Invasive versus non-invasive mapping of the motor cortex
1st September 2020
Carolin Weiss Lucas, Charlotte Nettekoven, Volker Neuschmelting, Ana-Maria Oros-Peusquens, Gabriele Stoffels, Shivakumar Viswanathan, Anne K. Rehme, Andrea Maria Faymonville, N. Jon Shah, Karl Josef Langen, Roland Goldbrunner, and Christian Grefkes
The main objective for surgery to treat brain tumours is to achieve total tumour resection whilst preserving motor function. Consequently, when planning and conducting a tumour resection, clinicians need a way to precisely and comprehensively map the motor cortex so they can limit the surgical impact on it. Currently, this is most often achieved using intraoperative direct cortical stimulation (DCS).
DCS is an invasive procedure that takes place during the resection surgery and uses small electrical pulses to stimulate individual cortical areas. The effect of the stimulation enables the surgeon to identify specific functional areas with a high degree of accuracy. However, although DCS is considered to be the gold standard, it has several drawbacks, the main one being that as the DCS takes place during the resection surgery itself, it cannot be included in preoperative decision making relating to the assessment of the function-lesion relationship. Moreover, the anaesthesiological regime required for DCS and the interoperative nature of the method affects the reliability of mapping motor representation, potentially leading to surgery-related deficits. Consequently, obtaining a detailed functional map of expressive functional areas, non-invasively, prior to surgery is of particular clinical interest.
Potential non-invasive methods for localising the motor cortex include functional magnetic resonance imaging (fMRI) and magnetoencephalography (MEG), or a combination of both methods. Whereas fMRI works by detecting the changes in blood oxygenation and flow that occur in response to neural activity, MEG uses very sensitive magnetometers to map the magnetic fields produced by naturally occurring electrical currents in the brain.
Both fMRI and MEG work using indirect stimulation to illicit neural activity, however, more recently, neuronavigated transcranial magnetic stimulation (nTMS) has gained increasing acceptance as a method for identifying the motor cortex. In this method, a hand-held coil is used to discharge a current through the scalp to the surface of the brain. When used in terms of the motor cortex, this stimulation causes the activation of neurons which control muscles, thus enabling the accurate mapping of functional areas.
The potential for using these methods in pre-surgical planning is clear. However, data are scarce and there is little research comparing these methods against the current gold standard, DCS.
In this study, the mapping of the motor cortex using neuronavigated transcranial magnetic stimulation (nTMS) and functional magnetic resonance imaging (fMRI) was assessed in comparison with DCS. The study used motor representations of the hand, foot and tongue regions of 36 patients with intracranial tumours. The tumours had been mapped pre-operatively using nTMS and fMRI and intraoperatively by DCS. The maps were compared in terms of euclidean distances (ED) between hotspots/centres of gravity and (relative) overlaps.
The results showed significantly smaller EDs (11.4 ± 8.3 vs. 16.8 ± 7.0 mm) and better spatial overlaps (64 ± 38% vs. 37 ± 37%) between DCS and nTMS compared with DCS and fMRI. In contrast to DCS, fMRI and nTMS mappings were feasible for all regions and patients without complications.
The figure below shows the euclidean distances (ED) between the map centres. The respective pairs of 3D coordinates represent (a) pooled data, (b) hotspots/local activation maxima and (c) CoG. Pairs of modalities are colour-coded: ED between fMRI and nTMS (light blue), between fMRI and DCS (blue) as well as between nTMS and DCS (dark blue). In A, statistically significant differences between ED according to post hoc tests are indicated by asterisks (*p < .05; ***p < .001). The error bars represent the standard error of the mean.
Although the study was not sufficiently powered to analyse factors on rare events such as permanent deficits, the results suggest that nTMS is the more promising non-invasive motor cortex mapping technique to approximate the gold standard DCS results and has the potential to improve pre-surgical planning of brain tumour resections.
Original publication: