Development and evaluation of a portable sham transcranial magnetic stimulation system

Abstract

Background: Transcranial magnetic stimulation (TMS) is a relatively noninvasive brain stimulation technology that can focally stimulate the human cortex. One significant limitation of much of the TMS research to date concerns the nature of the placebo or sham conditions used. When TMS pulses are delivered repetitively (especially prefrontal TMS), it is often experienced as painful. Most sham TMS techniques produce identical sounds to active TMS, but they do not cause much, if any, scalp or facial sensation or discomfort. This is a serious problem when investigators are attempting to evaluate the effects of TMS by using traditional sham techniques because of unintended systematic differences between real and sham TMS groups (ie, confounds). As long as traditional approaches to sham TMS are used, the validity of the inferences regarding the efficacy of TMS will be limited. Although some other sophisticated systems have been developed to address these concerns, they tend to be expensive and lack portability. Portability will likely become more and more important as TMS applications expand into different clinical areas (eg, TMS in the postanesthesia care unit after surgery).

Methods: This study describes a portable electrical TMS sham system (eSham system) modeled after the James Long System that was designed to produce similar scalp sensations as real TMS. Preliminary results are presented on 9 healthy adults who received both real and eSham 10 Hz repetitive TMS (rTMS) (at 80%, 100%, and 120% of resting motor threshold) over the prefrontal cortex and rated the sensation quality (pain, tingling, sharpness, piercing, electric, tugging, pinching), tolerability, and location.

Results: Real TMS and eSham TMS were rated similarly across all seven sensory dimensions examined. Real and eSham TMS were also rated similarly with respect to tolerability and perceived location of the TMS-induced sensations.

Conclusions: The eSham system may be a simple, affordable, and portable approach to providing convincing sham TMS for future clinical trials. This study provides preliminary evidence supporting the use of the eSham system. Future larger-scale studies are warranted.

Figure 1

Diagram of the eSham system. A constant electrical stimulus (a) is delivered to a specially designed switch box (b) where it is gated and prevented from continuing-on to the surface electrodes attached to the participant. Whenever a TTL pulse is (c) is delivered to the switch box (i.e., when the TMS machine fires), the gate is opened for ~250 μs allowing a very brief electrical pulse through to the participant’s scalp in sync with the audible TMS sham-coil pulse..

Figure 2

Photo of the eSham system attached to a TMS machine equipped with a sham TMS coil.

Figure 3

Screen shot of the software used to collect sensation ratings and to assess the areas of the face and scalp where the sensations were felt.

Figure 4

Mean (and 95% CI) visual analogue scale ratings for each of the sensory dimensions assessed during real and eSham TMS.

Figure 5

Mean face and scalp areas of activation during both real and eSham TMS