Determination of motor threshold using visual observation overestimates transcranial magnetic stimulation dosage: Safety implications

Abstract

Objective While the standard has been to define motor threshold (MT) using EMG to measure motor cortex response to transcranial magnetic stimulation (TMS), another method of determining MT using visual observation of muscle twitch (OM-MT) has emerged in clinical and research use. We compared these two methods for determining MT. Methods Left motor cortex MTs were found in 20 healthy subjects. Employing the commonly-used relative frequency procedure and beginning from a clearly suprathreshold intensity, two raters used motor evoked potentials and finger movements respectively to determine EMG-MT and OM-MT. Results OM-MT was 11.3% higher than EMG-MT (p<0.001), ranging from 0-27.8%. In eight subjects, OM-MT was more than 10% higher than EMG-MT, with two greater than 25%. Conclusions These findings suggest using OM yields significantly higher MTs than EMG, and may lead to unsafe TMS in some individuals. In more than half of the subjects in the present study, use of their OM-MT for typical rTMS treatment of depression would have resulted in stimulation beyond safety limits. Significance For applications that involve stimulation near established safety limits and in the presence of factors that could elevate risk such as concomitant medications, EMG-MT is advisable, given that safety guidelines for TMS parameters were based on EMG-MT.

Cerebellar repetitive transcranial magnetic stimulation (rTMS) for essential tremor: A double-blind, sham-controlled, crossover, add-on clinical trial

“…However, this can also be considered as a strength of this study regarding the fact that this setting might better reflect a real clinical practice situation, along with add-on nature of our study. Moreover, RMT measurement was based on visually perceptible muscle contraction which seems to be prone to over-estimation especially in elderly patients with joint problems [44]. Measuring tremor with more objective tools, such as accelerometer, could possibly allow detecting smaller changes in tremor severity.…”

Understanding tDCS effects in schizophrenia: a systematic review of clinical data and an integrated computation modeling analysis

“…Nonetheless, tDCS could theoretically have some advantages over rTMS such as lower cost -rTMS devices are more expensive to purchase and maintain than tDCS devices [27]; ease of use -rTMS requires more training for use than tDCS as tDCS can be applied by technicians whereas rTMS can only be applied by trained physicians and, further, optimal rTMS results usually require neuroimaging guidance [28] and electromyography devices for motor threshold determination [29]; portability -tDCS devices are portable and could be potentially used in primary care and even home use and safety and tolerability -direct tDCS effects are mild and well-tolerated [30], whereas direct rTMS effects cause facial twitching that can be unpleasant; in addition, rTMS can rarely induce seizures [31], whereas this severe adverse effect was not ever described for tDCS.…”

Intermittent theta-burst stimulation for upper-limb dysfunction and spasticity in spinal cord injury: a single-blind randomized feasibility study

“…This trial also lacked neurophysiological assessments of spasticity which can provide a more objective outcome [20]. Determining RMT by visual observation of a twitch is reliable [21], however, it can overestimate the threshold when compared to electromyographic determination [22]. Furthermore, the effects of iTBS on motor-evoked potentials (MEPs) in healthy participants shows variability and further research is required to determine whether this correlates with motor behavior [12,23].…”