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

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.

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.

OM-MT was 11.3% higher than EMG-MT (p<0.001), ranging from 0% to 27.8%. In eight subjects, OM-MT was more than 10% higher than EMG-MT, with two greater than 25%.

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.

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.