Corticospinal excitability measurements using transcranial magnetic stimulation are valid with intramuscular electromyography

Abstract

Objectives: Muscular targets that are deep or inaccessible to surface electromyography (sEMG) require intrinsic recording using fine-wire electromyography (fEMG). It is unknown if fEMG validly record cortically evoked muscle responses compared to sEMG. The purpose of this investigation was to establish the validity and agreement of fEMG compared to sEMG to quantify typical transcranial magnetic stimulation (TMS) measures pre and post repetitive TMS (rTMS). The hypotheses were that fEMG would demonstrate excellent validity and agreement compared with sEMG.

Materials and methods: In ten healthy volunteers, paired pulse and cortical silent period (CSP) TMS measures were collected before and after 1200 pulses of 1Hz rTMS to the motor cortex. Data were simultaneously recorded with sEMG and fEMG in the first dorsal interosseous. Concurrent validity (r and rho) and agreement (Tukey mean-difference) were calculated.

Results: fEMG quantified corticospinal excitability with good to excellent validity compared to sEMG data at both pretest (r = 0.77-0.97) and posttest (r = 0.83-0.92). Pairwise comparisons indicated no difference between sEMG and fEMG for all outcomes; however, Tukey mean-difference plots display increased variance and questionable agreement for paired pulse outcomes. CSP displayed the highest estimates of validity and agreement. Paired pulse MEP responses recorded with fEMG displayed reduced validity, agreement and less sensitivity to changes in MEP amplitude compared to sEMG. Change scores following rTMS were not significantly different between sEMG and fEMG.

Conclusion: fEMG electrodes are a valid means to measure CSP and paired pulse MEP responses. CSP displays the highest validity estimates, while caution is warranted when assessing paired pulse responses with fEMG. Corticospinal excitability and neuromodulatory aftereffects from rTMS may be assessed using fEMG.

Conflict of interest statement

Competing Interests: The authors have declared that no competing interests exist.

Figures

Fig 1. Electrode and ground placement.

sEMG was placed in a belly-tendon montage while fEMG was inserted near the active surface electrode. Both sets of electrodes were pre-amplified with two custom made amplifiers. The ground was wrapped around the wrist. sEMG = surface electromyography; fEMG = fine wire electromyography.

Fig 2. Simultaneous surface and fine-wire EMG trace.

Example of a single pulse motor evoked potential response in a single subject.

Fig 3. The relationship between sEMG and fEMG.

Pretest data (top row) and posttest (bottom row). Data from 10 subjects, 10 trials/session/person, total of 100 data point in each plot. Note, the boxes on SICI graphs: fEMG responses cluster near 0 (y-distribution) while surface EMG response is more varied (x-distribution). Each color represents a single subject.

Fig 4. Agreement between sEMG and fEMG.

Pretest data (top row) and posttest data (bottom row). Data from 10 subjects, 10 trials/session/person, total of 100 data point in each plot, 95% CI of limits of agreement (wide grey lines) and 95% CI of the mean (black lines).