Alterations in neurovascular coupling following acute traumatic brain injury

Optimal bispectral index exists in healthy patients undergoing general anesthesia: A validation study

PURPOSE

Continuous cerebrovascular reactivity monitoring in both neurocritical and intra-operative care has gained extensive interest in recent years, as it has documented associations with long-term outcomes (in neurocritical care populations) and cognitive outcomes (in operative cohorts). This has sparked further interest into the exploration and evaluation of methods to achieve an optimal cerebrovascular reactivity measure, where the individual patient is exposed to the lowest insult burden of impaired cerebrovascular reactivity. Recent literature has documented, in neural injury populations, the presence of a potential optimal sedation level in neurocritical care, based on the relationship between cerebrovascular reactivity and quantitative depth of sedation (using bispectral index (BIS)) - termed BISopt. The presence of this measure outside of neural injury patients has yet to be proven.

METHODS

We explore the relationship between BIS and continuous cerebrovascular reactivity in two cohorts: (A) healthy population undergoing elective spinal surgery under general anesthesia, and (B) healthy volunteer cohort of awake controls.

RESULTS

We demonstrate the presence of BISopt in the general anesthesia population (96% of patients), and its absence in awake controls, providing preliminary validation of its existence outside of neural injury populations. Furthermore, we found BIS to be sufficiently separate from overall systemic blood pressure, this indicates that they impact different pathophysiological phenomena to mediate cerebrovascular reactivity.

CONCLUSIONS

Findings here carry implications for the adaptation of the individualized physiologic BISopt concept to non-neural injury populations, both within critical care and the operative theater. However, this work is currently exploratory, and future work is required.

Multifaceted neural and vascular pathologies after pediatric mild traumatic brain injury

Dynamic changes in neurodevelopment and cognitive functioning occur during adolescence, including a switch from reactive to more proactive forms of cognitive control, including response inhibition. Pediatric mild traumatic brain injury (pmTBI) affects these cognitions immediately post-injury, but the role of vascular versus neural injury in cognitive dysfunction remains debated. This study consecutively recruited 214 sub-acute pmTBI (8-18 years) and age/sex-matched healthy controls (HC; N = 186), with high retention rates (>80%) at four months post-injury. Multimodal imaging (functional MRI during response inhibition, cerebral blood flow and cerebrovascular reactivity) assessed for pathologies within the neurovascular unit. Patients exhibited increased errors of commission and hypoactivation of motor circuitry during processing of probes. Evidence of increased/delayed cerebrovascular reactivity within motor circuitry during hypercapnia was present along with normal perfusion. Neither age-at-injury nor post-concussive symptom load were strongly associated with imaging abnormalities. Collectively, mild cognitive impairments and clinical symptoms may continue up to four months post-injury. Prolonged dysfunction within the neurovascular unit was observed during proactive response inhibition, with preliminary evidence that neural and pure vascular trauma are statistically independent. These findings suggest pmTBI is characterized by multifaceted pathologies during the sub-acute injury stage that persist several months post-injury.

Non-Invasive Continuous Optical Monitoring of Cerebral Blood Flow after Traumatic Brain Injury in Mice Using Fiber Camera-Based Speckle Contrast Optical Spectroscopy

Neurocritical care focuses on monitoring cerebral blood flow (CBF) to prevent secondary brain injuries before damage becomes irreversible. Thus, there is a critical unmet need for continuous neuromonitoring methods to quantify CBF within the vulnerable cortex continuously and non-invasively. Animal models and imaging biomarkers can provide valuable insights into the mechanisms and kinetics of head injury, as well as insights for potential treatment strategies. For this purpose, we implemented an optical technique for continuous monitoring of blood flow changes after a closed head injury in a mouse model, which is based on laser speckle contrast imaging and a fiber camera-based approach. Our results indicate a significant decrease (~10%, p-value < 0.05) in blood flow within 30 min of a closed head injury. Furthermore, the low-frequency oscillation analysis also indicated much lower power in the trauma group compared to the control group. Overall, blood flow has the potential to be a biomarker for head injuries in the early phase of a trauma, and the system is useful for continuous monitoring with the potential for clinical translation.

Targeting Astrocyte Signaling Alleviates Cerebrovascular and Synaptic Function Deficits in a Diet-Based Mouse Model of Small Cerebral Vessel Disease

Despite the indispensable role that astrocytes play in the neurovascular unit, few studies have investigated the functional impact of astrocyte signaling in cognitive decline and dementia related to vascular pathology. Diet-mediated induction of hyperhomocysteinemia (HHcy) recapitulates numerous features of vascular contributions to cognitive impairment and dementia (VCID). Here, we used astrocyte targeting approaches to evaluate astrocyte Ca2+ dysregulation and the impact of aberrant astrocyte signaling on cerebrovascular dysfunction and synapse impairment in male and female HHcy diet mice. Two-photon imaging conducted in fully awake mice revealed activity-dependent Ca2+ dysregulation in barrel cortex astrocytes under HHcy. Stimulation of contralateral whiskers elicited larger Ca2+ transients in individual astrocytes of HHcy diet mice compared with control diet mice. However, evoked Ca2+ signaling across astrocyte networks was impaired in HHcy mice. HHcy also was associated with increased activation of the Ca2+/calcineurin-dependent transcription factor NFAT4, which has been linked previously to the reactive astrocyte phenotype and synapse dysfunction in amyloid and brain injury models. Targeting the NFAT inhibitor VIVIT to astrocytes, using adeno-associated virus vectors, led to reduced GFAP promoter activity in HHcy diet mice and improved functional hyperemia in arterioles and capillaries. VIVIT expression in astrocytes also preserved CA1 synaptic function and improved spontaneous alternation performance on the Y maze. Together, the results demonstrate that aberrant astrocyte signaling can impair the major functional properties of the neurovascular unit (i.e., cerebral vessel regulation and synaptic regulation) and may therefore represent a promising drug target for treating VCID and possibly Alzheimer's disease and other related dementias.SIGNIFICANCE STATEMENT The impact of reactive astrocytes in Alzheimer's disease and related dementias is poorly understood. Here, we evaluated Ca2+ responses and signaling in barrel cortex astrocytes of mice fed with a B-vitamin deficient diet that induces hyperhomocysteinemia (HHcy), cerebral vessel disease, and cognitive decline. Multiphoton imaging in awake mice with HHcy revealed augmented Ca2+ responses in individual astrocytes, but impaired signaling across astrocyte networks. Stimulation-evoked arteriole dilation and elevated red blood cell velocity in capillaries were also impaired in cortex of awake HHcy mice. Astrocyte-specific inhibition of the Ca2+-dependent transcription factor, NFAT, normalized cerebrovascular function in HHcy mice, improved synaptic properties in brain slices, and stabilized cognition. Results suggest that astrocytes are a mechanism and possible therapeutic target for vascular-related dementia.

Neurocapillary-Modulation

OBJECTIVES

We consider two consequences of brain capillary ultrastructure in neuromodulation. First, blood-brain barrier (BBB) polarization as a consequence of current crossing between interstitial space and the blood. Second, interstitial current flow distortion around capillaries impacting neuronal stimulation.

MATERIALS AND METHODS

We developed computational models of BBB ultrastructure morphologies to first assess electric field amplification at the BBB (principle 1) and neuron polarization amplification by the presence of capillaries (principle 2). We adapt neuron cable theory to develop an analytical solution for maximum BBB polarization sensitivity.

RESULTS

Electrical current crosses between the brain parenchyma (interstitial space) and capillaries, producing BBB electric fields (EBBB) that are >400x of the average parenchyma electric field (ĒBRAIN), which in turn modulates transport across the BBB. Specifically, for a BBB space constant (λBBB) and wall thickness (dth-BBB), the analytical solution for maximal BBB electric field (EABBB) is given as: (ĒBRAIN × λBBB)/dth-BBB. Electrical current in the brain parenchyma is distorted around brain capillaries, amplifying neuronal polarization. Specifically, capillary ultrastructure produces ∼50% modulation of the ĒBRAIN over the ∼40 μm inter-capillary distance. The divergence of EBRAIN (Activating function) is thus ∼100 kV/m2 per unit ĒBRAIN.

CONCLUSIONS

BBB stimulation by principle 1 suggests novel therapeutic strategies such as boosting metabolic capacity or interstitial fluid clearance. Whereas the spatial profile of EBRAIN is traditionally assumed to depend only on macroscopic anatomy, principle 2 suggests a central role for local capillary ultrastructure-which impact forms of neuromodulation including deep brain stimulation (DBS), spinal cord stimulation (SCS), transcranial magnetic stimulation (TMS), electroconvulsive therapy (ECT), and transcranial electrical stimulation (tES)/transcranial direct current stimulation (tDCS).

Region-Specific Neurovascular Decoupling Associated With Cognitive Decline in Parkinson's Disease

Background: Cognitive deficits are prominent non-motor symptoms in Parkinson’s disease (PD) and have been shown to involve the neurovascular unit (NVU). However, there is a lack of sufficient neuroimaging research on the associated modulating mechanisms. The objective of this study was to identify the contribution of neurovascular decoupling to the pathogenesis of cognitive decline in PD.

Methods: Regional homogeneity (ReHo), a measure of neuronal activity, and cerebral blood flow (CBF), a measure of vascular responses, were obtained from patients with PD with mild cognitive impairment (MCI) and normal cognition (NC) as well as matched healthy controls (HCs). Imaging metrics of neurovascular coupling (global and regional CBF-ReHo correlation coefficients and CBF-ReHo ratios) were compared among the groups.

Results: Neurovascular coupling was impaired in patients with PD-MCI with a decreased global CBF-ReHo correlation coefficient relative to HC subjects (P < 0.05). Regional dysregulation was specific to the PD-MCI group and localized to the right middle frontal gyrus, right middle cingulate cortex, right middle occipital gyrus, right inferior parietal gyrus, right supramarginal gyrus, and right angular gyrus (P < 0.05). Compared with HC subjects, patients with PD-MCI showed higher CBF-ReHo ratios in the bilateral lingual gyri (LG), bilateral putamen, and left postcentral gyrus and lower CBF-ReHo ratios in the right superior temporal gyrus, bilateral middle temporal gyri, bilateral parahippocampal gyri, and right inferior frontal gyrus. Relative to the HC and PD-NC groups, the PD-MCI group showed an increased CBF-ReHo ratio in the left LG, which was correlated with poor visual–spatial performance (r = −0.36 and P = 0.014).

Conclusion: The involvement of neurovascular decoupling in cognitive impairment in PD is regionally specific and most prominent in the visual–spatial cortices, which could potentially provide a complementary understanding of the pathophysiological mechanisms underlying cognitive deficits in PD.

Attenuation of tonic inhibition prevents chronic neurovascular impairments in a Thy1-ChR2 mouse model of repeated, mild traumatic brain injury

Rationale: Mild traumatic brain injury (mTBI), the most common type of brain trauma, frequently leads to chronic cognitive and neurobehavioral deficits. Intervening effectively is impeded by our poor understanding of its pathophysiological sequelae.

Methods: To elucidate the long-term neurovascular sequelae of mTBI, we combined optogenetics, two-photon fluorescence microscopy, and intracortical electrophysiological recordings in mice to selectively stimulate peri-contusional neurons weeks following repeated closed-head injury and probe individual vessel's function and local neuronal reactivity.

Results: Compared to sham-operated animals, mTBI mice showed doubled cortical venular speeds (115 ± 25%) and strongly elevated cortical venular reactivity (53 ± 17%). Concomitantly, the pericontusional neurons exhibited attenuated spontaneous activity (-57 ± 79%) and decreased reactivity (-47 ± 28%). Post-mortem immunofluorescence revealed signs of peri-contusional senescence and DNA damage, in the absence of neuronal loss or gliosis. Alteration of neuronal and vascular functioning was largely prevented by chronic, low dose, systemic administration of a GABA-A receptor inverse agonist (L-655,708), commencing 3 days following the third impact.

Conclusions: Our findings indicate that repeated mTBI leads to dramatic changes in the neurovascular unit function and that attenuation of tonic inhibition can prevent these alterations. The sustained disruption of the neurovascular function may underlie the concussed brain's long-term susceptibility to injury, and calls for development of better functional assays as well as of neurovascularly targeted interventions.

Neurovascular-modulation: A review of primary vascular responses to transcranial electrical stimulation as a mechanism of action

BACKGROUND

The ubiquitous vascular response to transcranial electrical stimulation (tES) has been attributed to the secondary effect of neuronal activity forming the classic neurovascular coupling. However, the current density delivered transcranially concentrates in: A) the cerebrospinal fluid of subarachnoid space where cerebral vasculature resides after reaching the dural and pial surfaces and B) across the blood-brain-barrier after reaching the brain parenchyma. Therefore, it is anticipated that tES has a primary vascular influence.

OBJECTIVES

Focused review of studies that demonstrated the direct vascular response to electrical stimulation and studies demonstrating evidence for tES-induced vascular effect in coupled neurovascular systems.

RESULTS

tES induces both primary and secondary vascular phenomena originating from four cellular elements; the first two mediating a primary vascular phenomenon mainly in the form of an immediate vasodilatory response and the latter two leading to secondary vascular effects and as parts of classic neurovascular coupling: 1) The perivascular nerves of more superficially located dural and pial arteries and medium-sized arterioles with multilayered smooth muscle cells; and 2) The endothelial lining of all vessels including microvasculature of blood-brain barrier; 3) Astrocytes; and 4) Neurons of neurovascular units.

CONCLUSION

A primary vascular effect of tES is highly suggested based on various preclinical and clinical studies. We explain how the nature of vascular response can depend on vessel anatomy (size) and physiology and be controlled by stimulation waveform. Further studies are warranted to investigate the mechanisms underlying the vascular response and its contribution to neural activity in both healthy brain and pathological conditions - recognizing many brain diseases are associated with alteration of cerebral hemodynamics and decoupling of neurovascular units.

Dissecting the microvascular contributions to diffuse correlation spectroscopy measurements of cerebral hemodynamics using optical coherence tomography angiography

Significance: Diffuse correlation spectroscopy (DCS) is an emerging noninvasive, diffuse optical modality that purportedly enables direct measurements of microvasculature blood flow. Functional optical coherence tomography angiography (OCT-A) can resolve blood flow in vessels as fine as capillaries and thus has the capability to validate key attributes of the DCS signal. Aim: To characterize activity in cortical vasculature within the spatial volume that is probed by DCS and to identify populations of blood vessels that are most representative of the DCS signals. Approach: We performed simultaneous measurements of somatosensory-evoked cerebral blood flow in mice in vivo using both DCS and OCT-A. Results: We resolved sensory-evoked blood flow in the somatosensory cortex with both modalities. Vessels with diameters smaller than 10    μ m featured higher peak flow rates during the initial poststimulus positive increase in flow, whereas larger vessels exhibited considerably larger magnitude of the subsequent undershoot. The simultaneously recorded DCS waveforms correlated most highly with flow in the smallest vessels, yet featured a more prominent undershoot. Conclusions: Our direct, multiscale, multimodal cross-validation measurements of functional blood flow support the assertion that the DCS signal preferentially represents flow in microvasculature. The significantly greater undershoot in DCS, however, suggests a more spatially complex relationship to flow in cortical vasculature during functional activation.

Cannabidiol and Other Cannabinoids in Demyelinating Diseases

A growing body of preclinical evidence indicates that certain cannabinoids, including cannabidiol (CBD) and synthetic derivatives, may play a role in the myelinating processes and are promising small molecules to be developed as drug candidates for management of demyelinating diseases such as multiple sclerosis (MS), stroke and traumatic brain injury (TBI), which are three of the most prevalent demyelinating disorders. Thanks to the properties described for CBD and its interesting profile in humans, both the phytocannabinoid and derivatives could be considered as potential candidates for clinical use. In this review we will summarize current advances in the use of CBD and other cannabinoids as future potential treatments. While new research is accelerating the process for the generation of novel drug candidates and identification of druggable targets, the collaboration of key players such as basic researchers, clinicians and pharmaceutical companies is required to bring novel therapies to the patients.

Use of Medical Cannabis to Treat Traumatic Brain Injury

There is not a single pharmacological agent with demonstrated therapeutic efficacy for traumatic brain injury (TBI). With recent legalization efforts and the growing popularity of medical cannabis, patients with TBI will inevitably consider medical cannabis as a treatment option. Pre-clinical TBI research suggests that cannabinoids have neuroprotective and psychotherapeutic properties. In contrast, recreational cannabis use has consistently shown to have detrimental effects. Our review identified a paucity of high-quality studies examining the beneficial and adverse effects of medical cannabis on TBI, with only a single phase III randomized control trial. However, observational studies demonstrate that TBI patients are using medical and recreational cannabis to treat their symptoms, highlighting inconsistencies between public policy, perception of potential efficacy, and the dearth of empirical evidence. We conclude that randomized controlled trials and prospective studies with appropriate control groups are necessary to fully understand the efficacy and potential adverse effects of medical cannabis for TBI.

Cerebral Perfusion Effects of Cognitive Training and Transcranial Direct Current Stimulation in Mild-Moderate TBI

Background: Persistent post-traumatic symptoms (PPS) after traumatic brain injury (TBI) can lead to significant chronic functional impairment. Pseudocontinuous arterial spin labeling (pCASL) has been used in multiple studies to explore changes in cerebral blood flow (CBF) that may result in acute and chronic TBI, and is a promising neuroimaging modality for assessing response to therapies. Methods: Twenty-four subjects with chronic mild-moderate TBI (mmTBI) were enrolled in a pilot study of 10 days of computerized executive function training combined with active or sham anodal transcranial direct current stimulation (tDCS) for treatment of cognitive PPS. Behavioral surveys, neuropsychological testing, and magnetic resonance imaging (MRI) with pCASL sequences to assess global and regional CBF were obtained before and after the training protocol. Results: Robust improvements in depression, anxiety, complex attention, and executive function were seen in both active and sham groups between the baseline and post-treatment visits. Global CBF decreased over time, with differences in regional CBF noted in the right inferior frontal gyrus (IFG). Active stimulation was associated with static or increased CBF in the right IFG, whereas sham was associated with reduced CBF. Neuropsychological performance and behavioral symptoms were not associated with changes in CBF. Discussion: The current study suggests a complex picture between mmTBI, cerebral perfusion, and recovery. Changes in CBF may result from physiologic effect of the intervention, compensatory neural mechanisms, or confounding factors. Limitations include a small sample size and heterogenous injury sample, but these findings suggest promising directions for future studies of cognitive training paradigms in mmTBI.

Neuroprotection Following Concussion: The Potential Role for Cannabidiol

Cannabidiol (CBD) has been generating increasing interest in medicine due to its therapeutic properties and an apparent lack of negative side effects. Research has suggested that high dosages of CBD can be taken acutely and chronically with little to no risk. This review focuses on the neuroprotective effects of a CBD, with an emphasis on its implications for recovering from a mild traumatic brain injury (TBI) or concussion. CBD has been shown to influence the endocannabinoid system, both by affecting cannabinoid receptors and other receptors involved in the endocannabinoid system such as vanilloid receptor 1, adenosine receptors, and 5-hydroxytryptamine via cannabinoid receptor-independent mechanisms. Concussions can result in many physiological consequences, potentially resulting in post-concussion syndrome. While impairments in cerebrovascular and cardiovascular physiology following concussion have been shown, there is unfortunately still no single treatment available to enhance recovery. CBD has been shown to influence the blood brain barrier, brain-derived neurotrophic factors, cognitive capacity, the cerebrovasculature, cardiovascular physiology, and neurogenesis, all of which have been shown to be altered by concussion. CBD can therefore potentially provide treatment to enhance neuroprotection by reducing inflammation, regulating cerebral blood flow, enhancing neurogenesis, and protecting the brain against reactive oxygen species. Double-blind randomized controlled trials are still required to validate the use of CBD as medication following mild TBIs, such as concussion.

Proactive inhibition deficits with normal perfusion after pediatric mild traumatic brain injury

Although much attention has been generated in popular media regarding the deleterious effects of pediatric mild traumatic brain injury (pmTBI), a paucity of empirical evidence exists regarding the natural course of biological recovery. Fifty pmTBI patients (12–18 years old) were consecutively recruited from Emergency Departments and seen approximately 1 week and 4 months post‐injury in this prospective cohort study. Data from 53 sex‐ and age‐matched healthy controls (HC) were also collected. Functional magnetic resonance imaging was obtained during proactive response inhibition and at rest, in conjunction with independent measures of resting cerebral blood flow. High temporal resolution imaging enabled separate modeling of neural responses for preparation and execution of proactive response inhibition. A priori predictions of failed inhibitory responses (i.e., hyperactivation) were observed in motor circuitry (pmTBI>HC) and sensory areas sub‐acutely and at 4 months post‐injury. Paradoxically, pmTBI demonstrated hypoactivation (HC>pmTBI) during target processing, along with decreased activation within prefrontal cognitive control areas. Functional connectivity within motor circuitry at rest suggested that deficits were limited to engagement during the inhibitory task, whereas normal resting cerebral perfusion ruled out deficits in basal perfusion. In conclusion, current results suggest blood oxygen‐level dependent deficits during inhibitory control may exceed commonly held beliefs about physiological recovery following pmTBI, potentially lasting up to 4 months post‐injury.

Measurement of Neurovascular Coupling in Neonates

Neurovascular coupling refers to the mechanism that links the transient neural activity to the subsequent change in cerebral blood flow, which is regulated by both chemical signals and mechanical effects. Recent studies suggest that neurovascular coupling in neonates and preterm born infants is different compared to adults. The hemodynamic response after a stimulus is later and less pronounced and the stimulus might even result in a negative (hypoxic) signal. In addition, studies both in animals and neonates confirm the presence of a short hypoxic period after a stimulus in preterm infants. In clinical practice, different methodologies exist to study neurovascular coupling. The combination of functional magnetic resonance imaging or functional near-infrared spectroscopy (brain hemodynamics) with EEG (brain function) is most commonly used in neonates. Especially near-infrared spectroscopy is of interest, since it is a non-invasive method that can be integrated easily in clinical care and is able to provide results concerning longer periods of time. Therefore, near-infrared spectroscopy can be used to develop a continuous non-invasive measurement system, that could be used to study neonates in different clinical settings, or neonates with different pathologies. The main challenge for the development of a continuous marker for neurovascular coupling is how the coupling between the signals can be described. In practice, a wide range of signal interaction measures exist. Moreover, biomedical signals often operate on different time scales. In a more general setting, other variables also have to be taken into account, such as oxygen saturation, carbon dioxide and blood pressure in order to describe neurovascular coupling in a concise manner. Recently, new mathematical techniques were developed to give an answer to these questions. This review discusses these recent developments.

Advanced biomarkers of pediatric mild traumatic brain injury: Progress and perils

There is growing public concern about neurodegenerative changes (e.g., Chronic Traumatic Encephalopathy) that may occur chronically following clinically apparent and clinically silent (i.e., sub-concussive blows) pediatric mild traumatic brain injury (pmTBI). However, there are currently no biomarkers that clinicians can use to objectively diagnose patients or predict those who may struggle to recover. Non-invasive neuroimaging, electrophysiological and neuromodulation biomarkers have promise for providing evidence of the so-called "invisible wounds" of pmTBI. Our systematic review, however, belies that notion, identifying a relative paucity of high-quality, clinically impactful, diagnostic or prognostic biomarker studies in the sub-acute injury phase (36 studies on unique samples in 28 years), with the majority focusing on adolescent pmTBI. Ultimately, well-powered longitudinal studies with appropriate control groups, as well as standardized and clearly-defined inclusion criteria (time post-injury, injury severity and past history) are needed to truly understand the complex pathophysiology that is hypothesized (i.e., still needs to be determined) to exist during the acute and sub-acute stages of pmTBI and may underlie post-concussive symptoms.