Comparison of Muscle MEPs From Transcranial Magnetic and Electrical Stimulation and Appearance of Reflexes in Horses

Abstract

Introduction 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 p < 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 TMS – -MLT TES ), and −7.7 and −9.9 m/s (CV TMS -CV TES ). 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. Conclusion 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.

Trapezius Motor Evoked Potentials From Transcranial Electrical Stimulation and Transcranial Magnetic Stimulation: Reference Data, Characteristic Differences and Intradural Motor Velocities in Horses

“…The study protocol was approved by the animal ethics committee of the University of Groningen, the Netherlands and registered as DEC6440A and DEC6440B for the study of reference data of TES and TMS. Results of the TES and TMS studies for the TC and ECR MEPs were previously published ( Journée et al, 2018 , 2020a ).…”

“…After reaching the transcranial electrical motor thresholds (ET) the stimulation was continued to ET + 50V. The transcranial stimulation threshold was defined at stimulation intensity (V for TES and % of maximum output for TMS) at the first occurrence of the early muscle MEP after the latency jump from the late to the early MEP ( Journée et al, 2020a ).…”

“…TMS was performed twice at each step. When TMS motor thresholds (MT) were reached, which was detected at the stepwise latency reduction at the transition from extracranial to intracranial MEPs ( Journée et al, 2020a ), stimulation was continued to MT + 50%.…”

“…An attractive choice for this purpose, is the trapezius muscle. Trapezius MEPs are conducted via C2-C4 cervical nerve roots and are recorded with a similar convenience as MEPs from the TC and ECR muscles by means of surface or subcutaneous needle electrodes ( Journée et al, 2020a , b ). The electrodes can be placed at a sufficient distance from the TMS coil to avoid interference by magnetic pulses of varying strength leading to possible saturation and blockage of connected amplifiers.…”

“…Up until now, these data are unknown in horses. Although TES and TMS are interchangeable techniques for assessment of ECR and TC muscle MEPs, MEP latencies do show differences of at least one synaptic delay ( Amassian et al, 1989 ; Edgley et al, 1990 ; Burke et al, 1993 ; Ubags et al, 1999 ; Journée et al, 2020a ). However, it is not known whether both techniques are interchangeable for assessment of trapezius muscle MEPs.…”