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 points 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 point 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.

Feasibility of Interferential and Pulsed Transcranial Electrical Stimulation for Neuromodulation at the Human Scale

“…First, IS‐tDCS may have tolerable amplitudes above 2 mA. No adverse effects were reported for IS‐tDCS at 4.5 mA or less in its feasibility study (7), but it is unknown whether increasing or decreasing the duration of the pulsatile constituent fields alters the tolerability of IS‐tDCS (38). Second, IS‐tDCS mitigates amplifier saturation, facilitating simultaneous recordings during stimulation (7).…”

Safety and recommendations for TMS use in healthy subjects and patient populations, with updates on training, ethical and regulatory issues: Expert Guidelines

“…There is no evidence that pulse-width differences on this order affect the risk for seizure when the stimulation intensity is adjusted relative to the RMT. There is some evidence from single-pulse studies that the pulse width may affect scalp sensation, but the effect is small and it is unclear how it translates to rTMS ( Peterchev et al, 2017 ).…”

Modular multilevel TMS device with wide output range and ultrabrief pulse capability for sound reduction

“…More advanced devices allow some adjustment of the pulse duration and shape [8][9][10][11]. Such devices have enabled important findings regarding the effect of pulse shape and duration on neural activation thresholds [12], differential neural recruitment in the brain [13][14][15][16][17][18], lasting neuromodulatory effects [19,20], as well as the sensation of scalp stimulation [21]. However, these devices still have a restricted range of the shape (e.g., only sinusoidal or only rectangular), duration (e.g., lacking very brief and very long pulses), and amplitude (e.g., insufficient amplitude for suprathreshold brief or complex pulses) [5,7,22].…”