Magnetoencephalography for Mild Traumatic Brain Injury and Posttraumatic Stress Disorder

Dysregulation of the dopaminergic system secondary to traumatic brain injury: implications for mood and anxiety disorders

Traumatic brain injury (TBI) represents a public health issue with a high mortality rate and severe neurological and psychiatric consequences. Mood and anxiety disorders are some of the most frequently reported. Primary and secondary damage can cause a loss of neurons and glial cells, leading to dysfunction of neuronal circuits, which can induce imbalances in many neurotransmitter systems. Monoaminergic systems, especially the dopaminergic system, are some of the most involved in the pathogenesis of neuropsychiatric and cognitive symptoms after TBI. In this work, we summarize the studies carried out in patients who have suffered TBI and describe alterations in the dopaminergic system, highlighting (1) dysfunction of the dopaminergic neuronal circuits caused by TBI, where modifications are shown in the dopamine transporter (DAT) and alterations in the expression of dopamine receptor 2 (D2R) in brain areas with dopaminergic innervation, thus establishing a hypodopaminergic state and (2) variations in the concentration of dopamine and its metabolites in biological fluids of post-TBI patients, such as elevated dopamine (DA) and alterations in homovanillic acid (HVA). On the other hand, we show a large number of reports of alterations in the dopaminergic system after a TBI in animal models, in which modifications in the levels of DA, DAT, and HVA have been reported, as well as alterations in the expression of tyrosine hydroxylase (TH). We also describe the biological pathways, neuronal circuits, and molecular mechanisms potentially involved in mood and anxiety disorders that occur after TBI and are associated with alterations of the dopaminergic system in clinical studies and animal models. We describe the changes that occur in the clinical picture of post-TBI patients, such as alterations in mood and anxiety associated with DAT activity in the striatum, the relationship between post-TBI major depressive disorders (MDD) with lower availability of the DA receptors D2R and D3R in the caudate and thalamus, as well as a decrease in the volume of the substantia nigra (SN) associated with anxiety symptoms. With these findings, we discuss the possible relationship between the disorders caused by alterations in the dopaminergic system in patients with TBI.

The Injury Progression in Acute Blast-Induced Mild Traumatic Brain Injury in Rats Reflected by Diffusion Tensor Imaging and Immunohistochemical Examination

Diffusion tensor imaging (DTI) has emerged as a promising neuroimaging tool for detecting blast-induced mild traumatic brain injury (bmTBI). However, lack of refined acute-phase monitoring and reliable imaging biomarkers hindered its clinical application in early diagnosis of bmTBI, leading to potential long-term disability of patients. In this study, we used DTI in a rat model of bmTBI generated by exposing to single lateral blast waves (151.16 and 349.75 kPa, lasting 47.48 ms) released in a confined bioshock tube, to investigate whole-brain DTI changes at 1, 3, and 7 days after injury. Combined assessment of immunohistochemical analysis, transmission electron microscopy, and behavioral readouts allowed for linking DTI changes to synchronous cellular damages and identifying stable imaging biomarkers. The corpus callosum (CC) and brainstem were identified as predominantly affected regions, in which reduced fractional anisotropy (FA) was detected as early as the first day after injury, with a maximum decline occurring at 3 days post-injury before returning to near normal levels by 7 days. Axial diffusivity (AD) values within the CC and brainstem also significantly reduced at 3 days post-injury. In contrast, the radial diffusivity (RD) in the CC showed acute elevation, peaking at 3 days after injury before normalizing by the 7-day time point. Damages to nerve fibers, including demyelination and axonal degeneration, progressed in lines with changes in DTI parameters, supporting a real-time macroscopic reflection of microscopic neuronal fiber injury by DTI. The most sensitive biomarker was identified as a decrease in FA, AD, and an increase in RD within the CC on the third day after injury, supporting the diagnostic utility of DTI in cases of bmTBI in the acute phase.

Auditory driven gamma synchrony is associated with cortical thickness in widespread cortical areas

OBJECTIVE

Gamma synchrony is a fundamental functional property of the cerebral cortex, impaired in multiple neuropsychiatric conditions (i.e. schizophrenia, Alzheimer's disease, stroke etc.). Auditory stimulation in the gamma range allows to drive gamma synchrony of the entire cortical mantle and to estimate the efficiency of the mechanisms sustaining it. As gamma synchrony depends strongly on the interplay between parvalbumin-positive interneurons and pyramidal neurons, we hypothesize an association between cortical thickness and gamma synchrony. To test this hypothesis, we employed a combined magnetoencephalography (MEG) - Magnetic Resonance Imaging (MRI) study.

METHODS

Cortical thickness was estimated from anatomical MRI scans. MEG measurements related to exposure of 40 Hz amplitude modulated tones were projected onto the cortical surface. Two measures of cortical synchrony were considered: (a) inter-trial phase consistency at 40 Hz, providing a vertex-wise estimation of gamma synchronization, and (b) phase-locking values between primary auditory cortices and whole cortical mantle, providing a measure of long-range cortical synchrony. A correlation between cortical thickness and synchronization measures was then calculated for 72 MRI-MEG scans.

RESULTS

Both inter-trial phase consistency and phase locking values showed a significant positive correlation with cortical thickness. For inter-trial phase consistency, clusters of strong associations were found in the temporal and frontal lobes, especially in the bilateral auditory and pre-motor cortices. Higher phase-locking values corresponded to higher cortical thickness in the frontal, temporal, occipital and parietal lobes.

DISCUSSION AND CONCLUSIONS

In healthy subjects, a thicker cortex corresponds to higher gamma synchrony and connectivity in the primary auditory cortex and beyond, likely reflecting underlying cell density involved in gamma circuitries. This result hints towards an involvement of gamma synchrony together with underlying brain structure in brain areas for higher order cognitive functions. This study contributes to the understanding of inherent cortical functional and structural brain properties, which might in turn constitute the basis for the definition of useful biomarkers in patients showing aberrant gamma synchronization.

Rich-club reorganization of functional brain networks in acute mild traumatic brain injury with cognitive impairment

BACKGROUND

Mild traumatic brain injury (mTBI) is typically characterized by temporally limited cognitive impairment and regarded as a brain connectome disorder. Recent findings have suggested that a higher level of organization named the "rich-club" may play a central role in enabling the integration of information and efficient communication across different systems of the brain. However, the alterations in rich-club organization and hub topology in mTBI and its relationship with cognitive impairment after mTBI have been scarcely elucidated.

METHODS

Resting-state functional magnetic resonance imaging (rs-fMRI) data were collected from 88 patients with mTBI and 85 matched healthy controls (HCs). Large-scale functional brain networks were established for each participant. Rich-club organizations and network properties were assessed and analyzed between groups. Finally, we analyzed the correlations between the cognitive performance and changes in rich-club organization and network properties.

RESULTS

Both mTBI and HCs groups showed significant rich-club organization. Meanwhile, the rich-club organization was aberrant, with enhanced functional connectivity (FC) among rich-club nodes and peripheral regions in acute mTBI. In addition, significant differences in partial global and local network topological property measures were found between mTBI patients and HCs (P<0.01). In patients with mTBI, changes in rich-club organization and network properties were found to be related to early cognitive impairment after mTBI (P<0.05).

CONCLUSIONS

Our findings suggest that such patterns of disruption and reorganization will provide the basic functional architecture for cognitive function, which may subsequently be used as an earlier biomarker for cognitive impairment after mTBI.

Neuroimaging Biomarkers of New-Onset Psychiatric Disorders Following Traumatic Brain Injury

Traumatic brain injury (TBI) has traditionally been associated with cognitive and behavioral changes during both the acute and chronic phases of injury. Because of its noninvasive nature, neuroimaging has the potential to provide unique information on underlying macroscopic and microscopic biological mechanisms that may serve as causative agents for these neuropsychiatric sequelae. This broad scoping review identifies at least 4 common macroscopic pathways that exist between TBI and new-onset psychiatric disorders, as well as several examples of how neuroimaging is currently being utilized in clinical research. The review then critically examines the strengths and limitations of neuroimaging for elucidating TBI-related microscopic pathology, such as microstructural changes, neuroinflammation, proteinopathies, blood-brain barrier damage, and disruptions in cellular signaling. A summary is then provided for how neuroimaging is currently being used to investigate TBI-related pathology in new-onset neurocognitive disorders, depression, and posttraumatic stress disorder. Identified gaps in the literature include a lack of prospective studies to definitively associate imaging findings with the development of new-onset psychiatric disorders, as well as antemortem imaging studies subsequently confirmed with postmortem correlates in the same study cohort. Although the spatial resolution and specificity of imaging biomarkers has greatly improved over the last 2 decades, we conclude that neuroimaging biomarkers do not yet exist for the definitive in vivo diagnosis of cellular pathology. This represents a necessary next step for further elucidating causal relationships between TBI and new-onset psychiatric disorders.

Teasing apart trauma: neural oscillations differentiate individual cases of mild traumatic brain injury from post-traumatic stress disorder even when symptoms overlap

Post-traumatic stress disorder (PTSD) and mild traumatic brain injury (mTBI) are highly prevalent and closely related disorders. Affected individuals often exhibit substantially overlapping symptomatology - a major challenge for differential diagnosis in both military and civilian contexts. According to our symptom assessment, the PTSD group exhibited comparable levels of concussion symptoms and severity to the mTBI group. An objective and reliable system to uncover the key neural signatures differentiating these disorders would be an important step towards translational and applied clinical use. Here we explore use of MEG (magnetoencephalography)-multivariate statistical learning analysis in identifying the neural features for differential PTSD/mTBI characterisation. Resting state MEG-derived regional neural activity and coherence (or functional connectivity) across seven canonical neural oscillation frequencies (delta to high gamma) were used. The selected features were consistent and largely confirmatory with previously established neurophysiological markers for the two disorders. For regional power from theta, alpha and high gamma bands, the amygdala, hippocampus and temporal areas were identified. In line with regional activity, additional connections within the occipital, parietal and temporal regions were selected across a number of frequency bands. This study is the first to employ MEG-derived neural features to reliably and differentially stratify the two disorders in a multi-group context. The features from alpha and beta bands exhibited the best classification performance, even in cases where distinction by concussion symptom profiles alone were extremely difficult. We demonstrate the potential of using 'invisible' neural indices of brain functioning to understand and differentiate these debilitating conditions.