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

Transcranial magnetic stimulation and functional magnet resonance imaging evaluation of adductor spasmodic dysphonia during phonation

BACKGROUND

Reduced intracortical inhibition is a neurophysiologic finding in focal dystonia that suggests a broader problem of impaired cortical excitability within the brain. A robust understanding of the neurophysiology in dystonia is essential to elucidate the pathophysiology of the disorder and develop new treatments. The cortical silent period (cSP) is a reliable, non-invasive method to measure intracortical inhibition in the primary motor cortex associated with a muscle of interest. In adductor spasmodic dysphonia (AdSD), cSP of the laryngeal motor cortex (LMC) which directly corresponds to the affected musculature, the thyroarytenoid (TA), has not been examined.

OBJECTIVE

This work evaluated the cSP of the LMC and the relationship between cSP and functional magnetic resonance imaging (fMRI) blood-oxygen-level dependent (BOLD) activation in people with AdSD (n = 12) compared to healthy controls (CTL, n = 14).

RESULTS

Shortened LMC cSP were observed bilaterally in people with AdSD vs CTL (F(1, 99) = 19.5226, p < 0.0001), with a large effect size (η2 = 0.1834). Between-group fMRI analysis revealed greater activation in bilateral LMC in the AdSD > CTL contrast as compared to CTL > AdSD contrast. Correlation analysis showed that people with AdSD have positive correlation of left LMC BOLD activation and the cSP. Further, the right LMC cSP lacks either positive or negative associations with BOLD activation. CTL individuals displayed both positive and negative correlations between cSP and BOLD activation in the left LMC. In CTL, the LMC cSP and BOLD activation showed exclusively negative correlations in both hemispheres.

CONCLUSION

In AdSD, the cortical activation during phonation may not be efficiently or effectively associated with inhibitory processes, leading to muscular dysfunction. These findings may give insight into the maladaptive cortical control during phonation in people with AdSD.

Magnetic Motor Evoked Potential Recording in Horses Using Intramuscular Needle Electrodes and Surface Electrodes

To date, motor evoked potential (MEP) recording in animals is often performed using intramuscular monopolar needle electrodes. Their placement and use has several disadvantages. Adhesive surface electrodes appear to be attractive because they are painless and easy to place. Because these are not used in horses, a scouting study is performed to (1) explore the applicability of surface electrodes in horses (2) determine the repeatability of motor latency times (MLTs) and amplitude measurements, and (3) to investigate if MLTs and amplitude values of surface electrode recordings were similar to intramuscular needle electrode recordings. Transcranial MEP recordings were performed by both coated needle and surface electrodes on ten sedated warmblood horses. Mean MLTs for the thoracic limbs were 20.8 ± 1.5 ms for needle and 21.2 ± 1.4 ms for surface electrode recording and 39.4 ± 3.8 ms and 39.2 ± 3.8 ms for the pelvic limbs, respectively. Mean amplitude values were 8.3 ± 4.1 and 7.2 ± 4.7 mV for the thoracic limbs and 4.2 ± 3.1 and 3.8 ± 2.4 mV for the pelvic limbs, respectively. A good agreement and repeatability for MLTs but insufficient agreement and repeatability for amplitude between both recording types were determined by Bland-Altman plots and Passing-Bablok regression and coefficients of variation calculation. In conclusion, this preliminary study shows that surface electrode recording of MEP is possible and well tolerated in horses. Surface recordings were repeatable and look similar to the intramuscular recordings when regarding MLTs, but overshadowing effects of large test-to-test variations precluded a conclusion concerning amplitude.

Cerebellar Transcranial Direct Current Stimulation Modulates Corticospinal Excitability During Motor Training

Background: Cerebellar activity can be modulated using cerebellar transcranial direct current stimulation (ctDCS) and, when applied concurrently with task training, has been shown to facilitate cognitive and motor performance. However, how ctDCS facilitates motor performance is not fully understood.

Objective/Hypothesis: To assess the electrophysiological and motor performance effects of ctDCS applied during motor training.

Methods: Fourteen healthy adults (age 28.8 ± 10.5 years) were randomly assigned to complete one session of finger tracking training with either simultaneous bilateral anodal or sham ctDCS. Training was completed in two 15 min epochs with a 5-min break (total 30 min stimulation, 2 mA). Tracking accuracy and corticospinal and intracortical excitability were measured immediately before and after the training period. Motor cortical excitability measures included resting motor threshold (RMT), motor evoked potential (MEP) amplitude, cortical silent period (CSP) and short interval intracortical inhibition (SICI).

Results: There was a significant interaction of Group * Time for MEP amplitude and CSP duration (p < 0.01). Post hoc analysis revealed MEP amplitude was increased in the sham group (p < 0.01), indicating increased corticospinal excitability from baseline while the anodal group displayed a decrease in MEP amplitude (p = 0.023) and prolongation of CSP duration (p < 0.01). SICI and RMT remained unchanged following ctDCS and training. Task accuracy was improved in both groups at post-test with a significant effect of Time (p < 0.01); however, there was no effect of Group (p = 0.45) or interaction of Group * Time (p = 0.83). During training, there was a significant effect of Block (p < 0.01) but no significant effect of Group or interaction effect (p > 0.06).

Conclusions: ctDCS applied during task training is capable of modulating or interfering with practice-related changes in corticospinal excitability without disrupting performance improvement.

Shoulder Taping and Neuromuscular Control

CONTEXT

  Scapular taping can offer clinical benefit to some patients with shoulder pain; however, the underlying mechanisms are unclear. Understanding these mechanisms may guide the development of treatment strategies for managing neuromusculoskeletal shoulder conditions.

OBJECTIVE

  To examine the mechanisms underpinning the benefits of scapular taping.

DESIGN

  Descriptive laboratory study.

SETTING

  University laboratory.

PATIENTS OR OTHER PARTICIPANTS

  A total of 15 individuals (8 men, 7 women; age = 31.0 ± 12.4 years, height = 170.9 ± 7.6 cm, mass = 73.8 ± 14.4 kg) with no history of shoulder pain.

INTERVENTION(S)

  Scapular taping.

MAIN OUTCOME MEASURE(S)

  Surface electromyography (EMG) was used to assess the (1) magnitude and onset of contraction of the upper trapezius (UT), lower trapezius (LT), and serratus anterior relative to the contraction of the middle deltoid during active shoulder flexion and abduction and (2) corticomotor excitability (amplitude of motor-evoked potentials from transcranial magnetic stimulation) of these muscles at rest and during isometric abduction. Active shoulder-flexion and shoulder-abduction range of motion were also evaluated. All outcomes were measured before taping, immediately after taping, 24 hours after taping with the original tape on, and 24 hours after taping with the tape removed.

RESULTS

  Onset of contractions occurred earlier immediately after taping than before taping during abduction for the UT (34.18 ± 118.91 milliseconds and 93.95 ± 106.33 milliseconds, respectively, after middle deltoid contraction; P = .02) and during flexion for the LT (110.02 ± 109.83 milliseconds and 5.94 ± 92.35 milliseconds, respectively, before middle deltoid contraction; P = .06). These changes were not maintained 24 hours after taping. Mean motor-evoked potential onset of the middle deltoid was earlier at 24 hours after taping (tape on = 7.20 ± 4.33 milliseconds) than before taping (8.71 ± 5.24 milliseconds, P = .008). We observed no differences in peak root mean square EMG activity or corticomotor excitability of the scapular muscles among any time frames.

CONCLUSIONS

  Scapular taping was associated with the earlier onset of UT and LT contractions during shoulder abduction and flexion, respectively. Altered corticomotor excitability did not underpin earlier EMG onsets of activity after taping in this sample. Our findings suggested that the optimal time to engage in rehabilitative exercises to facilitate onset of trapezius contractions during shoulder movements may be immediately after tape application.

Short Interval Intracortical Inhibition Responses to Low-Frequency Repetitive Transcranial Magnetic Stimulation Under Multiple Interstimulus Intervals and Conditioning Intensities

BACKGROUND

The extent to which short interval intracortical inhibition (SICI) responds to low-frequency repetitive transcranial magnetic stimulation (rTMS) remains inconclusive with reports of increased, decreased and unchanged response following modulation. The aim of this study was to systematically investigate if the variability of SICI following rTMS is explained by the interstimulus interval (ISI) and/or the conditioning stimulus intensity (CSI).

METHODS

Two experiments with pretesting/posttesting and an rTMS session (1 Hz, 90% RMT, 900 pulses) were done. Experiment I (N = 15): SICI with multiple ISIs (1.0-4.0 msec, 0.2 msec increment). Experiment II (N = 15): SICI with CSIs (50-95% of RMT, 5% increment). In both experiments, the cortical silent period (cSP) was also collected.

RESULTS

After low-frequency rTMS, no significant change (p > 0.10) in SICI at any specific ISI or CSI was observed, nor did the optimal ISI or CSI change. However, a significant decrease was observed in SICI responses when assessed under the range of ISIs (p = 0.0001), but not CSIs. cSP inhibition increased significantly (p < 0.0015) for both experiments.

CONCLUSIONS

The optimal ISI or CSI did not shift or reveal SICI changes after inhibitory rTMS. However, when the whole curve of SICI responses were evaluated from a wide range of ISIs, a decrease in inhibition was found. The contrast between the results of individual ISI tests and the wide range of ISI assessment may be due to higher intersubject variability of SICI and/or sample size, rendering traditional SICI testing methods ineffective for measuring changes in inhibition. Further, it is possible that rTMS modulates GABA_A and GABA_B mediated inhibitory processes differently, which would explain the conflicting results for SICI and cSP.