Personalized TMS helmets for quick and reliable TMS administration outside of a laboratory setting

Abstract

•We built a custom TMS helmet to conduct TMS in non-laboratory environment.•This helmet consistently and reliably administers TMS to a desired brain target.•These helmets were used to administer TMS in parabolic flight.•Helmets like these can be used in other extreme, mobile, or at-home environments. Transcranial magnetic stimulation (TMS) administration currently necessitates trained operators and equipment to hold the coil in place over specific brain targets [[1]Nauczyciel C. Hellier P. Morandi X. Blestel S. Drapier D. Ferre J.C. et al.Assessment of standard coil positioning in transcranial magnetic stimulation in depression.Psychiatry Res. 2011; 186: 232-238Crossref PubMed Scopus (32) Google Scholar], limiting its application in ambulatory, hospital or home settings. We have developed custom, individually-tailored TMS helmets that allow one to administer TMS in diverse settings where trained TMS operators are unavailable or the experiment or treatment involves movement such as ambulatory, extreme environments, or potentially at home. Here we overview how we constructed helmets that fix a TMS coil into position over a participant’s motor hotspot (Fig. 1a and b) and describe its stepwise fabrication with video accompaniment (Supplemental Video). We also present within- and between-visit reliability resting motor thresholds (rMT) while wearing the helmets. Each helmet required: casting fiberglass (BSN Medical Delta-LITE Cast Tape), 3D printed components (custom fabricated), Epoxy adhesive (J-B Weld MinuteWeld), Velcro strap (18in), and chin strap (Pyramex elastic chin strap with chin cup). The estimated cost of these materials is $50. We describe our construction below. Wearing a cloth liner to protect hair, cover the top of the head with fiberglass and use the remaining roll to wrap the head of the participant, creating a base cast of the head. Hold downward pressure until the helmet cures. Mark the ears and midline of the helmet on the participant to ensure proper future positioning. Remove the cast and liner from the head and cut arches for the ears. Wear helmet and deliver supra-threshold TMS pulses to the motor cortex of the participant to determine the optimal “motor hotspot” (scalp target that elicits largest motor activation in contralateral thumb). Mark the front edge of the TMS coil and the coil “hole” position on the cap. Cut a window out of the helmet positioned under the center of the TMS coil to minimize the thickness of the helmet at the point of stimulation. 3D-print anchors to attach to the helmet that allow the attachment of a TMS coil. Our system is for use with a Magstim Fig. 8 coil. These anchors ensure the TMS coil sits precisely over the motor hotspot. Attach Velcro strap to anchor and strengthen helmet with additional fiberglass. Attach elastic chin strap to the helmet (1 cm anterior of ear). The TMS coil accessory is a plastic brick and provides a pressure point for the strap. This allows for increased downward force on the TMS coil while it is in the helmet and is critical to a “tight fit.” Final fitting determines whether modification is needed (sanding, expanding the TMS stimulation window, readjusting straps, etc). Note that we do visually observed thumb twitch as a demonstration, however the described experiment uses EMG as detailed below. We fabricated 10 of these custom TMS helmets (individually tailored to the heads of each participant). We then conducted a 2-visit study exploring the use of these TMS helmets to collect five resting motor thresholds (rMT) per visit using a closed-loop TMS system. Importantly, the TMS coils were simply placed into the helmet and were not held or guided by any administrator. Following written informed consent, TMS motor thresholds were collected using the closed-loop TMS system, utilizing the Magstim BiStim TMS system (70mm coil) connected to the Cambridge Electronics EMG system (CED 1401, 1902). This validated system uses electromyography (EMG) to measure the amplitude of the TMS motor evoked potentials in the contralateral abductor pollicis brevis (APB) to determine whether a thumb twitch occurred (>150μV) and is comparable to visually observed determination [[2]Badran B.W. Ly M. DeVries W.H. Glusman C.E. Willis A. Pridmore S. et al.Are EMG and visual observation comparable in determining resting motor threshold? A reexamination after twenty years.Brain Stimul. 2018; Google Scholar,[3]Pridmore S. Fernandes Filho J.A. Nahas Z. Liberatos C. George M.S. Motor threshold in transcranial magnetic stimulation: a comparison of a neurophysiological method and a visualization of movement method.J ECT. 1998; 14: 25-27Crossref PubMed Google Scholar]. This information is real-time assessed in Spike 2 software and triggers the subsequent TMS stimulus intensity to administer based on the parametric estimation via sequential testing (PEST) paradigm [[2]Badran B.W. Ly M. DeVries W.H. Glusman C.E. Willis A. Pridmore S. et al.Are EMG and visual observation comparable in determining resting motor threshold? A reexamination after twenty years.Brain Stimul. 2018; Google Scholar,[4]Mishory A. Molnar C. Koola J. Li X. Kozel F.A. Myrick H. et al.The maximum-likelihood strategy for determining transcranial magnetic stimulation motor threshold, using parameter estimation by sequential testing is faster than conventional methods with similar precision.J ECT. 2004; 20: 160-165Crossref PubMed Scopus (90) Google Scholar]. A repeated measures ANOVA was conducted to determine the stability of the motor threshold within visit for the group using Prism 8 (GraphPad, USA). Within- and between-visit intraclass correlation coefficients (ICC) were calculated to determine individual test-retest reliability and consistency of the five collected motor thresholds ((IBM SPSS 25, SPSS Inc). One participant had only four rMTs collected during their second visit due to coil overheating (subject 5 visit 2). This missing data point was interpolated from her 4 prior rMTs collected that visit. Helmet-facilitated rMTs were not significantly different over the five within-visit rMT attempts (Fig. 1c and d). Individual reliability of the within-visit reliability of five consecutive rMT attempts as measured by ICC is high (alpha, 95%CI) [(day 1 = 0.968, 0.925–0.991); (day 2 = 0. 979,0.949–0.994)], confirming consistent individual helmet rMT (p < 0.001) (Fig. 1e and f). Furthermore, helmets demonstrate high individual reliability between-visits (alpha, 95%CI) 0.956, 0.833–0.989. Custom, individually-fitted helmets are feasible, reliable and can help administer TMS in variable gravity or ambulatory, at-home, and other non-laboratory settings.

Topics & Concepts