Application of Navigated Transcranial Magnetic Stimulation to Map the Supplementary Motor Area in Healthy Subjects

Cortical stimulation depth of nTMS investigated in a cohort of convexity meningiomas above the primary motor cortex

BACKGROUND

In clinical routine, navigated transcranial magnetic stimulation (nTMS) is usually applied down to 25 mm. Yet, besides clinical experience and mathematical models, the penetration depth remains unclear. This study aims to investigate the maximum cortical stimulation depth of nTMS in patients with meningioma above the primary motor cortex, causing a displacement of the primary motor cortex away from the skull.

NEW METHOD

nTMS stimulation data was reviewed regarding the maximum depth of stimulations eliciting motor-evoked potentials (MEPs). Additionally, electric field values and stimulation intensity were analyzed.

RESULTS

Out of a consecutive cohort of 17 meningioma cases, 3 cases of meningioma located in motor-eloquent regions of the upper extremity and 3 cases of the lower extremity were analyzed after fulfilling all inclusion criteria. Regarding the upper extremity motor representations, the MEP could be elicited at a stimulation depth of up to 44 mm, with an electric field of 69 V/m. These results were found in 1 case with the maximum potential distance to the cortex being higher than the maximum stimulation depth eliciting MEPs. For the lower extremities, a maximum depth of 40 mm was recorded (electric field 64 V/m).

COMPARISON WITH EXISTING METHODS

None available CONCLUSIONS: The effect of nTMS is not limited to superficial cortical stimulation alone. Depending on electric-field intensity and focality, nTMS stimulation can be applied at a depth of 44 mm. In all cases, electric field strength was comparable and no superficial cortex with comparable electric field strength was observed to elicit MEPs.

Mapping of the supplementary motor area using repetitive navigated transcranial magnetic stimulation

Background

The supplementary motor area (SMA) is important for motor and language function. Damage to the SMA may harm these functions, yet tools for a preoperative assessment of the area are still sparse.

Objective

The aim of this study was to validate a mapping protocol using repetitive navigated transcranial magnetic stimulation (rnTMS) and extend this protocol for both hemispheres and lower extremities.

Methods

To this purpose, the SMA of both hemispheres were mapped based on a finger tapping task for 30 healthy subjects (35.97 ± 15.11, range 21-67 years; 14 females) using rnTMS at 20 Hz (120% resting motor threshold (RMT)) while controlling for primary motor cortex activation. Points with induced errors were marked on the corresponding MRI. Next, on the identified SMA hotspot a bimanual finger tapping task and the Nine-Hole Peg Test (NHPT) were performed. Further, the lower extremity was mapped at 20 Hz (140%RMT) using a toe tapping task.

Results

Mean finger tapping scores decreased significantly during stimulation (25.70taps) compared to baseline (30.48; p < 0.01). Bimanual finger tapping led to a significant increase in taps during stimulation (28.43taps) compared to unimanual tapping (p < 0.01). Compared to baseline, completion time for the NHPT increased significantly during stimulation (baseline: 13.6 s, stimulation: 16.4 s; p < 0.01). No differences between hemispheres were observed.

Conclusion

The current study validated and extended a rnTMS based protocol for the mapping of the SMA regarding motor function of upper and lower extremity. This protocol could be beneficial to better understand functional SMA organisation and improve preoperative planning in patients with SMA lesions.

Protocol for mapping of the supplementary motor area using repetitive navigated transcranial magnetic stimulation

Background

Damage to the supplementary motor area (SMA) can lead to impairments of motor and language function. A detailed preoperative mapping of functional boarders of the SMA could therefore aid preoperative diagnostics in these patients.

Objective

The aim of this study was the development of a repetitive nTMS protocol for non-invasive functional mapping of the SMA while assuring effects are caused by SMA rather than M1 activation.

Methods

The SMA in the dominant hemisphere of 12 healthy subjects (28.2 ± 7.7 years, 6 females) was mapped using repetitive nTMS at 20 Hz (120% RMT), while subjects performed a finger tapping task. Reductions in finger taps were classified in three error categories (≤15% = no errors, 15-30% = mild, >30% significant). The location and category of induced errors was marked in each subject's individual MRI. Effects of SMA stimulation were then directly compared to effects of M1 stimulation in four different tasks (finger tapping, writing, line tracing, targeting circles).

Results

Mapping of the SMA was possible for all subjects, yet effect sizes varied. Stimulation of the SMA led to a significant reduction of finger taps compared to baseline (BL: 45taps, SMA: 35.5taps; p < 0.01). Line tracing, writing and targeting of circles was less accurate during SMA compared to M1 stimulation.

Conclusion

Mapping of the SMA using repetitive nTMS is feasible. While errors induced in the SMA are not entirely independent of M1, disruption of the SMA induces functionally distinct errors. These error maps can aid preoperative diagnostics in patients with SMA related lesions.

Neurosurgery for eloquent lesions in children: state-of-the-art rationale and technical implications of perioperative neurophysiology

OBJECTIVE

In adult patients, an increasing group of neurosurgeons specialize entirely in the treatment of highly eloquent tumors, particularly gliomas. In contrast, extensive perioperative neurophysiological workup for pediatric cases has been limited essentially to epilepsy surgery.

METHODS

The authors discuss radio-oncological and general oncological considerations based on the current literature and their personal experience.

RESULTS

While several functional mapping modalities facilitate preoperative identification of cortically and subcortically located eloquent areas, not all are suited for children. Direct cortical intraoperative stimulation is impractical in many young patients due to the reduced excitability of the immature cortex. Behavioral requirements also limit the utility of functional MRI and magnetoencephalography in children. In contrast, MRI-derived tractography and navigated transcranial magnetic stimulation are available across ages. Herein, the authors review the oncological rationale of function-guided resection in pediatric gliomas including technical implications such as personalized perioperative neurophysiology, surgical strategies, and limitations.

CONCLUSIONS

Taken together, these techniques, despite the limitations of some, facilitate the identification of eloquent areas prior to tumor surgery and radiotherapy as well as during follow-up of residual tumors.

Mechanisms of Repetitive Transcranial Magnetic Stimulation on Post-stroke Depression: A Resting-State Functional Magnetic Resonance Imaging Study

We aimed to identify neural mechanisms underlying clinical response to repetitive transcranial magnetic stimulation (rTMS) in post-stroke depression (PSD) by the Resting-state functional magnetic resonance imaging (rs-fMRI). Thirty-two depressed patients after ischemic stroke were randomized in a 1:1 ratio to receive 20 min of 5 Hz rTMS or sham over left dorsolateral prefrontal cortex (DLPFC) in addition to routine supportive treatments. The clinical outcome was measured by the 17-item Hamilton Depression Rating Scale (HDRS-17), while the imaging results were acquired from rs-fMRI, including regional homogeneity (ReHo), fractional amplitude of low-frequency fluctuation (fALFF) and seed-based dynamic functional connection (dFC). HRSD-17 scores were improved in the two groups after treatment (P < 0.01), while greater mood improvement was observed in the rTMS group (P < 0.05). Compared with the sham group, the rTMS group demonstrated regions with higher ReHo and fALFF values locating mainly in the left hemisphere and highly consistent with the default mode network (DMN) (p < 0.05). Using the medial prefrontal cortex (mPFC) and posterior cingulate cortex (PCC) as seeds, significant difference between the two groups in dFC within the DMN was found after treatment, including 10 connections with increased connectivity strength and 2 connections with reduced connectivity strength. The ReHo, fALFF and dFC values within DMN in the rTMS group were negatively correlated with the HDRS scores after treatment (P < 0.05). Our results indicated reductions in depressive symptoms following rTMS in PSD are associated with functional alterations of different depression-related areas within the DMN.

Functional Mapping for Glioma Surgery, Part 1: Preoperative Mapping Tools

Although intraoperative mapping of brain areas was shown to promote greater extent of resection and reduce functional deficits, this was shown only recently for some noninvasive techniques. Yet, proper surgical planning, indication, and patient consultation require reliable noninvasive techniques. Because functional magnetic resonance imaging, tractography, and neurophysiologic methods like navigated transcranial magnetic stimulation and magnetoencephalography allow identifying eloquent areas prior to resective surgery and tailor the surgical approach, this article provides an overview on the individual strengths and limitations of each modality.

Electroencephalographic Functional Connectivity With the Tacit Learning System Prosthetic Hand: A Case Series Using Motor Imagery

We previously created a prosthetic hand with a tacit learning system (TLS) that automatically supports the control of forearm pronosupination. This myoelectric prosthetic hand enables sensory feedback and flexible motor output, which allows users to move efficiently with minimal burden. In this study, we investigated whether electroencephalography can be used to analyze the influence of the auxiliary function of the TLS on brain function. Three male participants who had sustained below-elbow amputations and were myoelectric prosthesis users performed a series of physical movement trials with the TLS inactivated and activated. Trials were video recorded and a sequence of videos was prepared to represent each individual's own use while the system was inactivated and activated. In a subsequent motor imagery phase during which electroencephalography (EEG) signals were collected, each participant was asked to watch both videos of themself while actively imagining the physical movement depicted. Differences in mean cortical current and amplitude envelope correlation (AEC) values between supplementary motor areas (SMA) and each vertex were calculated. For all participants, there were differences in the mean cortical current generated by the motor imagery tasks when the TLS inactivated and activated conditions were compared. The AEC values were higher during the movement imagery task with TLS activation, although their distribution on the cortex varied between the three individuals. In both S1 and other brain areas, AEC values increased in conditions with the TLS activated. Evidence from this case series indicates that, in addition to motor control, TLS may change sensory stimulus recognition.

Navigated transcranial magnetic stimulation of the supplementary motor cortex disrupts fine motor skills in healthy adults

Navigated transcranial magnetic stimulation (nTMS) over the supplementary motor area (SMA) may impact fine motor skills. This study evaluates different nTMS parameters in their capacity to affect fine motor performance on the way to develop an SMA mapping protocol. Twenty healthy volunteers performed a variety of fine motor tests during baseline and nTMS to the SMA using 5 Hz, 10 Hz, and theta-burst stimulation (TBS). Effects on performance were measured by test completion times (TCTs), standard deviation of inter-tap interval (SDIT), and visible coordination problems (VCPs). The predominant stimulation effect was slowing of TCTs, i.e. a slowdown of test performances during stimulation. Furthermore, participants exhibited VCPs like accidental use of contralateral limbs or inability to coordinate movements. More instances of significant differences between baseline and stimulation occurred during stimulation of the right hemisphere compared to left-hemispheric stimulation. In conclusion, nTMS to the SMA could enable new approaches in neuroscience and enable structured mapping approaches. Specifically, this study supports interhemispheric differences in motor control as right-hemispheric stimulation resulted in clearer impairments. The application of our nTMS-based setup to assess the function of the SMA should be applied in patients with changed anatomo-functional representations as the next step, e.g. among patients with eloquent brain tumors.