Even though the resection of a brain tumor has a potential for effectively treating brain cancer it poses a risk of harming healthy functional brain areas leading to the development of additional neurological deficits. In neurosurgical oncology, surgeons therefore use preoperative planning in order to identify functional brain regions which are set in relation to the tumor. This strategy proves to be highly advantageous for risk stratification and the preoperative planning process. However, this method of visualizing anatomical brain structures is more difficult and sometimes not possible to be performed if the patient’s brain anatomy is altered. Such changes in brain anatomy can occur due to tumor growth, tumor oedema or brain reorganization.
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Motor System Reorganization in Patients with Brain Tumors
In the past few years, the way the structure of the cerebral cortex is being perceived has drastically changed. The theory of it being a static and modular system has been replaced by a concept in which the cerebral cortex is seen as a dynamic and adaptive cortical and subcortical network (Hodotopic Concept, De Benedictis et al., Neurosurgery. 2011). Functional reorganization is based on synaptic plasticity and the activation of latent networks and enables compensation and rehabilitation of neurological deficits (De Benedictis et al., Neurosurgery. 2011).
As a result of these new research findings, the resection of several types of brain tumors which were believed to be inoperable a few years ago because of their location in functional brain areas and the therefore existing high risk of causing further neurological impairments is now possible. This method is applicable to recurring brain tumors as well, because the motor area of the brain can be shifted via cortical reorganization the same way cortical reorganization takes place in first-time discovered tumors, providing optimal circumstances for tumor surgery and lowering the risk of postoperative neurological complications.
Motor System Reorganization in Patients with chronic Cerebrovascular Ischemia
A stenosis that affects blood vessels in the brain (stenosis of the arteria carotis, Moya Moya Disease) often causes patients to suffer from repeated, temporary neurological deficits (Transient Ischemic Attack) and from hemodynamic strokes (Boundary Zone Infarction). Extracranial-intracranial bypasses (STA-MCA Bypass) are considered an effective surgical preventative measure against strokes.
In a pilot study, the motor networks of 39 patients who were suffering from chronic cerebrovascular ischemia were analyzed. After the patients had undergone surgical revascularization, another round of examination followed.
The study shows that the brain’s motor system has the ability to compensate for chronic hypoperfusion of the brain if the the brain cortex’ structure is being altered correspondingly. This functional reorganization that ensures adequate functionality of the motor cortex even at times of reduced oxygen supply can be best described with the term “hibernating brain”. Prior to the concept of the “hibernating brain”, neuronal function was believed to be irreversibly limited after reaching a certain, low threshold level of perfusion. However, our first study results indicate that, in fact, the brain gradually adjusts its activity to reduced cranial blood flow. This condition – the “neuronal hibernation” – represents the brain’s motor system’s potential to compensate imminent neurological deficits and could possibly serve as a prognostic parameter for the identification of patients who are very likely to benefit from a cerebral bypass.
The Corticospinal Reserve Capacity – Alteration of the Motor Network in Patients with Cervical Myelopathy
Injuries of the cervical spinal cord cause neurophysiological changes of the spine and the cerebrum that can vary depending on the extent of damage. Analyzing neurophysiological processes observable in spinal cord damages can improve the general understanding of pathophysiological processes and contributes to the objectification of possibilities of therapy by using individual neurophysiological parameters.
In a pilot study with 18 patients and 8 study participants our team was able to detect functional reorganization at the cortical and spinal level among patients who suffered from cervical myelopathy. Analogous to previous results among patients with traumatic or inflammatory spinal cord damage, our results suggested the existence of an association between frequent recruitment of supplementary motor areas as well as a low extent of corticospinal inhibition and a positive clinic course. Observed among patients with symptoms similarly far advanced, the analysis of navigated transcranial magnetic stimulation exhibited extensive functional reorganization among patients with milder symptoms. Congruently, those compensatory mechanisms were only poorly manifested in patients with severe neurological malfunctions.
In 2016, this project received a research prize in the field of spinal surgery (“Forschungsförderungspreis der Deutschen Gesellschaft für Wirbelsäulenchirurgie”) and in 2018, it was awarded first prize at the annual conference of the German Spine Congress (“Deutscher Wirbelsäulenkongress”) in Wiesbaden, Germany. The received subsidies are being used for augmenting the patient and study participant cohorts in order to research the concept of the “corticospinal reserve capacity” in a multicenter study called “CReMe” (Cerebral Reorganization in Myelopathy). The study sites of this project are the Charité in Berlin, the Technical University of Munich, the University Hospital of Cologne and the University Hospital of Bern “Inselspital”.
nTMS-based Tractography in Patients with Tumors at the Brainstem
Surgical intervention in the area of the brainstem are associated with a higher risk of morbidity and mortality, because the brainstem’s anatomical location is characterized by a dense arrangement of essential structures. If a tumor in this region is considered to be operable its resection has a beneficial impact on the oncological outcome. In some cases, patients might even be completely cured after the procedure.
This project focuses on the visualization of motor fiber tracts (pyramidal tracts) and its location in relation to the tumor and on testing the functionality of those tracts. The results of the nTMS mapping and the diffusion-weighted magnetic resonance imaging are being combined so nTMS-based tractography can be carried out (Frey et al, Neuroimage 2013). The goal of the study is to identify essential functional tracts with the collected nTMS data. As a result, the assessment of the risk of postoperative motor deficits could be more accurate. In addition, patients would be able to undergo surgery under optimal circumstances as their surgical treatment could be better adjusted to their individual condition.