Brain White Matter Impairment in Patients with Spinal Cord Injury

Distinct brain network patterns in complete and incomplete spinal cord injury patients based on graph theory analysis

AIMS

To compare the changes in brain network topological properties and structure-function coupling in patients with complete spinal cord injury (CSCI) and incomplete spinal cord injury (ICSCI), to unveil the potential neurobiological mechanisms underlying the different effects of CSCI and ICSCI on brain networks and identify objective neurobiological markers to differentiate between CSCI and ICSCI patients.

METHODS

Thirty-five SCI patients (20 CSCI and 15 ICSCI) and 32 healthy controls (HCs) were included in the study. Here, networks were constructed using resting-state functional magnetic resonance imaging to analyze functional connectivity (FC) and diffusion tensor imaging for structural connectivity (SC). Then, graph theory analysis was used to examine SC and FC networks, as well as to estimate SC-FC coupling values.

RESULTS

Compared with HCs, CSCI patients showed increased path length (Lp), decreased global efficiency (Eg), and local efficiency (Eloc) in SC. For FC, ICSCI patients exhibited increased small-worldness, clustering coefficient (Cp), normalized clustering coefficient, and Eloc. Also, ICSCI patients showed increased Cp and Eloc than CSCI patients. Additionally, ICSCI patients had reduced SC-FC coupling values compared to HCs. Moreover, in CSCI patients, the SC network's Lp and Eg values were significantly correlated with motor scores, while in ICSCI patients, the FC network's Cp, Eloc, and SC-FC coupling values were related to sensory/motor scores.

CONCLUSIONS

These results suggest that CSCI patients are characterized by decreased efficiency in the SC network, while ICSCI patients are distinguished by increased local connections and SC-FC decoupling. Moreover, the differences in network metrics between CSCI and ICSCI patients could serve as objective biological markers, providing a basis for diagnosis and treatment strategies.

Brain region changes following a spinal cord injury

Brain-related complications are common in clinical practice after spinal cord injury (SCI); however, the molecular mechanisms of these complications are still unclear. Here, we reviewed the changes in the brain regions caused by SCI from three perspectives: imaging, molecular analysis, and electrophysiology. Imaging studies revealed abnormal functional connectivity, gray matter volume atrophy, and metabolic abnormalities in brain regions after SCI, leading to changes in the structure and function of brain regions. At the molecular level, chemokines, inflammatory factors, and damage-associated molecular patterns produced in the injured area were retrogradely transmitted through the corticospinal tract, cerebrospinal fluid, or blood circulation to the specific brain area to cause pathologic changes. Electrophysiologic recordings also suggested abnormal changes in brain electrical activity after SCI. Transcranial magnetic stimulation, transcranial direct current stimulation, and deep brain stimulation alleviated pain and improved motor function in patients with SCI; therefore, transcranial therapy may be a new strategy for the treatment of patients with SCI.

Research on the Mechanism of Cognitive Decline in Patients With Acoustic Neuroma

Little is known about neuropsychological research on patients with acoustic neuroma (AN), especially cognitive neuropsychology. We aim to compare the cognitive function of patients with AN and healthy controls (HCs) and explore possible underlying mechanisms. Various neuropsychological assessments were performed on all participants. Tract-based spatial statistics (TBSS) was used to compare DTI metrics such as fractional anisotropy (FA), axial diffusivity (AD), radial diffusivity (RD), and mean diffusivity (MD). Correlation analysis was analyzed between DTI metrics and cognitive scales. Compared with the HC group, the AN group performed worse in the neuropsychological evaluations, and TBSS analysis showed widespread alteration of the FA, AD, RD, and MD, which correlated with the cognitive function. These white matter tracts include minor forceps, major forceps, anterior thalamic radiation, superior longitudinal fasciculus, corticospinal tract, and right inferior fronto-occipital fasciculus. Meanwhile, we found for the first time that cognitive decline was related to the decrease of FA in minor forceps, which can be used as a neurobiological marker of cognitive impairment in patients with AN. The occurrence of cognition impairment is common in patients with AN. Including neuropsychological evaluation in the routine clinical assessment and appropriate treatment may strengthen clinical management and improve the quality of life of patients.

Hippocampal Mitochondrial Abnormalities Induced the Dendritic Complexity Reduction and Cognitive Decline in a Rat Model of Spinal Cord Injury

Spinal cord injury (SCI) is a progressive neurodegenerative disease in addition to a traumatic event. Cognitive dysfunction following SCI has been widely reported in patients and animal models. However, the neuroanatomical changes affecting cognitive function after SCI, as well as the mechanisms behind these changes, have so far remained elusive. Herein, we found that SCI accelerates oxidative stress damage of hippocampal neuronal mitochondria. Then, for the first time, we presented a three-dimensional morphological atlas of rat hippocampal neurons generated using a fluorescence Micro-Optical Sectioning Tomography system, a method that accurately identifies the spatial localization of neurons and trace neurites. We showed that the number of dendritic branches and dendritic length was decreased in late stage of SCI. Western blot and transmission electron microscopy analyses also showed a decrease in synaptic communication. In addition, a battery of behavioral tests in these animals revealed hippocampal based cognitive dysfunction, which could be attributed to changes in the dendritic complexity of hippocampal neurons. Taken together, these results suggested that mitochondrial abnormalities in hippocampal neurons induced the dendritic complexity reduction and cognitive decline following SCI. Our study highlights the neuroanatomical basis and importance of mitochondria in brain degeneration following SCI, which might contribute to propose new therapeutic strategies.

Brain White Matter Abnormality Induced by Chronic Spinal Cord Injury in the Pediatric Population: A Preliminary Tract-based Spatial Statistic Study

Objectives: Tract-based spatial statistics (TBSS) is a diffusion tensor imaging (DTI)-based processing technique that aims to improve the objectivity and interpretability of analysis of multisubject diffusion imaging studies. This study used TBSS to measure quantitative changes in brain white matter structures following spinal cord injury (SCI). Methods: Eighteen SCI subjects aged 8-20 years old (mean age, 16.5 years) were scanned using a conventional single-shot EPI DTI protocol using a 3.0T Siemens MR scanner. All participants underwent a complete International Standards for Neurological Classification of Spinal Cord Injury (ISNCSCI) examination to determine the level and severity of injury. Five participants were classified as American Spinal Injury Association Impairment Scale (AIS) A, nine as AIS B, and four as AIS C/D. Imaging parameters used for data collection were as follows: 20 directions, b = 1000 s/mm², voxel size = 1.8 mm x 1.8 mm, slice thickness = 5 mm, TE = 95 ms, TR = 4300 ms, slices = 30, TA = 4:45 min. To generate TBSS, nonparametric permutation tests were used for voxel-wise statistical analysis of the fractional anisotropy (FA) skeletons between AIS groups. A two-tailed t test was applied to extract voxels with significant differences at p < .05. Results: Notable significant changes occurred throughout the corticospinal, spinothalamic, and dorsal column/medial lemniscus tracts. Altered regions in the temporal, occipital, and parietal lobes were also identified. Conclusion: These results suggest that white matter structures are altered differently between people with different AIS classifications. TBSS has the potential to serve as a screening tool to identify white matter changes in regions of interest.

Functional Hyperconnectivity and Task-Based Activity Changes Associated With Neuropathic Pain After Spinal Cord Injury: A Pilot Study

Neuropathic pain (NP) is a devastating chronic pain condition affecting roughly 80% of the spinal cord injury (SCI) patient population. Current treatment options are largely ineffective and neurophysiological mechanisms of NP are not well-understood. Recent studies in neuroimaging have suggested that NP patients have differential patterns of functional activity that are dependent upon the neurological condition causing NP. We conducted an exploratory pilot study to examine functional activation and connectivity in SCI patients with chronic NP compared to SCI patients without NP. We developed a novel somatosensory attention task to identify short term fluctuations in neural activity related to NP vs. non-painful somatosensation using functional magnetic resonance imaging (fMRI). We also collected high-resolution resting state fMRI to identify connectivity-based correlations over time between the two groups. We observed increased activation during focus on NP in brain regions associated with somatosensory integration and representational knowledge in pain subjects when compared with controls. Similarly, NP subjects showed increased connectivity at rest in many of the same areas of the brain, with positive correlations between somatomotor networks, the dorsal attention network, and regions associated with pain and specific areas of painful and non-painful sensation within our cohort. Although this pilot analysis did not identify statistically significant differences between groups after correction for multiple comparisons, the observed correlations between NP and functional activation and connectivity align with a priori hypotheses regarding pain, and provide a well-controlled preliminary basis for future research in this severely understudied patient population. Altogether, this study presents a novel task, identifies regions of increased task-based activation associated with NP after SCI in the insula, prefrontal, and medial inferior parietal cortices, and identifies similar regions of increased functional connectivity associated with NP after SCI in sensorimotor, cingulate, prefrontal, and inferior medial parietal cortices. This, along with our complementary results from a structurally based analysis, provide multi-modal evidence for regions of the brain specific to the SCI cohort as novel areas for further study and potential therapeutic targeting to improve outcomes for NP patients.

Inconsistency between cortical reorganization and functional connectivity alteration in the sensorimotor cortex following incomplete cervical spinal cord injury

The aim of this study was to explore whether there will be any alterations in sensorimotor-related cortex and the possible causes of sensorimotor dysfunction after incomplete cervical spinal cord injury (ICSCI). Structural and resting-state functional magnetic resonance imaging (rs-fMRI) of nineteen ICSCI patients and nineteen healthy controls (HCs) was acquired. Voxel based morphometry (VBM) and tract-based spatial statistics were performed to assess differences in gray matter volume (GMV) and white matter integrity between ICSCI patients and HCs. Whole brain functional connectivity (FC) was analyzed using the results of VBM as seeds. Associations between the clinical variables and the brain changes were studied. Compared with HCs, ICSCI patients demonstrated reduced GMV in the right fusiform gyrus (FG) and left orbitofrontal cortex (OFC) but no changes in areas directly related to sensorimotor function. There were no significant differences in brain white matter. Additionally, the FC in the left primary sensorimotor cortex and cerebellum decreased when the FG and OFC, respectively, were used as seeds. Subsequent relevance analysis suggests a weak positive correlation between the left OFC's GMV and visual analog scale (VAS) scores. In conclusion, brain structural changes following ICSCI occur mainly in certain higher cognitive regions, such as the FG and OFC, rather than in the brain areas directly related to sensation or motor control. The functional areas of the brain that are related to cognitive processing may play an important role in sensorimotor dysfunction through the decreased FC with sensorimotor areas after ICSCI. Therefore, cognition-related functional training may play an important role in rehabilitation of sensorimotor function after ICSCI.

Investigation of Cerebral White Matter Changes After Spinal Cord Injury With a Measure of Fiber Density

Remote neurodegenerative changes in supraspinal white matter (WM) can manifest after central lesions such as spinal cord injury (SCI). The majority of diffusion tensor imaging (DTI) studies use traditional metrics such as fractional anisotropy (FA) and mean diffusivity (MD) to investigate microstructural changes in cerebral WM after SCI. However, interpretation of FA readouts is often challenged by inherent limitations of the tensor model. Recent developments in novel diffusion markers, such as fiber density (FD), allows more accurate depictions of WM pathways and has shown more reliable quantification of WM alterations compared to FA in recent studies of neurological diseases. This study investigated if FD provides useful characterization of supraspinal WM integrity after SCI in addition to the traditional DTI readouts. FA, MD, and FD maps were derived from diffusion datasets of 20 patients with chronic SCI and compared with 19 healthy controls (HC). Group differences were investigated across whole brain WM using tract-based spatial statistics and averaged diffusion values of the corticospinal tract (CST) and thalamic radiation (TR) were extracted for comparisons between HC and SCI subgroups. We also related diffusion readouts of the CST and TR with clinical scores of sensorimotor function. To investigate which diffusion markers of the CST and TR delineate HC and patients with SCI a receiver operating characteristic (ROC) analysis was performed. Overall, patients with an SCI showed decreased FA of the TR and CST. ROC analysis differentiated HC and SCI based on diffusion markers of large WM tracts including FD of the TR. Furthermore, patients' motor function was positively correlated with greater microstructural integrity of the CST. While FD showed the strongest correlation, motor function was also associated with FA and MD of the CST. In summary, microstructural changes of supraspinal WM in patients with SCI can be detected using FD as a complementary marker to traditional DTI readouts and correlates with their clinical characteristics. Future DTI studies may benefit from utilizing this novel marker to investigate complex large WM tracts in patient cohorts with varying presentations of SCI or neurodegenerative diseases.

Robotic Rehabilitation in Spinal Cord Injury: A Pilot Study on End-Effectors and Neurophysiological Outcomes

Robot-aided gait training (RAGT) has been implemented to provide patients with spinal cord injury (SCI) with a physiological limb activation during gait, cognitive engagement, and an appropriate stimulation of peripheral receptors, which are essential to entrain neuroplasticity mechanisms supporting functional recovery. We aimed at assessing whether RAGT by means of an end-effector device equipped with body weight support could improve functional ambulation in patients with subacute, motor incomplete SCI. In this pilot study, 15 patients were provided with six RAGT sessions per week for eight consecutive weeks. The outcome measures were muscle strength, ambulation, going upstairs, and disease burden. Furthermore, we estimated the activation patterns of lower limb muscles during RAGT by means of surface electromyography and the resting state networks' functional connectivity (RSN-FC) before and after RAGT. Patients achieved a clinically significant improvement in the clinical outcome measures substantially up to six months post-treatment. These data were paralleled by an improvement in the stair-climbing cycle and a potentiating of frequency-specific and area-specific RSN-FC patterns. Therefore, RAGT, by means of an end-effector device equipped with body weight support, is promising in improving gait in patients with subacute, motor incomplete SCI, and it could produce additive benefit for the neuromuscular reeducation to gait in SCI when combined with conventional physiotherapy.

Divergent Findings in Brain Reorganization After Spinal Cord Injury: A Review

Spinal cord injury (SCI) leads to a general lack of sensory and motor functions below the level of injury and may promote deafferentation-induced brain reorganization. Functional magnetic resonance imaging (fMRI) has been established as an essential tool in neuroscience research and can precisely map the spatiotemporal distribution of brain activity. Task-based fMRI experiments associated with the tongue, upper limbs, or lower limbs have been used as the primary paradigms to study brain reorganization following SCI. A review of the current literature on the subject shows one common trait: while most articles agree that brain networks are usually preserved after SCI, and that is not the case as some articles describe possible alterations in brain activation after the lesion. There is no consensus if those alterations indeed occur. In articles that show alterations, there is no agreement if they are transient or permanent. Besides, there is no consensus on which areas are most prone to activation changes, or on the intensity and direction (increase vs. decrease) of those possible changes. In this article, we present a critical review of the literature and trace possible reasons for those contradictory findings on brain reorganization following SCI. fMRI studies based on the ankle dorsiflexion, upper-limb, and tongue paradigms are used as case studies for the analyses.

Disentangling the Effects of Spinal Cord Injury and Related Neuropathic Pain on Supraspinal Neuroplasticity: A Systematic Review on Neuroimaging

Background: Spinal cord injury (SCI) and its accompanying changes of brain structure and function have been widely studied and reviewed. Debilitating chronic neuropathic pain (NP) is reported in 53% of SCI patients, and brain changes have been shown to be involved with the presence of this secondary complication. However, there is yet a synthesis of current studies that investigated brain structure, resting connectivity, and metabolite changes that accompanies this condition. Methods: In this review, a systematic search was performed using Medical Subject Headings heading search terms in PubMed and SCOPUS to gather the appropriate published studies. Neuroimaging studies that investigated supraspinal structural, resting-state connectivity, and metabolite changes in SCI subjects with NP were included. To this end, voxel-based morphometry, diffusion tensor imaging, resting-state functional MRI, magnetic resonance spectroscopy, and PET studies were summarized and reviewed. Further inclusion and exclusion criteria allowed delineation of appropriate studies that included SCI subgroups with and without NP. Results: A total of 12 studies were eligible for qualitative synthesis. Overall, current studies that investigated NP-associated changes within the SCI cohort show primarily metabolite concentration alterations in sensory-pain processing regions, alongside bidirectional changes of brain structure. Moreover, in comparison to healthy controls, there remains limited evidence of structural and connectivity changes but a range of alterations in metabolite concentrations in SCI subjects with NP. Conclusions: There is some evidence suggesting that the magnitude and presence of NP following SCI results in both adaptive and maladaptive structural plasticity of sensorimotor regions, alongside altered metabolism of brain areas involved with descending pain modulation, pain perception (i.e., anterior cingulate cortex) and sensory integration (i.e., thalamus). However, based on the fact that only a few studies investigated structural and glucose metabolic changes in chronic SCI subjects with NP, the underlying mechanisms that accompany this condition remains to be further elucidated. Future cross-sectional or longitudinal studies that aim to disentangle NP related to SCI may benefit from stricter constraints in subject cohorts, controlled subgroups, improved pain phenotyping, and implementation of multimodal approaches to discover sensitive biomarkers that profile pain and optimize treatment in SCI subjects with NP.

White Matter Microstructure Alterations in Patients With Spinal Cord Injury Assessed by Diffusion Tensor Imaging

Compared to healthy controls, spinal cord injury (SCI) patients demonstrate white matter (WM) abnormalities in the brain. However, little progress has been made in comparing cerebral WM differences between SCI-subgroups. The purpose of this study was to investigate WM microstructure differences between paraplegia and quadriplegia using tract-based spatial statistics (TBSS) and atlas-based analysis methods. Twenty-two SCI patients (11 cervical SCI and 11 thoracic SCI) and 22 age- and sex-matched healthy controls were included in this study. TBSS and atlas-based analyses were performed between SCI and control groups and between SCI-subgroups using multiple diffusion metrics, including fractional anisotropy (FA), mean diffusivity (MD), axial diffusivity (AD) and radial diffusivity (RD). Compared to controls, SCI patients had decreased FA and increased MD and RD in the corpus callosum (CC; genu and splenium), superior longitudinal fasciculus (SLF), corona radiata (CR), posterior thalamic radiation (PTR), right cingulum (cingulate gyrus; CCG) and right superior fronto-occipital fasciculus (SFOF). Cervical SCI patients had lower FA and higher RD in the left PTR than thoracic SCI patients. Time since injury had a negative correlation with FA within the right SFOF (r = −0.452, p = 0.046) and a positive association between the FA of left PTR and the American Spinal Injury Association (ASIA) sensory score (r = 0.428, p = 0.047). In conclusion, our study suggests that multiple cerebral WM tracts are damaged in SCI patients, and WM disruption in cervical SCI is worse than thoracic injury level, especially in the PTR region.

Whether Visual-related Structural and Functional Changes Occur in Brain of Patients with Acute Incomplete Cervical Cord Injury: A Multimodal Based MRI Study

Visual-related cortex plays an important role in the process of movement. It is of great importance to clarify whether traumatic spinal cord injury (SCI), which is a typical disease that results in sensorimotor dysfunction, leads to the alteration of visual-related brain structure and function area. To address this issue, multimodality MRI was applied on eleven patients with acute incomplete cervical cord injury (ICCI) and eleven healthy controls (HCs) to explore possible structural and functional changes of the brain. Voxel-based morphometry (VBM) analysis was performed to investigate the changes in brain structure of ICCI patients. The fractional amplitude of low-frequency fluctuations (fALFF) was used to characterize changes in regional neural activities, and independent component analysis (ICA) was carried out to explore alterations in the resting-state networks (RSNs) after ICCI. We also investigated correlations among brain imaging metrics and between the metrics and clinical variables. Compared with HCs, ICCI patients exhibited significant gray matter atrophy in the left hippocampus and parahippocampal gyrus, right superior frontal gyrus (SFG), and middle frontal gyrus (MFG) and also a decrease in fALFF in the left orbitofrontal cortex (OFC). Moreover, ICCI patients exhibited decreased intra-network functional connectivity (FC) in the medial vision network (mVN). The mean fALFF value was correlated with clinical motor scores of the left extremities and the total motor scores. Our findings proved that ICCI can not only cause structural changes in visual-related brain regions, but also result in visual-related brain functional alterations, revealing the possible mechanism of the effects of visual feedback training in motor function rehabilitation of SCI patients.

Functional Brain Connectivity during Multiple Motor Imagery Tasks in Spinal Cord Injury

Reciprocal communication of the central and peripheral nervous systems is compromised during spinal cord injury due to neurotrauma of ascending and descending pathways. Changes in brain organization after spinal cord injury have been associated with differences in prognosis. Changes in functional connectivity may also serve as injury biomarkers. Most studies on functional connectivity have focused on chronic complete injury or resting-state condition. In our study, ten right-handed patients with incomplete spinal cord injury and ten age- and gender-matched healthy controls performed multiple visual motor imagery tasks of upper extremities and walking under high-resolution electroencephalography recording. Directed transfer function was used to study connectivity at the cortical source space between sensorimotor nodes. Chronic disruption of reciprocal communication in incomplete injury could result in permanent significant decrease of connectivity in a subset of the sensorimotor network, regardless of positive or negative neurological outcome. Cingulate motor areas consistently contributed the larger outflow (right) and received the higher inflow (left) among all nodes, across all motor imagery categories, in both groups. Injured subjects had higher outflow from left cingulate than healthy subjects and higher inflow in right cingulate than healthy subjects. Alpha networks were less dense, showing less integration and more segregation than beta networks. Spinal cord injury patients showed signs of increased local processing as adaptive mechanism. This trial is registered with NCT02443558.

Afferent input and sensory function after human spinal cord injury

Spinal cord injury (SCI) often disrupts the integrity of afferent (sensory) axons projecting through the spinal cord dorsal columns to the brain. Examinations of ascending sensory tracts, therefore, are critical for monitoring the extent of SCI and recovery processes. In this review, we discuss the most common electrophysiological techniques used to assess transmission of afferent inputs to the primary motor cortex (i.e., afferent input-induced facilitation and inhibition) and the somatosensory cortex [i.e., somatosensory evoked potentials (SSEPs), dermatomal SSEPs, and electrical perceptual thresholds] following human SCI. We discuss how afferent input modulates corticospinal excitability by involving cortical and spinal mechanisms depending on the timing of the effects, which need to be considered separately for upper and lower limb muscles. We argue that the time of arrival of afferent input onto the sensory and motor cortex is critical to consider in plasticity-induced protocols in humans with SCI. We also discuss how current sensory exams have been used to detect differences between control and SCI participants but might be less optimal to characterize the level and severity of injury. There is a need to conduct some of these electrophysiological examinations during functionally relevant behaviors to understand the contribution of impaired afferent inputs to the control, or lack of control, of movement. Thus the effects of transmission of afferent inputs to the brain need to be considered on multiple functions following human SCI.

Brain Gray Matter Atrophy after Spinal Cord Injury: A Voxel-Based Morphometry Study

The aim of this study was to explore possible changes in whole brain gray matter volume (GMV) after spinal cord injury (SCI) using voxel-based morphometry (VBM), and to study their associations with the injury duration, severity, and clinical variables. In total, 21 patients with SCI (10 with complete and 11 with incomplete SCI) and 21 age- and sex-matched healthy controls (HCs) were recruited. The 3D high-resolution T1-weighted structural images of all subjects were obtained using a 3.0 Tesla MRI system. Disease duration and American Spinal Injury Association (ASIA) Scale scores were also obtained from each patient. Voxel-based morphometry analysis was carried out to investigate the differences in GMV between patients with SCI and HCs, and between the SCI sub-groups. Associations between GMV and clinical variables were also analyzed. Compared with HCs, patients with SCI showed significant GMV decrease in the dorsal anterior cingulate cortex, bilateral anterior insular cortex, bilateral orbital frontal cortex (OFC), and right superior temporal gyrus. No significant difference in GMV in these areas was found either between the complete and incomplete SCI sub-groups, or between the sub-acute (duration <1 year) and chronic (duration >1 year) sub-groups. Finally, the GMV of the right OFC was correlated with the clinical motor scores of left extremities in not only all SCI patients, but especially the CSCI subgroup. In the sub-acute subgroup, we found a significant positive correlation between the dACC GMV and the total clinical motor scores, and a significant negative correlation between right OFC GMV and the injury duration. These findings indicate that SCI can cause remote atrophy of brain gray matter, especially in the salient network. In general, the duration and severity of SCI may be not associated with the degree of brain atrophy in total SCI patients, but there may be associations between them in subgroups.