Lessons from Brain Mapping in Surgery for Low-Grade Gliomas: Study of Cerebral Connectivity and Plasticity

Abstract

Radical surgical removal of WHO grade II glioma, when possible, appears currently the first treatment to delay the risk of anaplastic transformation. The goal of surgery is to maximize the quality of resection, while minimizing the risk of permanent neurological deficit. Thus, due to the frequent location of these lesions in eloquent areas, and because of an important interindividual anatomofunctional variability, it is important to study the cortical functional organization, the effective connectivity and the brain plastic potential for each patient, in order to taylor the resection according to both oncological and cortico-subcortical functional boundaries, then to optimize the benefit / risk ratio of surgery.

Consequently, in addition to preoperative functional neuroimaging, useful for the surgical indications and the surgical planning, intraoperative electrical stimulations can be used during the resection, under local anesthesia on awake patient if language and other cognitive functions need to be mapped. Indeed, this is an easy, accurate, reliable and safe technique of detection of both cortical and subcortical structures essential for the function.

Indeed, intraoperative electrostimulations have many interests:

– to understand the individual cortical functional organization of each patient before the beginning of the resection;

– to improve the knowledge of the pathophysiology of brain areas involved by the glioma, such as the supplementary motor area, insula, premotor cortex, supramarginal or angular gyri (i.e. participation in sensorimotor function, vision, language, spatial awareness, calculation, ...);

– to continue to map the subcortical structures all along the resection, allowing a direct study of the anatomo-functional connectivity (i.e. a tracking of the cortico-cortical and cortico-subcortical loops);

– to study the mechanisms of on-line short-term plasticity, using repeated cortical stimulations;

– to perform the resection according to functional boundaries, enabling to optimize the quality of tumor removal while minimizing the risk to induce a permanent postoperative neurological deficit.

Moreover, intraoperative stimulations can be combined with peri-operative functional neuroimaging methods (fMRI, PET, MEG, DTI), before and after surgery, in order to improve the reliability of the pre-operative planning (validation of the non-invasive techniques), but also with the goal to better understand both the short-term and long-term plasticity mechanisms based on functional cortical reshaping and connectivity changes – due initially to the tumor growth, then to its surgical resection. Such an individual knowledge may enable to increase the quality of tumor resection during a second surgery performed some years after the first procedure, while it was not possible to remove to entire glioma during the first surgery due its location in “eloquent” areas – reorganized following the initial operation.

Such a strategy has enabled both to increase the impact of the surgery on the natural history of low-grade glioma, and also to improve the quality of life of the patients.