (2) how TBI interferes with age-typical brain development and the effects of aging thereafter, and (3) how TBI results in greater frontotemporolimbic damage, results in cerebral atrophy, and is more disruptive to white matter neural connectivity. Neuroimaging quantification in TBI demonstrates degenerative effects from brain injury over time. An adverse synergistic influence of TBI with aging may predispose the brain injured individual for the development of neuropsychiatric and neurodegenerative disorders long after surviving the brain injury.