Comparison of figure-of-8 and circular coils for threshold tracking transcranial magnetic stimulation measurements - 17/01/21

Doi : 10.1016/j.neucli.2021.01.001

Søren Ørskov ^a, Hugh Bostock ^b, James Howells ^c, Kirsten Pugdahl ^a, Anders Fuglsang-Frederiksen ^a, Christina Shen-Zhuang Nielsen ^a, Bülent Cengiz ^d, Gintaute Samusyte ^e, Martin Koltzenburg ^b,^f, Hatice Tankisi ^a,⁎
^a Department of Clinical Neurophysiology, Aarhus University Hospital, Aarhus, Denmark
^b Department of Clinical Neurophysiology, National Hospital for Neurology and Neurosurgery, Queen Square, WC1N 3BG, London, United Kingdom
^c Central Clinical School, Faculty of Medicine and Health, University of Sydney, Sydney, Australia
^d Department of Neurology, Gazi University Faculty of Medicine, Beşevler, 06500, Ankara, Turkey
^e Department of Neurology, Medical Academy, Lithuanian University of Health Sciences, Kaunas, Lithuania
^f Sobell Department of Motor Neuroscience and Movement Disorders, UCL Institute of Neurology, Queen Square, WC1N 3BG, London, United Kingdom

^⁎Corresponding author at: Department of Clinical Neurophysiology, Aarhus University Hospital, Palle Juul-Jensens Boulevard no. 165, J209, 8200 Aarhus N, Denmark.Department of Clinical NeurophysiologyAarhus University HospitalPalle Juul-Jensens Boulevard no. 165, J209Aarhus N8200Denmark

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Abstract

Objectives

The transcranial magnetic stimulation (TMS) technique of threshold-tracking short-interval intracortical inhibition (T-SICI) has been proposed as a diagnostic tool for amyotrophic lateral sclerosis (ALS). Most of these studies have used a circular coil, whereas a figure-of-8 coil is usually recommended for paired-pulse TMS measurements. The aim of this study was to compare figure-of-8 and circular coils for T-SICI in the upper limb, with special attention to reproducibility, and the pain or discomfort experienced by the subjects.

Methods

Twenty healthy subjects (aged: 45.5 ± 6.7, mean ± SD, 9 females, 11 males) underwent two examinations with each coil, in morning and afternoon sessions on the same day, with T-SICI measured at interstimulus intervals (ISIs) from 1−7 ms. After each examination the subjects rated degree of pain/discomfort from 0 to 10 using a numerical rating scale (NRS).

Results

Mean T-SICI was higher for the figure-of-8 than for the circular coil at ISI of 2 ms (p < 0.05) but did not differ at other ISIs. Intra-subject variability did not differ between coils, but mean inhibition from 1−3.5 ms was less variable between subjects with the figure-of-8 coil (SD 7.2% vs. 11.2% RMT, p < 0.05), and no such recordings were without inhibition (vs. 6 with the circular coil). The subjects experienced less pain/discomfort with the figure-of-8 coil (mean NRS: 1.9 ± 1.28 vs 2.8 ± 1.60, p < 0.005).

Discussion

The figure-of-8 coil may have better applicability in patients, due to the lower incidence of lack of inhibition in healthy subjects, and the lower experience of pain or discomfort.

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Keywords : Circular coil, Figure-of-8 coil, Short interval intracortical inhibition, SICI, Threshold tracking, TMS, Transcranial magnetic stimulation