Original ArticleFirst United Kingdom Experience of Navigated Transcranial Magnetic Stimulation in Preoperative Mapping of Brain Tumors
Introduction
Surgery for lesions in eloquent brain areas remains challenging owing to the risk of causing a permanent neurological deficit. Direct electrical stimulation (DES) at the cortical and subcortical level represents the reference standard for minimizing these risks.1, 2, 3, 4, 5, 6 More recently, navigated transcranial magnetic stimulation (nTMS) has emerged as a noninvasive mapping tool to assist neurosurgeons in optimizing surgical planning for lesions in the eloquent brain.7, 8, 9, 10, 11 Using a high-precision coil, matched with neuronavigation and analytic software, nTMS delivers biphasic magnetic stimulation to the cortex.12 Its main application has been motor and language mapping. Single-pulse nTMS, applied to the primary motor cortex (PMC), generates muscle output that is recorded via a continuously running electromyogram.12
On application of repetitive nTMS to the cortex, a transient disruption of the areas responsible for language processing and execution will occur.13 Depending on the location within the brain and the type of surgery, preoperative nTMS can, therefore, elucidate the perilesional functional cortical organization. Although previous studies have focused on the role of nTMS compared with DES, more data are required to further validate this technique. In addition, a previous study suggested the utility of nTMS in refining and modifying the surgical approach to lesions in eloquent areas.12 These findings, although encouraging, have not been independently reassessed or replicated.
To the best of our knowledge, we report the first U.K. experience in the use of nTMS, correlating our nTMS results with the intraoperative findings and assessing their effect on surgical decision-making.
Section snippets
Patients
We retrospectively reviewed the data from patients who had undergone craniotomy for removal of a brain tumor at our institution with preoperative nTMS, intraoperative neuromonitoring (IOM), and DES from February 2017 to February 2018. The inclusion criteria were age ≥18 years, brain tumor involving motor or language eloquent area (assessed by anatomical location on structural magnetic resonance imaging [MRI] and/or clinical presentation), and performance status <2 using the Zubrod scale.14
Ethical Standard
The
Patients
Of the 35 patients, 24 (68.6%) had undergone preoperative nTMS mapping for motor function and 11 (31.4%) for language (Table 1). The mean age was 47 years with equal numbers of men and women in both groups. Right hand dominance was naturally distributed (91.4%).29, 30
Motor mapping was performed most commonly for lesions in the parietal lobe (n = 7) or central lobule (n = 7) and speech mapping for lesions in the frontal (n = 4) or temporal (n = 4) lobes. Of the 35 patients, 31 (88.6%) had a
Role of TMS in Neurosurgery
Although DES combined with IOM remains the reference standard for surgery in the eloquent brain,32, 33, 34, 35, 36 different modalities have been developed to assist in preoperative planning. This is important, both to obtain a risk estimate to help the process of informed consent and to aid in planning the surgical strategy before entering the operating room.
Functional MRI (fMRI), using a blood oxygen level-dependent technique as a metabolic surrogate of function, has been used to assess the
Conclusions
nTMS is a safe, noninvasive adjunctive tool for preoperative mapping of space-occupying lesions in eloquent areas. It reliably identified the PMC in our cohort with an accurate APB hotspot compared with the reference standard of DES. Our preoperative language mapping with nTMS confirmed a high NPV of 74.1% and specificity of 66.7%. Furthermore, the use of nTMS influenced surgical decision-making in up to one third of patients in our experience.
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Conflict of interest statement: The authors declare that the article content was composed in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
Keyoumars Ashkan and Francesco Vergani contributed equally to the present study.