Mapping of Motor Function with Neuronavigated Transcranial Magnetic Stimulation: A Review on Clinical Application in Brain Tumors and Methods for Ensuring Feasible Accuracy

Abstract

Navigated transcranial magnetic stimulation (nTMS) has developed into a reliable non-invasive clinical and scientific tool over the past decade. Specifically, it has undergone several validating clinical trials that demonstrated high agreement with intraoperative direct electrical stimulation (DES), which paved the way for increasing application for the purpose of motor mapping in patients harboring motor-eloquent intracranial neoplasms. Based on this clinical use case of the technique, in this article we review the evidence for the feasibility of motor mapping and derived models (risk stratification and prediction, nTMS-based fiber tracking, improvement of clinical outcome, and assessment of functional plasticity), and provide collected sets of evidence for the applicability of quantitative mapping with nTMS. In addition, we provide evidence-based demonstrations on factors that ensure methodological feasibility and accuracy of the motor mapping procedure. We demonstrate that selection of the stimulation intensity (SI) for nTMS and spatial density of stimuli are crucial factors for applying motor mapping accurately, while also demonstrating the effect on the motor maps. We conclude that while the application of nTMS motor mapping has been impressively spread over the past decade, there are still variations in the applied protocols and parameters, which could be optimized for the purpose of reliable quantitative mapping.

Minimum-Norm Estimation of TMS-Activated Motor Cortical Sites in Realistic Head and Brain Geometry

“…In the hand motor area, the FDI hotspots were located in the "hand knob" [49]. This implies that the initial mapping was accurate in localizing the musclespecific hotspot, which is crucial in the rMT determination and the subsequent motor mapping [13]. In most subjects, the motor maps were elongated in the direction of the E-field (Figs.…”

“…Navigated TMS (nTMS) enables stimulation with high localization accuracy [7], [8], which is crucial for modern neurosurgery in pre-surgical mappings that aim to delineate the eloquent motor areas [9]- [13]. Conventionally, these mappings stimulate the motor cortex with a slightly supra-threshold stimulation intensity (SI), e.g., 105% or 110% of the resting motor threshold (rMT), to outline the cortical sites associated with MEPs [13]- [15].…”

“…Navigated TMS (nTMS) enables stimulation with high localization accuracy [7], [8], which is crucial for modern neurosurgery in pre-surgical mappings that aim to delineate the eloquent motor areas [9]- [13]. Conventionally, these mappings stimulate the motor cortex with a slightly supra-threshold stimulation intensity (SI), e.g., 105% or 110% of the resting motor threshold (rMT), to outline the cortical sites associated with MEPs [13]- [15]. The pre-surgical nTMS maps have been shown to correspond well with the gold standard of direct electrical stimulation (DES) based on comparisons between hotspots, defined as the cortical sites eliciting the highest MEP amplitudes, or centers of gravity (CoGs) [16]- [18].…”

“…This tends to cause overestimation of the true motor representation area [20]- [22]. Furthermore, selecting the SI without the prior consideration of individual input-output (IO) characteristics may hamper inter-individual comparisons [13], [23].…”

Effects of Transcranial Direct Current Stimulation and High-Definition Transcranial Direct Current Stimulation Enhanced Motor Learning on Robotic Transcranial Magnetic Stimulation Motor Maps in Children

“…Transcranial magnetic stimulation (TMS) can produce maps of cortical representations of individual muscles within M1 (Cohen and Hallett, 1988;Wassermann et al, 1992;Wilson et al, 1993;Pascual-Leone et al, 1995b;Thickbroom et al, 1998) and is increasingly applied for clinical applications such as preoperative neurosurgical planning for brain tumor removal and epilepsy surgery (Picht et al, 2011;Lefaucheur and Picht, 2016), for review see Sollmann et al (2021). Common characteristics, such as map volume, area, hotspot magnitude, and centre of gravity (COG), may also potentially quantify changes in cortical neurophysiology following learning or stimulation-induced changes in motor performance (Cohen et al, 1993;Pascual-Leone et al, 1995a,b).…”