Addressing challenges in motor threshold measurement and frequency of assessment in transcranial magnetic stimulation protocols

Dear Editor,

The current use of neuromodulation techniques in psychiatry highlights the complexities involved in determining the motor threshold (MT) in transcranial magnetic stimulation (TMS), which is crucial for tailoring treatments and accounting for individual electrophysiological differences.

MT measurement serves as the primary mechanism to standardize TMS intensity across patients, thereby neutralizing variations in cortical excitability that arise from factors such as age, medication, and anatomical differences. However, the process is fraught with methodological challenges. Traditional approaches rely on either visual observation of muscle twitches or electromyographic (EMG) recordings to determine the minimum stimulus intensity that elicits a response. Studies indicate that visual methods often yield higher MT values compared to EMG, potentially leading to suboptimal or unsafe stimulation levels in certain individuals.[1] These discrepancies underscore the need for greater precision, as inconsistencies can compromise treatment efficacy and safety.

Furthermore, the frequency of MT assessment varies across contexts, ranging from every session in research settings to once per week in clinical practice, reflecting the balance between accuracy and practicality.[2] This periodic reassessment is essential due to documented day-to-day variability in MT, which can result in under- or over-stimulation if based solely on an initial measurement.[3] Studies have shown average variability of more than 5% in almost half the sessions, influenced by factors such as physiological state, medications, and treatment progression. In clinical populations with depression, MT fluctuations have been linked to treatment response, with increases observed in remitters and distinct patterns predictive of nonresponse.[4]

To address measurement difficulties, adaptive algorithms such as parameter estimation by sequential testing (PEST) have been introduced, which streamline MT estimation by reducing the number of required pulses while maintaining accuracy.[5] Free tools implementing PEST are available, offering a non-parametric approach suitable for research and clinical applications (https://www.clinicalresearcher.org/software.htm). Nonetheless, the cost of integrating such software into existing TMS systems, along with questions regarding its broad applicability in resource-constrained environments, remain a barrier.

Identification of MT also needs accurate localization of the motor cortex. Conventionally, manual measurements are conducted for estimating the MT; however, alternative techniques using neuronavigation may enhance localization efficacy, as the accuracy of motor hotspot localization determines MT accuracy.[3] MT change during TMS treatment can be considered as a predictor of response to TMS treatment in depression. It has been reported that the MT follows an increasing trend during the TMS treatment course[4] and probably the steadily increasing trend of MT predicts response to treatment, and a disruption in the steady increment of MT predicts nonresponse. Various electrophysiological, protocol-related, illness-related, and biological markers have been identified for predicting the response to TMS in various psychiatric disorders.[6,7] As various factors influence the variability in MT, clinicians and researchers need to consider those factors to minimize the variability of MT [Figure 1]. Clinicians need to consider these factors,[3,4,8,9] while estimating MT and delivering TMS therapy.

Figure 1.

Addressing challenges in motor threshold measurement

Establishing a link between these parameters and MT may give insight into the predictive value of MT in psychiatric disorders. We advocate further research into these practical solutions to improve TMS accessibility and efficacy in psychiatric care.

Conflicts of interest

There are no conflicts of interest.

Funding Statement

Nil.