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.
Highlights
Navigated transcranial magnetic stimulation combines the use of neuronavigation with TMS to target neurostimulation inductively to the brain cortex, utilizing views of the brain anatomy with sub-centimeter precision and enabling tracking of the coil during stimulation (e.g., using an infrared (IR) tracking system combined with the stimulator; Figure 1) [1]
The approach of Navigated transcranial magnetic stimulation (nTMS)-based tractography of the corticospinal tract (CST) was subsequently refined by investigating individually adapted adjustments for the fractional anisotropy (FA) that had to be defined for the deterministic tractography algorithm: the FA was increased stepwise until no fibers were displayed, followed by reducing the FA value by 0.01, delineating only a thin fiber course; the obtained FA value was defined as 100% FA threshold (FAT), and nTMS-based tractography was carried out with 50% and 75% FAT with motor-positive nTMS points and the manually delineated internal capsule or brainstem as regions of interest (ROIs) [52]
When added to the armamentarium of the preoperative workup of patients harboring brain neoplasms, the question arises whether nTMS motor mapping and derived nTMSbased tractography may be capable of improving the clinical outcome, as measured by an ideally increased extent of resection (EOR) combined with lowered rates of functional perioperative decline
Summary
Navigated transcranial magnetic stimulation (nTMS) combines the use of neuronavigation with TMS to target neurostimulation inductively to the brain cortex, utilizing views of the brain anatomy with sub-centimeter precision and enabling tracking of the coil during stimulation (e.g., using an infrared (IR) tracking system combined with the stimulator; Figure 1) [1]. In an early series of 20 patients with different entities of intracranial neoplasms, the motor hotspot was located on the same gyrus for nTMS and DES mappings in all enrolled cases, with distances of motor hotspots between techniques amounting to values
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