Navigated transcranial magnetic stimulation (nTMS) is a modification of TMS that combines magnetic resonance imaging-based (MRI-based), threedimensional cortical localization with TMS and simultaneous electromyography (EMG) measurement to locate brain areas capable of evoking muscle responses when stimulated.
Recent advances such as the co-registration of the subject’s head with structural MRI have allowed the application of stimulations with a precision of a few millimeters. Prior experiments have demonstrated a strong correlation between nTMS and intraoperative direct cortical stimulation (DCS) 1).
A study shows the feasibility of increasing the MEP detection threshold to 500 μV in rMT determination and motor area mapping with nTMS without losing precision 2).
While gaining importance in the preoperative planning process in motor eloquent regions, its usefulness for reliably identifying language areas is still being discussed.
Preoperative nTMS results correlate well with direct cortical stimulation (DCS) data in the identification of the primary motor cortex. Repetitive nTMS can also be used for mapping of speech-sensitive cortical areas.
It has become established as an accurate noninvasive technique for mapping the functional motor cortex for the representation areas of upper and lower limb muscles, facial muscles in the lower part of the face. Instead of using the motor threshold (MT) of the abductor pollicis brevis, the stimulus intensity during mapping should be proportioned to the MT of a facial muscle 3).
Its functional information benefits surgical decision making and changes the treatment strategy in one-fourth of cases 4).
Magnetic resonance images are being increasingly deployed in conjunction with navigated transcranial magnetic stimulation (nTMS) to account for inter-individual differences in brain anatomy as well as to reduce the variability of mapping findings.
Provides crucial data for preoperative planning and surgical resection of tumors involving essential motor areas. Expanding surgical indications and extent of resection based on nTMS enables more patients to undergo surgery and might lead to better neurological outcomes and higher survival rates in brain tumor patients 7).
The implementation of tractography based on nTMS increases the accuracy of fiber tracking. Moreover, this combination of methods has the potential to become a supplemental tool for guiding electrode implantation 8).
113 patients undergoing bihemispheric nTMS examination prior to surgery for gliomas in presumed motor eloquent locations. Multiple ordinal logistic regression analysis was performed to test for any association between preoperative nTMS-related variables and postoperative motor outcome.
A new motor deficit or deterioration due to a preexisting deficit was observed in 20% of cases after 7 days and in 22% after 3 months. In terms of tumor location, no new permanent deficit was observed when the distance between tumor and corticospinal tract was greater than 8 mm and the precentral gyrus was not infiltrated (p = 0.014). New postoperative deficits on Day 7 were associated with a pathological excitability of the motor cortices (interhemispheric resting motor threshold [RMT] ratio < 90% or > 110%, p = 0.031). Interestingly, motor function never improved when the RMT was significantly higher in the tumorous hemisphere than in the healthy hemisphere (RMT ratio > 110%).
The proposed risk stratification model, based on objective functional-anatomical and neurophysiological measures, enables one to counsel patients about the risk of functional deterioration or the potential for recovery 9).
Between 2010 and 2013, nTMS motor mapping was performed in a prospective cohort of 100 patients with brain tumors in or adjacent to the rolandic cortex. Spatial data analysis was performed by normalization of the individual motor maps and creation of overlays according to tumor location. Analysis of motor evoked potential (MEP) latencies was performed regarding mean overall latencies and potentially polysynaptic latencies, defined as latencies longer than 1 SD above the mean value. Hemispheric dominance, lesion location, and motor-function deficits were also considered.
Graphical analysis showed that motor areas were not restricted to the precentral gyrus. Instead, they spread widely in the anterior-posterior direction. An analysis of MEP latency showed that mean MEP latencies were shortest in the precentral gyrus and longest in the superior and middle frontal gyri. The percentage of latencies longer than 1 SD differed widely across gyri. The dominant hemisphere showed a greater number of longer latencies than the nondominant hemisphere (p < 0.0001). Moreover, tumor location-dependent changes in distribution of polysynaptic latencies were observed (p = 0.0002). Motor-function deficit did not show any statistically significant effect.
The distribution of primary and polysynaptic motor areas changes in patients with brain tumors and highly depends on tumor location. Thus, these data should be considered for resection planning 10).