Abstract: Transcranial electrical (TES) and magnetic stimulation (TMS) are both used for assessment of the motor function of the spinal cord in horses. Muscular motor evoked potentials (mMEP) were compared intra-individually for both techniques in five healthy horses. mMEPs were measured twice at increasing stimulation intensity steps over the extensor carpi radialis (ECR), tibialis cranialis (TC), and caninus muscles. Significance was set at < 0.05. To support the hypothesis that both techniques induce extracranially elicited mMEPs, literature was also reviewed. Results: Both techniques show the presence of late mMEPs below the transcranial threshold appearing as extracranially elicited startle responses. The occurrence of these late mMEPs is especially important for interpretation of TMS tracings when coil misalignment can have an additional influence. Mean transcranial motor latency times (MLT; synaptic delays included) and conduction velocities (CV) of the ECR and TC were significantly different between both techniques: respectively, 4.2 and 5.5 ms (MLT -MLT ), and -7.7 and -9.9 m/s (CV -CV ). TMS and TES show intensity-dependent latency decreases of, respectively, -2.6 (ECR) and -2.7 ms (TC)/30% magnetic intensity and -2.6 (ECR) and -3.2 (TC) ms/30V. When compared to TMS, TES shows the lowest coefficients of variation and highest reproducibility and accuracy for MLTs. This is ascribed to the fact that TES activates a lower number of cascaded interneurons, allows for multipulse stimulation, has an absence of coil repositioning errors, and has less sensitivity for varying degrees of background muscle tonus. Real axonal conduction times and conduction velocities are most closely approximated by TES. Conclusions: Both intracranial and extracranial mMEPs inevitably carry characteristics of brainstem reflexes. To avoid false interpretations, transcranial mMEPs can be identified by a stepwise latency shortening of 15-20 ms when exceeding the transcranial motor threshold at increasing stimulation intensities. A ring block around the vertex is advised to reduce interference by extracranial mMEPs. mMEPs reflect the functional integrity of the route along the brainstem nuclei, extrapyramidal motor tracts, propriospinal neurons, and motoneurons. The corticospinal tract appears subordinate in horses. TMS and TES are interchangeable for assessing the functional integrity of motor functions of the spinal cord. However, TES reveals significantly shorter MLTs, higher conduction velocities, and better reproducibility.
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The research article titled “Comparison of Muscle MEPs From Transcranial Magnetic and Electrical Stimulation and Appearance of Reflexes in Horses” explores the effectiveness of two techniques, Transcranial electrical stimulation (TES) and Transcranial magnetic stimulation (TMS), in assessing the motor function of the spinal cord in horses. The study concludes that while both methods are effective, TES provides significantly shorter muscle motor latency times, higher conduction velocities, and better reproducibility.
Study Overview
The study compared muscular motor evoked potentials (mMEPs), a method used to measure motor function, for both TES and TMS in five healthy horses. mMEPs were measured twice in increasing stimulation intensity over the extensor carpi radialis (ECR), tibialis cranialis (TC), and caninus muscles.
In order to support the hypothesis that both these techniques induce mMEPs extracranially, a literature review was also conducted.
Findings
The results showed that both TES and TMS techniques showed the presence of late mMEPs below the transcranial threshold, appearing as extracranially induced startle responses. The occurrence of these late mMEPs is particularly crucial when interpreting TMS tracings as coil misalignment can influence this.
The mean transcranial motor latency times (MLT) and conduction velocities (CV) of the ECR and TC were found to be significantly different between TMS and TES. TES showed shorter MLTs, higher CV, and greater reproducibility and accuracy.
Conclusion
The researchers concluded that both Intra-cranial and Extra-cranial mMEPs carry characteristics of brainstem reflexes, implying similar activation paths in both TMS and TES.
To prevent incorrect interpretations, the study suggests identifying transcranial mMEPs by a stepwise latency shortening of 15-20 ms when exceeding the transcranial motor threshold at increasing stimulation intensities. This strategy would reduce interference caused by extra-cranial mMEPs.
The researchers learned that mMEPs reflect the functional integrity of the route along the brainstem nuclei, extra-pyramidal motor tracts, proprio-spinal neurons, and moto-neurons. They also found that the corticospinal tract appears to be subordinate in horses.
Although TES and TMS are interchangeable for the assessment of the functional integrity of motor functions of the spinal cord, TES was found to be superior due to significantly shorter MLTs, higher conduction velocities, and better reproducibility.
Cite This Article
APA
Journée SL, Journée HL, Berends HI, Reed SM, de Bruijn CM, Delesalle CJG.
(2020).
Comparison of Muscle MEPs From Transcranial Magnetic and Electrical Stimulation and Appearance of Reflexes in Horses.
Front Neurosci, 14, 570372.
https://doi.org/10.3389/fnins.2020.570372
Department of Virology, Parasitology and Immunology, Research Group of Comparative Physiology, Faculty of Veterinary Medicine, Ghent University, Merelbeke, Belgium.
Journée, Henricus Louis
Department of Neurosurgery, University Medical Center Groningen, University of Groningen, Groningen, Netherlands.
Department of Orthopedics, University Medical Center Utrecht, Utrecht University, Utrecht, Netherlands.
Department of Orthopedics, University Medical Center Amsterdam, Amsterdam, Netherlands.
Berends, Hanneke Irene
Department of Orthopedics, University Medical Center Amsterdam, Amsterdam, Netherlands.
Reed, Steven Michael
Rood & Riddle Equine Hospital, Lexington, KY, United States.
M.H. Gluck Equine Research Center, Department of Veterinary Science, University of Kentucky, Lexington KY, United States.
de Bruijn, Cornelis Marinus
Wolvega Equine Clinic, Oldeholtpade, Netherlands.
Delesalle, Cathérine John Ghislaine
Department of Virology, Parasitology and Immunology, Research Group of Comparative Physiology, Faculty of Veterinary Medicine, Ghent University, Merelbeke, Belgium.
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