Pulse Width Affects Scalp Sensation of Transcranial Magnetic Stimulation

Abstract

Background: Scalp sensation and pain comprise the most common side effect of transcranial magnetic stimulation (TMS), which can reduce tolerability and complicate experimental blinding.

Objective: We explored whether changing the width of single TMS pulses affects the quality and tolerability of the resultant somatic sensation.

Methods: Using a controllable pulse parameter TMS device with a figure-8 coil, single monophasic magnetic pulses inducing electric field with initial phase width of 30, 60, and 120 µs were delivered in 23 healthy volunteers. Resting motor threshold of the right first dorsal interosseus was determined for each pulse width, as reported previously. Subsequently, pulses were delivered over the left dorsolateral prefrontal cortex at each of the three pulse widths at two amplitudes (100% and 120% of the pulse-width-specific motor threshold), with 20 repetitions per condition delivered in random order. After each pulse, subjects rated 0-to-10 visual analog scales for Discomfort, Sharpness, and Strength of the sensation.

Results: Briefer TMS pulses with amplitude normalized to the motor threshold were perceived as slightly more uncomfortable than longer pulses (with an average 0.89 point increase on the Discomfort scale for pulse width of 30 µs compared to 120 µs). The sensation of the briefer pulses was felt to be substantially sharper (2.95 points increase for 30 µs compared to 120 µs pulse width), but not stronger than longer pulses. As expected, higher amplitude pulses increased the perceived discomfort and strength, and, to a lesser degree the perceived sharpness.

Conclusions: Our findings contradict a previously published hypothesis that briefer TMS pulses are more tolerable. We discovered that the opposite is true, which merits further study as a means of enhancing tolerability in the context of repetitive TMS.

Keywords: Pulse width; Scalp; Sensation; Tolerability; Transcranial magnetic stimulation; cTMS.

Figure 1

TMS electric field waveforms for pulse widths of 30, 60, and 120 µs. The waveforms were measured with a search coil placed under the TMS coil [20,28]. The pulse amplitude was scaled by the average motor threshold for the respective pulse width in order to illustrate the relative pulse amplitude delivered in the three pulse width conditions.

Figure 2

Visual analog scale ratings of Discomfort (A), Sharpness (B), and Strength (C) for single-pulse TMS of left dorsolateral prefrontal cortex with pulse widths of 30, 60, and 120 µs and Low and High pulse amplitudes (nominally 100% and 120% of motor threshold, respectively). Markers and error bars correspond to mean and standard error across subjects, respectively. Discomfort and Sharpness were rated by all 23 subjects, and Strength was rated by 10 of these subjects. Significant differences between the pulse width conditions are indicated by ** (p < 0.0001) or * (p < 0.01). Effect of pulse amplitude is significant (p < 0.0001) for all scales (not marked in plots).

Figure 3

Individual visual analog scale ratings versus individual absolute TMS pulse amplitude. Columns correspond to Low (left) and High (right) pulse amplitude conditions (nominally 100% and 120% of motor threshold, respectively). Markers and error bars correspond to individual means and standard errors, respectively. Different marker shapes and colors correspond to the three pulse width conditions indicated in the legend in units of µs. The x-axis is individual TMS amplitude given as z-score within each of the six experimental conditions. Linear regression lines and corresponding correlation coefficients are given for the data within each plot, with significant results in bold.