Motor Potentials Evoked by Navigated Transcranial Magnetic Stimulation in Healthy Subjects

Functional neuronal anisotropy assessed with neuronavigated transcranial magnetic stimulation

BACKGROUND

Transcranial magnetic stimulation (TMS) can evaluate cortical excitability and integrity of motor pathways via TMS-induced responses. The responses are affected by the orientation of the stimulated neurons with respect to the direction of the TMS-induced electric field. Therefore, besides being a functional imaging tool, TMS may potentially assess the local structural properties. Yet, TMS has not been used for this purpose.

NEW METHOD

A novel principle to evaluate the relation between function and structure of the motor cortex is presented. This functional anisotropy is evaluated by an anisotropy index (AI), based on motor evoked potential amplitudes induced with different TMS coil orientations, i.e. different electric field directions at a cortical target. To compare the AI with anatomical anisotropy in an explorative manner, diffusion tensor imaging-derived fractional anisotropy (FA) was estimated at different depths near the stimulation site.

RESULTS

AI correlated inversely with cortical excitability through the TMS-induced electric field at motor threshold level. Further, there was a trend of negative correlation between AI and FA.

COMPARISON WITH EXISTING METHODS

None of the existing methods alone can detect the relationship between direct motor cortex activation and local neuronal structure.

CONCLUSIONS

The AI appears to provide information on the functional neuronal anisotropy of the motor cortex by coupling neurophysiology and neuroanatomy within the stimulated cortical region. The AI could prove useful in the evaluation of neurological disorders and traumas involving concurrent structural and functional changes in the motor cortex. Further studies on patients are needed to confirm the usability of AI.

Onset Latency of Motor Evoked Potentials in Motor Cortical Mapping with Neuronavigated Transcranial Magnetic Stimulation

Cortical motor mapping in pre-surgical applications can be performed using motor evoked potential (MEP) amplitudes evoked with neuronavigated transcranial magnetic stimulation. The MEP latency, which is a more stable parameter than the MEP amplitude, has not so far been utilized in motor mapping. The latency, however, may provide information about the stress in damaged motor pathways, e.g. compression by tumors, which cannot be observed from the MEP amplitudes. Thus, inclusion of this parameter could add valuable information to the presently used technique of MEP amplitude mapping. In this study, the functional cortical representations of first dorsal interosseous (FDI), abductor pollicis brevis (APB) and abductor digiti minimi (ADM) muscles were mapped in both hemispheres of ten healthy righthanded volunteers. The cortical muscle representations were evaluated by the area and centre of gravity (CoG) by using MEP amplitudes and latencies. As expected, the latency and amplitude CoGs were congruent and were located in the centre of the maps but in a few subjects, instead of a single centre, several loci with short latencies were observed. In conclusion, MEP latencies may be useful in distinguishing the cortical representation areas with the most direct pathways from those pathways with prolonged latencies. However, the potential of latency mapping to identify stressed motor tract connections at the subcortical level will need to be verified in future studies with patients.

Current and potential utility of transcranial magnetic stimulation in the diagnostics before brain tumor surgery

This article describes the evolution and state-of-the-art of navigated transcranial magnetic stimulation for evaluation of patients with brain tumors in presumed eloquent location. Alternative noninvasive technologies for functional brain mapping are described and assessed in the context of their usability and clinical needs. In addition to the description of the current validation level and clinical application of navigated transcranial magnetic stimulation for motor and language mapping, the manuscript highlights ongoing research efforts and provides an outlook on upcoming developments in the field of noninvasive brain mapping. Finally, the clinical rationale for presurgical noninvasive brain mapping is discussed in the light of current developments in neurosurgery.

Functional connectivity underlying postural motor adaptation in people with multiple sclerosis

A well-characterized neural network is associated with motor learning, involving several brain regions known to have functional and structural deficits in persons with multiple sclerosis (PwMS). However, it is not known how MS affects postural motor learning or the neural networks involved. The aim of this study was to gain a better understanding of the neural networks underlying adaptation of postural responses within PwMS. Participants stood on a hydraulically driven, servo-controlled platform that translated horizontally forward and backward in a continuous sinusoidal pattern across multiple trials over two consecutive days. Our results show similar postural adaptation between PwMS and age-matched control participants despite overall deficits in postural motor control in PwMS. Moreover, PwMS demonstrated better retention the following day. PwMS had significantly reduced functional connectivity within both the cortico-cerebellar and cortico-striatal motor loops; neural networks that subserve implicit motor learning. In PwMS, greater connectivity strength within the cortico-cerebellar circuit was strongly related to better baseline postural control, but not to postural adaptation as it was in control participants. Further, anti-correlated cortico-striatal connectivity within the right hemisphere was related to improved postural adaptation in both groups. Taken together with previous studies showing a reduced reliance on cerebellar- and proprioceptive-related feedback control in PwMS, we suggest that PwMS may rely on cortico-striatal circuitry to a greater extent than cortico-cerebellar circuitry for the acquisition and retention of motor skills.

Improved nTMS- and DTI-derived CST tractography through anatomical ROI seeding on anterior pontine level compared to internal capsule

Imaging of the course of the corticospinal tract (CST) by diffusion tensor imaging (DTI) is useful for function-preserving tumour surgery. The integration of functional localizer data into tracking algorithms offers to establish a direct structure–function relationship in DTI data. However, alterations of MRI signals in and adjacent to brain tumours often lead to spurious tracking results. We here compared the impact of subcortical seed regions placed at different positions and the influences of the somatotopic location of the cortical seed and clinical co-factors on fibre tracking plausibility in brain tumour patients.

The CST of 32 patients with intracranial tumours was investigated by means of deterministic DTI and neuronavigated transcranial magnetic stimulation (nTMS). The cortical seeds were defined by the nTMS hot spots of the primary motor area (M1) of the hand, the foot and the tongue representation. The CST originating from the contralesional M1 hand area was mapped as intra-individual reference. As subcortical region of interests (ROI), we used the posterior limb of the internal capsule (PLIC) and/or the anterior inferior pontine region (aiP). The plausibility of the fibre trajectories was assessed by a-priori defined anatomical criteria. The following potential co-factors were analysed: Karnofsky Performance Scale (KPS), resting motor threshold (RMT), T1-CE tumour volume, T2 oedema volume, presence of oedema within the PLIC, the fractional anisotropy threshold (FAT) to elicit a minimum amount of fibres and the minimal fibre length.

The results showed a higher proportion of plausible fibre tracts for the aiP-ROI compared to the PLIC-ROI. Low FAT values and the presence of peritumoural oedema within the PLIC led to less plausible fibre tracking results. Most plausible results were obtained when the FAT ranged above a cut-off of 0.105. In addition, there was a strong effect of somatotopic location of the seed ROI; best plausibility was obtained for the contralateral hand CST (100%), followed by the ipsilesional hand CST (>95%), the ipsilesional foot (>85%) and tongue (>75%) CST. In summary, we found that the aiP-ROI yielded better tracking results compared to the IC-ROI when using deterministic CST tractography in brain tumour patients, especially when the M1 hand area was tracked. In case of FAT values lower than 0.10, the result of the respective CST tractography should be interpreted with caution with respect to spurious tracking results. Moreover, the presence of oedema within the internal capsule should be considered a negative predictor for plausible CST tracking.

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Somatotopic CST tractography was done in 32 patients with eloquent brain tumours.

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Seeding ROIs were defined by navigated TMS of the M1 hot spot (hand, foot, tongue).

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Using the anterior pons as a second ROI yielded more plausible tracts than the PLIC.

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Low FAT and oedema of the internal capsule were negative predictors.

Functional brain mapping of patients with arteriovenous malformations using navigated transcranial magnetic stimulation: first experience in ten patients

BACKGROUND

Intracranial arteriovenous malformations (AVM) are known to be potent inductors of functional plasticity, and their vasculature makes standard functional imaging difficult. Here we conducted functional mapping of both primary motor cortex and speech related areas in patients with AVM using navigated transcranial magnetic stimulation (nTMS), which has been recently proven as a reliable noninvasive modality of preoperative functional brain mapping.

METHOD

nTMS mapping was performed in ten patients with unruptured intracranial AVMs located in or near eloquent areas. Motor mapping was conducted for six patients with AVMs near the rolandic region, and speech mapping was performed for four patients with left perisylvian AVMs. After the examination, all patients were treated with surgery, radiosurgery or observed with best medical treatment on case-by-case basis.

RESULTS

Motor mapping allowed for delineation of the primary motor cortex, even if the anatomy was severely obscured by the AVM in all cases with rolandic AVMs. No plastic relocation of the primary motor cortex was observed. Repetitive stimulation of the left ventral precentral gyrus led to speech impairments in all four cases that underwent speech mapping. Right hemispheric involvement was observed in one out of four cases and potentially indicated plastic changes. No side effects were observed.

CONCLUSION

nTMS allowed for detailed delineation of eloquent areas even within hypervascularized cortical areas. Our observations indicate that nTMS functional mapping is feasible not only in tumorous brain lesions, but also in AVMs.

Effect of single-session repetitive transcranial magnetic stimulation applied over the hand versus leg motor area on pain after spinal cord injury

BACKGROUND

Neuropathic pain often follows spinal cord injury (SCI).

OBJECTIVE

To compare the effect of repetitive transcranial magnetic stimulation (rTMS) applied over different motor cortex targets (hand vs leg area) versus sham stimulation on neuropathic pain and local neurophysiological changes in patients with SCI.

METHODS

A total of 16 patients with complete or incomplete motor SCI and chronic neuropathic pain participated in a double-blind, cross-over randomized study. Three single sessions of sham or active rTMS (10 Hz, total of 2000 stimuli) were applied in random order over the hand or leg area with a minimal 2-week interval. THE MAIN OUTCOME MEASURES: were the numeric rating scale for pain sensation and parameters derived from motor mapping of the first dorsal interosseous muscle, including maximal amplitude of evoked response as well as map area, volume, and location.

RESULTS

rTMS applied to either the hand or the leg area, but not sham stimulation, induced a significant but equivalent reduction in pain for the first 48 hours postintervention (P < .05). Participants with an incomplete lesion showed greater analgesia than those with a complete lesion (21% vs. 3%, respectively; P < .05). The main change observed for motor map measurements was an increase in corticospinal excitability after stimulation of the hand area (P = .04) but not for the other conditions.

CONCLUSION

rTMS applied over the hand or leg motor cortex decreased neuropathic pain regardless of any change in cortical excitability, suggesting that the analgesic effect is not associated with local changes at the motor cortex level itself.

Feasibility of automated analysis and inter-examiner variability of cortical silent period induced by transcranial magnetic stimulation

Cortical silent period (cSP) is a short interruption in electromyography (EMG) during active muscle contraction induced with transcranial magnetic stimulation (TMS). The cSP is a measure of cortical inhibition and is believed to represent inhibitory interneuron effects on excited motor cortical areas. Several pathological conditions and pharmacological manipulations induce changes to cSP duration indicating alterations in intracortical inhibition. At present, it is common to manually analyse the cSP duration from measured EMG. However, to avoid inter-examiner effects on cSP interpretation and detection, as well as to allow for quick measurement online, automated routine would be preferable. In this study, we evaluate the feasibility of a straight-forward cSP detection routine based on analysing the rectified first derivative of the EMG signal following TMS. Previously measured cSPs of 54 healthy subjects were reanalysed manually by two of the authors and using the automated routine. Furthermore, we recruited one subject for whom the cSPs were induced with several stimulation intensities, and those cSPs were analysed manually by two of the authors as well as using the automated routine. We found that cSPs were detected correctly by the automated cSP detection routine, and agreement with manually analysed subject-specific mean cSPs was excellent (ICC=0.992, p<0.001). The inter-examiner variability was similar to the variability between manual and automated analysis. Hence, we believe the introduced cSP detection routine would be feasible for online cSP detection, in such a way that is presently used to detect the motor evoked potentials.

Transcranial magnetic stimulation-electroencephalography responses in recovered and symptomatic mild traumatic brain injury

Mild traumatic brain injury (mTBI) may cause diffuse damage to the brain, especially to the frontal areas, that may lead to persistent symptoms. We studied participants with past mTBI by means of navigated transcranial magnetic stimulation (nTMS) combined with electroencephalography (EEG). Eleven symptomatic and 8 recovered participants with a history of single mTBI and 9 healthy controls participated. Average time from injury to testing was 5 years. The participants did not have abnormalities or signs of injury on brain magnetic resonance imaging, and they did not use any centrally acting medication. Left primary motor cortex (M1) and dorsolateral prefrontal cortex (DLPFC) were stimulated with nTMS and evoked potentials measured from the corresponding areas of both hemispheres. Delayed ipsilateral P30 and contralateral N45 peak latencies to left DLPFC nTMS were found in the symptomatic group, along with higher DLPFC N100 amplitudes compared with the control or recovered group. The recovered group had shorter P200 latencies in left DLPFC nTMS compared with the other groups. Both mTBI groups had higher motor thresholds compared with the control group. In left M1 nTMS, the mTBI groups showed less P30 amplitude increase, and the symptomatic group showed longer P60 interhemispheric latency difference with higher stimulation intensities. The results suggest altered brain reactivity and connectivity in mTBI. Some of the observed differences may be related to compensatory mechanisms of recovery. nTMS-EEG is a potentially useful tool for studying the effects of mTBI.

A Comparison of Language Mapping by Preoperative Navigated Transcranial Magnetic Stimulation and Direct Cortical Stimulation During Awake Surgery

BACKGROUND:

Navigated transcranial magnetic stimulation (nTMS) is increasingly used in presurgical brain mapping. 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.

OBJECTIVE:

The current cohort study compares the safety and effectiveness of preoperative nTMS with DCS mapping during awake surgery for the identification of language areas in patients with left-sided cerebral lesions.

METHODS:

Twenty patients with tumors in or close to left-sided language eloquent regions were examined by repetitive nTMS before surgery. During awake surgery, language-eloquent cortex was identified by DCS. nTMS results were compared for accuracy and reliability with regard to DCS by projecting both results into the cortical parcellation system.

RESULTS:

Presurgical nTMS maps showed an overall sensitivity of 90.2%, specificity of 23.8%, positive predictive value of 35.6%, and negative predictive value of 83.9% compared with DCS. For the anatomic Broca's area, the corresponding values were a sensitivity of 100%, specificity of 13.0%, positive predictive value of 56.5%, and negative predictive value of 100%, respectively.

CONCLUSION:

Good overall correlation between repetitive nTMS and DCS was observed, particularly with regard to negatively mapped regions. Noninvasive inhibition mapping with nTMS is evolving as a valuable tool for preoperative mapping of language areas. Yet its low specificity in posterior language areas in the current study necessitates further research to refine the methodology.

Transcranial electric and magnetic stimulation: technique and paradigms

Transcranial electrical and magnetic stimulation techniques encompass a broad physical variety of stimuli, ranging from static magnetic fields or direct current stimulation to pulsed magnetic or alternating current stimulation with an almost infinite number of possible stimulus parameters. These techniques are continuously refined by new device developments, including coil or electrode design and flexible control of the stimulus waveforms. They allow us to influence brain function acutely and/or by inducing transient plastic after-effects in a range from minutes to days. Manipulation of stimulus parameters such as pulse shape, intensity, duration, and frequency, and location, size, and orientation of the electrodes or coils enables control of the immediate effects and after-effects. Physiological aspects such as stimulation at rest or during attention or activation may alter effects dramatically, as does neuropharmacological drug co-application. Non-linear relationships between stimulus parameters and physiological effects have to be taken into account.

Assessing the functional status of the motor system in brain tumor patients using transcranial magnetic stimulation

BACKGROUND

Transcranial magnetic stimulation (TMS) is being used in the pre-operative diagnostics of patients with tumors in or near the motor cortex. Although the main purpose of TMS in such patients is to map the functional areas of the motor cortex in spatial relation to the tumor, TMS also provides some numerical neurophysiological measurements of the functional status of the patient's motor system. The aim of this paper is to provide reference values for these neurophysiological measurements from a large and varied clinical sample.

METHODS

TMS was used in the pre-operative work-up of patients with various types of tumors in or near the motor cortex during a 3-year period. Data was collected prospectively in 100 patients, yet this is a post hoc report.

RESULTS

Patient characteristics had no influence on the neurophysiological parameters. The response latency time was almost never different in the tumorous versus healthy hemisphere, so clinicians should be suspicious if they find interhemispheric differences for latency. A high interhemispheric ratio of resting motor threshold (RMT) or a low interhemispheric ratio of motor evoked potential (MEP) amplitude appear to suggest immanent deterioration of the patient's motor status.

CONCLUSION

In addition to topographic cortical mapping, TMS also serves as a neurophysiological assessment of the functional status of the patient's motor system. The results presented here provide clinicians with a set of reference values to contextualize findings in their own tumor patients. Further research is still needed to better understand the full clinical relevance of these neurophysiological parameters.

Long-lasting TMS motor threshold elevation in mild traumatic brain injury

OBJECTIVES

Mild traumatic brain injury (mTBI) is very common, and part of the patients experience persistent symptoms. These may be caused by diffuse neuronal damage and could therefore affect cortical excitability. The motor threshold (MT), measured by transcranial magnetic stimulation (TMS), is a measure of cortical excitability and cortico-spinal tract integrity.

MATERIALS AND METHODS

We used navigated TMS (nTMS) and electromyography to determine subjects' left hemisphere MTs. Nineteen subjects with mTBI (11 with persistent symptoms and eight fully recovered) and nine healthy controls were tested. The injuries had occurred on average 5 years earlier. All participants had normal brain MRIs, that is, no signs of injury. None used centrally acting medication.

RESULTS

The mean MT in controls was 43.0% (SD 2.5) of maximum stimulator output. The mTBI subjects mean MT was 53.4% (SD 9.7), being higher than the controls' threshold. Subjective recovery did not correlate with MT.

CONCLUSIONS

The results show chronic MT elevation in a sample of subjects with symptomatic or recovered mTBI. This suggests that mTBI may be compensated, although not fully recovered, years after the injury. While the cause for MT elevation cannot be concluded from these preliminary observations, possible explanations include decreased cortical excitability and impaired subcortical conduction.

Utility of presurgical navigated transcranial magnetic brain stimulation for the resection of tumors in eloquent motor areas

Object

Navigated transcranial magnetic stimulation (nTMS) is a newly evolving technique. Despite its supposed purpose (for example, preoperative central region mapping), little is known about its accuracy compared with established modalities like direct cortical stimulation (DCS) and functional MR (fMR) imaging. Against this background, the authors performed the current study to compare the accuracy of nTMS with DCS and fMR imaging.

Methods

Fourteen patients with tumors in or close to the precentral gyrus were examined using nTMS for motor cortex mapping, as were 12 patients with lesions in the subcortical white matter motor tract. Moreover, preoperative fMR imaging and intraoperative mapping of the motor cortex were performed via DCS, and the outlining of the motor cortex was compared.

Results

In the 14 cases of lesions affecting the precentral gyrus, the primary motor cortex as outlined by nTMS correlated well with that delineated by intraoperative DCS mapping, with a deviation of 4.4 ± 3.4 mm between the two methods. In comparing nTMS with fMR imaging, the deviation between the two methods was much larger: 9.8 ± 8.5 mm for the upper extremity and 14.7 ± 12.4 mm for the lower extremity. In 13 of 14 cases, the surgeon admitted easier identification of the central region because of nTMS. The procedure had a subjectively positive influence on the operative results in 5 cases and was responsible for a changed resection strategy in 2 cases. One of 26 patients experienced nTMS as unpleasant; none found it painful.

Conclusions

Navigated TMS correlates well with DCS as a gold standard despite factors that are supposed to contribute to the inaccuracy of nTMS. Moreover, surgeons have found nTMS to be an additional and helpful modality during the resection of tumors affecting eloquent motor areas, as well as during preoperative planning.

Effect of individual anatomy on resting motor threshold-computed electric field as a measure of cortical excitability

INTRODUCTION

Transcranial magnetic stimulation (TMS) is used for assessing the excitability of cortical neurons and corticospinal pathways by determining the subject-specific motor threshold (MT). However, the MT is dependent on the TMS instrumentation and exhibits large variation. We hypothesized that between-subject differences in scalp-to-cortex distance could account for the variation in the MT. Computational electric field (EF) estimation could theoretically be applied to reduce the effect of anatomical differences, since it provides a more direct measure of corticospinal excitability.

METHODS

The resting MT of the thenar musculature of 50 healthy subjects (24 male and 26 female, 22-69 years) was determined bilaterally at the primary motor cortex with MRI-navigated TMS using monophasic and biphasic stimulation. The TMS-induced maximum EF was computed at a depth of 25 mm from the scalp (EF(25 mm)) and at the individual depth of the motor cortex (EF(cortex)) determined from MRI-scans.

RESULTS

All excitability parameters (MT, EF(25 mm) and EF(cortex)) correlated significantly with each other (p<0.001). EF(cortex) at MT intensity was 95±20 V/m for biphasic and 120±24 V/m for monophasic stimulation. The MT did not correlate with the anatomical scalp-to-cortex distance, whereas the coil-to-cortex distance was found to correlate positively with the MT and negatively with EF(cortex) (p<0.05).

DISCUSSION

In healthy subjects, the scalp-to-cortex distance is not a significant determinant of the MT, and thus the use of EF(cortex) does not offer substantial advantages. However, it provides a purposeful and promising tool for studying non-motor cortical areas or patient groups with possible disease-related anatomical alterations.

New insights into Alzheimer's disease progression: a combined TMS and structural MRI study

BACKGROUND

Combination of structural and functional data of the human brain can provide detailed information of neurodegenerative diseases and the influence of the disease on various local cortical areas.

METHODOLOGY AND PRINCIPAL FINDINGS

To examine the relationship between structure and function of the brain the cortical thickness based on structural magnetic resonance images and motor cortex excitability assessed with transcranial magnetic stimulation were correlated in Alzheimer's disease (AD) and mild cognitive impairment (MCI) patients as well as in age-matched healthy controls. Motor cortex excitability correlated negatively with cortical thickness on the sensorimotor cortex, the precuneus and the cuneus but the strength of the correlation varied between the study groups. On the sensorimotor cortex the correlation was significant only in MCI subjects. On the precuneus and cuneus the correlation was significant both in AD and MCI subjects. In healthy controls the motor cortex excitability did not correlate with the cortical thickness.

CONCLUSIONS

In healthy subjects the motor cortex excitability is not dependent on the cortical thickness, whereas in neurodegenerative diseases the cortical thinning is related to weaker cortical excitability, especially on the precuneus and cuneus. However, in AD subjects there seems to be a protective mechanism of hyperexcitability on the sensorimotor cortex counteracting the prominent loss of cortical volume since the motor cortex excitability did not correlate with the cortical thickness. Such protective mechanism was not found on the precuneus or cuneus nor in the MCI subjects. Therefore, our results indicate that the progression of the disease proceeds with different dynamics in the structure and function of neuronal circuits from normal conditions via MCI to AD.

Reliability of the 'observation of movement' method for determining motor threshold using transcranial magnetic stimulation

The aim of this study was to establish the reliability of the observation of movement (OM) method for obtaining motor threshold (MT) in transcranial magnetic stimulation (TMS). MTs were obtained on separate days, following separate hunting procedures, for both left and right motor cortex (M1), with one or multiple estimates obtained from the same hemisphere within a single session. MTs obtained using the OM method were highly reliable and reproducible on different days (left M1: r=.98, p<.0001; right M1: r=.97, p<.0001). MTs were not influenced by the order of acquisition when two hemispheres were stimulated in the same session [F(1,22)=.12, p=.73] or by the collection of additional MTs as part of the distance-adjusted procedure [F(1,23)=.74, p=.40]. The results verify the reliability of the OM method and confirm its viability for the safe and efficient application of TMS to the left and right M1. The OM method is a reliable technique for obtaining MT and is relatively simple and quick to run. It therefore provides an effective procedure for research and clinical applications.

Navigated Brain Stimulation for Preoperative Cortical Mapping in Paretic Patients: Case Report of a Hemiplegic Patient

BACKGROUND AND IMPORTANCE:

Navigated brain stimulation (NBS) is an emerging technology that can be used for preoperative mapping of the motor cortex. It combines conventional transcranial magnetic stimulation with neuronavigation and achieves high precision by taking into account all relevant physical factors. In contrast to functional imaging technologies, NBS does not rely on voluntary patient movements for cortical mapping. Thus, NBS can be used even on patients with severe motor impairment.

CLINICAL PRESENTATION:

This article presents the case of a hemiplegic elderly woman with a brain tumor in the motor cortex. Preoperative NBS surprisingly demonstrated intact corticospinal tracts in the hemiplegic patient. The results modified the surgical strategy. Direct cortical stimulation was performed intraoperatively. The direct cortical stimulation results were in agreement with the preoperative NBS findings, and the clinical success of the surgery exceeded expectations.

CONCLUSION:

NBS can be used for preoperative mapping in plegic patients. Even more important, this case report discusses why tumor resection surgery based on NBS may sometimes lead to substantially better clinical outcomes than surgery planned according to functional imaging technologies.

Navigated Transcranial Magnetic Stimulation and Functional Magnetic Resonance Imaging: Advanced Adjuncts in Preoperative Planning for Central Region Tumors

BACKGROUND:

Tumor resection in the vicinity of the motor cortex poses a challenge to all neurosurgeons. For preoperative assessment of eloquent cortical areas, functional magnetic resonance imaging (fMRI) is used, whereas intraoperatively, direct cortical stimulation (DCS) is performed. Navigated transcranial magnetic stimulation (nTMS) is comparable to DCS in activating cortical pyramidal neurons.

OBJECTIVE:

To evaluate the reliability of nTMS compared with fMRI and DCS for preoperative resection planning of centrally located tumors.

METHODS:

In a prospective series, 11 patients (ages, 20-63 years; mean, 41.9 ± 14.9 years, 2 women) with tumors located in or adjacent to the motor cortex were evaluated for surgery. fMRI and nTMS were applied for preoperative assessment of the extent of tumor resection. A 3-dimensional anatomic data set with superimposed fMRI data was integrated in the eXimia Navigated Brain Stimulation station for ensuing motor cortex mapping by nTMS. Responses from nTMS were evaluated by electromyographic response. During surgery, the coordinates of each DCS site were unambiguously defined and integrated into neuronavigation. A post hoc comparison of the coordinates of nTMS, fMRI, and DCS was performed.

RESULTS:

Distances from nTMS to DCS (10.5 ± 5.67 mm) were significantly smaller than those from fMRI to DCS (15.0 ± 7.6 mm).

CONCLUSION:

nTMS anticipates information usually only enabled by DCS and therefore allows surgical planning in eloquent cortex surgery.

Long-term potentiation and long-term depression: a clinical perspective

Long-term potentiation and long-term depression are enduring changes in synaptic strength, induced by specific patterns of synaptic activity, that have received much attention as cellular models of information storage in the central nervous system. Work in a number of brain regions, from the spinal cord to the cerebral cortex, and in many animal species, ranging from invertebrates to humans, has demonstrated a reliable capacity for chemical synapses to undergo lasting changes in efficacy in response to a variety of induction protocols. In addition to their physiological relevance, long-term potentiation and depression may have important clinical applications. A growing insight into the molecular mechanisms underlying these processes, and technological advances in non-invasive manipulation of brain activity, now puts us at the threshold of harnessing long-term potentiation and depression and other forms of synaptic, cellular and circuit plasticity to manipulate synaptic strength in the human nervous system. Drugs may be used to erase or treat pathological synaptic states and non-invasive stimulation devices may be used to artificially induce synaptic plasticity to ameliorate conditions arising from disrupted synaptic drive. These approaches hold promise for the treatment of a variety of neurological conditions, including neuropathic pain, epilepsy, depression, amblyopia, tinnitus and stroke.

Group-level variations in motor representation areas of thenar and anterior tibial muscles: Navigated Transcranial Magnetic Stimulation Study

Navigated transcranial magnetic stimulation (TMS) can be used to stimulate functional cortical areas at precise anatomical location to induce measurable responses. The stimulation has commonly been focused on anatomically predefined motor areas: TMS of that area elicits a measurable muscle response, the motor evoked potential. In clinical pathologies, however, the well-known homunculus somatotopy theory may not be straightforward, and the representation area of the muscle is not fixed. Traditionally, the anatomical locations of TMS stimulations have not been reported at the group level in standard space. This study describes a methodology for group-level analysis by investigating the normal representation areas of thenar and anterior tibial muscle in the primary motor cortex. The optimal representation area for these muscles was mapped in 59 healthy right-handed subjects using navigated TMS. The coordinates of the optimal stimulation sites were then normalized into standard space to determine the representation areas of these muscles at the group-level in healthy subjects. Furthermore, 95% confidence interval ellipsoids were fitted into the optimal stimulation site clusters to define the variation between subjects in optimal stimulation sites. The variation was found to be highest in the anteroposterior direction along the superior margin of the precentral gyrus. These results provide important normative information for clinical studies assessing changes in the functional cortical areas because of plasticity of the brain. Furthermore, it is proposed that the presented methodology to study TMS locations at the group level on standard space will be a suitable tool for research purposes in population studies.

Navigated transcranial magnetic stimulation

Transcranial magnetic stimulation (TMS) is a unique method for non-invasive brain imaging. The fundamental difference between TMS and other available non-invasive brain imaging techniques is that when a physiological response is evoked by stimulation of a cortical area, that specific cortical area is causally related to the response. With other imaging methods, it is only possible to detect and map a brain area that participates in a given task or reaction. TMS has been shown to be clinically accurate and effective in mapping cortical motor areas and applicable to the functional assessment of motor tracts following stroke, for example. Many hundreds of studies have been published indicating that repetitive TMS (rTMS) may also have multiple therapeutic applications. Techniques and protocols for individually targeting and dosing rTMS urgently need to be developed in order to ascertain the accuracy, repeatability and reproducibility required of TMS in clinical applications. We review the basic concepts behind navigated TMS and evaluate the currently accepted physical and physiological factors contributing to the accuracy and reproducibility of navigated TMS. The advantages of navigated TMS over functional MRI in motor cortex mapping are briefly discussed. Illustrative cases utilizing navigated TMS are shown in presurgical mapping of the motor cortex, in therapy for depression, and in the follow-up of recovery from stroke.