Abstract
Background: The integrity of the motor system can be examined by applying navigated transcranial magnetic stimulation (nTMS) to the cortex. The corresponding motor-evoked potentials (MEPs) in the target muscles are mirroring the status of the human motor system, far beyond corticospinal integrity. Commonly used time domain features of MEPs (e.g., peak-to-peak amplitudes and onset latencies) exert a high inter-subject and intra-subject variability. Frequency domain analysis might help to resolve or quantify disease-related MEP changes, e.g., in brain tumor patients. The aim of the present study was to describe the time-frequency representation of MEPs in brain tumor patients, its relation to clinical and imaging findings, and the differences to healthy subject.Methods: This prospective study compared 12 healthy subjects with 12 consecutive brain tumor patients (with and without a paresis) applying nTMS mapping. Resulting MEPs were evaluated in the time series domain (i.e., amplitudes and latencies). After transformation into the frequency domain using a Morlet wavelet approach, event-related spectral perturbation (ERSP), and inter-trial coherence (ITC) were calculated and compared to diffusion tensor imaging (DTI) results.Results: There were no significant differences in the time series characteristics between groups. MEPs were projecting to a frequency band between 30 and 300 Hz with a local maximum around 100 Hz for both healthy subjects and patients. However, there was ERSP reduction for higher frequencies (>100 Hz) in patients in contrast to healthy subjects. This deceleration was mirrored in an increase of the inter-peak MEP latencies. Patients with a paresis showed an additional disturbance in ITC in these frequencies. There was no correlation between the CST integrity (as measured by DTI) and the MEP parameters.Conclusion: Time-frequency analysis may provide additional information above and beyond classical MEP time domain features and the status of the corticospinal system in brain tumor patients. This first evaluation indicates that brain tumors might affect cortical physiology and the responsiveness of the cortex to TMS resulting in a temporal dispersion of the corticospinal transmission.
Highlights
The human motor system consists of several cortical, subcortical, and spinal hubs
The aim of the present study is to describe the timefrequency representation of MEPs in brain tumor patients, its Abbreviations: ADC, apparent diffusion coefficient; Amp, amplitude; APB, abductor pollicis brevis muscle; AR, autoregression model; DTI, diffusion tensor imaging; Electromyographic Recordings (EMG), electromyography; ERSP, event-related spectral perturbation; EP, evoked potentials; FA, fractional anisotropy; first dorsal interosseous muscles (FDI), first dorsal interosseous muscle; ITC, inter-trial coherence; Lat, latency; MEP, motor-evoked potential; Medical Research Council Scale (MRCS), medical research council scale; nTMS, navigated transcranial magnetic stimulation; RMT, resting motor threshold; ROI, region of interest
Healthy subjects and patients differ in their power and ITC values, there were no significant differences in the standard time series values of MEPs
Summary
The human motor system consists of several cortical, subcortical, and spinal hubs. For unobstructed voluntary movements corticospinal integrity is required. The magnetic cortical input through TMS is suggested to activate excitatory and inhibitory neurons, transmitting their information in volleys (i.e., D- and I-waves) to the spinal cord and resulting in a synchronized activation of muscle cells, which can be measured as motor-evoked potentials (MEPs) [2,3,4]. The corresponding motor-evoked potentials (MEPs) in the target muscles are mirroring the status of the human motor system, far beyond corticospinal integrity. Frequency domain analysis might help to resolve or quantify disease-related MEP changes, e.g., in brain tumor patients. The aim of the present study was to describe the time-frequency representation of MEPs in brain tumor patients, its relation to clinical and imaging findings, and the differences to healthy subject
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