Grade II gliomas grow slowly and linearly (at rates about 4 mm/year) before undergoing anaplastic transformation. In order to analyze how surgery may affect radiological grade II glioma kinetics, we ...restrospectively reviewed our national database searching for patients operated on for a supratentorial grade II glioma between 1997 and 2007. We selected patients with at least two postoperative MRI with a minimal delay of 6 months. For each patient, postoperative residues were segmented on successive MRIs. Velocities of diameter expansion were estimated by linear regression of mean diameter evolution for each patient. Fifty-four patients fulfilled inclusion criteria. Median postoperative follow-up was 1.6 years with, on average, 3.4 MRI examinations per patient. Postoperative growth rates of mean diameter were normally distributed, around a mean value of 4.3 mm/year (SD = 3.2 mm/year). Statistical analysis showed no difference between this distribution and the distribution of preoperative growth rates in a previous series of 143 grade II gliomas. For a subset of 23 patients, delay between first MRI and surgery made it possible to estimate also preoperative growth rates. Intrapatient comparison revealed that growth rates were grossly unchanged for 80% of cases. In summary, inter- and intrapatient comparison of pre- and postoperative growth rates proves that surgery does not change grade II glioma dynamics, thus, acting as a cytoreduction.
Fatigue is a frequent symptom in many clinical conditions that is still poorly understood despite having a major impact on quality of life. Here, we propose a novel approach using model-based ...analysis of choice behaviour to extract fatigue markers. We applied this approach to the case of low-grade glioma, with the aim of testing the hypothesis that fatigability in this condition may manifest as limited control over choice impulsivity.
Patients with intact or resected glioma (n = 29) and matched healthy controls (n = 27) performed a series of behavioural tasks included in a 4 h-long neuropsychological assessment. Intertemporal choices, opposing smaller-sooner to larger-later monetary rewards, were intermixed with tasks designed to test cognitive and motor performance and to assess perceived fatigue with subjective ratings. All dependent variables were analysed with generalised linear models testing the main effects of group and time-on-task, as well as their interaction.
While absent in standard measures of fatigue (subjective rating and objective performance), a significant group-by-time interaction was observed in the rate of impulsive choices: contrary to controls, patients developed a preference for the smaller-sooner option in the course of neuropsychological assessment. This preference shift was captured by computational modelling as an increase in the present bias, a parameter that assigns an additive bonus to immediate rewards.
Thus, choice impulsivity was the only reliable marker that reflected the enhanced fatigability of patients relative to controls. These results suggest that the impact of glioma (or its resection) on brain functioning limits the exertion of cognitive control during decision-making. More generally, they pave the way to using model-based analysis of choice behaviour for future investigations of the many clinical conditions plagued with cognitive fatigue.
Despite better knowledge of cortical language organization, its subcortical anatomofunctional connectivity remains poorly understood. The authors used intraoperative subcortical stimulation in awake ...patients undergoing operation for a glioma in the left dominant hemisphere to map the language pathways and to determine the contribution of such a method to surgical results.
One hundred fifteen patients harboring a World Health Organization Grade II glioma within language areas underwent operation after induction of local anesthesia, using direct electrical stimulation to perform online cortical and subcortical language mapping throughout the resection.
After detection of cortical language sites, the authors identified 1 or several of the following subcortical language pathways in all patients: 1) arcuate fasciculus, eliciting phonemic paraphasia when stimulated; 2) inferior frontooccipital fasciculus, generating semantic paraphasia when stimulated; 3) subcallosal fasciculus, inducing transcortical motor aphasia during stimulation; 4) frontoparietal phonological loop, eliciting speech apraxia during stimulation; and 5) fibers coming from the ventral premotor cortex, inducing anarthria when stimulated. These structures were preserved, representing the limits of the resection. Despite a transient immediate postoperative worsening, all but 2 patients (98%) returned to baseline or better. On control MR imaging, 83% of resections were total or subtotal.
These results represent the largest experience with human subcortical language mapping ever reported. The use of intraoperative cortical and subcortical stimulation gives a unique opportunity to perform an accurate and reliable real-time anatomofunctional study of language connectivity. Such knowledge of the individual organization of language networks enables practitioners to optimize the benefit-to-risk ratio of surgery for Grade II glioma within the left dominant hemisphere.
•Two-steps computational modeling of extent of neural activation by white matter stimulation is presented.•Bipolar probes held parallel to the fibers provide the most focal activation.•At equal ...intensity, the depth of activation for anistropic tracts is much shorter than for isotropic white matter areas.
The aim of this study was to model how the different parameters of electrical stimulation (intensity, pulse shape, probe geometry) influence the extent of white matter activation.
The electrical potentials generated by the stimulating electrodes were determined by solving Laplace equation. The temporal evolution of membrane potentials at each nodes of Ranvier of an axon was then computed by solving the coupled system of differential equations describing membrane dynamics and cable propagation.
Regions of unilateral propagation were observed for monophasic pulses delivered with a bipolar probe aligned along the tract. For biphasic pulses, the largest activation areas and depths were found with a high inter-electrode-distance (IED) bipolar probe, oriented orthogonally to the tract. The smallest activation areas and depths were found for bipolar stimulations with the probe aligned parallel to the tract and low IED. For isotropic white matter regions, the activation area and depth were three times larger than for anisotropic white matter tracts.
Bipolar probes with biphasic pulses offer the greatest versatility: an orthogonal orientation acts as two monopolars (increased sensitivity when searching for a tract), whereas a parallel orientation corresponds to a single monopolar (increased specificity). Activation is more superficial when stimulating highly anisotropic tracts.
This knowledge is essential for interpreting the behavorial effects of stimulation and the recordings of axono-cortical evoked potentials.
Background
While the fundamental and clinical contribution of direct electrical stimulation (DES) of the brain is now well acknowledged, its advantages and limitations have not been re-evaluated for ...a long time.
Method
Here, we critically review exactly what DES can tell us about cerebral function.
Results
First, we show that DES is highly sensitive for detecting the cortical and axonal eloquent structures. Moreover, DES also provides a unique opportunity to study brain connectivity, since each area responsive to stimulation is in fact an input gate into a large-scale network rather than an isolated discrete functional site. DES, however, also has a limitation: its specificity is suboptimal. Indeed, DES may lead to interpretations that a structure is crucial because of the induction of a transient functional response when stimulated, whereas (1) this effect is caused by the backward spreading of the electro-stimulation along the network to an essential area and/or (2) the stimulated region can be functionally compensated owing to long-term brain plasticity mechanisms.
Conclusion
In brief, although DES is still the gold standard for brain mapping, its combination with new methods such as perioperative neurofunctional imaging and biomathematical modeling is now mandatory, in order to clearly differentiate those networks that are actually indispensable to function from those that can be compensated.
Although advances in diffusion tensor imaging have enabled us to better study the anatomy of the inferior longitudinal fasciculus (ILF), its function remains poorly understood. Recently, it was ...suggested that the subcortical network subserving the language semantics could be constituted, in parallel with the inferior occipitofrontal fasciculus, by the left ILF, joining the posterior occipitotemporal regions to the temporal pole, then relayed by the uncinate fasciculus connecting the anterior temporal pole to the frontobasal areas. Nevertheless, this hypothesis was solely based on neurofunctional imaging, allowing a cortical mapping but with no anatomofunctional information regarding the white matter. Here, we report a series of 12 patients operated on under local anaesthesia for a cerebral low-grade glioma located within the left temporal lobe. Before and during resection, we used the method of intraoperative direct electrostimulation, enabling us to perform accurate and reliable anatomofunctional correlations both at cortical and subcortical levels. In order to map the ILF. Using postoperative MRI, we correlated these functional findings with the anatomical locations of the sites where language disturbances were elicited by stimulations, both at cortical and subcortical levels. Our goal was to study the potential existence of parallel and distributed language networks crossing the left dominant temporal lobe, subserved by distinct subcortical pathways-namely the inferior occipitofrontal fasciculus and the ILF. Intraoperative stimulation of the anterior and middle temporal cortex elicited anomia in four patients. At the subcortical level, semantic paraphasia were induced in seven patients during stimulation of the inferior occipitofrontal fasciculus, and phonological paraphasia was generated in seven patients by stimulating the arcuate fasciculus. Interestingly, subcortical stimulation never elicited any language disturbances when performed at the level of the ILF. In addition, following a transient postoperative language deficit, all patients recovered, despite the resection of at least one part of the ILF, as confirmed by control MRI. On the basis of these results, we suggest that the "semantic ventral stream" could be constituted by at least two parallel pathways within the left dominant temporal lobe: (i) a direct pathway, the inferior occipitofrontal fasciculus, that connects the posterior temporal areas and the orbitofrontal region, crucial for language semantic processing, since it elicits semantic paraphasia when stimulated; (ii) and also possibly an indirect pathway subserved by the ILF, not indispensable for language, since it can be compensated both during stimulation and after resection.
PDF
