Altered corticospinal microstructure and motor cortex excitability in gliomas: an advanced tractography and transcranial magnetic stimulation study

Cortical resting motor threshold difference in asleep-awake craniotomy for motor eloquent gliomas: WHO grading influences motor pathway excitability

Developing neurophysiological tools to predict WHO tumor grade can empower the treating teams for a better surgical decision-making process. A total of 38 patients with supratentorial diffuse gliomas underwent an asleep-awake-sedated craniotomies for tumor removal with intraoperative neuromonitoring. The resting motor threshold was calculated for different train stimulation paradigms during awake and asleep phases. Receiver operating characteristic analysis and Bayesian regression models were performed to analyze the prediction of tumor grading based on the resting motor threshold differences. Significant positive spearman correlations were observed between resting motor threshold excitability difference and WHO tumor grade for train stimulation paradigms of 5 (R = 0.54, P = 0.00063), 4 (R = 0.49, P = 0.002), 3 (R = 0.51, P = 0.001), and 2 pulses (R = 0.54, P = 0.0007). Kruskal-Wallis analysis of the median revealed a positive significant difference between the median of excitability difference and WHO tumor grade in all paradigms. Receiver operating characteristic analysis showed 3 mA difference as the best predictor of high-grade glioma across different patterns of motor pathway stimulation. Bayesian regression found that an excitability difference above 3 mA would indicate a 75.8% probability of a glioma being high grade. Our results suggest that cortical motor excitability difference between the asleep and awake phases in glioma surgery could correlate with tumor grade.

Navigated Transcranial Magnetic Stimulation (nTMS) based Preoperative Planning for Brain Tumor Treatment

Transcranial Magnetic Stimulation (TMS) is a non-invasive technique for analyzing the central and peripheral nervous system. TMS could be a powerful therapeutic technique for neurological disorders. TMS has also shown potential in treating various neurophysiological complications, such as depression, anxiety, and obsessive-compulsive disorders, without pain and analgesics. Despite advancements in diagnosis and treatment, there has been an increase in the prevalence of brain cancer globally. For surgical planning, mapping brain tumors has proven challenging, particularly those localized in expressive regions. Preoperative brain tumor mapping may lower the possibility of postoperative morbidity in surrounding areas. A navigated TMS (nTMS) uses magnetic resonance imaging (MRI) to enable precise mapping during navigated brain stimulation. The resulting magnetic impulses can be precisely applied to the target spot in the cortical region by employing nTMS. This review focuses on nTMS for preoperative planning for brain cancer. This study reviews several studies on TMS and its subtypes in treating cancer and surgical planning. nTMS gives wider and improved dimensions of preoperative planning of the motor-eloquent areas in brain tumor patients. nTMS also predicts postoperative neurological deficits, which might be helpful in counseling patients. nTMS have the potential for finding possible abnormalities in the motor cortex areas.

Large-scale brain networks and intra-axial tumor surgery: a narrative review of functional mapping techniques, critical needs, and scientific opportunities

In recent years, a paradigm shift in neuroscience has been occurring from "localizationism," or the idea that the brain is organized into separately functioning modules, toward "connectomics," or the idea that interconnected nodes form networks as the underlying substrates of behavior and thought. Accordingly, our understanding of mechanisms of neurological function, dysfunction, and recovery has evolved to include connections, disconnections, and reconnections. Brain tumors provide a unique opportunity to probe large-scale neural networks with focal and sometimes reversible lesions, allowing neuroscientists the unique opportunity to directly test newly formed hypotheses about underlying brain structural-functional relationships and network properties. Moreover, if a more complete model of neurological dysfunction is to be defined as a "disconnectome," potential avenues for recovery might be mapped through a "reconnectome." Such insight may open the door to novel therapeutic approaches where previous attempts have failed. In this review, we briefly delve into the most clinically relevant neural networks and brain mapping techniques, and we examine how they are being applied to modern neurosurgical brain tumor practices. We then explore how brain tumors might teach us more about mechanisms of global brain dysfunction and recovery through pre- and postoperative longitudinal connectomic and behavioral analyses.

Clinical diagnostic utility of transcranial magnetic stimulation in neurological disorders. Updated report of an IFCN committee

The review provides a comprehensive update (previous report: Chen R, Cros D, Curra A, Di Lazzaro V, Lefaucheur JP, Magistris MR, et al. The clinical diagnostic utility of transcranial magnetic stimulation: report of an IFCN committee. Clin Neurophysiol 2008;119(3):504-32) on clinical diagnostic utility of transcranial magnetic stimulation (TMS) in neurological diseases. Most TMS measures rely on stimulation of motor cortex and recording of motor evoked potentials. Paired-pulse TMS techniques, incorporating conventional amplitude-based and threshold tracking, have established clinical utility in neurodegenerative, movement, episodic (epilepsy, migraines), chronic pain and functional diseases. Cortical hyperexcitability has emerged as a diagnostic aid in amyotrophic lateral sclerosis. Single-pulse TMS measures are of utility in stroke, and myelopathy even in the absence of radiological changes. Short-latency afferent inhibition, related to central cholinergic transmission, is reduced in Alzheimer's disease. The triple stimulation technique (TST) may enhance diagnostic utility of conventional TMS measures to detect upper motor neuron involvement. The recording of motor evoked potentials can be used to perform functional mapping of the motor cortex or in preoperative assessment of eloquent brain regions before surgical resection of brain tumors. TMS exhibits utility in assessing lumbosacral/cervical nerve root function, especially in demyelinating neuropathies, and may be of utility in localizing the site of facial nerve palsies. TMS measures also have high sensitivity in detecting subclinical corticospinal lesions in multiple sclerosis. Abnormalities in central motor conduction time or TST correlate with motor impairment and disability in MS. Cerebellar stimulation may detect lesions in the cerebellum or cerebello-dentato-thalamo-motor cortical pathways. Combining TMS with electroencephalography, provides a novel method to measure parameters altered in neurological disorders, including cortical excitability, effective connectivity, and response complexity.

Structure of the Motor Descending Pathways Correlates with the Temporal Kinematics of Hand Movements

Simple Summary

How hand motor behavior relates to the microstructure of the underlying subcortical white matter pathways is yet to be fully understood. Here we consider two well-known examples of our everyday motor repertoire, reaching and reach-to-grasp, by looking at their temporal unfolding and at the microstructure of descending projection pathways, conveying motor information from the motor cortices towards the more ventral regions of the nervous system. We combine three-dimensional kinematics, describing the temporal profile of hand movements, with diffusion imaging tractography, exploring the microstructure of specific segments of the projection pathways (internal capsule, corticospinal and hand motor tracts). The results indicate that the level of anisotropy characterizing these white matter tracts can influence the temporal unfolding of reaching and reach-to-grasp movements.

Abstract

The projection system, a complex organization of ascending and descending white matter pathways, is the principal system for conveying sensory and motor information, connecting frontal and sensorimotor regions with ventral regions of the central nervous system. The corticospinal tract (CST), one of the principal projection pathways, carries distal movement-related information from the cortex to the spinal cord, and whether its microstructure is linked to the kinematics of hand movements is still an open question. The aim of the present study was to explore how microstructure of descending branches of the projection system, namely the hand motor tract (HMT), the corticospinal tract (CST) and its sector within the internal capsule (IC), can relate to the temporal profile of reaching and reach-to-grasp movements. Projection pathways of 31 healthy subjects were virtually dissected by means of diffusion tractography and the kinematics of reaching and reach-to-grasp movements were also analyzed. A positive association between Hindrance Modulated Orientation Anisotropy (HMOA) and kinematics was observed, suggesting that anisotropy of the considered tract can influence the temporal unfolding of motor performance. We highlight, for the first time, that hand kinematics and the visuomotor transformation processes underlying reaching and reach-to-grasp movements relate to the microstructure of specific projection fibers subserving these movements.

Continuous Microdebrider-Based Dynamic Subcortical Motor Mapping: A Technical Advance in Tubular Retractor-Assisted Surgery

BACKGROUND

Transsulcal minimally invasive parafasicular (TsMIP) approaches to brain tumor resection use tubular retractors to minimize iatrogenic brain injury. Dynamic cortical and subcortical continuous neurophysiological mapping facilitates safer resection of motor-eloquent tumors.

OBJECTIVE

To describe a new technique to address the challenge of combining TsMIP with tubular retractors and dynamic subcortical mapping using a single electrified stimulating microdebrider instrument.

METHODS

We adapted the NICO Myriad microdebrider with continuous monopolar stimulation electrification using high-frequency stimulation with the train-of-5 technique. We performed continuous subcortical mapping using this device and compared it with standard dynamic monopolar subcortical mapping using a suction stimulation device. We found no significant difference in recorded stimulation response.

RESULTS

Using a single operating instrument that provides synchronous tumor resection and monopolar subcortical mapping with the NICO Brainpath tubular retractor, we observed increased degrees of movement, faster surgical resection times with an enlarged working channel down the retractor, and improved safety because the stimulating probe sits 2 mm deep to the resection window.

CONCLUSION

We show that the adapted device is reliable and provides similar stimulation response as conventional subcortical mapping. We advocate the use of our adapted microdebrider in TsMIP tubular retractor approaches.

Effect of neuroanatomy on corticomotor excitability during and after transcranial magnetic stimulation and intermittent theta burst stimulation

Individual neuroanatomy can influence motor responses to transcranial magnetic stimulation (TMS) and corticomotor excitability after intermittent theta burst stimulation (iTBS). The purpose of this study was to examine the relationship between individual neuroanatomy and both TMS response measured using resting motor threshold (RMT) and iTBS measured using motor evoked potentials (MEPs) targeting the biceps brachii and first dorsal interosseus (FDI). Ten nonimpaired individuals completed sham‐controlled iTBS sessions and underwent MRI, from which anatomically accurate head models were generated. Neuroanatomical parameters established through fiber tractography were fiber tract surface area (FTSA), tract fiber count (TFC), and brain scalp distance (BSD) at the point of stimulation. Cortical magnetic field induced electric field strength (EFS) was obtained using finite element simulations. A linear mixed effects model was used to assess effects of these parameters on RMT and iTBS (post‐iTBS MEPs). FDI RMT was dependent on interactions between EFS and both FTSA and TFC. Biceps RMT was dependent on interactions between EFS and and both FTSA and BSD. There was no groupwide effect of iTBS on the FDI but individual changes in corticomotor excitability scaled with RMT, EFS, BSD, and FTSA. iTBS targeting the biceps was facilitatory, and dependent on FTSA and TFC. MRI‐based measures of neuroanatomy highlight how individual anatomy affects motor system responses to different TMS paradigms and may be useful for selecting appropriate motor targets when designing TMS based therapies.

Impaired phase synchronization of motor-evoked potentials reflects the degree of motor dysfunction in the lesioned human brain

The functional corticospinal integrity (CSI) can be indexed by motor-evoked potentials (MEP) following transcranial magnetic stimulation of the motor cortex. Glial brain tumors in motor-eloquent areas are frequently disturbing CSI resulting in different degrees of motor dysfunction. However, this is unreliably mirrored by MEP characteristics. In 59 consecutive patients with diffuse glial tumors and 21 healthy controls (CTRL), we investigated the conventional MEP features, that is, resting motor threshold (RMT), amplitudes and latencies. In addition, frequency-domain MEP features were analyzed to estimate the event-related spectral perturbation (ERSP), and the induced phase synchronization by intertrial coherence (ITC). The clinical motor status was captured including the Medical Research Council Scale (MRCS), the Grooved Pegboard Test (GPT), and the intake of antiepileptic drugs (AED). Motor function was classified according to MRCS and GPT as no motor deficit (NMD), fine motor deficits (FMD) and gross motor deficits (GMD). CSI was assessed by diffusion-tensor imaging (DTI). Motor competent subjects (CTRL and NMD) had similar ERSP and ITC values. The presence of a motor deficit (FMD and GMD) was associated with an impairment of high-frequency ITC (150-300 Hz). GMD and damage to the CSI demonstrated an additional reduction of high-frequency ERSP (150-300 Hz). GABAergic AED increased ERSP but not ITC. Notably, groups were indistinguishable based on conventional MEP features. Estimating MEP phase synchronization provides information about the corticospinal transmission after transcranial magnetic stimulation and reflects the degree of motor impairment that is not captured by conventional measures.

Minimally invasive approach to a deep-seated motor eloquent brain tumour: a technical note

Deep-seated brain tumours represent a unique neurosurgical challenge as they are often surrounded by eloquent structures. We describe a minimally invasive technique using tubular retractors and intraoperative neurophysiology monitoring for open biopsy of a deep-seated lesion surrounded by the corticospinal tract. We used preoperative functional mapping with diffusion tensor imaging tractography and navigated transcranial magnetic stimulation to identify a safe surgical corridor. We also used 5-Aminolevulinic Acid induced fluorescence to identify the lesion intraoperatively and optimize tissue samples obtained for histopathological diagnosis. We found the use of these tools improved the safety of surgery and reduced the risk of surgical morbidity.

White matter variability, cognition, and disorders: a systematic review

Inter-individual differences can inform treatment procedures and—if accounted for—have the potential to significantly improve patient outcomes. However, when studying brain anatomy, these inter-individual variations are commonly unaccounted for, despite reports of differences in gross anatomical features, cross-sectional, and connectional anatomy. Brain connections are essential to facilitate functional organization and, when severed, cause impairments or complete loss of function. Hence, the study of cerebral white matter may be an ideal compromise to capture inter-individual variability in structure and function. We reviewed the wealth of studies that associate cognitive functions and clinical symptoms with individual tracts using diffusion tractography. Our systematic review indicates that tractography has proven to be a sensitive method in neurology, psychiatry, and healthy populations to identify variability and its functional correlates. However, the literature may be biased, as the most commonly studied tracts are not necessarily those with the highest sensitivity to cognitive functions and pathologies. Additionally, the hemisphere of the studied tract is often unreported, thus neglecting functional laterality and asymmetries. Finally, we demonstrate that tracts, as we define them, are not correlated with one, but multiple cognitive domains or pathologies. While our systematic review identified some methodological caveats, it also suggests that tract–function correlations might still be a promising tool in identifying biomarkers for precision medicine. They can characterize variations in brain anatomy, differences in functional organization, and predicts resilience and recovery in patients.

Corticospinal excitability and conductivity are related to the anatomy of the corticospinal tract

Probing the brain structure-function relationship is at the heart of modern neuroscientific explorations, enabled by recent advances in brain mapping techniques. This study aimed to explore the anatomical blueprint of corticospinal excitability and shed light on the structure-function relationship within the human motor system. Using diffusion magnetic resonance imaging tractography, based on the spherical deconvolution approach, and transcranial magnetic stimulation (TMS), we show that anatomical inter-individual variability of the corticospinal tract (CST) modulates the corticospinal excitability and conductivity. Our findings show for the first time the relationship between increased corticospinal excitability and conductivity in individuals with a bigger CST (i.e., number of streamlines), as well as increased corticospinal microstructural organization (i.e., fractional anisotropy). These findings can have important implications for the understanding of the neuroanatomical basis of TMS as well as the study of the human motor system in both health and disease.