Search for a command to run...
We would like to thank Yuchao Wang and colleagues, for their interest in using Magstim systems in their recently published study and acknowledge the authors' diligence in conducting a study on 38 healthy participants [[1]Wang Y. et al.Coils are not created equal: effects on TMS thresholding.Brain Stimulation Journal. 2023; 17: 1-3Google Scholar]. However, the authors claimed that the Magstim system showed significant within-subject resting motor threshold (MT) differences between assessment and treatment equipment by comparing threshold values obtained using the Magstim monophasic device (Bistim) with those acquired using the Magstim biphasic device (Rapid), without fully recognizing the inherent differences in pulse waveform. These findings present an opportunity to delve deeper into the consideration of pulse waveform. In early 2001, Kammer et al. utilized one Magstim 70 mm figure-of-eight coil for all measurements, assessing the MT using Magstim monophasic and biphasic devices. Their findings revealed significant main effects of waveform (F(1,7) = 221.8, P < 0.0001) [[2]Kammer T. et al.Motor thresholds in humans: a transcranial magnetic stimulation study comparing different pulse waveforms, current directions and stimulator types.Clin Neurophysiol. 2001; 112: 250-258Crossref PubMed Scopus (335) Google Scholar]. In a separate study, Sommer et al. assessed the MT values for both monophasic and biphasic pulses in the context of their 5 Hz repetitive transcranial magnetic stimulation protocols [[3]Sommer M. et al.Opposite optimal current flow directions for induction of neuroplasticity and excitation threshold in the human motor cortex.Brain Stimul. 2013; 6: 363-370Abstract Full Text Full Text PDF PubMed Scopus (46) Google Scholar]. Employing Magstim stimulators and one figure-eight coil, they observed that, during the baseline measurement and prior to applying the protocol, the MT was dependent on the pulse type (P < 0.0001, effect of type, ANOVA). Danner et al. employed MRI-navigated TMS to investigate and compare the MT in 50 healthy participants using both monophasic and biphasic stimulations with one figure-of-eight coil [[4]Danner N. et al.Effect of individual anatomy on resting motor threshold – computed electric field as a measure of cortical excitability.J Neurosci Methods. 2012; 203: 298-304Crossref PubMed Scopus (0) Google Scholar]. The measured values of resting MT were found to be significantly different for biphasic stimulation compared to monophasic stimulation (p < 0.001). Their conclusion strongly underscores the substantial influence of pulse form on the MT values, even when employing a consistent coil for both monophasic and biphasic MT measurements. The title of their paper, 'Coils are not created equal: Effects on TMS thresholding,' might be open to misinterpretation since it inadequately acknowledges the substantial variances in pulse shapes inherent to monophasic and biphasic devices. Based on TMS literature, as explained, the disparate motor threshold values observed in their study predominantly arise from variations in pulse shapes rather than distinctions in coils sharing similar geometry. Furthermore, the authors' assumption that TMS operators determine the motor threshold using one type of stimulator and subsequently apply that intensity (or any portion thereof) to the treatment protocol using a different stimulator type (‘coils and associated stimulator setup are switched within the same session.’) could benefit from further alignment with established principles in TMS. This approach is not substantiated by Magstim's Instructions for use and deviates from commonly accepted TMS guidelines/practices. Generally, when a biphasic device is used to determine the motor threshold, the same device should be employed to apply the treatment protocol, and the same holds true for a monophasic device. Within the same stimulator type, switching between similar coils for determining the motor threshold and applying the treatment protocol is possible. Lighter coils without active cooling facilitate easier movement over the patient's head, while actively cooled coils with appropriate coil holders deliver a sustainable treatment protocol. The difference in motor thresholds for coils with similar geometry and the same stimulator type remains within an acceptable range, irrespective of the presence or absence of active cooling for the coil. In the validation process of paired MT assessment and treatment coils at Magstim, we conducted point-by-point measurements of the induced electric field (E-field) values using a custom-made calibrated robotic device. The apparatus employed for this comparison is designed to move the 3D pickup coil in three dimensions with a precision of 0.1 mm. Our observations yielded no statistically significant differences in the induced E-field profiles between cooled and non-cooled paired coils, nor in the pulse widths. The authors' statements asserting that the 'Magstim system showed significant within-subject rMT differences between assessment and treatment equipment' and 'Therefore, applying rTMS interventions based on rMTs determined using assessment coils may lead to inaccurate dosing (i.e., underdosing)' are particularly susceptible to misinterpretation. This is attributed to their utilization of the Magstim Bistim monophasic stimulator for cortical excitability assessment and the Magstim Rapid biphasic stimulator for treatment, rather than differences in the coils used. In 2005, John Rothwell and his colleagues pioneered the establishment of the method for determining the motor threshold—a widely accepted procedure for the procedural steps in TMS treatment [[5]Huang Y. et al.Theta burst stimulation of the human motor cortex.Neuron. 2005; 45: 201-206Abstract Full Text Full Text PDF PubMed Scopus (2923) Google Scholar]. Their report included the separate determination of motor thresholds for both monophasic and biphasic pulses, with stimulation intensity defined individually for each Magstim machine, distinguishing between monophasic and biphasic devices. Also, none of the commonly accepted TMS guidelines and procedures [[6]Rossi S. et al.Safety, ethical considerations, and application guidelines for the use of transcranial magnetic stimulation in clinical practice and research.Clin Neurophysiol. 2009; 120: 2008-2039Crossref PubMed Scopus (3946) Google Scholar,[7]Rossi S. et al.Safety and recommendations for TMS use in healthy subjects and patient populations, with updates on training, ethical and regulatory issues: expert Guidelines.Clin Neurophysiol. 2021; 132: 269-306Crossref PubMed Scopus (517) Google Scholar], such as those outlined by IFCN [[8]Fried P.J. et al.Training in the practice of noninvasive brain stimulation: recommendations from an IFCN committee.Clin Neurophysiol. 2021; 132: 819-837Crossref PubMed Scopus (32) Google Scholar,[9]Rossini P. et al.Non-invasive electrical and magnetic stimulation of the brain, spinal cord, roots and peripheral nerves: basic principles and procedures for routine clinical and research application. An updated report from an I.F.C.N. Committee.Clin Neurophysiol. 2015; 126: 1071-1107Crossref PubMed Scopus (1787) Google Scholar], specify the categorization of monophasic devices for MT assessment and biphasic devices for treatment, contrary to the suppositions made by Wang et al. in their study [[1]Wang Y. et al.Coils are not created equal: effects on TMS thresholding.Brain Stimulation Journal. 2023; 17: 1-3Google Scholar]. Therefore it is essential to note that the same stimulator type should be consistently employed for both the MT assessment and treatment phases. In conclusion, the variations in motor thresholds result from using different pulse shapes with various stimulators, emphasizing the influence of pulse waveform on MT values rather than differences in coils with similar geometry. Magstim strategically designs each device to excel in specific applications, whether for single or paired pulses using monophasic pulses, or for rTMS and theta burst using biphasic pulses. In all magnetic devices, Magstim pairs assessment and treatment coils to ensure equivalent outputs, eliminating the possibility of insufficient or varying treatment dosage. Majid Memarian Sorkhabi: Writing – review & editing, Writing – original draft, Conceptualization. John Leedham: Writing – review & editing, Supervision. Majid Memarian Sorkhabi and John Leedham are employees of Magstim Co. Ltd. Together towards more accessible, standardized TMS protocols: Reply to Sorkhabi and LeedhamBrain Stimulation: Basic, Translational, and Clinical Research in NeuromodulationVol. 17Issue 2PreviewWe would like to thank Sorkhabi and Leedham for their careful consideration of our recent transcranial magnetic stimulation (TMS) study [1] and the Brain Stimulation Editors for this opportunity to clarify our findings further. Our study was motivated by a notable lack of clarity in many repetitive TMS (rTMS) papers regarding the issue of re-thresholding between non-cooled (typically used for hotspot search and motor thresholding, i.e., “assessment”) and cooled coils (typically for rTMS or theta-burst stimulation, i.e., “treatment”). Full-Text PDF Open Access