Letter: Altered Motor Excitability in Patients With Diffuse Gliomas Involving Motor Eloquent Areas: The Impact of Tumor Grading.

To the Editor: We have read and appreciated the recent article published in Neurosurgery by Lavrador et al1 on the association of abnormal cortical excitability and tumor grading in patients with gliomas. The authors retrospectively reviewed a case series of 45 patients submitted to preoperative cortical excitability evaluation by navigated Transcranial Magnetic Stimulation (nTMS) and found patients with higher WHO grading2 had lower resting motor threshold (RMT) in the hemisphere affected by the tumor. In addition, patients with high-grade gliomas had a higher prevalence of altered RMT interhemispheric ratio of both upper and lower limbs, presented as the concept of Cortical Excitability Score (CES). These findings confer a new role on nTMS beyond brain mapping, which was previously associated with a decreased risk of new postoperative neurological deficits and an increased extent of resection (EOR). 3 The authors1 suggest the relative spare of the cortical spinal fibers regarding the upper limbs is related to a potential mechanism preserving upper limb movements and making them more resilient, because upper limbs have a larger area in the precentral gyrus than the lower limbs.4 However, they do not detail the preoperative motor status of these patients. There might be a potential selection bias involved, since lower prevalence of upper limb weakness might explain the lack of correlation between RMT in this area and tumor grading, even though altered cortical excitability may coexist with a normal motor function, and motor evoked potential (MEP) may have high interindividual variation.5 Lastly, there is no information concerning size and location of the gliomas, what may be related to the spare of upper limbs area, as well. Unfortunately, the retrospective design of this study hindered the evaluation of motor outcome. The idea of abnormal preoperative cortical excitability predicting poor motor prognosis has been reported by other authors. Rosenstock et al6 found a high RMT interhemispheric ratio (iRMT) associated with worst motor score seven days postoperatively, but that there was no association three months postoperatively. The same group speculated that patients initially without hemiparesis but with high RMT interhemispheric ratio were at a higher risk of a decline in motor function in the future.7 High-grade gliomas, as rapidly growing infiltrative lesions, hinder motor function recovery. One might expect they consequently would be related with abnormal cortical excitability, such as increased iRMT, as previously described. However, the most interesting point of the Lavrador et al1 study was the finding of decreased RMT in the hemisphere affected— the numerator of the same ratio—in patients with high-grade gliomas. We encourage authors to follow-up patients motor function to verify if there is an association with RMT and outcome and if it disagrees to the previous reports. Finally, this article opens the possibility to expand the study with additional cortical excitability variables. Higher short interval intracortical inhibition (SICI) was found in patients with traumatic diffuse axonal injury after1yr from the onset.8 While RMT refers to the integrity of the cortical spinal tract (CST), SICI is exclusively mediated by intracortical GABAergic circuits and its alteration might be implicated with imbalance between excitatory and inhibitory circuits.9 The status of SICI in patients with tumors located on M1 would be of a great value to understand how gliomas behavior affects the surrounding cortical tissue. The study of Lavrador et al1 contributes to the knowledge about the neurophysiology of patients with gliomas surrounding the CST and brings up a new well-tolerated and noninvasive tool to predict histopathological diagnosis of these tumors. The most relevant aspect of this study is certainly the possibility to attend part of the patients’ concerns and expectations preoperatively.