Abstract and Introduction
Object. Navigated transcranial magnetic stimulation (nTMS) is a novel technology in the field of neurosurgery for noninvasive delineation of cortical functional topography. This study addresses the spatial accuracy and clinical usefulness of nTMS in brain tumor surgery in or near the motor cortex based on a systematic review of observational studies.
Methods. A systematic search retrieved 11 reports published up to October 2012 in which adult patients were examined with nTMS prior to surgery. Quality criteria consisted of documentation of the influence of nTMS brain mapping on clinical decision making in a standardized prospective manner and/or performance of intraoperative direct electrical stimulation (DES) and comparison with nTMS results. Cross-observational assessment of nTMS accuracy was established by calculating a weighted mean distance between nTMS and DES.
Results. ll studies reviewed in this article concluded that nTMS correlated well with the "gold standard" of DES. The mean distance between motor cortex identified on nTMS and DES by using the mean distance in 81 patients described in 6 quantitatively evaluated studies was 6.18 mm. The nTMS results changed the surgical strategy based on anatomical imaging alone in 25.3% of all patients, based on the data obtained in 87 patients in 2 studies.
Conclusions. The nTMS technique spatially correlates well with the gold standard of DES. Its functional information benefits surgical decision making and changes the treatment strategy in one-fourth of cases.
The goal of brain tumor surgery is to maximize the extent of tumor resection while preserving function. To achieve this goal is challenging, especially in glioma surgery, since these operations often involve structures that potentially carry essential function. To minimize the risk for new neurological sequelae as well as the risk of leaving residual tumor, precise knowledge of the individual functional topography is indispensable. Because of natural anatomical variation between all people, displacement of familiar anatomical landmarks by the tumor, and tumor-induced plastic reorganization of functional areas, functionally relevant brain tissue cannot be reliably predicted from standard anatomical imaging alone. Thus, it is essential to obtain case-specific knowledge of the location of functionally essential brain tissue. Intraoperative brain mapping by means of DES has been so far the most accurate and reliable gold standard to obtain such knowledge. However, if we could obtain functional maps that delineate resectable versus nonresectable brain tissue outside the operating theater with the same accuracy and reliability as DES, it would be of great use.
Transcranial magnetic stimulation is the only modality that is analogous to DES in that it allows for electrical stimulation of the brain and observation of the induced effect (Fig. 1). Although TMS was introduced in clinical neurology in 1985, its use in neurosurgery has been sporadic, and only recently an increasing number of reports from neurosurgical institutions on the use of nTMS for brain mapping in patients with rolandic brain tumors have been published.[3,4,6,8–11,13,14,21]
Upper: The "E-field navigation" TMS system in use. Reflective spheres are attached to the patient's head (modified elastic headband) and to the TMS coil. The stimulator coil is shown placed against the patient's head. The motor output is recorded by surface electrodes attached to the face, arm, and leg. The anatomical map of the patient's brain and the MEP output tracings are shown on the computer screens in the upper left corner. Lower: A 3D reconstruction of an MRI study, navigational view, obtained in a 63-year-old woman suffering from a mild hemiparesis of her right side due to a tumor in the central region of the left hemisphere. All spots stimulated with nTMS on the left hemisphere are displayed in the left panel. The image in the right panel displays only the spots where a muscle response was observed (MEP > 50 μV peak-to-peak amplitude). Three different hand muscles (APB, first dorsal interosseus, and abductor digiti minimi) and 1 leg muscle (TA) were recorded. The color coding corresponds to the intensity of the response, in which red indicates small responses (MEP 50–499 μV), yellow indicates medium responses (MEP 500–999 μV), and white indicates large responses (MEP ≥ 1000 μV). This mapping makes it evident that the precentral gyrus has been displaced frontally and that the center of gravity for the hand muscle representation is located immediately adjacent frontolaterally to the tumor. The responses close to the midline are from the leg (TA).
In the present article, based on a review of previous reports evaluating the spatial accuracy of nTMS in comparison with the gold standard of DES and/or assessing the clinical impact of the method, we address the clinical utility of nTMS in rolandic brain tumor surgery.
Neurosurg Focus. 2013;34(4):e3 © 2013 American Association of Neurological Surgeons