Comparison of figure-of-8 and circular coils for threshold tracking transcranial magnetic stimulation measurements

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.

Introduction

Transcranial magnetic stimulation (TMS) is the practice of generating a magnetic field with which an intracortical current is induced [15]. Among other methods, TMS is used to examine cortical excitability [15]. This can be done by activating desired parts of the motor cortex while performing peripheral measurements of the resulting compound muscle action potentials, in this case called motor evoked potentials (MEPs) [7]. Utilizing paired pulses, the activation of the motor cortex can be facilitated or inhibited by a sub-threshold conditioning stimulus given before the test stimulus. The test response is typically inhibited when the interstimulus interval (ISI) is between 1 and 5 ms with the highest inhibition at 1 ms and 3 ms, whereas the response is facilitated for ISIs between 7 and 20 ms - these phenomena are called short-interval intracortical inhibition (SICI) and intracortical facilitation, respectively [4], [8], [12], [15].

Patients with amyotrophic lateral sclerosis (ALS) elicit decreased SICI as demonstrated by both conventional paired pulse TMS [19] and the more recent method of threshold-tracking TMS (TT-TMS) [9], [16], [17], [18]. Advantages have been claimed for TT-TMS, such as higher reproducibility, shorter examination time and smaller required sample size for interventional studies [13]. In TT-TMS, one first determines the resting motor threshold (RMT) as the intensity of a single pulse stimulus that delivers a fixed MEP amplitude [15], [18]. Then, paired-pulse stimuli are applied with a conditioning stimulus set to a fixed percentage of the RMT, while the test stimulus is varied in order to obtain the fixed MEP amplitude [1], [5], [15]. The required changes in test stimulus intensity with ISI are recorded as being inhibition or facilitation, expressed as percentages of RMT.

The figure-of-8 coil has the advantage of producing a more focal stimulus that requires a lower stimulus intensity [2], [17], but most studies proposing TT-TMS as a potential diagnostic and prognostic biomarker for ALS have used a circular coil [6], [10], [11]. Only one study has compared upper limb SICI and short-interval intracortical facilitation (SICF) between circular and figure-of-8 coils, and reported stronger SICI with circular, but stronger SICF with the figure-of-8 coil [14]. That study did not compare reproducibility or degree of pain and discomfort. Another study on lower limb SICI found little difference between circular and figure-of-8 coils, but lower RMTs and greater intracortical facilitation with a double cone coil [3]. The aim of the present study was to make a direct comparison between the circular and figure-of-8 coils in TT-TMS reproducibility and SICI-parameters from the abductor pollicis brevis (APB) muscle. Additionally, the study aimed to compare the degree of pain and discomfort experienced by the two coil types.